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Centrifugal pumps are commonly used for most liquid pumping services. They can be employed for almost all liquids and services. For some very viscous liquids, they cannot be used. Positive displacement pumps should be used instead. Unfortunately, some engineers still specify positive displacement pumps for many water services or (relatively) low- or medium-viscosity liquids. This is perhaps due to some vague hope for better flexibility or possibly some perceived operational advantages.
But many current applications of positive displacement pumps can be properly served by variable- speed centrifugal pumps with far better performance and reliability. This also holds true for liquids with contamination or suspended solids.
As a rough indication: On liquids with contamination of up to 5%, centrifugal pumps like those used for clean liquids can be used with some modifications such as corrections on pump performance curves, and so forth. For contamination of up to about 12%, specially designed centrifugal pumps can be used. For contamination levels above about 16% to 18%, though, specially designed positive displacement pumps (such as screw pumps) may be better options.
Pump operation and reliability
The hydrodynamic unbalanced load is usually at its minimum when the pump is operated at the “Best Efficiency Point” (BEP). The BEP operating point is the most stable and suitable condition for the operation of a pump. To achieve the highest degree of reliability, the pump should operate near its BEP. If it operates far from the BEP, the hydrodynamic unbalanced load increases. This can affect performance and reliability. This load is usually at a peak at the shutoff point. Therefore, long-term operation near the shutoff point could reduce component life and pump reliability. Depending on the hydraulic design, a proper target should be pump operation within the range of 80% to 109% of the BEP. For a variety of reasons, this often cannot be achieved and an optimized operating point and range should be selected.
An important capacity-restricting factor could be the Net Positive Suction Head (NPSH) requirement. Usually at a flow significantly higher than the BEP flow, pressure drop (within the suction passages and the suction piping) could be high. In that case, the NPSH margin would vanish, leading to cavitation and pump damage.
When a pump is operating in closed-loop service (such as a recycle application), the operating point should be close to BEP or no more than 5% to 10% to the left of the BEP. In addition, the pump curve should be studied and evaluated with respect to all possible operating cases.
Many pump applications are known as batch transfer services. In these services, vessels or tanks with variable liquid levels are usually installed at the suction or discharge (or both). The pump should be able to operate well with varying liquid levels. Note that for batch transfer services, there can be two sets of extreme operating points: One for the highest head (highest differential pressure) and the other for the lowest head (lowest differential pressure). The traditional method used for sizing of pumps for batch transfer applications is to consider the operating point of the highest head as single rated point. The pump is selected based on matching this rated point with the BEP.
This approach often came with the justification, “if it is sized for highest head, it always has ample margin.” This out-of-date method is not usually true; it neglects other head requirements, and particularly the lowest head operating case. Thus, the selected pump will operate most of the time at the right side of BEP (sometimes at the end of curve, far from the BEP). This is unreliable and inefficient. Further, the selected pump is bigger than what is required. For low-head operating conditions, the operating point would be far to the right side of the BEP, which may offer operational issues and instability. Poor pump selection, then, results in a large pump, high power consumption, lower efficiency, more vibration and noise, a shorter seal and bearing life, and lower reliability.
Making the best selection
The correct pump selection for a batch transfer service should be based on locating the operating point of the highest head at the left of the BEP and the operating point of the lowest head at the right of the BEP. Selection should carefully consider details, such as efficiency and NPSH. The selected pump should operate most of the time near the BEP. As an indication, the mid-point between the highest head and the lowest head should be located at or close to the BEP. A small deviation to the left of the BEP would be preferred for many applications.
In other words, the mid-point could be a little to the left of the BEP. Generally, all operating points should be identified and pump operation should be evaluated for all possible operating points. For some pump applications, such as batch transfer services with great differences between the highest and the lowest head points, variable speed driven (VSD) centrifugal pumps should be used.
Amin Almasi is a Chartered Professional Engineer in Australia and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.