With spring around the corner, many irrigation professionals are beginning to check their systems for repair and planning how they will operate over the next season. We offer some helpful tips covering a variety of pumping topics for making this a great year for your irrigation endeavors. Remember, these are quick reminders. Each of these subjects could be further detailed on their own in multiple books.
1. Understand NPSH
There are two measurements of net positive suction head that we need to know for proper operation of your system: NPSH available (NPSHa) and NPSH required (NPSHr).
NPSHa is determined by a property of your irrigation system. It is a calculated value, and in simplest terms, it is the suction-side pressure minus the vapor pressure of the liquid pumped to the measurement point, most often the eye of the impeller.
NPSHr is the required minimum pressure at the suction inlet that keeps the pump from cavitating. It is a tested value/function of the pump that the pump manufacturer determines.
There are some general guidelines to keep in mind when working with NPSH in your irrigation system. NPSHa must be greater than NPSHr or the pump will cavitate. Cavitating pumps reduces life and efficiency and can harm the rest of your system. Also, you should allow a safety factor of 2 feet of head so that NPSHa does not cross below the NPSHr value. It’s better to be safe than sorry.
Atmospheric pressure, suction lift and liquid temperature affect NPSHa. Be sure you know the range of values when you make your calculations, and prepare for the worst operating conditions. On the suction side of the pump, increases in friction loss such as corrosion of a pipe or partially closing a valve also affect NPSHa. Check what you’re doing as you work so the pump does not cavitate.
A simple formula to determine NPSHa in an installation where there is suction lift is: NPSHa = atmospheric pressure - suction lift - suction pipe friction loss - vapor pressure - entrance loss - other losses. For entrance loss, we use 3 feet, typically. Other losses include foot valves, elbows and fittings.
To make it easy on yourself you may want to invest in a vacuum gauge that will show you at a glance the total of all the head losses on the suction side. If the vacuum (converted to feet of water) is much lower than it should be, you could have air leaks that you have not identified. If the vacuum reads much higher than it should, then there is probably some problem in the suction pipe system that is causing excessive head loss and thus lowering your NPSHa.
2. There’s an app for that
If this talk about NPSH has made your eyes gloss over, fear not. You have an NPSH estimator in your pocket if you carry a smartphone. Various free applications in the Apple App Store and Google Play Store walk you through step-by-step to estimate the NPSHa for your system.
The apps consider whether the system is open to atmosphere or if it is connected to a pressurized tank in a closed system. They also ask you if have a flooded suction (liquid flowing to the pump above the eye of the impeller) or if you are pulling water up into the pump (suction lift). You input flow and select the length of pipe in the system, the size of the pipe and the pipe material of construction, plus call out valves, fittings, elbows and other friction loss points. The app will then return an estimation of your operating conditions. You can also check on total dynamic head in the system and see what different sizes pipes or fittings would do to your friction loss.
The apps make a handy check for conditions.
There are dozens of apps offered, so you can try several to see which one you like best. We are partial to the free Cornell Mobile Toolkit.
3. Check suction first
When troubleshooting a pump station, more than 90% of the issues we see are on the suction side. We strongly suggest you start there.
If you’re trying to find an issue with the pump station, there are some common problem areas. Start with NPSHa. If the pump is starved to liquid cavitation, then a slew of other problems can ensue. Loose fittings and valves, incorrect pipe slope, or use of a concentric reducer (when you should be using an eccentric reducer) can allow air to entrain and bind the system. The presence of enough air in the system will affect pump performance.
Sometimes a change in conditions can cause a problem. If the vertical suction lift increases or you increase the flow in the system, pump performance can rapidly deteriorate. If everything performed well last season and you have performance problems at start-up this season, the first place to start is finding what’s different. Was the suction pipe removed at any time during the offseason? Is the foot valve in good operating order? Do you need to tighten flanged bolts or replace flange gaskets in the suction-side piping? Make note and adjust the system as necessary.
Improperly supported pumps can vibrate, and that vibration can affect the suction and operation. Too much weight on the pump casing can also cause premature wear. Also, the suction pipe system should be sized for a water velocity of less than 7 feet per second, should approach the pump inlet with a straight run of at least four times the pipe diameter, and should reduce down at the pump inlet with an eccentric reducer with the flat side on top to prevent providing a spot for any entrained air to accumulate.
Remember that for a pump to perform as it was designed to perform, it must be given uninhibited access to all the water your system needs.
4. Simplify with remote monitoring
The Industrial Internet of Things has exploded during the last four years. You can now receive data from your pump remotely, checking conditions such as temperature, vibration, flow, pressure, drive, hours run or out-of-parameter conditions. Even GPS location can be accessed from hundreds of miles away. With the right components, users are able to start and stop the system with an IIoT module without traveling to a client’s property to turn on a pump.
Many of the IIoT systems also alarm if conditions are not where they are supposed to be, allowing a user to save a pump, motor or system before a problem becomes catastrophic.
With machine learning, IIoT allows predictive maintenance recommendations. Making downtime less frequent provides savings costs. Catching problems when they are minor and easy to fix is always better than when they’ll mean large time and capital expenses.
And hey, we’ve beat the NPSH drum a bunch. IIoT can give your conditions and plot it right away so you can see what’s going on. Check with the pump manufacturer about the IIoT solutions they provide.
5. Check for proper submergence
Proper submergence is required to stop vortexing, which can make the pump lose prime, reduce head, reduce flow, lower efficiency and cause noise. The amount of submergence needed is a function of pumping flow and suction pipe inlet diameter. Remember that it is possible to be drawing air into the suction inlet even if you don’t see a vortex, and other issues can increase the possibility of entraining air such as turbulence in the water source.
A suction bell reduces inlet losses and helps suppress vortexing as well. A basket screen may be used.
The open area should be at least four times the pipe area. The screen opening size must be less than the maximum solid passing capacity of the system. Avoid use of very fine screen; it will plug easily and possibly collapse. If very small particles must be avoided, consider the use of another type of protection. The screen is also a safety feature should someone fall or swim close to the suction inlet.
6. Stay on the curve
Pump manufacturers publish curves that detail the performance of their pumps, and there should be one available for every pump you have installed. The manufacturers will call out a best efficiency point. This is the area where a pump operates optimally for its design. However, on many pumps BEP is a very small part of the operating range, and you may not run near that point. If you are not operating at BEP, you should stay in a window within 70% to the left (generally higher heads/low flows) through 120% to the right (generally lower heads and higher flows) of BEP. If you stay within that operating envelope, you will generally have lower fuel costs due to pump efficiency, smoother operation performance and less wear on parts.
While operating a pump at flow too far above BEP can cause cavitation, operating a pump at a flow too far below BEP can cause cavitation in some cases as well due to recirculation in the impeller. If you are not sure if a zone you want to run alone is too low on flow for the pump, listen to your pump. If the pump begins to run rough or vibrate when you drop down to that flow, then you know you need to increase the flow somehow. The closer to the BEP the better.
We hope that 2021 is a great irrigating year for everyone and that our tips help keep your pumps running smoothly.
Bob Jansen is recently semiretired from Cornell Pump Company and has worked in the pump industry for more than 40 years. Eric Holtan, agricultural market manager for Cornell Pumps, has more than 18 years of experience.