Gene Vogel
EASA Pump and Vibration Specialist
When a motor fails to perform, we know what to check: the voltage, whether it’s balanced, the current, whether there is a ground, etc. When a pump fails to perform, many service centers are at a loss on how to troubleshoot it. If the pump has just been repaired and fails to perform, it will be hard to convince the customer that the pump is not the problem. The fact is, there are three areas of possibilities: It could be the pump, or it could be the fluid that is being pumped (the pumpage), or it could be the system of vessels, pipe and fittings connected to the pump (the system).
Understanding a little bit about pump curves and pump performance parameters, and using the process of elimination, will allow the service center technician to narrow the possibilities — especially those that are pump related.
First, we will limit this discussion to centrifugal pumps, which are the most common in industrial and commercial applications. Normal centrifugal pump performance is illustrated with a pump curve. The pump curve relates head (related to pressure), flow rate, efficiency and power.
The pump curve illustrates that as head increases, the flow decreases, and vice versa (see Figure 1). For any certain flow rate, there is a certain amount of head. By the design of the impeller, the pump has a certain flow rate at which the performance is most efficient. This is called the Best Efficiency Point (BEP). Many problems with the pump, and some problems with the system, will cause the pump to operate at a point below the pump curve line. When a technician understands that relationship, and can measure the head and flow of the pump, the problem can be isolated to the pump, to the system or to the pumpage.
Is it the pump?
To determine if the problem is the pump, measure Total Dynamic Head (TDH) and flow at the pump and compare them to the pump curve. The TDH is the difference between discharge and suction pressure, converted to feet or meters of head. If the operating point is on the pump curve, the pump is operating properly; the problem is the system or possibly the fluid. If the operating point is below the pump curve, the problem may be the pump, the system, or possibly the fluid. If, on startup, there is little or no head or flow, shut off the pump immediately and verify there is sufficient fluid in the pump (pump primed). Running the pump dry may damage the seal.
Is it the fluid?
Verifying the fluid properties is a good place to start. Tests for viscosity, specific gravity and temperature of the fluid are readily available and inexpensive. Ambient conditions (temperature) can affect viscosity. When pumpage is a strong acid or base, dilution can change the specific gravity.
Is it the system?
With fluid properties verified, the problem must now be either the pump or the system. Again, if the pump operating point is on the pump curve, then the pump is working properly and the problem must be the system to which the pump is connected. If the pump is not operating on its curve, there still are some system problems that need to be eliminated before the pump is identified as the culprit. If vapor is getting into the pump by “air entrainment” or by cavitation, the pump will not operate on its curve, but there may be nothing wrong with the pump. So the next step is to test for cavitation and for air entrainment. A live time vibration analysis, while varying the suction conditions on the pump, will help identify cavitation and air entranment. If cavitation and air entrainment can be eliminated and the pump is not operating on its curve, then 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 damage to the impeller, blockage in the pump impeller (see Figure 2), blockage in the pump 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 driver speed can be verified externally, investigating the other causes will involve opening the pump.
What could be wrong with the system?
When a pump is operating on its curve and fluid properties have been verified, the problem must be with the system. There are two possible cases: 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. So if one is too low the other must be too high. Of course one other possibility is that it is the wrong pump, either by improper design or by mistaken installation, including installing the wrong impeller.
Low flow (head too high)
A low flow condition most often indicates a restriction in a line. If the restriction is in the suction line there will likely be cavitation; otherwise the restriction is likely in the discharge line. Other possibilities are that the suction static head is too low, or the discharge static head is too high. A suction tank may have a float switch to shut off the pump at some level below which the pump should not operate. Similarly, a discharge tank may have a high level switch which has malfunctioned.
Low head (flow too high)
A low head condition indicates there is too much flow. And it’s likely that the flow is not going where it should. System leaks can be internal or external. A diverter valve that is allowing 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).
Searching for system problems
Looking for blockage or leaks in a hydraulic system is very similar to looking for opens and shorts in an electrical system. However, you can’t just touch test leads to the pipe to measure pressure as you would to measure voltage. Just as with electrical circuits, though, it is easier to measure pressure than flow (like voltage being easier to check than current). Where there is a blockage or a leak there will be an abnormal pressure differential across the area or component involved. Checking pressure is the easiest way to locate a problem.
Measuring pump operating parameters
Often, the most difficult part of troubleshooting pump performance is measuring the pressure and flow. To determine pump TDH, a suction and discharge gauge is required at both the pump suction and discharge. Since pump suction pressure can be below atmosphere (a vacuum), the suction gauge should read vacuum or pressure.
Measuring the flow can be more difficult. Ultrasonic flow meters will measure flow from the outside of the pipe (see Figure 3). There are several types that may work depending on the type of pumpage. Care must be taken to choose the correct type and install and calibrate it properly. An alternate method that works well on pumps that draw from an open sump is to measure the change in liquid depth of the sump for a short period of time (15 – 60 seconds). All other flows into and out of the sump must be closed. Compute the flow as the volume of the change in sump level, converted to gallons (m3) per unit time.
Troubleshooting pump performance is not too much different than troubleshooting motor performance. The performance parameters are just head and flow, rather than voltage and current.
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