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JUST AS CENTRIFUGAL PUMPS SHOULD BE OPERATED NEAR THEIR BEST EFFICIENCY
POINT (BEP), ADEQUATE ATTENTION SHOULD BE PAID TO THEIR ASSOCIATED PIPING
A pump set should be placed as near as possible to the source of liquid supply to reduce the length and simplify the suction piping. When installing a pump system, its location should be considered in relation to the suction source to assure that sufficient Net Positive Suction Head (NPSH) is available. This should be done with respect to pump’s details, such as type, size, speed and NPSHR. Also, a pump system should be positioned in a way that permits enough space for inspection, maintenance, repair and service.
Additionally, pump suction piping should be designed and installed to prevent the formation of gas or air pockets. Suction piping should be as straight as possible and as short as possible. In other words, pump suction piping should be arranged such that the flow is as smooth and uniform as practicable at the pump suction nozzle. Suction piping sizes are usually one, two or three sizes larger in diameter than the pump suction nozzle size. If a relatively long suction piping is required (and all attempts to reduce the piping length have failed), suction piping may be three sizes (or even more) larger than the pump-flange-nozzle size depending on piping length and complexity.
The total dynamic suction lift (static lift plus friction losses in the suction line) should not exceed pump design limits at any operating point on the pump curve. Special attention is needed for the end of curve where NPSHR and liquid velocity are high, which result in low NPSHA. The NPSH margin (NPSHA minus NPSHR) of 1.5m, 2.0m, 3.0m or more is specified and should be guaranteed for the operation at any point. Even the proper NPSH margin should be provided for the right-hand side of any rated point for points near end of the curve. Suction piping for all pumps should have a straight length of pipe between the suction flange and the first elbow, tee, permanent strainer or other obstruction in the upstream.
Elbows installed in any position at the suction piping have a tendency to cause operational problems in pumps, such as to distribute the liquid unevenly in the pump’s impeller chamber, causing a reduction in capacity and creating an undesirable thrust condition. As an indication, this straight length (immediately upstream of the pump nozzle) is five-to-ten times the suction nozzle diameter, depending on pump type, speed, details and size. For some small, low-speed pumps, a lower limit might be used of say four-to-five times the suction nozzle diameter. For large, sensitive, high-speed and critical pumps, a higher straight length (say eight-to-twelve times the suction nozzle diameter) might be required. This straight length in suction piping is needed to reduce the turbulence of the suction by straightening out the flow before it enters the pump; this is also to ensure stable and uniform flow at the pump suction nozzle.
For horizontal pumps with side suction, when reducing the suction piping to the pump’s suction nozzle, an eccentric reducer with the flat side on top should be used to avoid air and gas pockets. For top suction pumps, the reducer is concentric. Discharge piping can be designed with bends, elbows and fittings as needed. This is different with suction piping and all the necessary changes of direction and loops would be provided. Complexity should only be to the required level; and minimum necessary elbows, fittings and valves should be used to reduce head loss from friction. The size of piping is most often calculated based on hydraulic analysis. As a rough indication, discharge piping diameter is usually one size larger than the pump discharge nozzle size. In special cases, for long and complex discharge piping systems, this might be two sizes larger than the pump nozzle size.
High spots, such as vertical loops, should be avoided as they might collect air and gas and throttle the system or lead to erratic pumping. A check valve and gate valve are usually installed in the pump’s discharge piping. The check valve, placed between pump and gate valve, protects the pump from excessive back pressure, and prevents liquid from running back through the pump in case of sudden pump stop (for instance, due to power failure). The gate valve can be used in priming and starting, and when shutting the pump down; it is most often installed with the stem in a horizontal or downward position to prevent formation of an air and gas pocket (in the valve).
Elbows should not be installed immediately at the pump’s discharge nozzle as they can impose operational issues, such as the turbulence that can affect pressure gauge readings. Flexible piping designs are always encouraged; they have many benefits such as helping to suppress slight piping misalignment. Many pumping system designs have used rubber expansion joints to dampen vibration or shaking forces. These rubber items have a tendency to deteriorate, making frequent replacement necessary; they should be avoided if possible. One of the most common conditions affecting pump efficiency is the formation of air and gas pockets in the suction piping, which can cause damage to pumps. Air and gas pockets are usually a result of high points and improper installation of elbows, reducers and valves in the suction piping. Accordingly, all measures and precautions should be taken to prevent air and gas pocket formation in pumping systems.
Author: Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.