Selecting a Pump for Your Existing Irrigation System:

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

If you have an irrigation system that currently has a pump on it, and everything has worked good right up to the point the existing pump died, then your best bet is to replace the pump with the exact same model as the old one.  This is a much, much easier process than trying to find a different make or model of pump.  However if the old pump wasn’t really getting the job done, or isn’t available anymore, this is a good time to reevaluate your pump needs.  Perhaps there is a better pump out there for your system.  This tutorial will teach you how to do that.

Replacing it with the Same Exact Pump:

Just a quick tip on swapping out pumps, then we’ll move on to selecting a new one.

Make sure the new pump really is the exact same pump as the old one.  Many centrifugal pumps (the most likely pump type for an irrigation system) have custom impellers in them that are cut and sized for a very specific flow and pressure.  This means even if you get the same pump model, it may not have the exact same performance as your old one.   Do NOT rely on a tag on the pump that says the GPM and PSI.  These tags are often based on the basic model’s default performance, while your pump may be customized.  The pump should have a serial number on it that hopefully will allow the manufacturer to find the original pump specifications from their records (but don’t hold your breath, records are often lost.)  So get the serial number off the pump as well as the model number.   Take photos of all the name plates and labels on the pump, as well as the entire pump from various angles.  Take lots of photos of the pump and bring them with you (or email them) to the pump dealer to help their tech department figure out what actual pump customization you might need.


Getting a Different Pump Make and Model:

We will assume from here on that you want a  whole new pump model, maybe even a different brand.

Warnings:

  • Do not select a replacement pump based on horsepower alone. Two pumps can have the same exact horsepower and produce radically different flows and pressures.  We’ll discuss this more later.
  • Keep in mind that if your old pump is over 5 years old it is likely worn, and as pumps begin to wear out their performance decreases. So while you may be thinking you need a bigger pump because the irrigation isn’t working as good as it should, this may not be true.  You may just need a newer, less worn out pump!  When you install a new pump you may find it performs much better than the old worn out one did, even though they are identical.

Step-by-Step:

Here’s the basic procedure to follow if you’re selecting a pump for an existing irrigation system.

 

1. Determine your flow and pressure requirements.

If you have the original irrigation design the plans should tell you this information.  If so go on to step 2.  If not, you will need to figure out what the requirements are for your irrigation system by “reverse engineering” it.

Go to the page ->    Reverse Engineering A Sprinkler System  for step-by-step instructions.  Then return to this page.

2. Miscellaneous Stuff to Go with a New Pump Installation. 

You may need to add some new pipe and other equipment to the existing irrigation system to accommodate the new pump. For example if you are switching a irrigation system from city water to water from a lake or stream you will probably need to add a intake pipe or manifold, as well as an intake screen and filter.  You may require pump control valves (ie; anti-cycle valves), filters, and/or new mainline pipe to attach the new pump into the old irrigation system.  Each one of these new items causes some additional water pressure loss, so you need to add pressure to the PSI value you previously calculated above for each of them.   You may be able to get exact pressure requirements for many of these items like filters and valves from the item manufacturer’s website.  If not you can figure 5 PSI for each as a “guesstimate” value.  For long lengths of new pipes the pressure loss can be calculated using the pressure loss calculators on the Landscape Irrigation Formulas page of this website.

Example: If you’re just adding a short intake pipe (20 feet or less) and a filter you could figure out exact pressure losses using the spreadsheets and the filter manufacturer’s data.  If you don’t have that information you can “guesstimate” and add about 10 PSI additional pressure (5 PSI for the filter plus 5 PSI for the intake pipe.)  So if you had previously determined you needed 65 PSI to run the sprinklers, you would add the 10 PSI to that value, resulting in a new pressure requirement of 75 PSI.

 

3. Finish this tutorial to find your replacement pump.

Use the link below to continue the tutorial.  It will teach you about pumps and guide you through selecting one.

Go to the next page –>   Types of Water Pumps

 


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