Pinning down possibilities for pump problems
Problems in pumping systems can, seemingly logically, be blamed on the pump. There could however be other hidden issues; ambient conditions can affect pumpage, or perhaps there are problems in the system itself. In this article we discuss a systematic approach to help determine where the problem really lies.
Troubleshooting should start by looking at the pump, the fluid and the system
When a newly repaired pump performs poorly, it seems logical that something is wrong with it. While that might be true, good troubleshooting procedures should also eliminate several other possibilities, including problems with the fluid being pumped (the pumpage), or with the pipes, fittings and vessels that are connected to the pump (the system). Fortunately, a savvy technician with just a basic understanding of pump curves and performance parameters can quickly narrow down the possibilities–especially those associated with the pump.
Pump curves
This discussion is limited to the most common pumps in industrial and commercial applications– centrifugal pumps. The performance curves in Figure 1 illustrate how the parameters of head, flow rate, efficiency and power relate to one another for a typical centrifugal pump. Note that as head increases, flow decreases, and vice versa (Head-Flow Pump Curve, Figure 1).
For any certain flow rate, there is a corresponding amount of head. The impeller design dictates a specific flow rate at which the pump will perform most efficiently–i.e., its Best Efficiency Point
Many pump problems, and some system problems, will cause the pump to operate at a point below its normal pump
curve line.
(BEP). Many pump problems, and some system problems, will cause the pump to operate at a point below its normal pump curve line. A technician who understands this relationship can measure the pump parameters and isolate the problem to the pump, the pumpage, or the system.
Figure 1. The pump curve plots normal head and flow performance and includes efficiency and power data.
Is it the pump?
To determine if the problem is the pump, the Total Dynamic Head (TDH) and flow should be measured at the pump and compared to the pump curve for that pump (see sidebar on page 49). The TDH is the difference between discharge and suction pressure, converted to feet or meters of head. (Caution: If there is little or no head or flow on start-up, the pump should immediately be shut off to verify that there is sufficient fluid in it–i.e., that the pump is primed. Running a pump dry may damage the seal.)
• If the operating point is on the pump curve, the pump is operating properly. Therefore, the problem is with the system or possibly the pumpage.
• If the operating point is below the pump curve, the problem could be the pump, the system, or possibly the pumpage.
Is it the pumpage?
Ambient conditions like temperature can change the viscosity of the pumpage, which in turn may change the head, flow and efficiency of the pump. Mineral-based oil is a good example of a liquid that changes viscosity with temperature. When the pumpage is a strong acid or base, dilution can change its specific gravity, which may affect the power curve.
To find out if the pumpage is the problem, its properties need to be verified. Tests for viscosity, specific gravity and temperature of the fluid are readily available and inexpensive. Standard conversion charts and formulas from the Hydraulics Institute and elsewhere can then be used to determine if the pumpage is adversely affecting the pump’s performance.
Is it the system?
Assuming the fluid properties have been ruled out, the problem must be with the pump or the system. Again, if the pump is operating on the pump curve, it is working properly. In that case, the problem must be the system to which the pump is connected.
There are two possibilities here. Either the flow is too low (and therefore the head is too high), or the head is too low (indicating the flow is too high). When considering head and flow, remember that the pump is operating on its curve. Therefore, if one is too low, the other must be too high.
Low flow (head too high). A low flow condition usually indicates a restricted line. If the restriction is in the suction line, there will likely be cavitation; otherwise, it is probably in the discharge line.
Other possibilities are that the suction static head is too low, or that the discharge static head is too high. For example, a suction tank may have a float switch that fails to shut off the pump when the fluid level drops below the set point. Similarly, a discharge tank may have a “high level” switch that has malfunctioned. a discharge tank may have a “high level” switch that has malfunctioned.
Figure 2. An ultrasonic flow meter measures flow by transmitting a signal through the pipe and into the pumpage.
Low head (flow too high). A low head condition indicates too much flow. And it is likely that the flow is not going where it should. System leaks can be internal or external. A diverter valve that allows too much flow to bypass, or a failed check valve that lets flow circulate backwards through a parallel pump, would result in too much flow and low head. On a municipal water system with buried water mains, a major leak or line break will allow too much flow and result in low head (low line pressure).
Blockages and leaks. Looking for blockage or leaks in a hydraulic system is like looking for opens and shorts in an electrical system, except that the parameters to measure are pressure and flow rather than voltage and current. Where there is a blockage or a leak, there will be an abnormal pressure differential across the area or component involved. It is easier to locate a problem by checking pressure (THD) than by measuring flow (see sidebar).
Other system problems. Even if the pump is not operating on its curve, there are some system problems that must be ruled out before the pump can be identified as the culprit. For example, if vapor is getting into the pump by air entrainment or cavitation, the pump will not operate on its curve, even if there is nothing wrong with it. Performing vibration analysis in real time while varying the pump suction will help identify cavitation and air entrainment. If the pump does not operate on its curve after these conditions have been eliminated, there is very likely a problem with the pump.
What could be wrong with the pump?
When a pump does not operate on its curve and cavitation and air entrainment have been eliminated, the most likely causes are a damaged impeller, blockage in the impeller (see Figure 3), blockage in the volute, or excessive wear ring or impeller clearance. Other causes would be related to the speed of the pump, such as the shaft spinning in the impeller, or an incorrect drive speed. While drive speed can be verified externally, investigating the other causes will involve opening the pump.
Figure 3. A clogged impeller causes pump performance that falls below the pump curve.
Conclusion
Troubleshooting pump performance is straight-forward. By measuring the pump’s head and flow and comparing the results to the manufacturer’s pump curve, and checking for air entrainment and cavitation, the technician can readily determine if the problem is with the pump or the system. The properties of the pumpage can also be tested to rule them out as the cause of the problem.
Measuring pump operating parameters
Often the most difficult parts of troubleshooting pump performance is measuring the pressure and flow. To determine pump TDH (i.e., the difference between the suction and discharge pressures), a suction gauge and a discharge gauge are required at both the pump suction and discharge. Pump suction pressure can be below atmosphere (a vacuum), so the suction gauges should read vacuum or pressure.
Although flow can be harder to measure than pressure, ultrasonic flow meters can accomplish the task from the outside of the pipe (Figure 2). There are several kinds that may work, depending on the type of pumpage, so it is important that the correct type is selected, and that it is installed and calibrated properly.
Another way to determine flow is a method which works well on pumps that draw from an open sump. Simply measure the change in depth of the liquid during a short interval (15–60 seconds) with all other flows into and out of the sump closed. Compute the flow as the volume of the change in sump level, converted to gallons (m3) per unit time.
EASA is an international trade association of more than 1,700 firms in nearly 70 countries that sell and service electromechanical apparatus. For more information, visit www.easa.com .
Text: Gene Vogel, pump and vibration specialist at EASA
Images: EASA and SHUTTERSTOCK
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