NPSH (Net Positive Suction Head)

It is the responsibility of the system designer to establish the NPSHa, which is the head available characteristic of the system that provides flow of liquid to the pump. The normal practice is to select a pump with NPSHr less than NPSHa.

The difference between NPSHa and NPSHr is the margin.

To express the quantity of energy available in the liquid entering into the pump, the unit of measure for NPSH is feet of head or elevation in the pump suction. The pump has its NPSHr while the system has the NPSHa. There should always be more NPSHa in the system than the NPSHr of the pump.

NPSHr (Net Positive Suction Head Required)

This is the energy in the liquid required to overcome the friction losses from the suction nozzle to the eye of the impeller without causing vaporization. It is a characteristic of the pump. In other words, NPSHr may be considered to be the minimum suction pressure necessary to keep the pumped fluid in a liquid state. Pump manufacturers publish the NPSHr values on their pump curves.

NPSHa (Net Positive Suction Head Available)

This is the energy in the fluid at the suction connection of the pump, above the liquid’s vapor pressure. It is a characteristic of the system and should be greater than the NPSHr. This value depends on the arrangement / configuration of the piping and pumping system.

To increase the NPSHa:

• Raise the level in the tank.
• Elevate the tank and / or lower the pump.
• Pressurize the tank.
• Reduce the losses of connections and fittings in the suction piping by substituting fittings that are capable of obstructing flow paths with fittings which pose minimal losses.

If the NPSHa of the system cannot be increased, you could reduce the NPSHr by:

• Change to a pump with a larger suction diameter but with the impeller diameter. Retaining the impeller diameter helps to maintain the discharge head and pressures. The larger suction diameter would reduce the fluid velocity entering into the pump, and therefore the losses due to friction.
• Install a small booster pump into the suction piping.
• Increase the diameter of the eye of enclosed impellers.
• Increase the diameter of the suction nozzle and pathway to the impeller either by machining on a lathe or by reaming out and polishing.
• Convert to a pump with double suction impeller.
• Use two smaller pumps in parallel.
• Use a larger / slower pump.

Inadequate NPSHa causes stress, vibration and maintenance on pumps because there isn't enough energy in the liquid for the pump to perform its work.