Sprinkler systems need flow and pressure to operate. Sometimes you might need to know the flow and pressure requirements of an existing sprinkler system. For example the flow and pressure are needed if adding on to the existing irrigation system or adding or replacing a pump. This article contains instructions on how to determine those values. This article uses the USA measurements GPM for flow and PSI for pressure, however conversion to metric is easy using standard conversion formulas.
How to Reverse Engineer the Flow (GPM)
This is going to take some time and a lot of crawling around on your hands and knees… sorry, that’s just how it is.
- Look to see the sprinkler brand and which nozzles are installed in the existing sprinklers on a single valve circuit. Write them all down. You may have to look very close to find the nozzle number, it is usually imprinted using tiny text on the nozzle itself. It may be right next to where the water comes out, or it may be on the top of the nozzle. It is often hard to see. Each sprinkler may have a different nozzle, so you will need to look at every one of them and write them all down. Rotor type sprinklers (streams of water that rotate) tend to have numbered nozzles, #1, #3, #9, etc.. For rotors you often need to pull the pop-up riser up from the sprinkler body to see the nozzle outlet and the number. Spray type sprinklers (steady spray like a shower head) tend to have a number followed by a letter that indicates the arc, like 10F, 12H or 15Q.
- Look up the flow requirement (GPM) for each sprinkler and nozzle using charts you should be able to find on the sprinkler manufacturer’s website. Add all the GPM values together to determine the total GPM for the valve circuit. For older discontinued sprinkler models you may have to contact the manufacturer’s consumer help department and ask them to email you performance charts.
- The sprinkler manufacturer’s website will probably give you a chart that shows different flow values for the nozzles depending on the “PSI”. To determine the PSI to use measure the distance between adjacent sprinklers in feet. Measure several and determine the average distance. Now use that average distance between sprinklers as the “PSI” value– but do NOT use a value less than 30. Example: If the average distance between adjacent sprinklers is 45 feet, use 45 PSI to find the GPM in the chart. If the average distance between sprinklers is 25 feet, use 30 PSI. Do not use less than 30 PSI!
- Repeat for each valve circuit.
- Assuming you run one valve circuit at a time, your flow requirement for the pump will be the same as the single valve circuit with the highest flow value (GPM.) If you run more than one circuit at a time, add together the GPM values for the ones you run together as they essentially become one big circuit when run together.
- To recap, calculate the total GPM of each group of sprinklers that run at the same time. The flow (GPM) value to use when determining the pump size that of the group of sprinklers that has the highest total flow (GPM).
Determining the pressure requirements can be difficult. First off, pump pressure in the USA is measured in either “Feet of Head” (Ft.Hd.) or “PSI”. (The rest of the world uses “bars” but this article is USA based. ) We’ll talk more about this later, but for now you can use either “Ft.Hd.” or “PSI.” You can switch back and forth between these two values using simple conversion formulas:
____ PSI x 2.31 = ____ Feet of Head (ft.hd.)
____ Feet of Head x 0.433 = ____ PSI
How to Reverse Engineer the Pressure
(3 different methods)
One quick rule of thumb- too much water pressure is much easier a problem to deal with than too little pressure. If you are uncertain at all, always go with the higher value. It is very easy to throttle down the pressure if you have too much, if you don’t have enough pressure it is very hard and expensive to increase the pressure. So if you are looking at a sprinkler head and asking yourself: “should this sprinkler head use 30 PSI or 35 PSI, I’m just not sure?”, the answer is easy, use the higher pressure, 35 PSI.
Method #1: Measure It with A Gauge.
If the irrigation system is still operational install a pressure gauge on the pipe as close as possible to where the water source, turn on the system, and the gauge will tell you the pressure the existing system uses. You may need to cut the water supply pipe and install a tee for the gauge to connect to, or possibly you can drill a hole in the pipe, thread the hole with a 1/4″ NPT thread tap, and then screw the gauge into the hole (most gauges have 1/4″ male NPT thread connections on them, but you might want to make sure before you drill and tap!)
If the irrigation system uses a pump to run it you typically would measure the pressure within a foot or two of the pump outlet. Remember that if you do have an existing older pump the pressure you measure may be lower than it should be due to the pump being tired, old, and worn out. You might want to add another 10 PSI to the pressure you measure at the pump to compensate for age.
Method #2: Calculate the Pressure losses.
If you can’t take a pressure reading on the existing system, the next best method is to completely redo all the calculations for the sprinkler system. Let’s be honest, this is too difficult and time consuming for most people, but it is the best way. If you want to try it you need to read through the Sprinkler System Design Tutorial to learn how pressure requirements for a sprinkler system are calculated. You should be able to then reverse engineer your existing sprinkler system by calculating the pressure loss in each section of pipe, each valve, each sprinkler head, etc. to figure out the PSI it requires to operate. You may have to dig up some pipes to determine what size they are. It is a lot of work, you will have to basically learn how to design irrigation systems to do it. Seriously, it is unlikely you are going to do this unless you are an engineer or just very anal-retentive… let’s move on.
Method #3: Guesstimate It.
If you really can’t figure out the pressure needed you can use a guesstimated pressure value. Most of you are likely to use this method. Obviously this method is not optimal and there are no guarantees it will work but it’s a lot less effort that the first two methods above. The idea here is to guesstimate high as previously mentioned in the introduction. Not sure? Add 5 PSI! Really feeling uncertain? Add 10 PSI. You get the idea. Instead of measuring with a pressure gauge, the guesstimate method is going to require you to grab a tape measure (which most people have) and you won’t need to cut into any pipes or dig anything up.
FYI: This guesstimate method is based on some hydraulic principles that define the water pressure needed at a sprinkler head in order to shoot water a specific distance, along with some very rough assumptions of how much pressure is typically needed to move water through all the various pipes, valves, etc.
Sprinkler spacing in FEET + 30 = guesstimated PSI required at water source.
If the resulting total is less than 50, use 50.
Do not use a guesstimated water pressure value less than 50 PSI !
To guesstimate the pressure needed take the largest distance in feet between adjacent sprinkler heads, and add 30 to it. If the answer is less than 50, use 50. Do not use a pressure lower than 50, very few sprinkler systems will function well at a pressure below 50 PSI. Which sprinkler heads to measure? Generally you should measure and write down the distance between all your sprinkler heads that are next to each other. It’s not uncommon to have two heads that are way further apart than all the others. If so you can choose to ignore that set of sprinklers. The examples below will help this make more sense.
Example: let’s say you have 6 sprinklers and you measure the distances between the adjacent ones and you get 50′, 32′, 29′, 33′, 35′, and 29′. In this case 50′ is way out of line with the other values so let’s ignore it and use the next greatest distance, which is 35′. So then 35′ + 30 = 65 PSI. So in this case 65 PSI is the guesstimated pressure required for the system. Understand there may be a dry spot between those two heads that are 50′ apart (there probably already is a dry spot there!) That 50′ distance is really a design error on the original sprinkler system and there should probably be another sprinkler head in the middle of that space.
More Guesstimate Method Examples:
Sprinklers 15 feet apart: 15 + 30 = 45 PSI. This is less the 50, so use 50 PSI.
Sprinklers 35 feet apart: 35 + 30 = 65 PSI. Use 65 PSI.
Sprinklers 45 feet apart: 45 + 30 = 75 PSI. Use 75 PSI.
Remember, 50 PSI is the minimum!
Remember: No guarantees, this method gives you a guesstimate! You understand that you are taking a risk using this. Buying a pump? It is strongly recommended that if you guesstimate the pressure you buy your pump from someplace with a generous return/exchange policy. You may need to return or exchange the pump.