COLUMN | Focus on Fundamentals
Pump Selection: How to Manage System Curve Uncertainty, Part 2
Sotirios Christofi | Industrial Machinery Consultant
In Part 1, we discussed pump purchase procedures, calculating system curve and what happens when a pump with the lowest allowable best efficiency point (BEP) flow rate is selected.
A word of warning: An actual operating point is different than the calculated operating point, which means that not only the flow rate, but also the actual head, will probably be different. Engineers should check whether the actual head is acceptable for the process. With a pump with a steeper curve, the flow rate fluctuation will be less than with a pump with a flatter curve.
Consequences
The graph in Image 1 shows what happens when a pump does not operate close enough to the BEP flow rate. The result is many failures in both directions, depending on whether the actual flow rate is lower or higher than the BEP flow rate.
| IMAGE 1: Pump failures at various operating points (Image courtesy of the author)
Outside the region of recommended operation flow, several types of failures appear, the first symptom usually being higher vibrations.
How to Avoid This Problem Before Pump Purchase
In order to have a reliable system curve, it is highly recommended that effort be given by the project team so suction and discharge piping routing are determined as early as possible—even if only as draft isometric drawings—before the pump data sheet is filled in. The reason this is not done is not technical but purely procedural, as the piping engineer starts working on the project only after the basic design is completed. All it takes is the project manager’s commitment and decision.
There is no safety margin in calculating the system curve, as either too steep or too flat a system curve can cause problems. Therefore, in constructing the system curve, make your best estimate. Do not add a safety margin, because it will lead to the pump operating far away from its BEP.
Managing the Issue After Pump Startup
The exact operating point should always be known to operators installing and regularly checking manometers at the suction and discharge of the pump, as well as the flow meter. Manometers alone are not enough, since their measuring tolerances—along with the tolerance of the pump curve (3% allowed by American Petroleum Institute [API])—can lead to a wide range of flow rates, particularly when the curve tends to be horizontal (API specifies a minimum total declination of 10%). Flow meter installation is usually worth the cost.
- Monitor filter pressure differentials on the pump lines—on the ∆P manometer, indicate the allowable pressure difference, i.e., the point at which the filter elements should be cleaned or replaced.
- Beware of pump line modifications that alter the system curve. Before any modifications are planned, check what the new system curve will look like and where the operating point will lie.
- Pump reliability, mean time between failures (MTBF) and maintenance costs heavily depend on the pump BEP position relative to the actual operating point. It is more than worth the effort to table all plant pumps (at least API pumps) along with their relative operating positions as follows:
This table can be a valuable tool for several plant disciplines. It provides operators with a clear indication of the limits in controlling flow through valve throttling. The distance of the operating point to the BEP can explain pump failure record—e.g., unusually low MTBF/frequent repairs—and provide valuable feedback for decision-making of the maintenance personnel.
MTBF is a key performance index for maintenance managers these days. This table can be used to indicate that a low MTBF is probably not due to maintenance performance but to incorrect pump selection and/or operation.
Sotirios Christofi, after a career of 38 years in the petrochemical industry, provides consulting and training services for technical personnel on the design, construction, inspection, selection, condition monitoring and maintenance of industrial machinery. He is a mechanical engineer with a master's degree in business administration from the University of Warwick, U.K.
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