MechChem Africa September-October 2024

⎪ Water, wastewater and pumping solutions ⎪

from the original pump curve and data sheet. Add 1.0 m for a margin, and con vert from absolute pressure to gauge pressure. Move the suction gauge in line with the pump. If this is a problem with respect to access and being able to view the gauge easily, reduce the NPSHR value by the gauge height – in this case this would reduce NPSHR to -71 kPa. Make sure this value is clearly shown in the pump station, by marking up the pres sure gauges with red and green zones as shown in the Figure 1. This clearly shows when the pump begins to experience suc tion problems. When the gauge reads less than the minimum or inches towards the red zone, then something must be done. This could be happening for a number of reasons: Over time, friction increases through the suction pipe due to organic growth. The solution is to clean out the pipe, using either a high pressure wash or manually. Spring low tides cause unusually low suction levels, causing the suction pres sure to drop into the red. Stop the pump at these times. It only happens twice per month for a couple of hours. Build ad ditional storage into the system to allow for this and other times like load shedding when you cannot run your pumps Regularly clean out the strainers and cages around the pump inlets. Once again, the suction needle inching towards the red zone will tell you when this needs to be done. As with all difficult pumping applications, the starting point is designing the system and se lecting the pump that best suits the site-specific conditions. In this case the negative suction head and low NPSHA was key. But a well-designed system is not enough: conditions vary over time so it is essential to continuously monitor pump operation. This will prevent the pumps from destroying themselves, even under the harshest unforeseen circumstances. And finally, a negative pressure on the suction side is not necessarily a problem. This happens in many applications with a positive suction head that we are not even aware of. It is only when this value falls below the absolute pressure required by the pump, the NPSHR, that we have a real problem. www.tasonline.co.za • • • • • •

One of the more scenic views of a pump station, the sea water intake pump station for an Abalone farm showing the suction lift conditions.

4.0 m. Including a margin of 1.0 m, we get a total positive suction head requirement of 5.0 m, or 50 kPa. So long as our NPSHA exceeds this amount, we will not expect the pumps to cavitate. If pumping systems were this simple, though, I would probably be out of a job and have nothing interesting to write about. However things change, sea levels rise and fall, organic growth builds up inside our large diameter pipes reducing their effective di ameter, kelp and seaweed lifted up through storms clog the inlets to the pumps, etc. The pumps were running fine last week but sud denly three weeks later we have problems. Ongoing monitoring At the time of my visit the suction pressure gauge was reading -61 kPa. This sounds seri ous, but what does it mean – are we pulling a vacuum, is the pump cavitating, what now? This is where successful abalone farmers need to understand the relationship between gauge pressure and absolute pressure. NPSH is expressed in absolute pressure – which includes atmospheric pressure – whereas the suction gauge is reading gauge pressure. These are not the same thing, like comparing apples with oranges! P absolute = P gauge + P atmospheric If we switch the terms around we get: P gauge = P absolute - P atmospheric Our NPSH required for the pump was 5.0 m or

50 kPa absolute pressure. If we subtract the 101 kPa of atmospheric pressure, this gives us gives us -51 kPa gauge pressure. Now we have something useful to work with – if the gauge pressure on the suction side of the pump is less than -51 kPa, then our pump is in trouble. But our suction gauge is reading -61 kPa, so our pump must be cavitating: Right? Wrong! The suction gauge was installed 2.0 m above the level of the suction pipe or pump centreline. This gauge height correction must be added back to the suction pressure reading as the liquid loses pressure as it rises in the tube connected to the gauge. The gauge pressure at the pump suction is therefore -41 kPa, and even though it is way less than zero – or what many people would call pulling a vacuum – our pump is not cavitating. This is the power of a compound pressure gauge on the suction side that can read nega tive pressure. If we used a standard discharge gauge on the suction it would probably read zero all the time, not telling us anything useful. If the gauge pressure dropped below -50 kPa we know our pump is close to cavita tion and we need to stop the pump, fix the suction conditions and only then restart the pump. A pump cavitating under these condi tions could destroy the impeller in a matter of weeks. What were the recommendations? • Work out the NPSHR for the pump –

September-October 2024 • MechChem Africa ¦ 13