Suction Specified Speed (NSS)

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 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 from the higher pressure areas of the pump into the impeller eye. 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 in the impeller eye where the pressure has increased.

Getting back to the NSS, the Hydraulic institute recommends a number less than 175 (metric) with design values higher than this increasing 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.

System head curves: the forgotten asset

Over the past year the profile of delegates attending courses offered by JTA have ranged from professional engineers right through to pump operators. During the courses, many were exposed to the features and benefits of systems head curves for the first time. The level of interest shown in this particular subject has been amazing. Most graduate engineers say something like "yes we did this at university but very briefly and we were not shown where and how to apply this". When the theory associated with system curves was related to real life case studies then the light bulbs really started coming on! For a trainer to see eyes lighting up and comments such as "so THAT's why that station is giving us so much trouble!" coming out, the rewards are hard to express.

What is a system head curve? Simply put, it is a graph which shows the total dynamic head (TDH) in a pipework system at various flow rates. Often this is superimposed on the performance curve of an appropriate pump and the duty point will be at the intersection of the system curve and the pump's performance curve. If the static head varies for any reason, a second system curve can be drawn which shows the increased TDH. The two intersection points show the range of flows and heads the pump will "see" as the static head changes.

Where does this all originate from? Thanks must go to a colleague who took the time to take this trainer through the process (too) many years ago. At that time the light went on and, yes, somebody was found to be at home. The lesson stuck and whenever the suspicion arose that the system was the culprit, out came the graph paper (this was before Excel and other dedicated software) and the friction loss charts. Once the plotting process began the story unfolded before some eager eyes. Yes the pipe was too small or in some cases too big!

Once all the plotting was complete, there it was, in black and white or in colours, the whole sad story. No more arguing or debate. The solution(s) became pretty self evident and the feeling of achievement and satisfaction were palpable.