Busting some common myths about pumps
|By Kurt Thompson|
There can be a lot of misunderstandings surrounding this important part of an irrigation system.
Irrigation system pumps can be confusing, especially for less experienced contractors or technicians. For one thing, there are so many different kinds of pumps commonly used in landscape irrigation that are known by many names, formal and informal: lake, jet, shallow well, multistage, submersible, vertical turbine, variable frequency drive and centrifugal. In actuality, all irrigation pumps are centrifugal pumps, and they all work by the same principles.
Because of this confusion, there are many inaccurate perceptions and assumptions about pump selection and operation. If a contractor or technician selects a pump based on one or more of these faulty perceptions, it can lead to significant problems for the irrigation system in which it is installed. Some of the more common “myths” or misperceptions are presented here with a basic explanation of the science behind the facts.
Myth #1. The higher the horsepower, the more pressure the pump will produce.
It is a common practice to replace a pump with a higher horsepower motor when more pressure is needed. In order to determine if this is fact or fiction, we need to look at the relationship between motor horsepower and pump discharge pressure.
The curve shown in the graphic is for a model of pump that is widely used for irrigation systems. It is available with four different horsepower motors. This pump’s performance curve shows the typical relationship between horsepower and pressure. This curve is only to show how pumps behave under different circumstances; always refer to the manufacturer’s specifications for the specific pump being considered.
If the flow of an irrigation system is 30 gallons per minute, the curve shows that the discharge pressure of the 1-horsepower model will be approximately 35 pounds per square inch. This pressure at the source is too low for most irrigation components except possibly drip; therefore, it probably will not be a common pump choice for an irrigation system.
If the 1.5-horsepower motor is selected for this pump, at 30 gpm, the pump pressure will be approximately 45 psi. If this isn’t enough psi for the sprinkler system to cover, then a contractor might be motivated to use the 2-horsepower model. At 30 gpm, the 2-horsepower model will produce about 49 psi. This 5-psi increase will probably not be enough to make a difference in improving the sprinkler’s coverage.
If the 2-horsepower pump is replaced by a 2.5-horsepower pump, the pressure would be about 54 psi. Again, the 5-psi increase might not be enough to make the sprinklers perform better than the smaller horsepower pump would.
Even if the 1.5-horsepower pump is replaced with a 2.5-horsepower model, the increase in pressure would only be about 10 psi. Most of the time a pump is changed out because a system needs more than 10 psi of increased pressure.
So, the solution to getting more pressure is not to increase the horsepower, unless you only need a few more pounds per square inch. That’s because applying more power to the impellers does not increase pressure very much; rather, it’s the diameter of the impellers and/or the number of impellers in a pump that creates significant changes in pressure.
There is only one way to get more pressure out of an existing pump and that is to reduce the flow demand of the pump. All pumps will deliver higher pressure at lower flows and lower pressure at higher flows. In other words, pumps will always sacrifice pressure to deliver flow.
One common way to reduce flow demand is to split a single zone into two smaller ones. Sprinkler flow can also be reduced by retrofitting conventional sprays with lower-flow nozzles. Be aware that the precipitation rate will also be reduced, so run times may need to be increased.
If it is not practical to make such changes, or if doing so will not achieve the desired pressure and flow rate, then replacing the existing pump with one that will is the only solution. You will be surprised at how many times a pump that is properly sized for the flow and pressure requirements will have the same or maybe even a lower horsepower than the pump being replaced.
Myth #2. A 2-horsepower pump will produce 30-40 gpm. That means I can have 10-12 rotors with 3-gpm nozzles on a single zone.
A common belief is that the flow of an irrigation system is the primary factor when selecting a pump or checking if an existing pump is the correct one. This belief is half right; the other essential factor is the pressure the pump needs to produce that flow in order to give the sprinklers the pressure they need to operate adequately. When sprinklers do not receive water at the adequate pressure, the coverage will be poor and more sprinklers will be needed to cover the same area.
All of the major irrigation component manufacturers design their nozzles to operate best at a certain optimum pressure. Understanding this is important, because the further away from optimum pressure you get, the worse the coverage will be. The table below shows the approximate high, low and optimum operating pressures of different sprinkler types.
That does not mean the sprinklers will not work at pressures that are lower or higher than optimum; it just means that the nozzles will not work as well as they were designed to.
There are several things a contractor or technician can do to remedy this. You can place sprinklers closer together, or in the case of rotors, adjust their breakup screws, replace the sprinklers or use a combination of those techniques. Sprinklers should not be used below the lowest or above the highest operating pressures listed in the manufacturers’ charts.
This graphic shows a 2-horsepower pump curve that is similar to most single-stage, 2-horsepower irrigation pumps. You can see that if the demand is 30 gpm, this pump will produce that flow at 50 psi. If the demand is 40 gpm, the pump will produce that flow at 45 psi. Remember, pumps will always sacrifice pressure to produce more flow.
Whatever a pump’s discharge pressure is at the required flow, that pressure must overcome the main two causes of pressure loss from the water source to the last sprinkler: 1) pressure loss due to friction caused by water flowing through the components, and 2) pressure loss from forcing the water to go uphill from the water source to the last sprinkler.
To demonstrate this, look at a sprinkler system that gets water via a pump out of a lake or other body of water, as shown in the graphic below.
The next step is to estimate how much pressure the pump will need to produce at the required 30 gpm flow, add the following: the optimum sprinkler pressure, plus the total friction losses, plus the pressure loss due to elevation. For this example: 45 psi + 12 psi + 6 psi = 63 psi required at the pump’s discharge point.
The 2-horsepower pump shown in the pump curve on p. 36 will produce 30 gpm at 50 psi. That means that the rotors in this example will have 13 psi less than is optimum, or about 32 psi (50 18 = 32). Even though a psi of 32 is not optimum, the rotors will still pop up and throw water. The spacing between rotors will need to be closer than it would if they were getting 45 psi, and the breakup screw will have to be inserted further into the rotor stream to compensate for the lower sprinkler pressure.
However, if the flow for this pump is 40 gpm, its discharge will be 45 psi. That means the rotors will be operating at just 27 psi (45 18 = 27). That is very close to 25 psi, which is the lowest pressure manufacturers recommend for rotors. Even with close spacing and other major adjustments to the rotors, the coverage will be far worse than if they were operating at optimum pressure.
If the largest zone requires only 20 gpm, the pump would produce that flow at 54 psi, and the sprinklers would be operating at about 36 psi. Rotors operating at 36 psi, while not optimum, are at the lower end of acceptable operating pressure for adequate coverage.
Myth #3. A pressure relief valve is not needed when using a pump start relay
It is needed. The purpose of a pressure relief valve on a pump is to provide an exit point for the water, and therefore the pressure, should something happen to prevent the water from flowing downstream from the pump.
There are several reasons why water may fail to flow through an irrigation system even when a pump is turned on:
Merely having a pump start relay does nothing to stop any of these scenarios from happening. The pump start relay simply turns the power on and off to the pump when the controller turns on any valve.
“Dead heading” is the term that describes when a pump is pushing water but there is no place for that water to go. Dead heading a pump will cause it to become very hot, which will damage the seals inside, causing them to leak or the bearings to fail.
Excess heat can also cause the PVC pipe on either the suction or the discharge sides of the pump to soften (as shown on the right). In turn, the softened PVC pipe could expand under the pressure of the discharge side and possibly tear open or collapse from the vacuum on the suction side.
A pressure relief valve (as shown on the left) should be installed on the discharge side of any pump with a pump start relay. The discharge of a pressure relief valve should be aimed or piped away from a pump’s motor to prevent water from flooding it. If the pump is located in a garage or other building where water discharge from the pressure relief valve could cause damage, a pipe should be connected to the discharge opening to channel the water to a safe location.
Myth #4. A pump’s pressure can be increased by turning up the pressure switch.
A pump’s pressure switch has nothing to do with the pressure the pump will produce. Only the pump’s design, the flow demanded by an irrigation system and the properties of the suction line determine the pump’s discharge pressure.
What, exactly, does a pressure switch do, then? Think of a pressure switch as a cousin to a pump start relay. Both are switches that turn power on to the pump. The difference between the two is that a pump start relay is turned on and off by the controller, and a pressure switch is turned on and off by the pressure in the irrigation system.
Pressure switch settings simply designate the lowest pressure at which the switch will turn the pump on and the highest pressure that will turn the switch off. But changing these settings will not change how much pressure a pump can create.
Avoid adjusting the pressure switch on a pump unless you’ve had the proper training, as an incorrect setting can damage the pump. Unfortunately, there are no markings on the set screws of the pressure switch (see arrow at right) to indicate the original settings, adding to the probability of mistakes, especially for a novice technician.
Myth #5. Adding a booster pump will increase the amount of flow and the pressure.
Here’s another misperception that’s half right. A booster pump does add additional pressure to that created by the first pump, but it does not increase the flow. If the water source cannot supply enough flow to satisfy the demand, a booster pump can’t help.
In reality, all pumps are booster pumps, in that they take the water being pushed into their inlets and add pressure to it. Looking at the graphic below left, the submersible pump and the well must be able to supply all the flow needed by the system. The booster pump will add the remaining pressure the system needs at the same flow rate as the submersible.
As the graphic below right shows, the meter supplies all the flow the system needs and some pressure. The booster pump adds the rest of the pressure at that same flow.
A booster pump does add pressure, but that’s all it adds. If greater flow is required, an additional or larger water source is the solution. In either case, the booster pump must be sized properly to deliver the additional pressure required by the system.
Myth #6. It’s always good to replace a dead pump with another of the same model and size.
This appears to be a sound assumption, and many times it’s correct. However, if a pump failed because it was simply the wrong one to use, replacing it with the same type of pump will only repeat the error.
There are two ways to avoid this. The first is to have a good record of the system’s history in terms of the original pump and any changes to the system’s hydraulics. This will allow anyone to assess if the dead pump was actually the right pump. This is important because using an oversized pump is just as bad as using an undersized one.
The second is considerably more complicated; it involves evaluating the irrigation system’s hydraulics to determine the flow and pressure needed for each of the zones, and then making sure the suction-
The lesson to be learned here is to always be sure that the replacement pump is the correct one for the pressure and flow demand of every one of the zones, and that its suction-side components can support the pump.
I hope this discussion has given you a clearer picture of the physics involved in how irrigation pumps work and dispelled some of the common myths about these important components. Hopefully, this will prevent you from making pump decisions based on faulty information and improve your installation and troubleshooting productivity. That should help your bottom line — and prevent a lot of frustration for you and your clients going forward.
Kurt K. Thompson, CIC, CID, CLWM, CLIA, CGIA, CIT, CSWP, has 39 years of experience in the irrigation industry. He is a consultant, an educator and the owner of K. Thompson & Associates and director of educational programs and an instructor at IrriTech Training. He has been honored as an Irrigation and Water Management Trailblazer by the National Association of Landscape Professionals.