While some obvious reasons may cause a pump to run off of its factory-issued performance curve, they are often overlooked on initial investigation.
The plant is having process issues and the culprit appears to be a process centrifugal pump. The area production manager has asked you to check the pump to see if it is operating on its curve. “Pump won’t pump, make pump pump,” he says.
Sounds like a reasonable request. However, other questions need to be answered, such as: Where is the pump running on its curve? If it is not running on its curve, why? Most pump problems can be attributed to system pipe and system friction changes.
To determine if the pump is operating on its curve, separate performance parameters (head, flow, or motor load) must converge near the same point. If there is a contradiction of data, then the pump, system or the performance parameter being measured is likely the issue. The pump is running on a particular operating curve, but the question is which one, and likely, that curve is not readily available.
While some obvious reasons may cause a pump to run off of its factory-issued performance curve, they are often overlooked upon initial investigation. For example, a pump might run off of its curve for any of the following reasons:
- Wrong Pump Rotation
- Pump Speed Differences
- Incorrect Impeller
- Fluid Property Changes
- Pump Wear
- Pump Plugging
- Entrained Air
Each cause for pump maintenance is addressed in this article.
Why Is Your Pump Operating Off the Curve?
1. Wrong Pump Rotation
Identifying the wrong pump rotation sounds easy, but most pumps will still produce a head and flow while running in the wrong direction. In fact, some pumps may produce as much as 50 percent flow and 50 percent head while operating in the wrong rotation. Checking the motor rotation is routine for new installations, but it should also be standard practice when a motor is changed, a motor is disconnected, or any motor control center disconnect work is done. Wrong pump rotation also risks some impellers unscrewing from the pump shaft. Checking motor rotation uncoupled is ideal, but this may not be practical, in which case a running shaft rotation check is sufficient.
2. Pump Speed Differences
Any speed difference in the actual pump speed will cause a variation with the published curve. Different motor slips may cause small differences but typically only in the 15 to 20 RPM range. If a variable frequency drive (VFD) is used, then determining the actual running speed and pump performance must be matched to the correct curve. If the connecting power transmission element (coupling, belt, clutch, etc.) from the driver (motor) to the pump has any slip, then decreased pump performance relative to the curve will result.
3. Incorrect Impeller
Having an incorrect impeller is an obvious reason why a pump might run off of its curve, but what might not be so obvious is how many ways an impeller can be incorrect. Incorrect impellers could be caused by documentation errors, trimming errors (measurement, taper, etc.), wrong diameter, wrong impeller pattern number (geometry), wrong suction wear plate, etc.
No published curve accounts for wrong impeller design (pattern numbers) or other gross impeller errors on a particular pump. The most important impeller parameter is diameter, so this should be one of the first parameters verified. If the impeller is not available for direct diameter measurement, then the impeller diameter can be estimated by briefly dead-heading the pump, and using the following estimation:
Shut Off Head, ft = D2(N/1750)2
where D = actual impeller diameter in inches
N = pump speed in RPM
4. Fluid Property Changes
Changes in fluid properties can affect pump performance in a way that may force the pump off of its curve. Fluid viscosity changes can affect pump head and pump brake horsepower (BHP). A higher viscosity will result in a higher HP and a reduction in developed head. A new curve for viscous fluids will have to be calculated to more accurately reflect the actual pump performance. As a rule, corrections may be needed if the viscosity changes > 10 centistokes.
Fluid density or specific gravity (SG) can also affect BHP as seen from the pump horsepower equation. Specific gravity is the fluid density relative to water. Both fluid viscosity and density can be affected by fluid temperatures, so a change in process temperature can also affect pump performance.
Brake Horsepower = (GPM × Head Feet × Specific Gravity)/(3.960 × Pump Efficiency)
5. Pump Wear
Table 1. Impeller wear material chart
Pump impeller and casing wear can significantly affect pump performance when compared to the original conditions that produced the factory pump curve. Wear around the cutwater, suction wear plate, and impeller are critical issues that could affect performance. Impeller wear on the front edge (suction side of impeller vanes) will directly affect the capacity of the pump as the impeller gets thinner. This wear is not always obvious and is often overlooked, since the target impeller thickness may not be known or can’t be easily verified.
The size, concentration and type of abrasive materials being pumped will affect the wear rate. Pump wear will be a gradual impact on pump performance, but unless the process has trending to observe this decrease in performance (flow or pressure), it may go unnoticed until one day when the pump will no longer effectively support the process. Pump materials and optimizing the operating point can reduce abrasive wear on a pump. See Table 1.
6. Plugged Pump
Pump plugging will instantly cause a decrease in performance that will throw the pump off of curve. Pumps may plug in many areas, such as pump suction, casing passages, or between impeller vanes (see photo). Some pump curves will list the max solids that the pump can handle or pass without plugging. Specially designed impellers and pumps (recessed impeller or chopper pump) are typically selected for handling process fluids that may have plugging type material. An enclosed impeller cannot handle much solid material.
7. Entrained Air
Figure 1. Air entrainment total head correction. (Courtesy SCA)
Pumps were designed to pump fluids in liquid phase and not anything in a gas phase. Anything greater than 2 percent air may require pump corrections, because pump performance continues to decline as air entrainment increases. Air collecting at the impeller eye chokes off the pump causing a decrease in pump head and pump BHP. If the pump is a paper stock pump, high consistencies can entrain high amounts of air. Several actions can be taken to offset the negative impact of air entrainment, such as larger impeller (offset head correction), larger impeller clearances, or install a suction inducer, which helps to collapse air at the impeller eye (Figure 1).
Figure 2. NPSHr values for factory performance curves. (Courtesy SCA)
Cavitation is the implosion of vapor bubbles when the suction pressure has fallen below the vapor pressure at the pump suction. The fluid vaporization and implosion energy disrupts pump performance. Cavitation decreases the pump head which will alter pump performance. The factory performance curve actually documents this drop in head for the net positive suction head required (NPSHr) curves (Figure 2).
The NPSHr is recorded as the suction head that produces a 3 percent drop in pump head so NPSHr is sometimes called NPSH3. Significant cavitation may drop the pump performance much more — to the point where it does not appear to be running on the curve since it is starved at the suction.
There are many different variables that can cause a pump to perform outside the best efficiency range of its factory performance curve. However, before troubleshooting what seems to be a performance curve issue, there has to be some initial investigation to determine that the pump is actually running off of its curve. If it is not, then more invasive steps will have to be taken to determine the parameter that is causing the performance issue.
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