Tag Archives: pvc fittings

Making Leak-Free Threaded Connections

Standard threaded fittings as used in most irrigation and plumbing are very slightly tapered.  This creates a more positive seal as the male thread is twisted into the female thread, to prevent leaks.  So the diameter of the male threads increases slightly at the back of the threaded portion, while the diameter of the female threads decreases further inside the fitting.  In most cases this taper is not sufficient to create a reliable seal, so a sealer is used.  For irrigation systems the recommended sealer is PTFE thread seal tape (often called Teflon® tape.)  Do not use “pipe dope” (liquid or paste type sealers) on irrigation systems!  If it comes in a tube or bottle, don’t use it.  Look on the bottom of many sprinklers and you will see the warning “Don’t use pipe dope”.   This is not an insult aimed at people who use pipes rather than tubes!  When pipe dope or paste works it’s way inside the pipe (which it will) the water will carry it to the valves, drip emitters, and/or sprinklers and clog them up and ruin them!  So the sprinkler manufacturer is warning you not to use them.  Only use PTFE tape type sealers.  If tape gets into a valve or sprinkler it can be removed and the damage is not permanent.

Using PTFE Thread Sealant Tape (Teflon® Tape)

When joining male and female threaded fittings, put a nice thick layer of PTFE thread seal tape on the male threads before you screw them into the fittings. Pull the tape tight onto the male thread so that the tape molds into the threads.  Wrap it in the direction of the threads so it doesn’t unwind off when you screw the fitting on. (If you are looking at the end of the male fitting that would be clockwise.)

How much tape to use?  The old standard was “3 wraps”.  However now they are selling low-cost PTFE tape that is thinner and requires more wraps.  When you have enough tape on the male threads the shape of the threads will be just barely visible through the tape.  I personally prefer to err on the side of using too much tape, it is not fun to find you didn’t use enough after the water is turned on and you discover a leak.

 

Wrap Teflon tape on the male threads in the direction shown. If wrapped the oppsite direction it will unravel as you attempt to screw on the female fitting.
Wrap PTFE tape on the male threads in the direction shown. If wrapped the opposite direction it will unravel as you attempt to screw on the female fitting.

 

Connecting the Fittings

Once the tape is on the male thread screw the male thread into the female threaded fitting. If the joint is between two metal pieces put a wrench on it and tighten it as tight as you can get it.  If one or both of the fittings is plastic just tighten it by hand. If you are an average guy or gal you can add one more full turn using a wrench after it is hand tight.  If you have ever been called a gorilla for your strength or grip, stop at hand tight. Over-tightening plastic fittings splits the female fitting, resulting in a leak. But not tightening them enough also gets you a leak.

Avoid joining a male metal threaded fitting to a plastic female threaded fitting.  This will be a disaster if you are not very careful.  The male metal end does not give at all, and the female plastic fitting is likely to split open unless it has heavy reinforcement.  If you must join male metal to female plastic, use lots of PTFE tape and hand tighten only.  No wrenches!!

High density polyethylene (HDPE, trademark name is Marlex®), street ells often don’t need PTFE tape to seal, but I still use a little on them.  It will not hurt to use a couple of wraps of PTFE tape on HDPE fittings.  HDPE is a shiny, black, plastic that is slightly softer than PVC and feels slightly “oily”.  HDPE is good for places where the fitting needs to be able to rotate and not “seize up” over time.  Metal, PVC and PBS plastic threads will seize up and not turn easily once assembled.  The HDPE fittings are idea for sprinkler risers where you want the threaded joints to remain pliable and able to move and absorb impacts.  Warning: The black barbed insert fittings used to connect to flexible tubing risers (often called “Funny Pipe®”) are not HDPE.   HDPE is seldom, if ever, used for any fitting with barbs.  This is because the slippery surface of HDPE makes the tube slide off the barbs!  If it has barbs, be sure to use PTFE tape on any threads.

Rule of thumb: If you can’t scratch the plastic with your fingernail, use PTFE thread seal tape on the threads!

A "Marlex" or high density polyethylene street ell.
A High Density Polyethylene (HDPE) street ell.  It should feel a little “oily” if it is HDPE.  If not it may be ABS or PVC plastic.

This article is part of the Sprinkler Irrigation Installation Tutorial Series
<<< Previous Page ||| Tutorial Index ||| Next Page >>>
By using this tutorial you agree to be bound by the conditions and limitations listed on the Terms of Use page.

 

A Guide to Irrigation Pipe Fittings

Basic Terminology and Definitions Used for Pipe Fittings:

Every industry has it’s own “code language”, sometime terms are a short-hand way to communicate faster and more accurately, some are just a way to identify “insiders” from “outsiders.”   The following will help you to speak “Irrigation” (as well as “Plumbing” which uses most of the same terms.)

SSS, SST, SS, ST, etc…
These are terms used in describing the connections for PVC fittings. “S” stands for “SLIP” “socket”, or “spigot”, which means that the connection is a solvent weld (or glued) type. By the way, both sockets and spigots CAN be threaded also, which is why we use the term SLIP to specify that the connection is solvent welded. I’ve been told that slip really only means a socket, but common usage is for both sockets and spigots. In case you hadn’t guessed by now, there is a bit of confusion on this subject! (Webster’s Unabridged Dictionary doesn’t have the plumbing definition among it’s 69 definitions for the word slip.) At any rate, if you go to the hardware store to get a 3/4″ tee with solvent weld inlet and outlet and a 1/2″ threaded side outlet, you would ask for a “three quarter by three quarter by half tee, slip, slip, thread” and you would write it as “3/4 x 3/4 x 1/2 TEE SST”. (You can say “one-half” if you want to be technically correct, but most of us lazy people just say “half”.)
Lock, Loc, Push-Fit, etc…
There are an assortment of different terms used for these easy to assemble, glue-free fittings, most often the names in some way imply that the tube is “locked” into place.  In irrigation most PEX connections are made using these push in and lock type fittings, where the tube is pushed into the fitting and a sharp metal clip locks it in place.  At the time I am writing this article, the lock type fittings are starting to be made for all types of plastic tube and pipe, including  poly tubing, PVC, and drip tubing.    One note, when assembling using these locking fittings, be sure you press the tube or pipe very firmly into the fitting forcing it all the way in, it needs to go fully into the fitting in order to form a tight connection and not leak.
Spigots, Sockets, and Slip.
A spigot is the equivalent of a male end. A socket is a female end. In other words a spigot fits into a socket. Slip can mean either a spigot or socket, but normally means socket, and normally means the connection is solvent welded. For more on the subject see “SSS, SST, SS, ST, etc..” above.
Male and Female.
Oh come on now. Surely you can figure this one out! (Hint: many plumbing terms were originated by sex-crazed males.) Well all-right, male means spigot, female means socket. What do you mean that didn’t help?
Solvent Weld
You can say “glued” if you want, but that’s not totally correct (you do get partial credit). The cement (glue) used for connecting PVC parts is sticky like standard glue, but in addition it actually melts the plastic, creating a true weld. Thus the term “solvent weld”. By the way, the solvent for PVC is acetone (nail polish remover), so you can remove the PVC cement if you spill it with acetone.  (I saved the front seat of my truck from disaster this way after knocking over a complete can of purple color pvc cement onto it.  It was a lot of work and took a lot of acetone, but it all came out of the seat fabric eventually.)  You can also use acetone to clean PVC pipe, but be careful as the acetone will melt the pipe if you use too much!
Galv.
Abbreviation used for galvanized steel. Always avoid screwing a male metal fitting into a female plastic fitting. The plastic fitting will split when you tighten the joint. Always work male plastic into female metal when going from metal to plastic parts. If you absolutely must use a plastic female fitting place a worm-gear clamp around the very end of the female fitting and tighten it slightly before you insert the male end. Use lots of Teflon tape or pipe dope and don’t over tighten!
Cu, Type M, Type L, Type K.
All are abbreviations used for copper tube and fittings.  Type M, L, and K describe the thickness of the copper pipe wall.  M has the thinnest wall and K has the thickest.  K is the strongest of the three.  K is the only one that should have threads cut into it, the others are weakened too much by cutting threads.  K is a good choice for use where physical strength is required, such as supporting the weight of a backflow preventer, although L will support a smaller 3/4″ or 1″ backflow preventer in most situations. L is the most common choice for standard uses, especially if the tube will be in or under buildings.  Type M is more likely to develop pin holes in it over time.  Widespread use of type M in homes during the 60’s and 70’s has lead to the creation of an entire industry devoted to replacing or rehabilitating failed copper pipe in homes.
Insert
A type of fitting used with polyethylene pipe. Insert fittings have barbs that are inserted into the open end of the tube. You must use a clamp placed over the tube and tightened to physically hold barbed fittings in place. Do not rely on the barbs to hold the tube on the insert fitting.  Crimp and worm gear clamps are both used.
SCH 40, SCH 80.
Terms used with PVC fittings that indicate the specification standard the fitting was constructed to meet. SCH 80 is usually a gray color and is stronger than SCH 40 which is usually a white color.
UPC, NSF
Uniform Plumbing Code, National Sanitation Foundation. Indicates that the PVC fitting complies with standard code requirements. An indication of a top-quality PVC fitting.
Fipt, Mipt
Female Iron Pipe Thread, Male Iron Pipe Thread. Specifies the end is threaded and the thread pattern used is standard iron pipe style.
PVC
Poly Vinyl Chloride. The material most plastic fittings are constructed out of. See SCH 40, SCH 80 above.
Marlex, HDPE
The other plastic that some fittings are made of. Marlex is actually a brand name of a specific High Density Poly Ethylene. Normal PVC threaded joints “seize up” and will not turn freely. The HDPE has a oily surface which acts as a lubricant. HDPE fittings are used in situations where the threaded connection needs to remain flexible, such as swing joint risers. (A swing joint allows the sprinkler to move up and down for adjustment and to protect it from being damaged if run over by a vehicle. Swing joints are the standard type of sprinkler riser used in parks and golf courses.)
Teflon Tape, Pipe Dope
Most threaded joints need a sealer placed on the threads before the connection is made. The sealer serves two purposes. First it seals the joint (like you couldn’t figure that out). Second, it lubricates the joint, which makes it much easier to thread the pieces together. Teflon tape is my preferred product , it is easy to use and clean. Wrap 3 layers around the male threads, wrapping in the same direction as the threads (so it doesn’t unwrap when you start threading the fittings together). Don’t cover the end thread, that will help avoid “cross-threading” the joint. Be careful not to allow “strings” of Teflon tape to get into the pipes where they can clog sprinklers or emitters. Never use pipe dope with sprinklers, it can gum up the nozzles, and in gear-driven sprinklers it jams the turbines. Most sprinklers say “no pipe dope” on them, the manufacturer is not calling you a derogate name and saying not to use pipe!

The Basic Fittings (drawings are of PVC fittings but names also apply to polyethylene and PEX.)

Bell Reducer
Bell Reducer
Bell Reducer. A bell reducer has female threads on both ends. Bell reducers are generally not available in PVC (so, despite what the caption says above, this drawing is not of a PVC fitting).

Cap
Cap
Cap. A cap may have a solvent weld socket end or a female threaded end. The other end is closed off. If a solvent weld cap is used to provide for a future connection point, be sure to leave several inches of pipe before the cap! When the cap is cut off for the future connection there will need to be enough pipe present to glue a new fitting onto! I can’t begin to tell you how many times I’ve seen a solvent weld cap butted right up against another fitting, making it impossible to ever use the capped connection again!

Coupling
Coupling
Coupling. A coupling connects two sections of pipe together. Couplings may have solvent weld socket ends or female threaded ends.

Cross
Cross
Cross. A cross connects four pipe sections together. Crosses may have solvent weld socket ends or female threaded ends (no female threads available for PVC). Crosses are special order parts at many suppliers. Crosses create a great deal of stress on the pipe because they have four connection points. In theory this is the same principle that makes a 3 leg stool (a “tee”) more steady than a 4 leg stool (a “cross”). I recommend that you avoid using crosses in most situations. Use two tees.

Female Adapter
Female Adapter
Female Adapter. Female adapters are used to add a female threaded pipe connection on a solvent welded pipe. Never use female adapters when converting to a metallic pipe. The metal male pipe threads tend to split the PVC fittings. Place a metal coupling on the metallic pipe then use a PVC male adapter. Metal male threads should never be inserted into any female threaded PVC fitting!

Male Adapter
Male Adapter
Male Adapter. Male adapters are used to add a male threaded pipe connection to a solvent weld pipe section.

Plug
Plug
Plug. Used to plug a unused fitting outlet. May have female threads or a solvent weld spigot. In most cases a threaded plug is used to provide a connection point for future use. If solvent welded in place the plug is never going to be removed!

Side Outlet Ell
Side Outlet Ell
Side Outlet Ell. Side outlet ells are an ell with a side outlet. (Well duh…) They most commonly have two 3/4″ or 1″ solvent weld sockets, with a 1/2″ side outlet having female threads. Side outlet ells are common in residential sprinkler systems, but are seldom used in commercial installations. The side outlet is listed last when stating the side outlet ell size. Example: 1x1x1/2 SO ELL SST has a 1/2″ threaded side outlet.

Tee
Tee
Tee.The most common fitting! Available with all female thread sockets, all solvent weld sockets, or with opposed solvent weld sockets and a side outlet with female threads. Many configurations of “reducer tees” are available, meaning that one or more of the sockets is smaller than the others. Tees are always labeled as “NxNxN TEE with the side outlet as the last size. The largest of the other two sockets is always listed first. Thus a 1×3/4×1/2 TEE SST has a 1/2″ threaded side outlet (T for threaded) with the remaining sockets being 1″ and 3/4” solvent weld sockets (SS for slip, slip). On a “bullhead tee” the side outlet is the largest socket on the tee (thus it looks somewhat like a bull’s head I guess). The side outlet is referred to as the “bullhead”.

This article is part of the Sprinkler Irrigation Installation Tutorial Series
<<< Previous Page ||| Tutorial Index ||| Next Page >>>
By using this tutorial you agree to be bound by the conditions and limitations listed on the Terms of Use page.

Pressure Loss in Sprinkler Risers

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:

OK, I realize that didn’t answer your question, you are asking about a non-standard riser that uses a long pipe to hold the sprinkler high above the ground.  In that case you must calculate what the friction loss will be in the longer-than-normal riser pipe. (In this case that would be the 3 ft long pipe you described in your question above.)  To do that you simply use the same friction loss spreadsheets that you use to calculate the friction loss in any other pipe.  Just use this link to get the proper spreadsheet from my website for the type of pipe you are using.  Then open the spreadsheet and on the first line enter the pipe size, GPM of the sprinkler you will install on the riser, and the length of the riser.  Enter an error factor of 1.4 rather than the default 1.1.  This is because even your “longer” riser is shorter than the typical pipe length that the default error factor is based on.  Now read the friction loss.  That’s it, you have the friction loss for your non-standard riser!  Don’t worry about the fittings like ells and couplings that are part of the riser, that is part of what the error factor is compensating for.

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.

 

Calculating Sprinkler System Pipe Size Using a Spreadsheet

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:

  1. Download and open the Friction Loss Calculator.
  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.

 Previous Page of Tutorial      Sprinkler Design Tutorial Index      Next Page of Tutorial

Is Bending PVC Pipe OK?

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!