## Posts Tagged ‘GPM’

### Pressure Loss in Sprinkler Risers

Saturday, September 8th, 2012

Q.  How do I calculate sprinkler risers losses in a sprinkler zone where the risers are extra long, 3 ft or more above ground?  I have 10 risers in a zone for my proposed sprinkler irrigation system.

A.  If you are using my Sprinkler System Design Tutorial and a standard riser of the recommended size, then you don’t need to worry about pressure lose in the riser, the tutorial has friction loss for the risers built-in to the formulas it uses.  So you can ignore the riser pressure loss.  Some standard risers are shown on the page on Sprinkler Risers in the Irrigation Installation Tutorial.  The recommended size for a riser?  In most cases it should be the same size as the threaded inlet on the sprinkler.  But please actually read that page on risers, as there are some exceptions to that rule for certain types of standard risers!

### Non-Standard Risers:

When adding the riser friction loss into the total friction loss calculations for your whole sprinkler system, just add in the loss for a single riser.  Use the friction loss value for the riser that has the highest friction loss.  (This is most likely the one with the highest GPM sprinkler, or it may be the longest riser if you have different riser lengths.  You may have to calculate the friction loss for several different risers to figure out which of them has the highest loss.)  Why do you add in the friction loss for only one sprinkler, rather than the combined loss for all of them?  Because as a single drop of  water goes through the sprinkler system it only goes through one sprinkler, not all of the sprinklers.  You have to think about the water as a collection of millions of drops, not as one solid body.  So the pressure loss is what a single drop would experience as it travels through the system.  As a drop of water enters the sprinkler system it travels through a water meter, lots of pipe, a valve or two, then it finally blows out through a single sprinkler onto  the landscape.  The pressure loss  calculation for the whole sprinkler system is determined by what the worst case pressure loss values would be for a single drop of water traveling through the sprinkler system.

OK, so you calculated the friction loss, but what if it is a really high value, or maybe the calculator complained about the velocity being to high.  In this case you need to use a larger size pipe for your riser.  For the velocity in a riser you can go all the way up to the 7 ft/sec maximum without too much risk.  Velocities in the marginal “use caution” zone are generally OK for risers.  High velocity in a riser will seldom cause a water hammer problem, unless you are using a special type of sprinkler that has a solenoid valve built in to it.  Those sprinklers are called “valve-in-head sprinklers”, they are very expensive, and are mostly used for golf course greens.

### Sprinkler Coverage, Nozzle Selection, & Sprinkler Spacings

Thursday, January 12th, 2012

## Sprinkler Coverage:

The area watered by each sprinkler must overlap substantially the area watered by the adjacent sprinkler. This overlap may seem like a waste at first, but it is a very important necessity. Without this overlap it would be impossible to design sprinkler systems that provided uniform water coverage.

Have Doubts?  See for yourself, it only takes a couple of minutes to prove! Grab a piece of paper and draw circles on it so that all areas of the paper are inside a circle, but no circles overlap. You can’t do it, can you?

Important!
Sprinklers are intentionally designed to require 100% overlap of watered areas. That means each sprinkler throws water ALL the way to the next sprinkler in each direction. READ THAT AGAIN!

That’s right, 100% overlap of watered areas is REQUIRED or you will get dry spots! This is known in the industry as “head-to-head coverage or head-to-head spacing”.  A lot of those free design guides you find in stores and on the Internet get this wrong.  They don’t show enough overlap!  The writers of those brochures think you are going to look at the overlap and buy the brand of sprinkler that shows the least sprinkler heads.  So they try to make it look like you can use less sprinklers with their brand.  After you’ve bought the sprinklers if you have dry spots, well hey, it’s YOUR problem now!  You’ll probably just buy a few more of their sprinklers to get rid of the dry spots. In fact, it will probably take more sprinklers to fix the dry spots than it would have to do it right the first time. \$\$\$ Ching, ching!

### Rule: Sprinkler Radius = distance between sprinklers

One more time: The water from any single sprinkler should actually get the sprinklers on each side of it wet!

(Optional reading for those who need explanations.) Back when I designed my first sprinkler system in High School I wondered why they wanted so much overlap of the sprinklers. It seemed to me to be nothing more than a ploy to sell more sprinkler heads! I was smarter than that, so I stretched them out to save my folks some money! The result was big dry spots, and my parents wound up replacing the sprinkler system a few years later. (They never said anything about it to me, I just noticed the new sprinklers a few years later on a visit home from college.) Ouch! Not a good start for a future irrigation expert! Now that I’m a bit wiser and more knowledgeable I realize there is a good reason behind the head-to-head coverage. Unfortunately, it’s rather hard to explain. The perfect sprinkler would put out a pattern of water that is heaviest right next to the sprinkler, then uniformly declines out to the radius. So the farther you move away from the sprinkler, the less water falls on any given patch of ground. When we test sprinklers for water coverage we set up a series of cups between the sprinklers to collect the water that falls. That way we can see how much water falls at various distances from the sprinkler. In the diagram below you can see what happens when there are various distances between the sprinklers.

Close to 100% sprinkler overlap is important for good water application uniformity

In example “A” the sprinklers are just barely overlapping and much more water is falling in the cups next to the sprinkler heads. But the middle 3 cups are only getting ½ the water of the cups next to the sprinkler. If you watered long enough to keep the middle green, the areas around the sprinklers would turn to mud! In example “B” we see that moving the sprinklers closer together has evened up the amount of water a bit more. However the areas near the heads are still getting 25% more water than the other areas. Not enough to cause mud, but you would definitely see rings of greener grass around the sprinklers! Example “C” shows almost head-to-head spacing. The cups are almost all uniformly full! So don’t stretch the distance between sprinklers.

What if you need a smaller radius than the sprinklers available?

Almost all sprinklers have a radius adjustment device on them so that you can reduce the radius of the water throw. This is one way you can adjust for narrower areas. Keep in mind that for most sprinklers you can’t reduce the radius by more than 50% without causing problems. The other solution for smaller areas is to use nozzles made to spray less far, or that spray a special pattern. An example of a special pattern would be the nozzles that spray a 4′ x 30′ rectangular pattern. These are commonly used in long, narrow areas.

Remember if you reduce the radius of the sprinkler you must reduce the distance between sprinklers by the same distance! Keep the coverage head-to-head!
Calculating the GPM for sprinklers when you reduce the radius is easy:

Warning for rotors only:
When designing systems with rotors do NOT rely on the manufacturer’s stated radius for design. They get those distances by testing the rotors inside a building with no wind. The real world is harsher! If the gallonage of the rotor is less than 6 GPM the maximum spacing should never be more than 35′ between rotor type sprinklers.

Stryker’s Rule: the spacing in feet between rotors can never exceed the operating pressure in PSI at the sprinkler inlet (So a rotor with a 30 PSI operating pressure = 30 foot maximum spacing between rotors.  Yes, I know the package says you can space them farther apart.)

Ignore the rule above and you will be very sorry!

## Sprinkler Precipitation Rate and GPM

The precipitation rate is the amount of water the sprinkler throws onto the area it waters, measured in inches per hour. (Inches per hour is how deep, in inches, the water would be after one hour if it didn’t soak into the ground or run-off.) Precipitation rate must be considered when selecting your sprinkler heads to eliminate water application uniformity problems (dry spots).

Spray Heads: Almost all sprinkler manufacturers make their spray heads so that you can mix and match nozzle patterns and the precipitation rates will still match for all the heads. This is referred to as “matched precipitation rates”. Look for this feature when selecting your sprinklers. Important: do not mix different brands of spray heads and nozzles together on the same valve circuit without checking to see that they have the same performance specifications. Just because the nozzle will screw into the sprinkler body doesn’t mean it’s designed to work with that sprinkler!

Rotors: Rotor-type heads aren’t quite as easy. You must select the appropriate nozzle size for each rotor in order to match the precipitation rates. A simple illustration will help explain. Rotor heads move back and forth across the area to be watered. The rotation speed is the same regardless of whether the rotor is adjusted to water a 1/4 circle or a full circle. So the stream from a 1/4 circle head will pass over the same area 4 times in the same amount of time that it takes for a full circle head to make one pass over the area it waters. With the same size nozzle in both, a 1/4 circle rotor will put down 4 times as much water on the area under the pattern as a full circle rotor will. (Remember that after every quarter turn the 1/4 circle rotor reverses direction and covers the same area again!) To match the precipitation rates between these sprinklers, the quarter circle rotor must have a nozzle that puts out 1/4 the amount of water that the full circle nozzle puts out! A half circle rotor must have a nozzle that puts out 1/2 the water of a full circle. This is why when you buy a rotor-type sprinkler head they often include a handful of different size nozzles with it. Wait, there’s more (don’t panic yet, there is a simple solution forthcoming)!

If you have rotors that are adjusted for different radii you will need to adjust the nozzle size to compensate for the radius change also! For example if most of the rotors are set for a 30 foot radius, but one is adjusted down to 20 ft., the 20 ft. one will need a nozzle 1/2 the size. (Remember: when you reduce the RADIUS by 1/3 you reduce the AREA by a little more than half.)

Avoid using rotors with nozzle flows that are less than 2.5 GPM, except in corners (quarter circle patterns). Flows under 2.5 GPM give very poor coverage due to the tiny water stream. Even a slight breeze will distort the watering pattern and give you dry spots. I strongly suggest that you stick to using nozzles as close as possible to the GPM of those in the cheat chart below.

O.K. Now that you understand the principles, let’s simplify this a bit by using a cheat chart…

Jess Stryker’s

### Quick & Dirty Guide for Rotor Nozzle Selection

1. Find the section of the chart with your desired spacing.
2. Find the pattern (1/2, full circle,etc.) of the sprinkler.
3. The chart tells you the GPM the nozzle must have.
4. Use a nozzle size that comes close to matching both the PSI – GPM combination.
5. Ignore the radius given by the manufacturer.
6. Be sure to read the notes below the chart!

For 20-29′ spacing between sprinklers-
1/4 circle . . . 30 PSI – 0.8 GPM
1/2 circle . . . 30 PSI – 1.6 GPM
3/4 circle . . . 30 PSI – 2.4 GPM
full circle . . 30 PSI at 3.2 GPM
Important: see notes below!

For 30-39′ spacing between sprinklers-
1/4 circle . . . 40 PSI – 1.5 GPM
1/2 circle . . . 40 PSI – 3.0 GPM
3/4 circle . . . 40 PSI – 4.5 GPM
full circle . . 40 PSI – 6.0 GPM

For 40-55′ spacing between sprinklers-
1/4 circle . . .55 PSI – 3.0 GPM
1/2 circle . . . 55 PSI – 5.5 GPM
3/4 circle . . . 55 PSI – 8.0 GPM
full circle . . 55 PSI – 11.0 GPM

Important Notes:

It is critical that the water pressure (PSI) at the sprinkler be as high, or higher, than the distance between the sprinklers in feet (per Stryker’s Rule). For example, if you space the sprinklers 45′ apart, you must have at least 45 PSI of pressure at the sprinkler inlet. That’s the pressure at the sprinkler inlet, not the total pressure available. Remember, you will lose pressure in the pipes and valves, so the pressure at the sprinkler inlet will be lower than your available pressure! Go back to the tutorial pressure loss pages to figure out how much pressure will be lost in your sprinkler system.

Select the nozzle size closest to these GPMs without regard to the radius the manufacturer gives. For example, if you are looking at a 25′ radius, the chart above says to use a 1.6 GPM nozzle for a half-circle rotor. But you happen to notice that the rotor manufacturer’s literature says that at 25 PSI, a 1.6 GPM nozzle has a radius of 32 feet. So why am I telling you to space it at 25′? When the manufacturer tested the rotor on their test range (inside a large building with no wind) they measured a few drops of water 32′ from the rotor. When you install it out in your yard it will not perform as well. You may still get a few drops of water 30′ or even 32′ from the head, but not enough to grow anything. You need to trust me on this one! Remember, if the sprinkler sprays too far, most rotors have a radius reduction screw that will allow you to very easily reduce the radius. But, if the rotor does not spray far enough there is nothing you can do about it without a major expense! Best to play it safe.

You may want to make additional adjustments to nozzle sizes after installation to compensate for your specific conditions. Most rotors now come with a “nozzle tree” that contains most of the different nozzles for the rotor, so you can change the nozzle sizes if you need to. Some manufacturer’s don’t offer nozzles sizes larger than 3.0 GPM for their economy-priced heads (providing those extra nozzles would probably cost them at least another nickel in costs!). You may need to upgrade to the next better model line if you have a large yard! The larger size nozzles for 40′ spacing are not available with most of the “mini-rotor” models sold for residential use. You will need to upgrade to the next model. Also, sometimes other nozzle sizes are available separately from the manufacturer, for example low angle nozzles. You will probably need to get these from a store that specializes in irrigation sales, rather than a hardware or home store. Look in the yellow pages under “Irrigation” or “Sprinklers”, or try one of the online stores listed in the tutorial links pages.

There is a conflict between the nozzles recommended for the 20-29′ spacing range of the chart and my previous advice to “avoid using rotors with nozzle flows that are less than 2.5 GPM”. This is because the Nozzle Selection Guide assumes you will be mixing 20-29′ radius rotors together on the same valve with 30′ plus radius rotors. To keep from having enormous nozzles on the larger radius rotors I am recommending that you use smaller nozzles than I would otherwise consider for the smaller radius rotors. This is essentially a compromise. Sometimes it is not practical to obtain perfection! If all or a majority of your rotors will be spaced at 20-29′ apart, then you should probably use larger nozzles than I recommend in the chart. In other words, use those listed in the chart for 30-39′ spacing for the 20-29′ spacing. This will help avoid problems caused by the wind blowing the spray out of the irrigated area. However, if your sprinkler system will be located in an area with little or no wind you can go ahead and use the smaller nozzles in the chart. What is little or no wind? Go outside in the evening or early morning when you will likely be irrigating. If you can feel the wind blowing even gently against your face, I would consider that enough wind to need the larger nozzles.

If you calculate the precipitation rates you will notice that the shorter spacings result in a higher precipitation rate than the larger spacings. This is because the smaller heads with lower GPM rates are more susceptible to wind and evaporation, and thus it is assumed less of the water is actually reaching the ground. The higher precipitation rate compensates for this.

## Windy Locations

If you are designing a sprinkler system for an area where the wind blows a lot you should look at the Irrigation and Wind FAQ.

If you haven’t started shopping for sprinklers yet, now’s the time to start checking out what’s available.  Check out which sprinklers are available and look them over.  Write down a list of the heads you think will work well for your irrigation system on your Design Data Form. Be sure to list the PSI and GPM for each head as given in the manufacturer’s literature, along with the maximum spacing between heads.

### One last warning!!!

Do not blow-off my advice on sprinkler spacing in order to save a few bucks on sprinkler heads! Right now you may be feeling pretty smug about how much money you saved by stretching the sprinkler spacing. But next summer you’re going to look pretty stupid to the neighbors, standing out there with a hose watering the yellow spots your new sprinklers don’t cover!  I have a collection of “wish I’d listened to you” letters from people who didn’t take this advice. I get a few more of these every year, and these are just the brave folks willing to confess they messed up. They all say you should listen to me on this!

Later on you will need to know the flow rate for each sprinkler you use, so it might be helpful to make some notes on the back of your Design Data Form showing the nozzle size and GPM you will need for each different sprinkler you plan to use. Otherwise you’ll wind up having to look the information up over, and over, and over…

You’re now ready to pencil in the sprinkler head locations on your drawing. Hallelujah! I know it seems like it took a long time to get here, but to do a good job we needed to cover a lot of background information! Use a pencil to draw in the sprinkler heads so you can easily make adjustments to the locations later. Many people find it helpful to use a compass to draw a light pencil line showing the radius of water throw for each head.

Remember these tips:

• Keep the distance as uniform as possible between heads. To the extent possible a sprinkler should be equal distance from the adjacent sprinkler in each direction (forming a triangle if possible). Changes in spacing between adjacent sprinklers should be made as a gradual transition when possible.
• Try to position heads so that if you were to draw a straight line between adjacent heads they would form an equilateral triangle (each side of triangle is same length). This is called “triangular spacing” and creates more even water coverage than “square spacing” (ie; lines between 4 heads form a square). That said, you will often be unable to form a triangle so don’t panic if you can’t.
• Don’t stretch the spacings, use “head to head” spacing. Using too many sprinkler heads is seldom a problem, using too few sprinklers heads is ALWAYS a disaster!
• Start by drawing a sprinkler in each corner. Next, draw sprinklers around the perimeter of the irrigated area, watching that they are not too far apart (one more time, better too many than too few!). Adjust the locations to make the spacing between sprinklers as even as possible. After the perimeters are done, then draw the sprinklers in the interior area.
• If the sprinklers need to overlap so that the spray from one head goes over and beyond the next that’s OK. While you don’t want to over-water, it is always easier to correct an over watered area than a under watered one. For example, you can use the radius adjustments on the sprinklers to cut down the water in the over-irrigated areas. If need be you can even remove or relocate a sprinkler later. It’s much easier to remove one than to add one!
• Sprinklers that are placed closer than 6 feet apart need some special consideration. Standard spray-type sprinklers don’t work well if the radius is adjusted below 6 feet. (The opening the water goes through is so tiny that the normal expansion of the plastic or metal on a warm evening can close off the water flow!) If the area is long and narrow (4′ wide or less), use the strip pattern nozzles. I prefer the so called “side-strip” type that you place along the edge of the area, they have better patterns than the center strip nozzles. End-strip nozzles have notoriously bad patterns, they shouldn’t be more than 10′ from the next head! When using standard spray sprinklers (like quarter, half, and full circles) in areas where the radius must be adjusted to less than 6 feet use a “pressure compensating device” to reduce the radius. The pressure compensating device is normally installed under the nozzle where it reduces the flow and pressure through the nozzle. The good news is that by using a under sized pressure compensating device you can also reduce the nozzle radius! Unlike the adjustment screw on the nozzle these devices work well regardless of the temperature. However, you will need to select the proper size pressure compensating device for each nozzle. It is possible that every nozzle will need a different size! To select the right device you use a special chart provided by the pressure compensating device’s manufacturer. The chart will tell you exactly which device you must use with each different nozzle in order to get the radius you want. Most major sprinkler manufacturer’s make pressure compensating devices for their spray sprinklers, and the charts you need can be found in their catalogs. You may need to go to a commercial sprinkler supplier to find them.

Study the example drawing below.

Again, notice that the radius of each sprinkler’s spray goes all the way to the next sprinkler! This is critical.

Note that in the example above only the lawn area outlined with a green curving edge is being watered. The area between the lawn (green line) and the edge of the property (brown line) would most likely be planted with shrubs and irrigated separately from the lawn. In most cases a drip system would be considered for watering the shrubs as it is less expensive and more efficient. See the separate guidelines for designing drip irrigation systems.

Bonus landscape design tip: Creating a border of shrubs around the perimeter of your yard is, in most cases, a good landscape design practice. A shrub border helps to reduce the visual impact of the fence (assuming that like most residential properties you have a fence.) Shrubs also typically use less than half the water of lawn areas of the same size, saving money spent for water. Once a month you need to weed and trim shrub areas, as opposed to the lawn that needs to be mowed every other week at the least in summer. A border using shrubs of various sizes, textures and colors can add greatly to the attractiveness of your yard. Place smaller shrubs near the lawn, with larger growing varieties behind them next to the fence.

### Sprinkler Layout for Narrow Planters:

Sample sprinkler head layout for narrow planters using strip nozzles

This example shows the typical placement for sprinkler heads in a narrow planter. In this example, special spray sprinkler nozzles called “end-strips” and “side-strips” are used. These nozzles spray a long, but narrow, pattern. A typical pattern is 4′ x 30′ (4′ out and 15′ in either direction from the head). There are also spray nozzles called “center-strips” which don’t work as well. Be careful when using end-strips. They tend to have a weak coverage area on either side of the nozzle (the yellow area in the drawing above). Avoid using 2 end-strips facing each other in a lawn area. If possible always install a side-strip in the middle between 2 end-strips. The sprinkler layout above is for lawn. In a shrub area you can eliminate the sprinklers on one side as long as the width of the planter is 4 feet or less- so you can install the sprinklers on one side only. Shrubs don’t need as even a watering pattern. Lawns require heads on both sides. Note the triangular arrangement of the sprinklers, which gives more even coverage. Yes, it takes an extra head to create the triangle pattern, and you need to space the heads a little closer together than the normal maximum on one side to create the “triangle pattern”, but it’s worth the cost.

For narrow strips wider than 5′ you would use regular half circle heads on both sides. The distance between the sprinkler heads should not be more than 1 foot greater than the width of the planter. In other words, if the planter is 8 feet wide you would install half circle heads on both sides of the planter, not more than 9 feet apart from each other. As with the example above, it is best if you arrange the sprinklers in a triangular pattern.

## Sprinkler GPM

As we saw previously, the flow rate in gallons per minute (GPM) of each sprinkler head is determined by the nozzle installed in the head. It is necessary to know the GPM for each head in order to determine which heads will be connected to each valve and in order to determine the size of each pipe in the sprinkler system.

You will probably need to dig up the sprinkler manufacturer’s literature again. In the literature the manufacturer shows different GPM and radius information for each sprinkler nozzle based on the operating pressure (PSI). Now we can use that information to find the GPM for each sprinkler head. First, determine what the SPACING is between each head and the others around it. Next, look for the radius closest to that spacing and use the corresponding GPM as the flow for the head.

Write down on your plan the GPM for each sprinkler next to the sprinkler symbol.

Hint: You will find the GPM and radius data for many of the popular sprinklers in the product reviews .

Example: You note that a spray type head on your plan is a 1/2 circle pattern and the distance to the 3 closest adjacent heads are 13 feet, 12 ft., and 14 ft.. So the spacing for this head is 14 ft. (the highest of the 3). Looking at the manufacturer’s literature you note that a radius of 14 ft. for the 1/2 circle nozzle in this sprinkler requires a pressure of 25 PSI and a flow of 1.65 GPM. Write down the flow of 1.65 GPM next to the sprinkler head on your drawing. You then repeat this procedure for each sprinkler head on your drawing.

Write the GPM of each sprinkler on the plan next to the sprinkler

### Irrigation Lateral Sprinkler Pipe Size

Monday, November 28th, 2011

### Step #5 of the Landscape Sprinkler System Design Tutorial

Friction Loss:
As water moves through a pipe it loses pressure due to a phenomenon commonly called “friction loss”. Much of this loss is caused by turbulence, but we call it friction loss for simplicity. The amount of friction loss is determined by the type of pipe, the diameter of the pipe, the amount of water flowing through the pipe, and the length of the pipe. A complex formula (called the Williams/Hazen Formula) predicts the amount of pressure that will be lost due to friction loss.  The water also loses pressure each time it passes through a valve, a backflow preventer, or anything else it encounters on it’s way to the sprinkler head. Even a bend in the pipe causes pressure loss!  Don’t panic over the formula, we’ll use a pipe sizing chart or a friction loss calculator!

You need lots of pressure at those sprinkler heads!
The sprinkler head needs a minimum amount of water pressure to work properly. The manufacturer’s performance charts tell you how much pressure is required to achieve a specific radius for the water.  As the pressure increases so does the flow (GPM) and the radius of the throw. So in order to assure that there is enough pressure to make the sprinklers operate as they should, we need to calculate the pressure losses between the water source and the sprinkler head.  If the pressure loss is found to be too great, then we must reduce.  The easiest way to do that is to use a larger size pipe.

## DETERMINING THE SPRINKLER PIPE SIZE

There are several methods used to determine pipe sizes of sprinkler system lateral pipes.  I’m going to explain two methods.  One method is faster but less accurate, the other is very accurate but takes more time.

### Chart Method:

Pros: The fastest and easiest method.  Requires a single, simple calculation and uses a chart to determine the sizes.
Cons:  The learning curve to use it is a bit more difficult to understand.  It uses an averaging system to arrive at pipe sizes.
See step-by-step tutorial for the Chart Method

### Trial & Error:

Pros:  Very accurate, calculates the pressure loss in each pipe section using a spreadsheet.  Easier to understand.
Cons:  Time consuming, need to enter data into the spreadsheet, uses trial and error to establish pipe sizes.
See step-by-step tutorial for the Trial & Error method

Which method should you use?  For a beginner with a small irrigation system probably the Trial & Error system will be easier.  Below are overviews of each method for experienced designers to use.  Unless you are experienced you should probably read the full tutorial for the method you select.

## Overview: CHART METHOD FOR LATERAL IRRIGATION PIPE SIZING

See Determining Sprinkler Pipe Size Using a Pipe Sizing Chart for detailed step-by-step instructions.

### Calculate the PSI/100 value:

( ____ PSI x 100) / ____ Feet Total Length = PSI/100

____ PSI.  Insert the maximum PSI loss for the valve circuit laterals into the formula where it says “____PSI.”

____ Feet.  Insert the distance from the valve to the farthest sprinkler on the valve circuit  in the space labeled “____ Feet Total Length” in the formula.

Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.

### The Pipe Size Table or Chart:

Sprinkler Pipe Sizing Chart for Laterals
PSI/100 = Desired PSI Loss in Lateral x 100 / Total length of Lateral

 PSI/100 0.2 0.5 0.8 1.0 1.5 2.0 3.0 4.0 5.0 6.0 SIZE 2.2 3.3 4.4 5.0 6.2 7.1 8.5 10 11 13 ¾” 3.8 6.3 8.1 9.2 11 13 17 20 22 24 1″ 7.1 12 15 18 22 25 31 36 37 37 1¼” 11 16 22 24 31 35 44 48 49 49 1½ 18 30 40 44 57 65 76 76 76 76 2″ 28 46 60 67 83 96 114 114 114 114 2½” 46 75 100 112 140 162 165 170 170 170 3″ 87 140 185 208 250 280 280 280 280 280 4″ 255 410 540 600 600 600 600 600 600 600 6″

Flows shown red are over 5 feet/second. Use caution!

Instructions:

1. Find your PSI/100 value in the top blue row.
2. Read down the column to the value equal to, or higher than, the GPM in the pipe section.
3. Read across to the pipe size for that section in the right column.
4. Repeat steps 2 & 3 for next pipe section.

This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%. This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.

Notes about the Pipe Sizing Chart:

• Warning:  The sprinkler pipe sizing chart is based on using Cl 200 PVC pipe.   It also works for Class 125 (not recommended) and Class 160 (hard to find).
• Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40″) for the laterals you need to make an adjustment before using the chart. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.
• Polyethylene, Polybutylene: After you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc.
• Where’s the 1/2″ pipe?  See “why not 1/2″?”

CONFUSING?  DON’T PANIC:   For detailed instructions see the page Irrigation Pipe Sizing Chart for Laterals.

## Overview: TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE

See Calculating Sprinkler System Pipe Size Using a Spreadsheet for detailed step-by-step instructions.

This method involves trying various pipe sizes until a good combination is found.  A spreadsheet does the calculations.

You will need a spreadsheet friction loss calculator:  Friction Loss Calculator Spreadsheets

Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.

Use the spreadsheet friction loss calculator to determine the pressure loss:

2. There is a line on the spreadsheet for each section of pipe.
3. Start with the pipe section after the control valve and work out to the farthest sprinkler.
4. Select 3/4″ pipe for the pipe or tube size. (See “why not 1/2″?”)
5. Enter the GPM for the section of pipe.
6. Enter the length of the section of pipe.
7. Use an error factor of 1.1
8. Go to the next line down and repeat steps 4-7 for the next pipe section.
9. The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
10. At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
11. Change the pipe size if the velocity or total pressure loss is too high.

You must calculate the pressure loss for each of the possible water paths in the valve circuit.

Here is an example of the possible water paths for a valve circuit, shown in red, blue, and magenta.

Start with the water route that is the longest.  In this case that would be the red route.   There are 9 pipe sections in this route labeled 1-9.  Enter the data from this route into the calculator.  Use a larger pipe size if the velocity is not safe.   Check that the friction loss “Total of All Sections” does not exceed your maximum allowable amount.

Write the pipe size for each section on your plan.
Now repeat the process for the blue water route and then the magenta color route.

Confused? For detailed instructions on using the spreadsheets see What Size Pipe for Sprinkler System Laterals?

### Calculating Sprinkler System Pipe Size Using a Spreadsheet

Saturday, November 26th, 2011

### THE GROUND RULES

First a tip that just may save your behind!

### When in doubt, always use a larger diameter pipe!

It will not harm anything to use a larger pipe size.  Period.  If you are uncertain whether to use a 3/4″ or 1″ pipe, then you should use the 1″.   Using a larger size pipe is ALWAYS the safest choice.

No, I don’t own stock in an irrigation pipe manufacturer and I’m not getting kickbacks for pushing bigger pipe! Unlike clothing, pipe can never be “too large”. Contrary to what might appear to be true, forcing water into a smaller pipe REDUCES the water pressure, and hurts sprinkler performance. This is because the smaller pipe creates more pressure loss due to friction and turbulence as the water flows through it. It’s another of those hard to grasp hydraulic principles! Just remember that when it comes to pipe, bigger is better! I’m always amazed at how many irrigation equipment sales people don’t know this most basic of irrigation rules. I’ve had clients tell me they were told to use a smaller pipe to keep the pressure up by tech support people at some of the major sprinkler manufacturer’s. That’s an industry disgrace!

So one more time to drill it into your head– You don’t decrease the pipe size to keep the pressure up- or down for that matter. That is totally, completely, wrong. The reason we use smaller pipe is to save money. Which of course, is a good reason!  For those who want more specifics on this, there is a very boring scientific explanation at the bottom of this page.

Is it Pipe or Tube?  For the most part I use the term “pipe” rather than “tube” on this page and elsewhere.  Bad habit of mine (note that by reading carefully, you have found one of my faults!)  The difference is the material they are made from.  Steel and PVC plastic are generally called pipe.  Polyethylene, PEX, and copper are usually referred to as tube.  I often screw up and call tube pipe!

## TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE USING A SPREADSHEET

This method involves trying various pipe sizes until a good combination is found.

Definitions you need to know:

Lateral pipe: all the pipes between the control valve and the sprinkler heads.

Mainline: The pipe that goes from the water source to the control valves.

Control Valve: The valve that turns on and off a group of sprinklers. Most often it is an electric valve operated by a timer.

Valve circuit: a single valve, and all the pipe, fittings and sprinkler heads downstream from it. In other words, all the sprinkler heads that start working when you turn on the valve are part of the same valve circuit.

GPM:  Gallons per minute, a measure of water flow rate.  Use primarily in the United States.

PSI:  Pounds per square inch, a measure of water pressure.  Use primarily in the United States.

You will need a spreadsheet Friction Loss Calculator.
Here’s a page with calculators for almost every type of pipe: Friction Loss Calculator Spreadsheets
Grab the appropriate spreadsheet for the type pipe you plan to use.

Now you just enter the appropriate data for each section of pipe into the calculator and then read the total pressure loss at the bottom of the spreadsheet. If the pressure loss is too high, then try making one of the lengths of pipe larger. The calculator will also give you the water velocity in each section of pipe, and warn you if the velocity is too high.

### Detailed Instructions:

The best way to teach this is probably to walk you through a couple of examples.

If I may make a suggestion, download the spreadsheet for Cl200 PVC now, and open it up in a separate window.  Then think about each step, enter the values I show into the spreadsheet, and actually try to duplicate what I do in the examples below.  Something about actually doing this helps engage people’s brains.  People tell me they read it twice and still don’t get it when they just read it, but as soon as they actually TRY it, then it suddenly makes sense.  It’s called learning by doing, and it is considered the best teaching method.  This process is simple, HOWEVER, it is not obvious and sounds illogical to those not trained in hydraulics.

#### A Simple Example:

Example Plan

The sketch above is an example of a very simple valve circuit with 5 sprinkler heads. In this example the sprinklers are 15′ apart and each sprinkler uses 3.7 GPM of water. The red numbers on the sketch are the total water flow for each pipe section in GPM.
Let’s assume we want to use Cl200 PVC pipe and we want a maximum total of 4 PSI of pressure loss in our lateral pipes.

If you are working through the Sprinkler Design Tutorial the maximum total pressure loss is entered on your Design Data Form in the Pressure Loss Table section.  There you will see a figure you entered called “_____ PSI – Laterals”.   That is the maximum PSI loss for the laterals, use that number here.  If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems.

If you don’t understand how to calculate the water flow in each section (the red numbers) you should take a look at the Sprinkler Pipe Layout page.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.
Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
If you don’t understand pressure losses in irrigation, see Pressure Loss & Selecting Your Sprinkler Equipment.
For advice on types of pipe (Cl200, poly, etc.) see Irrigation System Lateral Pipes.

To use the spreadsheet friction loss calculator to determine the pressure loss:

2. There is a line on the spreadsheet for each section of pipe. So for this example you will enter data for 5 pipe sections.
3. Start with the pipe section closest to the valve as section #1, and work out to the farthest sprinkler head.
4. Start by selecting 3/4″ pipe for the pipe or tube size for all the sections. (See “why not 1/2″?”)
5. Enter the GPM for the section of pipe.
6. Enter the length of the section of pipe.
7. Use an error factor of 1.1
8. Go to the next line down and repeat steps 4-7 for the next pipe section.
9. The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
10. At the bottom of the calculator it will tell you the pressure loss total of all sections combined.

Here’s what the spreadsheet calculator looks like after we enter the data requested for each of the pipe sections using the example in the sketch above.

Note that the “Total of all Sections” shown at the bottom exceeds the 4 PSI maximum limit we set for pressure loss. Also notice that the velocity in two of the sections (highlighted in red) exceeds the safe level. The marginally high velocity highlighted in yellow is considered acceptable by most experts, since these are lateral pipes. (The marginal velocity level would not be as acceptable in mainlines.) Start by fixing the velocity problems.  To decrease the velocity in those sections we will need to increase the pipe size.  So let’s increase the pipe size for the two sections highlighted with red to 1″. Here’s what it looks like after the change:

Now the velocities are all within acceptable levels. Also note that increasing the pipe sizes reduced the pressure loss “Total of All Sections” shown at the bottom to 2.9 PSI, which is well below our maximum level of 4 PSI. That’s good, no more changes are needed.    It is not possible for the pressure loss to be “too low.” As long as it is under the maximum it is fantastic.  So what would happen if the pressure loss was still too high? If there was still too much pressure loss we would need to try increasing the size of some of the pipes to lower the friction loss.

So we now have pipe sizes that will work for each section of pipe in our lateral. I’m often asked at this point if it would be OK to make some of the pipes 1/2″ since the pressure loss is so low? The answer is yes, but you might not want to do it.   See my explanation of the problems associated with the use of 1/2″ pipe.

#### A More Complex Example:

Now lets look at a more complex valve circuit. (Please note that this circuit is much larger than that found on a typical residential irrigation system. It would require much more water than most residences have available and is just used to show you an example of a much more complex layout.)  As with the previous example we will assume that our maximum pressure loss value for the valve circuit is 4 PSI.

This valve circuit involves numerous paths the water may take. This makes the calculation a bit more complex, as a separate calculation is needed for each possible route that the water might take through the laterals on it’s way to the last sprinkler at the end of a pipe. If you look at the example above you will notice there are 3 sprinklers that are at the end of pipes, each sprinkler at the end of a pipe represents a different route the water can take. So this circuit has 3 and will therefore require 3 separate pressure loss calculations.  The next drawing shows the possible water routes in magenta, blue, and red colors.

It may help to think of each path as the shortest route that a single drop of water could take to go from the valve to the last sprinkler on a pipe branch.  For some people it helps to think of it as a road map and your looking for the shortest route to each of the dead ends at the end of the roads.

Start your calculations with the water route that is the longest. In this case that would be the route highlighted in red. There are 9 pipe sections in this route, I have labeled them 1-9 for clarity. Just as before, enter the data from this route into the calculator, and make all the pipe 3/4″ size.   Here’s what the spreadsheet looks like:

As you can see there are a number of pipe sections highlighted red due to unsafe velocity.  Change those pipe sections to larger pipe sizes until all the velocities are within safe levels.

Here’s the resulting spreadsheet calculator with the smallest possible pipe sizes.  However, notice the Total of All Sections is 4.4 PSI, which is more than our 4 PSI maximum:

So we need to make some of the pipe sections larger in order to reduce the pressure loss (or friction loss.)  Start by increasing the size of one of the the smaller pipe sections.  Changing a 3/4″ pipe to a 1″ size is a lot less expensive than changing a 1″ pipe to 1 1/4″.  So for the example lets change section 7 from 3/4″ to 1″.  Doing that drops the Total of all Sections value to 3.77 in our example, below the 4 PSI maximum we set earlier.  So now everything is good, these sizes will work for the “red” highlighted water route.

Now we add the pipe sizes from the spreadsheet to our circuit drawing (note that the pipe sizes for the red highlighted sections have been added on the next drawing below.)

Now we relabel our sections to follow the blue highlighted route.

Using the blue highlighted water route, repeat the same process used for the red one. Enter the GPM and pipe lengths for each section in the spreadsheet. This time we already know the sizes for sections 1-6, they were entered into the spreadsheet when we did the red section.  So we just enter those for the new blue sections 7 and 8, again using 3/4″ size pipe.   And it looks like this on the spreadsheet calculator:

Using 3/4″ pipe size for our two new sections works good.  The velocity is safe and the Total of All Sections is 3.29 PSI, so the pressure loss for this route is also within the 4 PSI maximum we set.  Write the size of the two new sections on the drawing and the blue water route is done.

Now all that remains is to do are the calculations for the magenta highlighted water route. That is done the same way, entering the data into the spreadsheet calculator for each pipe section. Start with 3/4″ then change to larger sizes until the velocity is safe.  Then check that the total of all Sections is less than 4 PSI as before.  Here’s the data entered into the spreadsheet calculator:

Now all that remains is to insert our lateral pipe sizes from the spreadsheet calculators into the drawing of the valve circuit.

All done! So the pressure loss for the entire circuit is the same as that for the highest water route.  In this case the red route was highest at 3.77 PSI.  So the pressure loss for the lateral circuit shown here is 3.77 PSI.

Often I get asked at this point why the “Total of all Sections” pressure losses for all 3 routes wasn’t added together?  The pressure loss for the red route was 3.77 PSI, for the blue section it was 3.29 PSI, and for the magenta route it was 3.02 PSI.  So the confusion is that it seems like there should be a total loss of 10.08 PSI!  Nope, the pressure loss for the entire lateral is 3.77 PSI, the loss of the highest route.  To understand this think of a single drop of water again.  It can only travel on one route from the valve to the farthest sprinkler.  It is not going to go backwards and try another route!  So the pressure loss for the entire valve circuit is equal to the pressure loss from the valve to the farthest sprinkler.

### Irrigation Pipe Sizing Chart for Laterals

Saturday, November 26th, 2011

### (Sometimes called a Pipe Sizing Table.)

This method is based on the assumption that you are using Cl 200 PVC pipe for the lateral pipes.  With minor adjustments this method will also work reasonably well for SCH 40 PVC pipe or polyethylene irrigation tube.  For other types of pipe or tube you will need to use the Trial & Error method to determine the pipe sizes.

While the Pipe Sizing Chart method described here seems rather complex when you read it the first time, it is actually extremely fast and easy once you figure it out.  You will start with a simple calculation to obtain a “PSI/100″ value.  Then you will use that value in the Pipe Sizing Chart to figure out the maximum flow for various sizes of pipe.  You will only do this once for each sprinkler system.  Once you have that schedule you will fly through inserting pipe sizes into your plan.  Most designers who “design in their heads” are using this method or a close variation of it.   It is the method I use when designing my systems.

Definitions:

Lateral pipe:  The pipes between the control valve and the sprinkler heads are called “laterals”.

Mainline:  The pipes that go from the water source to the control valves are called “mainlines”.

Control Valve:  The control valve is the valve used to turn on and off a group of sprinklers.  Often it is an electric solenoid valve operated by a timer.

Valve circuit:  A valve circuit consists of  a single control valve, and all the fittings, pipes, and sprinkler heads that it turns on.

GPM:  Gallons per minute, a measure of water flow rate.  Use primarily in the United States.

PSI:  Pounds per square inch, a measure of water pressure.  Use primarily in the United States.

### When in doubt, always use a larger diameter pipe!

You may always use a larger size pipe.  No, I don’t own stock in a irrigation pipe manufacturer.  But using a larger size of pipe will not cause any harm to how well your sprinkler system works.  Using a larger pipe will NOT noticeably reduce the water pressure.  (Yes, I did condition that statement with a “noticeably”.)  The only damage done by using a larger size of pipe is to your pocketbook.  Larger pipe generally costs more.  But from a irrigation system performance perspective you will NEVER hurt anything by using a larger size pipe.  Now I realize that somewhere out there, somewhere will tell you this is not true.  They are going to tell you that you need a smaller pipe to squeeze the water and create more pressure.  They are totally wrong of course, but as you read this you are probably uncertain who is right, since they will claim I am wrong!  Ask them to provide you with a scientific, documented explanation of why they are right.  I will also provide both a basic and a very scientific explanation with references for you.  Here’s mine: Using A Smaller Pipe to Increase Water Pressure.  OK, sorry, I’ll climb down off my soapbox now.

Is it Pipe or Tube?  I tend to call everything pipe.  Habit, since here in La La Land (Los Angeles, California) we use mostly PVC pipe for irrigation.  However some types of “pipe” are technically defined as “tube”.    The difference is the material they are constructed of.  Steel and PVC plastic are generally called pipe.  Polyethylene, PEX, and copper are usually called tube or tubing.  If I say pipe where I should say tube, please accept my apologies.

### CALCULATING THE PSI/100 VALUE:

The PSI/100 value is a value used in the Pipe Sizing Chart (we’ll get to the chart in a moment.)  The PSI/100 value determines which column of the chart you will use when finding the pipe sizes.   A simple calculation will give you the PSI/100 value.

### The PSI/100 formula:

( ____ PSI x 100) / ____ Feet Total Length = PSI/100

For those who prefer variables, this is the same formula written using variables:    (LPSI * 100) / FTL = PSI/100

Here are the values to insert in the blank spots (“____” ), or variables, in the formula:

____ PSI.  (LPSI)  Insert the maximum PSI loss for all laterals on the valve circuit into the formula where it says “____PSI.”  .

If you are working through the Sprinkler Design Tutorial look on your Design Data Form for the Pressure Loss Table. There you will see a figure you entered called “_____ PSI – Laterals”.   That is the maximum PSI loss for the laterals, use that number here.  If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems.  If you don’t understand pressure losses in irrigation, see the Pressure Loss & Selecting Your Sprinkler Equipment and Lateral Pressure Loss pages.  Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.  Example:  20 PSI sprinkler operating pressure.  20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.

____ Feet Total Length.  (FTL)   Insert the distance from the control valve to the farthest sprinkler  in the space labeled “____ Feet Total Length” in the formula.

For this value you need to figure out the total length of pipe (in feet) that the water needs to travel through in order to get from the valve to the farthest sprinkler.   Measure only the pipe sections that the water would pass through on the way from the control valve to that farthest sprinkler.  Don’t add in the length of any side spurs going off to other heads that aren’t on the longest route.  In the example below, the route from the control valve to the farthest sprinkler that you would measure the distance of is shown in red.  Totaling each of the pipe sections along that route results in 118′.  So 118 feet would be the ___ feet value you would use in the PSI/100 formula.

Example of A Typical Valve Circuit

Now use the PSI/100 formula above to calculate the PSI/100 value.  ( ____ PSI x 100) / ____ Feet Total Length = PSI/100

Write down the PSI/100 value.  ____________

Example: Let’s say the value “____ PSI – Laterals” is 4 PSI. Let’s also assume that the total length of the lateral as measured above is 118 feet. Those values inserted in the formula would look like this:  (4 PSI x 100) / 118 feet     Now do the math.  4 times 100 = 400.  Then 400 divided by 118 = 3.389    Round that number to 3.4.    Therefore when using this example your PSI/100 value to use in the Pipe Sizing Chart would be 3.4 PSI/100 .

You can repeat this procedure for each valve circuit. But the usual method is–

It is possible to use the same PSI/100 value for all the valve circuits. That’s how most professionals (myself included) do it. The only catch is that you must use the “worst case” PSI/100 value. In other words you need to figure out which of the valve circuits on your entire sprinkler system has the longest “Feet Total Length” between the valve and last sprinkler.   Then use that valve circuit to calculate your worst case PSI/100 for the entire sprinkler system.  The advantage of using the same PSI/100 value for everything is uniformity of design and, obviously, doing only one PSI/100 calculation for the entire sprinkler system saves time.  For example, a pipe with five half circle spray heads downstream would always be the same size pipe. This is much less confusing for the installer, which is the main reason we do it this way.

### Pipe Sections and GPM:

Each section of lateral pipe may be a different size. For example, the first section of pipe leading away from the valve might be 1 1/4″. The next two sections might be 1″, and the rest of the sections might be 3/4″. The pipe size to each section is based on the actual GPM flow passing through that section of pipe, so you will need to know what the GPM flow is for each section.  If you have been working through the Sprinkler Design Tutorial you have already figured this out and written these GPM values down on your plan in an earlier step.  If not, you will need to take a few minutes to do this now.  See the page on Sprinkler Pipe Layout for instructions on figuring out the GPM for each pipe section.

### THE PIPE SIZING TABLE or CHART:

before you use the chart…

Warning:  The sprinkler pipe sizing table /chart is based on using Cl 200 PVC pipe.  For other pipe types you will need to make an adjustment if you want to use the chart.

Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40″) for the laterals you need to make an adjustment before using the chart below, because SCH 40 PVC pipe has a much less water capacity than other PVC pipes. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.

Example for SCH 40 PVC pipe: In the example above you calculated a value of 3.4 PSI/100. But you have decided to use SCH 40 PVC pipe for the laterals, rather than Cl 200 PVC pipe. So you will need to reduce the PSI/100 value by half. 3.4 x 0.5 = 1.7 PSI/100. So your new value is 1.7 PSI/100. As you will see, this will result in much larger lateral pipes! This is why most people do not use SCH 40 PVC for laterals, and why I recommend you use Class 200 PVC. It makes a big difference in cost!

Class 125, Class 160, or Class 200 PVC pipe: The chart below is based on the use of Class 200 PVC pipe. It also works for Class 125 (not recommended) and Class 160 (hard to find).

Class 100 and 315 PVC pipe: As a general rule, these types of PVC pipe are not used for laterals.

Polyethylene, Polybutylene: Use the chart below. Then, after you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc.  Note: PEX pipe is not the same thing as polyethylene irrigation pipe.

PEX: Do not use the chart for PEX pipe.  PEX has extremely limited flow.  Use the Trial & Error Sizing Method for PEX!

To use the chart you will use the PSI/100 value you calculated along with the GPM flow in the pipe section.

### Sprinkler Pipe Sizing Chart for Laterals

 PSI/100  (round down) 0.2 0.5 0.8 1.0 1.5 2.0 3.0 4.0 5.0 6.0 SIZE 2.2 3.3 4.4 5.0 6.2 7.1 8.5 10 11 13 ¾” 3.8 6.3 8.1 9.2 11 13 17 20 22 24 1″ 7.1 12 15 18 22 25 31 36 37 37 1¼” 11 16 22 24 31 35 44 48 49 49 1½ 18 30 40 44 57 65 76 76 76 76 2″ 28 46 60 67 83 96 114 114 114 114 2½” 46 75 100 112 140 162 165 170 170 170 3″ 87 140 185 208 250 280 280 280 280 280 4″ 255 410 540 600 600 600 600 600 600 600 6″

Flows shown in red are over 5 feet/second.
Permission is granted for reuse for any purpose and in any media, provided the copyright notice is maintained.

### Sprinkler Pipe Sizing Table /Chart Instructions:

2. Find the PSI/100 value in the top row (blue text, directly under the heading PSI/100.)
3. Read down that column and find a value equal to, or higher than, the GPM in the pipe section.
4. Now read across to the right column to find the pipe size to use for the pipe section.
5. Repeat steps 3-5 for the other pipe sections in the lateral valve circuit.

Notes:

• Flows over 5 ft/second are considered marginal (shown in red on chart.)  Most experts believe that flows up to 7 ft/sec are acceptable for laterals.  However flows over 7 ft/sec velocity are not considered safe, so they are not shown on the chart.
• This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%.
• This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.
• No 1/2″ pipe?  See my explanation of why I don’t use half-inch size pipe.

### Example Using the Pipe Sizing Chart:

Example Sketch of a Sprinkler System

In the example above the flows for each pipe section are noted in gray text with an arrow pointing at the pipe section.  The red pipe circuit has the longest distance between the control valve and the farthest sprinkler head.  So for our example let’s use the red pipe circuit.

First we need to calculate the PSI/100 value.

We start with the maximum pressure loss we want in our lateral pipes.  For this example we will use 4 PSI.
Now we measure the total pipe distance from the valve to the farthest head.  I showed this route using a bold red line.  It is 96 feet from the control valve to the farthest head when following this bold red route.
Now the PSI/100 formula with the values from this example inserted:  ( _4_ PSI x 100) / _96_ Feet Total Length = _4.2_ PSI/100

Now we start using the chart to find the pipe sizes.

Our PSI/100 value is 4.2, so we look on the chart.  Rounding down we see that 4.0 is the closest PSI/100 value on the chart, so we use the 4.0 column.
Now read down the 4.0 column.  The numbers will tell us the maximum flow for each pipe size.
So the first number we see is 10.  That would mean 10 GPM.  Reading across to the right we see that 10 GPM is the maximum flow for 3/4″ size pipe.
Continuing in the 4.0 column, the next number is 20.  Again we read across and see that 20 GPM will be the maximum flow for 1″ pipe.
Reading down one more line we see that 36 GPM is the maximum flow for 1 1/4″ pipe.  And we can continue this on down the chart.
So now we can create a simple pipe size schedule to use for our plan, based on the values we took from the pipe sizing chart:

Up to 10 GPM = 3/4″ size pipe
Up to 20 GPM = 1″ size pipe
Up to 36 GPM = 1 1/4″ size pipe
Up to 48 GPM = 1 1/2″ size pipe
Up to 76 GPM = 2″ size pipe

Now  go back and look at the flow for each section of pipe on your plan.  Then based on the GPM flow, insert the pipe size from the schedule you made.

So the section with a flow of 2.5 GPM will be 3/4″ pipe.
The section with 1.3 GPM will also be 3/4″ pipe.
The section with 3.8 GPM will be 3/4″ pipe.
The section with 6.4 GPM will be 3/4″ pipe.
The section with 1.3 GPM will be 3/4″ pipe.
The section with 2.6 GPM will be 3/4″ pipe.
The section with 9.0 GPM will be 3/4″ pipe.
The section with 11.5 GPM will be 1″ pipe.

I’ve inserted these pipe sizes on the example sketch above.

See how fast and easy that is?  Once you have the initial PSI/100 calculations done you can use the pipe sizing chart to create a custom pipe schedule for your plan.  Then it is really fast to simply look at the flow in a pipe section, look it up on the schedule, and write in the pipe size!  You can see why pros use this method, it allows them to fly through a large design with hundreds of sprinklers.

### COMMON PROBLEMS AND QUESTIONS REGARDING USING THE PIPE SIZING CHART

Is your PSI/100 value off the chart? If your PSI/100 value is 6.0 or higher you should use the 6.0 column. At 6.0 you have reached the maximum safe capacity of the pipe sizes used on the chart.

Is the pipe size larger than the valve size? It is fairly normal for the first pipe after the valve to be one size larger than the valve. So you may have a 1″ mainline going into a 3/4″ valve and then have a 1″ lateral pipe coming out of the 3/4″ valve.  This is very common, and is not a problem at all.  So don’t worry if the pipe size you get from the chart is larger – or smaller – than the valve size.

Write the pipe size down next to the pipe on your plan. Repeat for each pipe section.  Repeat for each valve circuit.

### Using A Smaller Pipe to Increase Water Pressure

Saturday, November 26th, 2011

There is a very persistent misconception in the lawn sprinkler industry that using progressively smaller pipe sizes in a sprinkler system will help keep the water pressure high.   The argument is that as the water moves through the pipes past the sprinklers, the pipe must get smaller in order to squeeze the water so that the pressure stays high enough to operate the sprinklers.  Unfortunately, it’s not true.  It would be nice if it was, because we could eliminate pumps.  Plus think of all the money you would save on pipe.  The smaller the pipe you used, the better your system would work!  So why not use 1/4″ or even 1/8″ tube for the pipes?  That would really pump up the pressure!  Sounds a little silly when you look at it that way, right?  OK, enough with the sarcasm.  I’ll explain this whole mess.

### Squeezing the water into a smaller pipe will not increase the water pressure!

Part of the reason this misconception persists is that it does seem logical.  The example most often given to support this idea is what happens when holding your thumb over the end of a hose.  As you press your thumb over the opening, making it smaller, you can feel the water pressure against your thumb increase.  Pushing your thumb even tighter against the end of the hose, makes the opening even smaller, and you feel the pressure increase even more.  That would seem to prove that decreasing the opening size is increasing the water pressure.  So logically, using a smaller pipe would also increase the water pressure.

Unfortunately there is a lot more happening with this “thumb over the hose end” example than you realize.   As water moves through a hose or pipe there is a lot of resistance caused by the hose or pipe surfaces.   The water moves through the hose at the maximum speed it can while still overcoming this friction.  When the water reaches the end of the hose it has close to zero pressure left as it exits.  So if you have, say, 50 PSI of water pressure at the hose faucet, the  water will move as fast as it can through the hose, such that it will use up almost all that 50 PSI of pressure by the time it reaches the end of the hose.  If there were 60 PSI of pressure, the water would just move a little faster through the hose so that it used up almost all 60 PSI by the time it exits.  So basically regardless of the pressure, almost all the water pressure is used up by the time the water flows through the hose.  The nature of water is that it will reach the most efficient balance between flow rate and pressure loss that it can.  (Note, I am oversimplifying this to make it digestible for the average person.  If you have a degree in hydraulics you already know all the other related stuff about open vs. closed channels and nozzling effects.)

When you put your thumb over the end of the hose you change the flow dynamics in the hose.  Your thumb restricts the flow of water through the hose.   With your thumb over the end, the water is flowing much slower through the hose, and as a result, there is a lot less pressure loss due to friction.  So with less pressure being lost in the hose, the pressure at the end of the hose where your thumb is increases.  The tighter you squeeze your thumb, the more the flow is reduced, and the greater the pressure you feel will be.  But you haven’t created any NEW pressure.  You have simply traded reduced flow for increased pressure.    You can easily test this yourself.  Take a bucket and time how long it takes to fill it using an open end hose.  Now time how long it takes to fill the same bucket with your thumb firmly pressed over the hose end.  It will take longer to fill, because your thumb has reduced the flow!  The same thing would happen in your sprinkler system if you used smaller pipe to increase the pressure.  The smaller pipe would restrict the flow of water.   The reduced flow would reduce the pressure loss in the pipes, resulting in more pressure.  But of course the sprinklers would not work because they won’t be getting the flow they require!    Sprinklers require both flow and pressure.

OK, that’s the layman’s explanation.  But there are also some much more complex scientific theory that I have been asked about in relation to this topic.   So here’s some very scientific explanations.

### Bernoulli’s Principle, Venturi Effect, & Flying Pigs

Grab your thinking caps for this.  As you well know, Bernoulli’s Principle essentially says (paraphrased) that as the speed of a fluid increases, the pressure of that fluid decreases. If it didn’t, pigs wouldn’t fly.*   Obviously as you force a given amount of water through a smaller size pipe, the velocity of the water must increase for it to get through the smaller pipe.  According to Bernoulli’s Principle that will decrease the water pressure!  This is called the Venturi effect.  By suddenly forcing the water through a narrow passage you can actually create enough of a pressure decrease that it creates suction.  This is how many fertilizer injectors work.  It also is another reason why using a smaller pipe would not increase the pressure– it would actually decrease it!

Another less common argument is the pipe size must be decreased because the flow is decreasing at each sprinkler head location along the pipe route. Thus if the pipe were to remain the same size, the velocity in the pipe would decrease, resulting in an increase in pressure (according to Bernoulli’s Principle again.)  This is actually a good, scientifically based point, and accurate too!  So the argument is that the pipe sizes must become smaller in order to keep the velocity constant and avoid an increase in water pressure. (Are you bored yet?) Unfortunately when used as an argument for using smaller pipe, this one falls flat when you do the actual math.  At a flow of 7 feet per second, which is the maximum recommended safe flow for PVC pipe, the maximum possible pressure increase  due to velocity change would be a whopping 1/3 PSI.  So in theory, using a smaller pipe would eliminate that 1/3 PSI pressure gain.  But using a smaller pipe probably would also increase the pressure loss due to friction, as previously mentioned.   The drop in pressure due to friction loss likely will offset most if not all of any gain that might have occurred due to decrease in velocity.  Even if it didn’t the maximum possible pressure gain of 1/3 PSI is simply not significant and would not be noticed.  So I stand by my statement that the only reason to decrease pipe size is to save money.

*Oh, by the way, Bernoulli’s Principle is why airplane wings create lift, which helps airplanes fly.  Therefore, it is also the reason that people, and yes, even pigs, can fly!

### Spreadsheets for Calculating Pipe Pressure Loss

Monday, October 10th, 2011

Here are some spreadsheets I have created to help you calculate the capacity and water pressure loss through pipes and tubes of various types and sizes. These should be useful for both figuring pressure loss in mainlines and laterals. Each spreadsheet allows for multiple sections of pipe of various sizes and flows. All you do is select the proper spreadsheet for the type of pipe you are going to use, select the pipe size from a drop down list, enter the flow through the pipe in GPM, then enter the length of the pipe in feet. The spreadsheet calculator will then do the math to give you the water velocity in the pipe along with the pressure loss in PSI for that section of pipe. If there are multiple sections of pipe the spreadsheet will also total all of them for the total pressure loss.

Remember, you can un-install Open Office when you are done if you don’t like it.

If you try to open the spreadsheets directly using your Internet browser they will probably open as Read Only and the spreadsheet won’t work.  This is because the browser will not open an executable file directly.  (It is trying to protect you from possible viruses.)
Solution:  Right click on the link and “Save” the file to your hard drive.  The actual wording varies, so depending on your browser you may select “Save link as..”, “Save Target as…”, etc.   This should save the spreadsheet file to your hard drive.  Then open it directly from your computer without using the browser plug-in.  Tablet and phone users:  you may need to get to a real computer to use the spreadsheets.

### If the spreadsheets don’t work for you..

1. Do you have Open Office installed?  If not, install it.
2. Are you using Open Office to read the spreadsheet?  Sometimes another spreadsheet program will try to open it instead of Open Office.
3. Have you tried saving the spreadsheet file to your desktop, starting up Open Office, then opening the spreadsheet with Open Office?
4. If the spreadsheet says it is “Read Only” you probably are using a non-compatible plug in.  Install & use Open Office.
5. Just dragging the spreadsheet link to your desktop may not actually save the file.  You need to right click on the link and select Save as…
6. If you have an older version of Open Office you may need to upgrade it.
7. Try rebooting.  I’ve experienced a problem where if I try to open one of these spreadsheets in a browser the computer gets messed up and won’t start Open Office.  Rebooting fixed the issue.

## SPREADSHEET CALCULATORS FOR PVC PIPE

Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

Cl 125 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 160 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 200 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 315 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

SCH 40 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

SCH 80 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

## SPREADSHEET CALCULATOR FOR POLYETHYLENE TUBE

Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

Polyethylene Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

## SPREADSHEET CALCULATOR FOR PEX TUBE

Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

PEX Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

## SPREADSHEET CALCULATORS FOR COPPER TUBE

Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

Type K Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Type L Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Type M Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

## SPREADSHEET CALCULATOR FOR SCH 40 STEEL PIPE

Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

SCH 40 Steel Pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.  (also use for galvanized steel)

### Huge Grass Yard with Minimal Water Supply

Sunday, April 24th, 2011

Q.  My pump produces 10 GPM at 45 PSI or 7 GPM at 65 PSI.  Do you think the flow is decent for my yard?   When I had pro’s quote my job (which is why I’m doing it my self as the numbers were huge) they all said I’d need about 40 sprinkler heads.

A.  My gut feeling is that you don’t have enough water capacity from your pump and/or well to irrigate the size of area you have planted in lawn.  (Don’t panic yet, keep reading for some suggestions.)  40 sprinklers would be a lot to try to run off of 7-10 GPM of flow.  But it depends greatly on your climatic location and water needs.  If you only need irrigation for periodic supplemental watering you may be OK.  The rule of thumb is that 10 GPM will water about 1/2 an acre of lawn, assuming you need to water about 3 times a week to keep the grass lush.  So if you need to water only twice a week, then you could water more area with 10 GPM of water flow.  Also, the rule of thumb assumes you only wish to water during the night hours.  You could water more area if you are willing to water 24/7 during the peak hot season.  Keep in mind that if you share the water use with your house that 24/7 watering might not be a good idea as you won’t have any water left over for use in the house.  When your spouse gets in the shower and no water comes out because the sprinklers are using all of it, things are not going to be pretty!

The rule of thumb is that it takes 20 GPM to water an acre of lawn in a hot climate area.  So if you have a larger lawn you may need to think about adding a new pump and/or well.  I really don’t have enough info to say for sure since I don’t know the size of your yard or your climate location.  Take a look at this article which will show you how to calculate how much water you will need for your exact situation:

Options to consider if you don’t have enough water:
An option a lot of people in rural areas use is to create watering zones around the property.  They heavily water the area right around the house, possibly 25′ or so out from the foundation, sufficient to create a really lush green lawn.  Then for the next 25-50′ out they apply just supplemental water, watering maybe once (or twice) a week in hot weather.  This supplemental water area would tend to yellow a bit and show stress during the hottest part of the year.  Then they have a “no irrigation” area at the far reaches of the property that gets no irrigation water at all.

Another option: If you don’t have the water supply or money to do all of it right, then install it in phases.  Start with the area around the house.  Then add more sprinkler zones to water the areas farther out from the house each year as you have funds and time.  Add another pump and/or well later when you need the water.   The critical thing is to “do it right” in regards to the sprinkler spacing and resist the temptation to stretch spacings between the heads to stretch the water supply or save money.  The results are always disappointing if you do that.

### Can I Run Two Irrigation Valves at the Same Time?

Tuesday, March 29th, 2011

Q.  Is it possible to have two valves on at the same time or to run two irrigation valves at once?

A. Yes, it is often possible to run two valves at once.  However there are several problems that can occur.

You must have a sufficient water supply for both valves to run at once.  If the performance of the sprinklers suffers and you start seeing dry spots in the landscape, you obviously don’t have enough water.  You may need to do some adjusting of the sprinklers as the water pressure operating them is likely to be less when two valves are on.

Both valves running at the same time may require more water than the pipe supplying them can reasonably handle.  This can result in water hammer, or premature pipe wear/failure, due to high water velocity.

Water Hammer: Listen for a loud water hammer “thump” or “bang” noise when the valves close.  A gentle thump is fine, but if the pipes reverberate from it that is not good.  Run just one valve and listen to the sound when it closes.  Assuming the irrigation is properly designed, that should be the “normal” closing sound.  Now listen to the sound when both valves are closed together to see if it is significantly louder.  If it is significantly louder, that is not good.  You can possibly reduce or eliminate the water hammer problem by closing the valves separately, one at a time.

High Velocity: Premature wear due to velocity is harder to figure out.  It generally isn’t a problem unless the water is really flowing fast through the pipe, like 8 feet per second or higher.  The only way to determine if it is a problem is to do a couple of calculations.  Start with the sprinklers.  On top of each sprinkler is an identifying names and part numbers that tell you the brand, model, and hopefully the nozzle size. Write down that information for each sprinkler, then look up the water use (GPM value) for that sprinkler and nozzle at the sprinkler company’s website.  (You may need to call the company’s help line to assist you, each brand and model is different so I can’t give exact instructions.)  Now add together the GPM values for all the sprinklers that are running at the same time when two valves are turned on.  This will tell you how much water the two valves require when running together.  Next find the size and type of the water pipe that leads to the valves.  (For example it might be a 3/4″ copper tube, or maybe a 1″ PVC pipe.  It may be several different sizes and types of pipe, in which case you would use the smallest pipe size and type.)  Using that information you can calculate the velocity of the flow in the pipe using the Friction Loss Calculator at http://www.irrigationtutorials.com/formulas.htm#sec8.  Just enter the pipe type, size, and GPM into the calculator and it will give you the velocity.

If you decide to use a controller to operate the valves the controller must be a brand that provides sufficient amperage to run two valves at the same time (most do.)  If you want the controller to run the valves at the same time, but start and stop them about one minute apart to reduce water hammer, you will need a controller that allows you to run two separate valve zones at the same time.  Most controllers have a “stacking feature” that prevents them from doing this.  You will need a controller that allows you to turn off the stacking feature.  Most controllers can’t do this.  You will probably need to enlist a knowledgeable controller salesman at a professional irrigation supply store to assist you in finding a controller that will work for this unique situation.

### Can I Pump my Irrigation Water from a River, Creek, or Pond?

Thursday, March 24th, 2011

Q.  We live on a river.  I would love to plant some interesting things on the bank below our home but with the price of water these days I would love to be able to pump some river water up to do the job. Do you think that that is something we could do without spending a fortune?  It would be great to have a soaker system.

A.   First, you must have the right to take water from the creek, river,. pond, etc..   This almost always means you need to talk with the US Fish & Game Department, State regulators, and possibly the Environmental Protection Agency (or equivalent agencies for whatever country you are located in.)  If you take water from a creek or pond or any other natural body of water in the USA without checking on the legal rights and requirements you can get into a lot of hot water, fast.  The fines penalties and restitution costs can be enormous.  So before you do anything, start doing some calling around.  Be safe, not sorry.  If you don’t know who to call, try calling the local County or Parrish Planning Department, they should be familiar with the agencies that regulate water and be able to point you to the right people.

Yes, from a physical standpoint it is not difficult to pump the water.  The cost depends on how fancy you make it.  My parents had a cabin on a river in Oregon.  They simply had a small portable pump that sat on a concrete block and was chained to a tree.  One end of a 15′ garden hose was attached to the pump intake, the other end of the hose had a piece of window screen tied around it to create a home-made filter and keep out small fish and junk.  The end of the hose with the screen filter was tied to a concrete block and dropped into the river.  The pump outlet was attached to a second garden hose, this one was 150 feet long.  A long extension cord went from the pump to the power outlet at the cabin.  They put a sprinkler on the end of the hose, placed the sprinkler where they wanted water, then plugged in the pump.  Simple, cheap.  You could easily semi-automate that by simply plugging the pump’s power cord into a timer to turn it on and off.

A fancier system is certainly possible.  The pump still needs to be portable in most cases.  The pump has to be mounted less than 8 feet above the water level (the closer the better.)  You need a pad of some sort to put the pump on, but it is best if the pump can be easily moved, especially if the water level fluctuates in the creek or floods.   There is also the possibility of using a submersible pump.   A submersible should not sit on the bottom of the stream if there is a lot of mud and silt in the water that would get sucked into the pump.  If you have a floating dock or a pier an alternative is to place the pump on it (or hang it below the dock in the case of a submersible pump.)  Submersible pumps are often strapped to the side of pier pilings.  Be sure to read installation instructions for the pump, many pumps have very specific positioning requirements, some submersibles must be installed inside a special sleeve.

You can get about as fancy as you want- using automatic controls to start and stop the pump and also to open and close multiple irrigation valves.  Many irrigation controllers have built in circuitry that will start and stop the pump for you using a electrical relay.  If you do it yourself, and you need only something similar to my parent’s  small pump  you could probably install a pump  for around \$200.00.  The price can go up fast as you get bigger and fancier, \$1000.00 is not an out of line figure for a pump system capable of watering an acre or so of yard.  The wiring for the pump automated controls is a bit tricky, so most people would want to have that part done by a electrician.  How much that costs depends on the length of wire needed to reach the pump.  One option to look at when you get to larger irrigation systems is a pre-constructed pump unit.  This consists of the pump and all of the needed controls for it pre-installed and pre-tested on a metal frame.  You just hook up the pipes and wires to it and turn it on.

You may also need a storage tank for the water, especially if you have a small water supply (like a creek.)   That way you could pump a small flow continuously from the creek  to fill the tank.  Once in the tank the irrigation water would either be pumped out of the tank to the irrigation system by a second pump, or if the tank can be located 30′ or so higher than the level of the irrigated area, you could use gravity flow from the tank.  (If you want to use sprinklers the tank would need to be at least 60 feet higher to create enough pressure for a small sprinkler.)  The tank will probably need to be a lot larger than you think.  Typically they are 5,000 gallons or larger.  To find out what size tank you will need you need to determine how much water it will take to irrigate your area.  See How to Estimate Irrigation Water Quantity Needed for instructions on estimating your water requirements.

One last word of warning before you start:  PLAN FIRST, BUY LATER!   Don’t run out and buy an “irrigation pump” first!  Most pumps sold with the description “irrigation pump” are designed to operate a single sprinkler on the end of a hose.  You need to design the irrigation first, then you will now how much water volume AND water pressure the pump will need to produce.  The Sprinkler System Design Tutorial takes you through the process of irrigation system design and finding the right pump size.  It’s at  http://www.irrigationtutorials.com/sprinkler00.htm

### Outlet Pipe Size for Pump- is a Bigger Pipe Better?

Friday, February 4th, 2011

Q.  I’m designing a pump system from a lake and have read and understand your calculation of FT HD needed for pump selection but it seems that the upstream (uphill) pipe diameter would be a factor in the calculation.  I was going to use larger pipe to reduce pipe resistance and valve pressure drop but it seems to me the weight of the additional water (back pressure) would be higher for a  larger diameter pipe than a smaller one.  It must be easier to push water up a 3/4″ column than a 1 1/2 inch column.  You mention nothing about this.  Excluding pipe resistance, does the pipe diameter play a roll in taxing the FT HD required?  Rephrased –  Does a larger diameter column of water have any effect on the static pressure or force required to move it?

A.  The short answer is that the larger pipe would be better because there would be less pressure loss in the pipe.  This is due to less “friction loss” as the water flows through the larger size pipe.  The larger amount of water in the bigger pipe has no impact on the water pressure.   A smaller pipe may create more friction loss however, so it can be worse than a larger pipe.    To find out, you need to calculate the friction loss in the different sizes of outlet pipe based on the flow and pipe size.  See the Friction Loss Calculators to calculate the friction loss in pipes.

One of the really hard to grasp principles of hydraulics is the relation of volume of flow, pressure, and the weight of water.  Odd as it seems a larger pipe will actually be easier for the pump.  It’s not the volume of water, but the height it is lifted that matters.  In a way this is a variation on the old saying “which weighs more, a pound of feathers, or a pound of lead?”  Obviously both weigh a pound!  This version could be phrased “which is easier for the pump, 5 GPM in a 1/2″ pipe or 5 GPM in a 2″ pipe?  Neither because 5 GPM is still 5 GPM regardless of the pipe size!  Yes, you would need more power if you were actually lifting more water, also we would need more power to lift the water higher, but neither is not what is happening.  The amount of water nor the height we are lifting hasn’t changed.

The other issue here is flow through a pipe.  This is the issue that actually makes the smaller pipe potentially worse than the larger.  Because the smaller pipe is smaller it is harder to force the water through it.  The resistance of the walls of the smaller pipe causes pressure loss as water flows through.  this is commonly called “friction loss”.  How much friction loss occurs depends on the flow rate and pipe size.  Both higher flows and smaller pipes sizes result in greater friction loss.  This is the only reason a smaller pipe would be worse than the bigger pipe.  How much worse is dependent upon the actual flow rate and pipe size.

As a general rule (ie: not always true, but is most of the time)  the pipe size of the pump outlet is almost always smaller than the size of pipe that will provide optimal flow from the pump.  In other words, if a pump has a 1″ threaded outlet, it is very likely that a 1 1/2″ pipe would be attached to the 1″ outlet for use as the outlet pipe.  Pump manufacturer’s tend to use smaller size inlets and outlets to save money.

Think about feet of head.  As discussed in the Pump Tutorial, the number of feet of water depth determines the water pressure.  So 80 feet of water depth equals a pressure of 80 ft. hd.  This pressure will be the same regardless of the pipe size.  The water pressure at the bottom of an 80′ high 1/2″  pipe is exactly the same as the water pressure at the bottom of an 80′ high 6″ pipe, even though the 6″ pipe holds a lot more water.  A pump actually works by creating water pressure.  So for the pump there is no difference between pumping into either size pipe, the water pressure required to move the water into the bottom of both pipes is the same.  Now the pressure lost as water moves through the two pipes will be different.  Assuming a high rate of flow, a lot more pressure will be lost due to friction in the smaller pipe.    So for that reason using a larger pipe will be better.  Depending on the flow, however, it may be only very marginally better.  To find out you need to calculate the friction loss in the outlet pipe based on the flow and pipe size.  See the Friction Loss Calculators to calculate the friction loss in pipes or tubes of various types.

### How to Estimate Water Useage Required for an Irrigation System

Monday, January 31st, 2011

The amount of water needed for irrigation depends on many different factors.  A reasonably accurate estimate of the amount of irrigation water needed can be made using Eto data for your actual zip code.  “Eto” is the amount of water needed for irrigation, based on scientific research.  You can find the historic Eto for any zip code in the USA at the website www.rainmaster.com/historicET.asp courtesy of the Rainmaster irrigation controller company, who makes very good “Smart” irrigation controllers.   I use one of their Eagle model controllers on my own home.  (Rainmaster get a plug from me as well as a big “thank you” for providing the ETo look up service online.)  Unfortunately the Eto value only tells you how many inches per day are needed, which for most folks is a meaningless value. It makes more sense if you think about rainfall which is often also measured in inches.  If you find you need 0.20 inches of irrigation, then 0.20 inches of rainfall would provide the required water.  But most people in the USA want a value in gallons, which requires you to provide a little more information about your yard.  Then you plug the values into a simple formula, and do a little multiplication and division on any calculator.

Formula to calculate the gallons of irrigation water needed per day:
(Eto x PF x SF x 0.62 ) / IE  =  Gallons of Water per day

Values for the formula:
Eto: Get this from http://www.rainmaster.com/historicET.asp .  Enter your zip code, or a nearby zipcode, and the website will give you the average daily ET value for each month of the year.  Use the highest value or the “suggested reference value”.  Usually they are the same thing.

PF: This is the plant factor.  Different plants need different amounts of water.  Use a value of 1.0 for lawn.  For water loving shrubs use .80, for average water use shrubs use 0.5, for low water use shrubs use 0.3.

SF: This is the area to be irrigated in square feet.  So for a 30 foot x 50 foot lawn you would use 1500.

0.62: A constant value used for conversion.

IE: Irrigation efficiency.  Some irrigation water never gets used by the plant, this value compensates for that.  I suggest using 0.75 as the value for this.  Very well designed sprinkler systems with little run-off that using efficent sprinklers can have efficiencies of 80% (use 0.80).  Drip irrigation systems typically have efficiencies of 90% (use 0.90).

Example:
A 1500 square foot grass lawn in zip code 85232 (Central Arizona)
Start by looking up the Eto for zip code 85232 at the Rainmaster website, which displays a suggested reference value of 0.3 inches per day using June, the driest month of the year in that area.

Now rewrite the formula inserting your values into it:

0.3 (Eto value)  x  1.0 (grass value)  x 1500 (sq ft)  x 0.62 ÷ 0.75 (efficency factor) = gallons of water per day

Now do the math, just punch the values into a calculator and get your answer:
0.3   x   1.0   x   1500   x   0.62   ÷   0.75   =   372 gallons per day

We could figure out the average daily water use for other months of the year also.  Just use the same formula but insert the Eto value from the Rainmaster website for the month you want to get a valve for.

Remember this calculation just gives you an estimated value.  There are many other factors that could make this value higher or lower.  When planning for how much water a system that has not yet been designed or installed will use, it would be very wise to allow for  error by adding 10% or more to the daily water use needed.  It is generally better to have too much water, than to have too little!  Play it safe!

A common related question is “how much water pressure will my irrigation system need?”  The answer depends on a lot of factors, but as a rule of thumb, I would suggest 50 PSI of water pressure as a good starting point for sprinklers, 45 PSI for drip systems.  If you have a large yard and want to put the sprinklers farther than 30 feet apart you will need more pressure.  For example, if you want your sprinklers 45 feet apart you will probably need 65 PSI of water pressure.  To get a real value you will need to create an actual sprinkler system design. See the Landscape Sprinkler Irrigation System Design Tutorial .

Never buy a pump, sprinklers, or any other materials before your sprinkler design is completed!

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