## Posts Tagged ‘pipe’

### 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.

### PVC Pipe: SCH vs. Class

Thursday, May 10th, 2012

### SCH Rated Pipe

PVC pipe types labeled “schedule” (abbreviated “SCH“) are made based on the traditional dimensions used for steel pipe.  Unfortunately steel has very different strength characteristics from plastic, so it is a system that isn’t very logical for use with PVC pipe.  But when plastic first came along it was made to the same size standards that were already in use for steel.  The common PVC pipe schedules you will see in stores are SCH 40 and SCH 80.  As the pipe sizes rated SCH increase, the strength and pressure rating of SCH pipe decreases.  So 1/2″ SCH 40 PVC pipe is very strong, while 2″ SCH 40 PVC has comparatively a low pressure rating, and is more easily damaged.  In sizes 1/2″ to 1 1/2″ SCH 40 is a thick wall pipe with a reasonably high pressure rating and good resistance to physical damage.  It is often used for mainlines and other situations where a tough high pressure pipe is needed.  Sch 80 is generally used for making threaded plastic nipples because the plastic walls are thick enough to have threads cut into them (although most now have molded threads rather than threads “cut” with a die.)

Pressure ratings of SCH 40 PVC pipe:

1/2″  =  600 PSI
3/4″ = 480 PSI
1″ = 450 PSI
1 1/4″ = 370 PSI
1 1/2″ = 330 PSI
2″ = 280 PSI
2 1/2″ = 300 (not a typo, 2.5″ pressure is an oddity)
3″ = 260 PSI
4″ = 220 PSI

As you can see, the pressure ratings drop as the pipe size increases.  Note that the industry standard rule is that your normal operating pressure should not exceed 1/2 of the rated pipe pressure.  In other words, you shouldn’t use 1 1/2″ pipe for pressures higher than 165 PSI (330 x 0.5 = 165 PSI).  This is because pressure surges created by closing valves can easily double the water pressure in the pipe.  This rule applies to all PVC pipe, including that labeled SCH and CL.

### Class rated pipe

PVC pipe types labeled “Class” (abbreviated “CL“) are based on the pipe’s pressure rating.  So Cl 200 PVC pipe is rated for 200 PSI of water pressure.  Cl 315 PVC pipe is rated for 315 PSI of water pressure.  The strength of CL labeled pipe is directly related to the pressure rating.  The standard “Cl” pipes are Cl 125, Cl 160, Cl 200 and Cl 315.  Of these Cl 200 and Cl 315 are most common.  Cl 125 is sold as a low cost pipe for use in sprinkler laterals for those for whom low price is everything.  It has a very thin wall and breaks easily if not handled carefully or nicked with a digging tool.

1/2″ size pipe is generally only available in SCH 40.  This is because of the thin wall of 1/2″ pipe makes it very easy to break.  I don’t recommend using 1/2″ PVC pipe at all, however if you must, you should use SCH 40.  Sometimes you will find 1/2″ Cl 125 PVC pipe at discount stores due to the very low price.

The Class system is obviously a more logical system for labeling pipe as you know immediately how strong the pipe is based on the label.  Unfortunately the more confusing “SCH” system became entrenched in the industry and remains.

### What Pipe Type to Use

All PVC pipe labeled for a given size in the USA has the same outside diameter.  So any pipe labeled as 3/4″ will be the same diameter, whether it is SCH 40 or Cl 200 or any other type.  That allows the same fittings to be used to join the various pipe types together.  Most fittings are made to SCH 40 standards, although SCH 80 fittings are available, typically only at specialty plumbing and irrigation stores.  Technically most codes require SCH 80 fittings for pipe sizes 2″ or over.  In practice I’ve noticed that  SCH 40 fittings are often used up to 3″ size.  When dealing with sizes 4″ and above the use of non-glued “rubber ring-joint” fittings is recommended and usually required by code as well.  Glueing joints on 3″ and larger PVC pipe is very, very difficult.

“Mainlines” are all of the pipes that are under constant pressure, that is, the pipes that are before the sprinkler zone valves.   In most of the industry SCH 40 PVC pipe is used for irrigation mainlines up to 1 1/2″ size.  For 2″ size and larger Cl 315 PVC is used.  Most building codes prohibit the use of 2″ and larger SCH 40 PVC pipe for pressurized water lines.  Depending on the jurisdiction, this rule may or may not be applied to irrigation systems.  Those same codes generally require that all pressurized PVC pipes (mainlines) be buried at least 18″ deep to protect them from accidental damage, regardless of the type or size of pipe used.

“Lateral” pipes are the pipes after the sprinkler zone valve.  These pipes are only pressurized when the sprinklers are operating.   For lateral pipes the standard is to use Cl 200 PVC pipe.  Where budget is a concern and you can find it, sometimes Cl 160 is used.  As previously mentioned I recommend you avoid Cl 125 PVC pipe.  Laterals can be buried any depth, but I generally recommend at least 10″ deep to avoid a lot of maintenance problems with broken pipes.

### Why Not Use 1/2″ Pipe or Tube for Irrigation

Saturday, December 3rd, 2011

Well, if you have been reading my tutorials, I guess it’s pretty obvious that I’m not recommending you use 1/2″ pipe. I have a number of reasons for this.

• 1/2″ PVC pipe is generally not available in most areas except in SCH 40 type.
• The water capacity of 1/2″ pipe or tube is very low.
• 1/2″ PVC pipe is hard to glue together without using too much glue. The glue piles up on the inside of the pipe when you insert it into the fitting and blocks the water flow. Too much glue also weakens the wall of the pipe and a leak develops after a few years.
• Using 1/2″ pipe means you have to have another size of spare pipe and fittings on hand for repairs.
• But the biggest reason is that 1/2″ pipe leaves no flexibility for future changes or additions to your sprinkler system. If you ever need to add another sprinkler to the pipe you’re screwed.

My conclusion: The small amount of money saved by using 1/2″ pipe just isn’t worth the hassle and risk.

### 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)

### How to Use Pressure and Flow Switches with Irrigation Controls

Thursday, May 26th, 2011

Almost any major maintenance problem in an irrigation system will cause a unusual pressure level or flow level in your irrigation system.  Therefore pressure and/or flow monitoring is a good way to detect problems.  Most of the time the response to a abnormal pressure or flow level would be to shut down the system, or possibly to shut down the current valve zone  and try another one.  Irrigation systems are typically shut down using what is called a master valve.  A master valve is a single valve located at the water source that can shut off all the flow of water into the irrigation system.  For more details see my article on master valves. On systems with a pump you will probably want to shut off the pump.  Sometimes, as with booster pumps, you will need to both shut down the pump and close a master valve.

So what problems might an abnormal pressure or flow indicate? A very low pressure may indicate that perhaps the pump is broken (if you have a pump), an intake screen is clogged, a filter is dirty, a valve failed to open, or a pipe has broken.  Abnormally high pressure could be the result of  a valve not opening when it should, a dirty filter (if the pressure is measured upstream of the filter rather than downstream) or some obstruction in the pipes.  Low flow could indicate a valve failed to open, a filter is dirty, or that a pump isn’t working as it should.  High flow could indicate a broken pipe, a broken sprinkler, or a valve that is stuck open.   In most cases monitoring either flow or pressure is sufficient as opposed to monitoring both.

### How to Monitor Your Irrigation System

There are a number of different ways to detect and respond to abnormal pressure or flows.  Following are a few or these.  If you would like to suggest other methods, please contact me.  I realize this is not an exhaustive list.

Use a Smart Irrigation Controller that has a Sensor Input and Response Feature:
This is probably the easiest way to add pressure detection and response.   It is also what I consider to be the preferred method, as it is reliable and gives you the most control.  Some high-end irrigation controllers can use an electronic sensor hooked up to the mainline pipe to monitor the water in the irrigation system. Some of these controllers use flow sensors, some use pressure sensors, some can use both types.   These controllers with advanced features are typically sold as Smart Controllers and are expensive compared to ones typically found on a residential irrigation system.  Prices for these controllers typically start around \$300.00 and go up into the thousands for ones that handle dozens of stations.  But then you get a lot more with them too.  They are sold through professional irrigation supply stores, both online and locally.

WARNING: Be sure the controller will do exactly what you want BEFORE you purchase it!  Not all controllers marketed as “Smart Controllers” have these sensor input features, many only work with specific types or even models of sensors, and some controllers may not provide the response options you want or need.  You need to research the controller carefully.  Don’t rely on a simple check list of features!  “Sensor input” can mean almost anything, you need details!  I have seen controller feature lists where the unit sounded fantastic and ultra flexible, only to discover after closer examination that the actual response features don’t do what I need or want.   Read the actual owner’s manual (most controller manufacturer’s have them available on their websites) to see what the true capability of the controller is.  Read the sections of the manual on how to hook up the sensor, then there will also be a separate section on how to program the sensor you should look through.  Some controllers allow for time delayed responses, some don’t.  If you have a pump you will almost always need a time delay feature to bypass the sensor when the pump is starting up.  Even those controllers that do allow you to add delay times may not allow as much or little time as you need.  It is critical that you do as much research as possible before you go to the expense and effort of purchasing, installing and programming the controller.

For example, I have a Rainmaster Eagle Smart Controller on my own irrigation system, as well as using it on the majority of the commercial systems I design.  This particular Smart Controller has flow sensing capabilities, but it does not have built-in pressure sensing capability.  It does have a delayed response allowing delays of 1-6 minutes, but only in one minute intervals.  It will also allow the use of one additional simple on/off type sensor (most controllers have a circuit for this type of very simple sensors.  A simple rain switch is an example of this type of sensor.)    It has an audible “chirp” alarm that alerts you that a sensor response has been activated.  While this particular controller meets my needs, it certainly will not meet everyone’s.  Almost every major irrigation company makes a Smart Controller, and each has different features and capabilities.  Be sure you are using up-to-date resources when checking out models.  Smart Controller models are introduced each year, and often the capabilities of existing models change from year to year, so it is hard to keep up with them.

When using a controller with a pressure and/or flow sensor you start by installing the actual sensor on  the mainline pipe.  The method varies with the brand and model of sensor, most are pretty easily installed.  The sensor is wired to a special terminal on the irrigation controller.  Typically the wire used must be a special shielded communications cable, rather than standard irrigation valve wire.  Consider installing communications cable in PVC conduit to protect it, as it is very sensitive to even the smallest nicks from shovels, animals digging it up, or rodents chewing on it.  Most pressure sensors work by sending a reading of the current pressure to the controller every few seconds.  A typical flow sensor has a small paddle that turns as the water flows through the pipe.  Flow sensors normally send a signal based on the amount of flow, for example they might send a signal each time 5 gallons of water has flowed past the sensor.  The controller then interprets that data from the sensor and responds.   In most cases you will pre-decide what the response will be when you set up the controller.  For example; if you have a system with a pump, you could program the controller to shut down the irrigation system if the pressure was below 10 PSI for more then 2 minutes during the set irrigation period.  The 2 minute qualifier (delay) for shut down would allow the pump time to pressurize the system during start up and also avoid “false alarms” caused by brief dips in pressure.

Using a Simple Pressure Switch with a Pump Operated System:
This method is for those with pumps.  What I am describing here is for emergency shut off only.  I’m assuming you already have something set up to turn on or off the pump during normal irrigation operation.  That might be a standard pressure tank with a pressure switch to control it.  Or you may be using the pump start feature on the irrigation controller to actually start and stop the pump using a 120v relay.  The new pressure switch we are talking installing in this case is used only to detect pressures that indicate a problem and turn off the pump.  So if all is hooked up properly, in the event of blockage or no water going into the irrigation system the pressure will drop and the new pressure switch will shut the pump off.

This method requires that your irrigation system is leak free and can hold pressure for days between irrigations.  If the system is not leak free see #4 below.

1. Make sure you have a really good quality spring-loaded check valve on the irrigation mainline pipe.  The check valve goes someplace after the pump, but before the pressure switch.  A good quality check valve is needed to keep the water from leaking backwards out of the system through the pump.  Typically the self-priming feature of the pump is not good enough by itself to do this, you need a separate check valve.

2. You will need to use a pressure switch that works backwards from normal ones used for household water systems, since you want the switch to shut off the pump at low pressure (standard switches used on household water systems turn on the pump at low pressure.)  Some switches can be wired to work either way, others can’t.  Keep in mind that the low end on many common pressure switches in around 25-30 PSI.  That might be a bit higher than you want for a low end shut off, especially if your system will be operating at less than 45 PSI.  You don’t want accidental “false” shut offs since the only way to get the system back on will be to manually start the pump and hold it on until the pressure is back above the shut-off level.

3. There a problem to be dealt with.  The problem is that valves close slowly, taking as much as a minute or two to close after the controller tells them to.  At the end of the last irrigation cycle a typical controller closes the last valve and immediately shuts off the pump.  But it takes the valve several seconds up to a minute or two to actually close.  During this closing period the system will depressurize.  With no pressure in the system the pump will not restart for the next irrigation cycle, because the low pressure shut-off switch is detecting low pressure and shutting off the power to the pump.  There are two ways to deal with this.

A. You can fool the controller into keeping the pump running after the last valve circuit has finished watering.  Your controller needs to have the capacity for one extra valve on it to do this, so if you have 10 valves you will need a controller with 11 stations.  The last station on your controller needs to not have a valve attached to it.  Program 1 minute of time on that last station.  Now the controller thinks it is operating one last valve, so it keeps the pump running.   That will keep the system pressurized while the final valve closes.  If one minute is not enough time for the final valve to close then add another minute of run time to that last empty station.

B. Some controllers have a built in delay feature that keeps the pump running after the last valve closes.  This feature keeps the pump start circuit energized, which keeps the pump running for a minute or two after the last valve is signaled to close.  This gives the valve time to close before the pump is shut off.   Some less expensive controllers have this feature.  But typically only high-end controllers have this feature, so this method isn’t very practical.  If you are going to buy an expensive controller you might as well forget about using a pressure switch and use a Smart Controller and a sensor to shut the system down, as described in the first section of this article.

4. Often a small leak will cause the system to depressurize between irrigation runs.  This can be a major problem.  The pump will not start if the pressure is low, the low pressure switch is going to shut off the power to it.

If the leak is very small you can install a pressure tank, just like on a typical house water system.  Assuming a small leak, the tank keeps the system pressurized.  But that only works with a very small leak and it can take a huge pressure tank to supply enough water to keep the system pressurized.   If your system has a larger leak you will need to find and repair the leak.  If you can’t get the system leak free, you will need to take a different approach, as described below.

You can use a timer to over-ride the low pressure switch, and allow the system to start even with no pressure.  You will need a “Time Delay Relay”.  The time delay relay needs to be the type that allows the power to flow when energized, then shuts it off after a minute or two of delay.  It needs to have an automatic reset.  You then install the relay on a bypass wire around the low pressure switch.  That way the pump can start even when the pressure switch is “off” due to low pressure.  You will need to work with someone knowledgeable when ordering the time delay relay to be sure you get the correct relay, as they make many different kinds.

Using a Pump Controller with a Sensor:
This is essentially the same method as the Smart Controller method I described earlier.  Only the “smarts” are in the pump controller rather than in the irrigation controller.  Some of the newer digital pump controllers (don’t get confused here, we’re talking about a separate pump controller, not the sprinkler controller) are programmable, they are simply a small computer that operates a relay that starts and stops the pump.  You hook them up to a pressure sensor, also to the irrigation controller, and to any other sensor you want (wind, rain, temperature, light, flow, you name it.)  Then you can program them to do just about anything using that information input.  They can turn off the pump if a low pressure occurs for more than x number of seconds, turn off the pump if a high pressure occurs for x number of seconds, turn on the pump at a given time of day, etc.  Pretty much any input you want can cause the pump to turn on or off.  The capability depends on the brand and model of the pump controller. The downside is it takes electronics know-how to set the thing up and someone tech savvy to program it.  Typically you hook up a laptop to the pump controller to program in the logic, then once it is programmed it runs by itself.  The laptop just gives you an interface that is easier to work with.  I really can’t give you much more details beyond that, this type of pump control is beyond my expertise, I just have seen pump system experts use them to do amazing things.

### Is Bending PVC Pipe OK?

Wednesday, May 18th, 2011

PVC fittings only come in 90 degree and 45 degree angles.  Sometimes you need a smaller bend.  A website reader asked if it is safe to bend PVC pipe and if so, how much can PVC pipe be bend without damaging the pipe?

The answer is that, yes, it is OK to bend PVC pipe, but don’t bend it too sharp or too much.  Each pipe manufacturer has rules on what degree curve you can bend the pipe to based on the type and size of pipe.  You could look that up but it would take a lot of time and even then figuring out how much a 15% bend is out in the yard is not very practical for the average homeowner.  So here is a simpler “rule of thumb” that I basically just made up.  But it seems to work reasonably well, it’s easy to do, and it gives you a nice, visual answer!

To determine how much is the maximum bend you should allow grab one end of a length of the pipe you plan to bend and hold it so the other end is off the ground.  The amount the pipe bends on it’s own is about the maximum amount of bend you should allow.

You can also make any angle you want simply by using two 45 degree ells.  This is easier to demonstrate than to explain.  Get two 45 degree PVC ells.  Lightly push them together onto either end of a very short piece of pipe.  (Don’t glue them for now, this is just a learning experience.  If you do ever use them on a irrigation system then you can glue them!)  Now start twisting them in different directions.  You will see that you can make any angle curve from 0 degree up to 90 degree!  Add another 45 degree ell and you can make even more angles.  Have fun.  It’s cool!

### Using a Looped Mainline for Irrigation

Wednesday, April 27th, 2011

Looping your mainline often allows you to use a smaller pipe size for it, so using a loop system can be financially advantageous on a large irrigated area.  A looped mainline also provides maintenance advantages on larger sprinkler systems, and almost all large landscape irrigation systems, like parks and golf courses, utilize a loop mainline layout.  For smaller sprinkler irrigation systems they often provide little or no advantages.   As a general rule if you have less than 10 valve zones you are not going to get much of an advantage from a looped mainline.

## What is a Looped Mainline?

The irrigation mainline is the pipe that runs from your water source to the individual valves that turn on a group of sprinklers or a drip irrigation circuit.   When the mainline is looped that simply means that all or part of it creates a continuous loop.  Typically a looped mainline starts with a single pipe coming out from the water source (pump, water meter, etc.) then the single pipe splits into two pipes.  the two pipes loop around the irrigated area and then rejoin each other to create the “loop”.   Zone valves would be located at various points along the loop to supply groups of sprinklers.   Normally you would put a isolation ball valve on each leg of the loop at the “split”, the location where the pipes separate into the loop.  A third isolation ball valve is placed on the far side of the loop, allowing the loop to be divided into sections.  (see the sketch of a looped mainline below)   Additional isolation valves may be added anywhere along the loop if desired, to divide it into more segments.  The isolation valves allow you to shut down sections of the mainline for repairs while the rest of it may still be operational.  For a large irrigation system being able to shut down only a portion of the system for repairs can be very advantageous.   Very large irrigation systems may have multiple loops, sometimes one loop will even be inside of another loop.

Plan View Sketch of a Looped Mainline

## How to Design a Looped Mainline

When using a loop you should use the same size pipe for the entire looped portion of the mainline.  (This is not a hard and fast rule, just a strong suggestion unless you really understand hydraulics!)  The pipe leading from the water source to the loop may need to be a larger size than the loop pipe.  It is also OK to have mainline “spurs” off of the loop leading to other valves or faucets.  While unusual, the pipe size of the spurs may also be larger than the size of the loop pipe if they need to be.  Valves for sprinkler zones, faucets, quick coupler valves or any other equipment may be placed anywhere along the loop, as well as on the mainline leading to the loop or even on spurs off of the loop. Normally drinking fountains would be on a separate pipe and not connected to the irrigation system due to the possibility of water contamination from the sprinklers.  See the article on use of backflow preventers for more information on contamination.

### Multiple Loops

You can have multiple loops, but I suggest that if you do, you size the pipe using the outside perimeter (largest) loop, as if there were not any cross pipes within the loop (using the method that follows below.)  Then after you have determined what size the outside loop pipes need to be,  use the same pipe size for any smaller loops or cross pipes inside the perimeter loop.  (Technical note: The pipe sizes of inside loops and cross pipes do not necessarily need to be the same size as the outside loop, it is just that using the same pipe size will almost always work.  Using a smaller size pipe may work, but it may not.  So it is  advisable for non-experts to stick with the same size!)

## Loop Mainline Calculations

You must calculate both the friction loss AND the velocity for a looped mainline.  We’ll go through the process step-by-step.  You calculate the pressure loss for the non-looped mainline section from the water source to the beginning of the loop in the normal way (as described in the Irrigation Mainline Tutorial), using the pipe size, flow rate, and length of the pipe section.  I suggest that you use one of the  the Friction Loss Calculator Spreadsheets I’ve created, they are easier for non-mathematically oriented folks to use than the old manual calculations using charts.  Choose the proper spreadsheet for the type of pipe or tube.   Then enter the  size, the GPM, and the length of the loop.  Pressure loss for spurs off of the loop are also calculated using this same method.

### Pressure Loss in the Loop:

To calculate pressure loss for the looped section we simply will assume that the water flow splits, and 1/2 of the water is going around 1/2 of the loop and the rest of the water is going the other way around.  To do this start by determining the “highest flow GPM” that is found on the looped section.  If you are planning to operate only one valve at a time that would be the flow for the largest zone valve to be installed on the loop section.  If you will be running more than one valve at a time then the “highest flow GPM” will be the combined flow rate for the largest group of valves that will operate at the same time. Once you have the “highest flow GPM” you calculate pressure loss in the pipe using the calculator.  But for the looped portion you will enter 1/2 of that “highest flow GPM” for the flow in the looped pipe and 1/2 of the total loop length (all the way around and back to the beginning) as the length of the pipe.  The calculator result is the pressure loss in PSI for the entire looped section.

### Total Pressure Loss for the Mainline:

Just total up the pressure loss for the mainline leading to the loop, the pressure loss for the loop, and if you have any spurs add to your total the pressure loss for the single spur having the largest loss value.  The total of those is the pressure loss for your entire mainline network.

### Velocity Problems:

As mentioned, this method of calculating pressure loss uses an averaging system that assumes that half the water goes one direction to the valve, and the other half goes the other direction.  While this is not a perfect method, it works good enough for figuring out the pressure loss.  However, the flow doesn’t really split evenly in both directions.  In reality the flow balances in each direction based on pressure loss, with most of the flow in the loop going the shortest distance to a valve.  So if one of the zone valves is just a few feet from the split point on the loop, almost all the flow will go through that short distance rather than going the long way around and back to the valve.  Also in the event you make a repair you may close those isolation valves mentioned earlier.  This will force all of the flow in a singe direction through one side of the loop.  So we come to the second rule of loops, the pipe size for the looped section must be large enough to handle ALL of the flow in one direction.  If it is not, you may create excessive velocity of flow in a section of pipe which can cause major, and very expensive, problems.  This means you must check the velocity of the flow while assuming all the flow may go in a single direction around the loop.  If the velocity is over 7 feet per second you may create excessive pipe wear and water hammer.  Pipe wear can seriously shorten the life of your system, water hammer is much worse, let’s just say you don’t want it.  (Look it up if you really are curious.)

### Calculate the Velocity:

Using the Friction Loss Calculator Spreadsheets mentioned above enter the pipe size, the “highest flow GPM”, and any random value for the length of the loop.    Ignore the pressure loss it gives you, just check the ft/sec velocity result it gives you.  The velocity MUST be less than 7 ft/sec.  If it is not, you will need to use a larger size pipe for the loop.

That’s it!  You should now know how to create a looped mainline.  If it seems confusing try rereading it, it is admittedly a bit confusing after the first reading!  It really is simpler than it first sounds, be patient, grab some coffee, take your time.  Go back and actually work through it one sentence at a time, study the sample sketch of a looped mainline and try sketching your own on paper.    Practice a bit with the Pressure Loss Calculator to see what happens when you change the input values.  It should start to make more sense.

### 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.

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