What is NPSH?
An overview of the pump’s cavitation prior to the detailed discussion about NPSH would be helpful. For example, when we pump liquid, the local pressure inside the pump (suction pressure) can drop below the vapour pressure of the liquid Pv. If P (suction pressure) is less than Pv, the liquid boils up on the suction side of the impeller.
This partial boiling forms vapour bubbles and the impeller transports them to higher pressure regions. Consequently, the vapour bubbles get collapsed in high-pressure regions. The sudden collapsing can generate shock waves which put stress on the metal, causing it to wear, also called cavitation.
To avoid cavitation, we always try to keep local pressure inside the pump greater than the vapour pressure. Therefore, a flow parameter called Net Positive Suction Head (NPSH) is quite helpful in this regard.
Reference: Fluid Mechanics by Yunus A. Cengal & John M. Cimbala
The Net positive suction head (NPSH) is the absolute suction head minus vapour pressure head.
NPSH = (P/ρg+V2/2g)pump inlet – Pv/ρg (1)
Suppose for a pump, the suction head is 60 feet and the vapour pressure head is 45 feet. Its NPSH would be 15 feet.
NPSH available and required
The pump manufacturers perform cavitation tests at various flow rates and suction pressures. They decrease suction pressure for a fixed flow rate and liquid temperature until cavitation occurs. Then by using equation (1) they calculate the value of NPSH. The measurements are taken for several other flow rates, after which they draw them on the pump characteristic curves as NPSH required. It is thus, the minimum suction pressure required to avoid cavitation in the pumps.
It depends upon the volumetric flow rate of the pump. Therefore, it is plotted on the same characteristic curves of the pumps.
NPSH available totally depends upon the operating environment. For example, the type of liquid being pumped, its temperature, flow rate, the irreversible head losses in the piping system, etc.
Effect of pressure drop & other factors on cavitation
We know that when a liquid flows its pressure converts into velocity, causing a drop in the pressure. In order to increase the flow rate, we have to compromise a more pressure drop. Since the pressure drop acts as a driving force for the fluid to flow.
On the other hand, fluid experiences friction during its flow which is a resiting force. In order for the fluid to flow, the driving force must be greater than the resisting force. The resisting force or the friction depends upon the flow rate and piping system. Increasing the flow rate increases the friction which means a greater pressure drop (driving force) would be required, thus reducing NPSH available.
Moreover, a pipe with a smaller diameter offers more resistance than one with a larger diameter. It is important to note that NPSH required is the built-in property of the pump as it is first manufactured. But NPSH available depends on the pumping environment as discussed above. While operating a pump we must keep NPSH available higher than NPSH required to avoid cavitation.
The impeller of the centrifugal pump pulls liquid to its centre. As a result, the fluid speeds up towards its eye consequently increasing the pressure drop at suction. Irrespective of how small the suction line is, the pressure drop can be much greater. This drop in pressure can cause cavitation. We normally provide pumps with a suction head by locating them far below the vessels. Thus the suction head due to the height of fluid gets high enough to compensate for the pressure drop. The NPSH available, therefore, should be kept 10% greater than NPSH required.
Reference: API 1071WB-Centrifugal Pumps
Maximum flow rate of the pump
I’ve taken the above figure from the book, “Fluid Mechanics by Yunus A. Cengal & John M. Cimbala for reference. It gives us a better understanding of how and when does cavitation occur. It shows that cavitation occurs at and beyond the flow rate where both the curves of NPSH available and required, intersect each other. This is the maximum volumetric flow rate, which can be supplied by a particular pump without cavitation.
The region beyond the intersection is where the cavitation dominates. However, to increase the flow rate we would’ve to increase the net positive suction head available. We can increase the net positive suction head available by:
- raising the source reservoir to increase static head
- assuring to use minimum elbows
- increasing the diameter of the pipe (to decrease major losses)
- decreasing the surface roughness
- using a ball valve instead of globe (to decrease minor losses)