How to Select the Right Booster Pump

This page is part of the Pump Tutorial Series.  The 1st page is at  Pumps: Selecting-a pump step-by-step.

Booster pumps are used to increase the water pressure.  Increasing the water pressure may increase the flow as well, more pressure makes the water move faster through a pipe, so more flow usually results as well.  If the water pressure is too low a irrigation system will perform poorly.  For example, doughnut shaped dry areas around the sprinklers are a visual indication of insufficient water pressure.  (Hint: take a look at Google Maps using the aerial view, the yellow colored doughnuts really show up on the aerial photos.  You’ll likely spot a few at school playgrounds and parks if they are watered with sprinklers.)   One solution to low water pressure problems is to add a booster pump to increase the water pressure.

A quick warning here.  Increasing the water pressure is not an automatic solution for any and all irrigation coverage problems.  If your sprinkler heads are too far apart no amount of water pressure will get rid of the dry spots.  Your irrigation system must also be able to handle the increased water pressure and flow as well.  Creating too much water pressure can do very serious and expensive damage to an irrigation system.  If the irrigation system is connected to your home water supply it can also damage the plumbing inside your house by creating high pressure surges that travel through the pipes known as “water hammer“!  We are talking about very expensive damage, it can result in the need to replace all the pipes in your house.  As a general rule you should not boost the pressure over 70 PSI unless you really know what you are doing.

Most booster pumps are end-suction centrifugal type pumps simply because that type works best (we’ll talk more about that on the next page.)  Here’s the process of determining the required pressure and flow for a booster pump:

  1. Figure out how much water pressure the irrigation system optimally needs.
  2. Measure how much pressure the system actually has now.
  3. Subtract the actual pressure from the optimal pressure, and that gives you the amount of pressure the booster pump needs to create.  This is simple addition.  The existing pressure plus the booster pumps pressure gives you the final irrigation system pressure.
    Existing PSI + Booster Pump PSI = Irrigation System PSI.
  4. You will need to know how much flow will occur in the irrigation system once the pressure is increased to the optimal level.  Remember what we said above, when the pressure is increased the flow will also increase. If this is a new system your design will have determined this in the design process.  It this is an older existing system you probably don’t know the flow, so you will need to reverse engineer your sprinkler system.
  5. You now know the required boost in pressure and the flow so you can select and purchase a booster pump using that information.

Pressure is measured one of two ways in north America, PSI or feet head.  Pumps used to be always rated for pressure using the feet head system, while sprinklers are typically rated for pressure using PSI.  Recently pumps have started using PSI like the sprinklers.  Thankfully both PSI and Feet Head measure the exact same thing, pressure, and it is easy to convert back and forth between the two:

PSI x 2.31 = feet head
Feet head x 0.433 = PSI

Okay, let’s work through an example as that is the best way to understand this.

Example: The existing pressure in the water company mainline you will use to supply water for your sprinkler system is 35 PSI static. Static pressure means the water pressure when measured with all water flows shut off; no faucets running, ice maker is off, no sprinklers on, nobody taking a shower (don’t turn off the water if someone is in the shower!!!), etc. To measure static water pressure just get a pressure gauge at the hardware store and attach it to a water outlet someplace reasonably close to the irrigation system. Make one last check to be sure all other water outlets are turned off, then turn on the water to the gauge only. The gauge will show the static water pressure.

So let’s say you measured 35 PSI existing static water pressure.  You check your irrigation plans and discover your irrigation system is designed to require 50 PSI to operate correctly. So you decide to add a booster pump to create more pressure.  But what size pump?

We determined above that the booster pump needs to raise the pressure to 50 PSI.  We measured the existing static pressure and it was 35 PSI.  So simple math reveals how much additional pressure the pump needs to create:  50 PSI – 35 PSI = 15 PSI.  So we need a booster pump that produces 15 PSI of pressure at whatever flow rate the irrigation system requires.

Now you need to know the required flow as well.  Again, it’s nice if you have a set of irrigation plans for the system that tell you.  But if you don’t then you will need to reverse engineer the system to determine the flow.  Remember the flow will be higher after the pressure is boosted, so when you reverse engineer it use flow values based on the higher new pressure.  This is a bit tricky.  The best way is to calculate the increase is to use flow data for your sprinklers that you will find on the manufacturer’s website.   So you look up your sprinkler model and discover that the flow for the sprinkler is 2.7 GPM at 35 PSI.  Then you look up what the flow for that same sprinkler is at 50 PSI is 3.2 GPM.   Now determine the percent increase in flow.  3.2 / 2.7 = 1.19.   We have discovered the flow will increase by 119%.  Lets say the current flow is 20 GPM.  The flow for the booster pump will need to be 119% of that.  So 20 GPM x 1.19 = 24 GPM (23.8 rounded up to 24.)  So the booster pump needs to have a flow of 24 GPM.

One last possible problem…  some brands of pumps rate pressure in feet head, not PSI!  So convert just in case lets convert the PSI to feet head. 15 PSI * 2.31 = 35 feet head (round the result up to the next whole number.)  That wasn’t difficult at all!

 

There are many companies that build and sell pre-packaged booster pump systems.  These are really good solutions for most people because setting up a pump is not within most people’s talent skill range!  These pump packages come with everything you need pre-assembled and ready to go. Typical assemblies include the pump, electrical controls, any needed control valves, a frame to hold everything and an enclosure to protect it from the weather.  All you do is install it on a concrete pad, connect the pipes, and connect it to the power source. For most people this is the best way to buy a booster pump.

Now that you know what the process is, let’s move on and start learning about pumps.  First up there are lots of different types of pumps used for irrigation systems: centrifugal, submersible, turbine, jet, etc..  Let’s determine which is the best type for your irrigation system.

Go to the next page –>   Types of Water Pumps


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