4 Reasons For Low Flow in Pumps

            A pump cannot be deemed to be “efficient” if it is not pumping its intended level of flow. When a centrifugal pump is not pumping enough liquid, the impediment is usually due to issues relating to the impeller within. This article attempts to briefly describe the four major causes of low flow in pumps.

            The first major cause of low flow in a pump relates to having too much clearance. “If clearances are too wide for the type of fluid pumped, excessive slip will occur. Fluid will continue to recirculate inside the pump, yielding lower flow out of the pump.”1 Pumps require specific amounts of clearance because excessive clearance will result in more area for fluid to become dispersed in, thereby reducing the overall flow rate toward the discharge valve.

A very worn impeller can also cause low flow. “Wear that affects the impeller only (and not wear ring clearances or other leak paths) would result in system flow decrease pump power decrease, pump efficiency decrease.”2 Our previous articles are more in-depth on the critical importance of an impeller in a centrifugal pump. Similar to a worn impeller, debris in an impeller is just as damaging to a pump.

            Though debris in any area of a pump is very problematic, debris will typically build up around the suction valve of a pump. “Less flow into the pump, will obviously yield less flow out of the pump.”3 Hence, impellers can be subject to being clogged. “A clog at the eye of the impeller can prevent it from creating an area of low pressure.”4 Still, this hardly ever occurs in pumps such as self-priming pumps – AKA “trash” pumps due to their ability to pump fluids and debris. Having an impeller which is too small is also an issue.

            Though uncommon, installation of the wrong impeller size can also be an issue which causes too little or too much flow. “Using the wrong sized impeller for the pump causes the pump to produce too much, or too little flow, running off its best efficiency point (BEP)…An impeller that’s too small will most likely not generate enough flow or head.”5 Certainly, installation of an improperly-sized impeller is not due to following the manufacturer’s specifications. Worse than an improperly-sized impeller when a closed impeller is installed when an open or semi-open impeller should have been installed (depending on the fluid to be pumped) – For the wrong type of impeller can cost much more than a slight error in the size of impeller.

            Again, the main reasons for low flow in pumps are primarily due to issues with an impeller. The impeller is a key component to the centrifugal pump, thus if there are any issues with the impeller, then the whole pump will suffer and be inefficient.


1             https://blog.craneengineering.net/8-reasons-your-centrifugal-pump-has-low-flow

2             https://www.pumpsandsystems.com/how-impeller-wear-affects-pump-curve

3             Same as 1

4             https://visserssales.com/Blog/problems-with-a-low-flow-pump.html

5             “36 Ways to Kill Your Pump” by Crane Engineering (eBook)