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Home » Pumps » Difference between your Pump’s Performance and the Actual Published Curve?

Difference between your Pump’s Performance and the Actual Published Curve?

Posted: 29/09/2017
Category: centrifugal Pumps , Pump Maintenance , Pumps

One of the most asked questions by pump owners is, “Why is my pumps performance different from the actual published curve?” While the answer is often pretty simple, it’s often difficult to pinpoint the exact reason for the difference in performance. Here, in this article, we give you a few solutions that will help you in finding out the reasons behind the difference in your pumps performance. First, begin with the Basics. Start by checking the basics. Is your pump operating in the right direction? Is your suction line free of restrictions? Is it properly primed? Remember that it isn’t possible to vent a running pump, as the lighter air tends to stay in the middle of the casing, while the heavier dense liquid moves to the outside. Air entrainment, even if it’s as low as 2 to 4%, will restrict the operations of a standard pump. Hence, it’s vital that you maintain a sufficient NPSHA (Net Positive Suction Head). Are there air leaks? If you’re using a suction-based system, where the fluid is below the impeller’s centrepiece, then it’s essential to check for air leaks in the system. Remember that it isn’t the fluid leaking out of the system, but rather enters the system during an air leak. The ideal suction lift at sea-level is around 34 feet. But, in reality, due to vapour pressure and friction in the system, the level is often less, around 26 feet. If you’re looking to lift warmer liquids above 30 feet at higher altitudes vertically, then it’s natural to expect a drop in performance. How old is your pump? Age can take a toll on the performance of your pump. If the front cover of the impeller clearance is not set correctly or has opened due to wear and tear, then there is often a decrease in the efficiency of the pump and its performance will fall below the curve. Another component that wears down with age is the casing. The cutwater (the point in the housing that converts centrifugal force into pressure) is often washed out with extended usage, which in turn results in a performance drop. Have you installed the right impeller? Very often, most pumps are fitted with the wrong impeller, causing the performance to fall below the curve. Also, you may have used the right impeller but in the wrong diameter. Check out if this is the case. Accuracy is Key when Measuring Performance Another significant factor that contributes to the difference in performance is the method you’re using to check the pumps performance. Are you measuring it the right way? Are you using the correct calibrations? If the measurement data is skewed, then the results will be way off the mark. Ensure that all the flowmeters and gauges are calibrated correctly. Take the readings as close to the pump as possible. Remember that a single gauge on the discharge outlet measures just the discharge pressure and not the differential pressure. The discharge pressure depends heavily on the suction levels (the increasing or decreasing levels of the supply tank). Don’t forget to consider friction when measuring the performance of a pump. If you pump fluids of higher density than water, then the curve must be altered accordingly to allow for viscosity. Additionally, the power requirements of a pump also vary with different specific gravities. Also, when you operate the pump at either end of the curve, you can expect issues like separation, cavitation, and recirculation. It’s Time to Record the Performance of the Pump Now, that you have made the corrections suggested above, it’s time to put the pump to test by operating it at different points of the curve. You can do this by altering the discharge value. Field-test the pump at the maximum head (shutoff or zero flow) and make a note of the discharge pressure. Now, increase the flow bit by bit and track the discharge pressure at varying levels. If the performance doesn’t meet the curve at any point on the curve, then you can determine that there is a problem with the impeller size, opened clearance, incorrect speed or wear and tear. However, if the performance matches at Shut-off but differs as the flow increases, then you can conclude that there is an issue of insufficient NPHSA, starved suction, air binding or inadequate submergence. The Last Word While in an ideal world, the pumps performance should meet the actual curve, the reality is often different. Some differences can be tolerated. However, if you see a significant difference in performance, then it’s time to call in experts and fix the issue as early as possible. Need help figuring out what’s wrong with your pump or require assistance in improving its performance? Get in touch with our pump experts at +91-120-4176000 or drop in a line at

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