Some thoughts about NPSHR and Suction Specific Speed Nss…

Suction Specific Speed: It represents the Suction behavior of the Pump and is calculated by below equation.

Where N = Operating Speed (RPM) ; Q = Flowrate at Best Efficiency Point (cu.m / s) and NPSHR = Net Positive Suction Head Required (m).

Now, here Q will be fixed depending upon the Process requirement and N could be estimated by the Type of Pump, in other words, Specific Speed.

Once, these 2 parameters are fixed, the only variable remains, is the NPSH required, which will be governed by the Suction Geometry of the Impeller. Now if we consider, the limit imposed by the studies done by J L Hallam, which indicated that the NSS should not be more than 11000 rpm in US units or 213 rpm in ISO units, it should mean that we are reducing NPSHR which will go on increasing NSS.

Fig.1 General representation of drop in suction energy at impeller eye

So, as indicated in Fig.1, this drop in suction energy from Suction pressure Ps to some value above Vapor pressure Pv line, shall decide the NPSH required value at the flowrate of interest. This will certainly be the function of Suction geometry at impeller eye. However, now with the help of modifications in Suction geometry, what we could work out is to reduce this difference i.e. drop in suction energy shall be as minimum as possible. This is just a rough representation. Also one more notable fact is that, the NPSHR value published by Pump manufacturer will generally correspond to NPSHR3% i.e. the value of NPSH when discharge pressure of the pump has shown a drop of 3% at given flow rate. This will be exclusively mentioned in most of the cases on the NPSH curve published by Pump manufacturer.

Fig.2 Comparison of Improvement in NPSHR with modifications in Suction geometry

However, through various technological improvements and improved understanding of flow through the impeller, it is now possible to reduce further the NPSHR.

Fig.3 Improved NPSH with modifications in Suction geometry

Now, as seen from fig.2 and fig.3, improved NPSHR can allow the Pump to be operated at lower amount of Suction energy (NPSHA) as compared to earlier. This is obviously good from Pump user’s point of view. However this shall also indicate that, the pump will always operate close to vapor pressure of liquid as compared to earlier, if the NPSHA is maintained just appropriate as per the guidelines.

It means that inherently, irrespective of what is happening inside the Impeller eye, the margin between the vapor pressure and suction pressure is reduced since, we have reduced NPSHR.

The effect of Cavitation is visible or more pronounced when certain amount of drop in discharge head is seen.(1% /2% / 3%). However, the formation of 1st bubble starts well beyond this range. The Point which is known as NPSHinc (Cavitation incipient). Hence, it is quiet certain that, every pump could be operating with some amount of cavitation but it differs from pump to pump or application to application, that whether is it damaging or fine to go with it.

Suction pressure or NPSHA play a vital role in the selection, as depending upon the NPSHR, the value of NPSHA will be decided and if the value of NPSHR is low then obviously the value of NPSHA is also going to be relatively low. This will reduce the margin between vapor pressure and the suction energy. And hence, the operating flexibility in terms of preferred operating region for a given pump will be a concern. Hence one may has to be careful while deciding the allowable range to operate the pump.