SOME DISCUSSIONS CONCERNING THE NPSH
The net positive suction head problem is still very persistent. It is now compounded for large high head pumps by a new need for erosion avoidance at low flow rates. Admittedly, there have been few or rare problems arising from centrifugal pumps failing to deliver rated capacity and pressure in the field due to a NPSH shortcoming. Paradoxically, while NPSH requirements of the average pumps have been so well established, there are some new findings about cavitation effects on larger, high-head pumps that need considerable further research. Purchasers Users, and Contractors have all been “bearing down” on the pump manufacturers to achieve lower NPSH requirements on larger pumps to the point where other things are suffering, the pendulum has sewing too far. This was resulted in large pumps operates at low speed.
Also one of the recommendations usually used to avoid cavitation is the use of a larger pump than the size that would normally be chosen, one merely offers a “4 inches” pump instead of “3 inches” or one offers a double-suction impeller type pump instead of a single-suction, Figure. This practice thereby reduces the required NPSH to a significantly lower value and in this relatively painless manner pump application engineers have been falling into a trap lately.
Cavitation constant σ versus specific speed for best efficiency points
To understand the trap, it is necessary to show the shortcoming of the present definition of NPSH (required). Here is the issuance of the combined API and ASME definitions: “Net Positive Suction Head Required is determined by the vendor and is the total absolute head at the pump inlet less the vapor pressure head corresponding to the liquid temperature at the inlet required for the specified capacity” also “a drop 269 in head of 3 percent is usually accepted as evidence that cavitation is present”. It is important to note that 3 percent head drop point is not always the start-of-cavitation point, neither is a 1 percent head drop point.
At these head drop points, cavitation has already progressed sufficiently to affect the flow in the impeller passages.
The aforementioned “trap” is the excessive cavitation that can occur at low flows on high-energy pumps. Also the dangers of pulsation or surges due to rotating stall or unstable H-Q curve, the oversizing may cause the rated capacity to be below 60% the b.e.p. capacity for that size pump.
What should pump application engineers do to avoid partial-flow cavitation on high head pumps? If it becomes necessary to use an oversized pump for NPSH reasons, give more thought to the provisions to assure a higher minimum flow than usual. Table 2 gives approximate figures for minimum flows.
Recommended minimum flows for high-energy pumps