Hi Friends!!!
In this post , I will explain basic terminologies which are related to selection of correct pump. I have tried to explain it in layman terms so that everyone can understand it.
Application : Let us consider that in our society we want Water pump to transfer water from underground tank to Overhead tank which is on Building terrace.
- Flow/Capacity : It is the amount of liquid a pump is required to transfer in specific time. General Units : m3/hr , lph , lpm ,lps etc
eg : Overhead water tank is of 12000 Liters capacity and we have to fill it in 2 hours. So we can choose pump with Flow of 6000LPH.
Pump Flow = 6000lph ( Liters per hour) and the application of this pump is to transfer water from undergound tank to overhead tank in building.
So 6000lph means that the selected pump should fill the overhead tank at the rate of 6000 liters in 1 hour. So if the tank is of 12000 liters capacity then this pump would take 2 hours to fill the tank.
2. Head/Differential Pressure : Pump Head is generally the pressure a particular pump has to develop to transfer the liquid. In layman terms it is how far the liquid has to be transfered.
There are complex formulas in calculation of head which i will discuss in a separate blog. However generally Pump Head : Discharge Pressure – Suction Pressure.
In calculation of the head we have to also consider Frictional losses , Pipe diameters and many other factors.
eg: Considering our example of transferring liquid from underground tank to overhead tank , The distance from underground tank till overhead tank which the liquid has to travel is called head.
3. Shut Off Pressure/Shut off Head : It is the maximum pressure the pump can develop. It is the pressure which is achieved when the flow is 0. Shut off pressure can be checked by shutting off the discharge valve.
4. NPSH ( Net Positive Suction head) — NPSH Available and NPSH Required.
It is a very wide topic. Infact major problems in pumps arise due to ignoring NPSH while selecting the pump.
In short it is the capacity of the pump to suck/lift water when the suction is negative. Below this depth the pump will start cavitating.
NPSH Available at site :
Eg : a) If the sump depth is 2 meter , then considering atmospheric pressure of 10m , the NPSH available would be 10-2- Frictional loss(Assume 0m) = 8m
NPSH Required : If the NPSH Required of the pump is 3m.
That means the pump can suck the liquid or lift the liquid upto depth of 10 – 3 = 7m .
In this case if the depth of the sump is 9 meters then this pump will not be able to suck liquid. The liquid will start vaporizing resulting in cavitation problem.
As a thumb rule : NPSH-A > NPSH-R
5. Efficiency : No pump can give 100% efficiency. Since there are many losses in the pump hence depending on the flow and head , the pump gives specific efficiency.
The best efficiency the pump can at a particular flow and head is called BEP(Best Efficiency Point). Ideally pump should be selected in such a way that the pump operates at BEP.
6) Material Of construction(MOC) : It is the material from which the pump components are made.
MOC of the pump is generally selected based on the liquid properties. Wrong MOC selection can lead to pump failure , parts corrosion , impeller tip-off , shaft breakage etc.
7) Motor RPM(Revolutions per minute) : It is the shaft rotation speed of motor. It is determined by Frequency and number of poles of Motor.
RPM Formula= Hz* (2 / poles) * 60
eg : 4 pole motor with 50 hz frequency will have RPM of 1500 ie RPM = 50*(1/2)*60 = 1500RPM
8) BKW ( Brake Kilo Watt) = It is the power required by the pump to drive the pumpset or pump the liquid.
Pump should be selected with highest efficiency so that power consumption is less. Lesser the power consumption better is the pump performance.
If there are any other topics which you need me to cover in this post then please comment/email so that i can cover it.
Hope this blog is useful to you.
Thanks and Regards
Siddharth Bhimani.
Sial Valves Co…Authorised Dealer of Kirloskar Brothers Ltd.
Centrifugal Pump Basics. 