Part 1: NSS practical aspects and calculation
During a recent visit to a large chemical plant, I was intrigued by the unusually loud knocking noise which was emanating from three reasonably large (350kW) cooling water end suction pumps. A non-technical colleague who was with me at the time even remarked that the noise was unusual.
Closer inspection revealed an intermittent but severe knocking noise in both the suction and discharge, to the degree that the rubber compensators were showing evidence of rapid internal pressure transients. Classic cavitation symptoms caused by operation too far left of the Best Efficiency Point (BEP) combined with a high pump suction specific speed number(NSS)!
The plant supervisor confirmed that it was non-standard practice to run all three pumps at one time as the system was designed for a two operating one standby basis. That at least explained the operation on the left hand side of the pump's curve (see last posting on system head curves).
The pumps in question were designed for a high capacity/low head type of operation where NPSH required becomes something of an issue. In order to meet the challenge, the pump designers enlarged the impeller inlet area to reduce velocity and use Bernoulli's theorem to increase the pressure in the eye of the impeller. This solved the problem of having an NPSH required which was too high, but created another problem in the form of recirculation of flow within the eye of the impeller. The resultant drop in pressure often reaches a point which is less than the liquid's vapour pressure. This allows the formation of vapour pockets which implode as they reach areas where the pressure has increased.
Getting back to the NSS, the Hydraulic institute recommends a number less than 175 (metric). Design values higher than this will increase the chance of pump failure exponentially. The formula used for calculating NSS is:
Where: Q = flow rate in cubic metres per sec at BEP
NPSHr = Net Positive suction head at the same impeller diameter and flow rate
Part 2 of this blog will cover the relationship between impeller eye area and NPSHr.