Last month I reviewed an Excel-based Radial Thrust Calculator and showed how it can predict the magnitude of unbalanced radial thrust when a centrifugal pump is operated to the left of BEP. This month we will explore another calculator that can help identify pumps that may be problematic when operated on either side of BEP.
I suspect that most readers will agree that Terry Henshaw's year-long series on NPSH (Understanding NPSH) has been extremely useful and informative. His September, October and November 2009 columns addressed an NPSH topic that may have been new to many of us. Suction Specific Speed (S or Nss) and its effect on NPSH margin can be a useful parameter in identifying pumps that could have a high potential for suction recirculation-induced cavitation. As Henshaw said, S is an indication of the aggressiveness of an impeller eye design. As the ratio of eye diameter to peripheral diameter increases, NPSHR typically decreases, but a reduction in a pump's stable window of operation can be an unexpected byproduct.
As shown below, the equation for S is similar to the equation for Pump Specific Speed (Ns). The only difference is that Head (H) is replaced with NPSHR.
S = N √Q / NPSHR0.75
S is directly proportional to the pump speed in RPM (N) and the square root of pump flow in GPM (Q). It is inversely proportional to NPSHR to the three quarter power. Therefore, S will increase with an increase in speed and/or flow and decrease with an increase in NPSHR. NPSHR is also an indication of the aggressiveness of the impeller eye design. Lower values of NPSHR usually indicate a larger eye diameter ratio. As Henshaw mentioned in his September article, a flow reduction at the entrance of a large eye can result in a partial reversal of flow toward the suction pipe. The vortices created by such a reversal can lead to the onset of cavitation.
An Excel-based Suction Specific Speed calculator is shown to the left of Figure 1 and is available for download from my website. Entering the required data in the three yellow cells will result in the calculation of S. The chart below the calculator shows the stable window of operation for several values of S. Normally, suction recirculation occurs at low flows, but with increasing values of S, suction recirculation will begin at higher than expected flow rates. Many experts specify that pumps with a value of S greater than 9,000 may require a sizeable increase in NPSH margin if they are to maintain a stable window of operation.
Another indicator of potential suction recirculation is a quantity known as Suction Energy (SE). (Reference: H.P. Bloch and A.R. Budris, Pump Users Handbook, Life Extension/Fairmont Press). SE is an indication of a liquid's momentum at the impeller eye and takes S a step further. As shown in the equation below, it is the product of the eye diameter (De), pump RPM (N), suction specific speed (S) and specific gravity (SG).
SE = De x N x S x SG
The SE calculator appears to the right of Figure 1. Entering the required data in the highlighted cells will result in the calculation on SE. The table below the calculator shows the values for the start of high and very high suction energy for several pump designs. Pumps that exhibit a high SE can experience vibration, noise and minor cavitation damage. Those with an extremely high SE can experience more severe erosion due to cavitation.
The example used in both calculators is a dry pit non-clog with a 12 in suction and a two vane, 18.5 in impeller. BEP flow is 5,000 gpm @ 105 ft and requires a NPSH of 10 ft. Both calculators confirm very high values of S and SE, which indicate that a large NPSH margin is required for this pump to operate at off BEP flows.
Igor Karassik was the major force behind the development of suction specific speed. In the mid-80s he wrote a three article series titled "Centrifugal Pump Operation at Off Design Conditions." They are written in a simple Pump Ed 101 style and are available on my website under the "Other Pump Topics" tab. They are definitely worth reading.