The Truth About NPSH & Priming

I am frequently asked whether a priming system can increase the NPSHA (Net Positive Suction Head Available) of a pump installation to help solve cavitation problems due to the pump’s high NPSHR (Net Positive Suction Head Required).

While priming systems perform a critical function, unfortunately this is not one of their benefits. The NPSH of an installation is defined as follows and is usually expressed in Feet of water column:

NPSHA = HA + HZ – HF + HV – HVP

Where:

HA =    Absolute pressure acting on the liquid source, or atmospheric pressure for open sources such as a reservoir, clear well, sump pit, etc.

HZ =    Vertical distance ((+) below, or (-) above) between the source and the pump’s shaft centerline.

HF =    Friction losses in the suction piping.

HV =    Velocity head (typically sufficiently small to ignore).

HVP =   Fluid’s vapor pressure at the operating temperature.

Centrifugal pumps are capable of creating a vacuum to pull up liquid from a lower source, provided the pump starts with a flooded suction. The density of the pumped fluid is directly proportional to the pump’s pressure – head relationship. Let us use water as an example. Since water is more than 800 times as dense as air, centrifugal pumps are simply not capable of developing sufficient vacuum to self-prime from a dry state.

When the HZ component is negative (i.e. pump installed above the source) it represents the static lift requirement for the pump. Vacuum priming systems are used to lift the liquid up to the pump, so that the centrifugal pump can then create sufficient vacuum to overcome the static lift during operation. A gauge reading of the pressure at the inlet to an operating pump represents the NPSHA minus HVP (vapor pressure component), and is commonly referred to as the Total Dynamic Suction Head. Once the pump is running, this pressure exists whether a priming system is connected or not. At this point the priming system is merely removing any gas that accumulates to maintain a liquid level above the suction piping high point where it is connected into the process.

A typical system uses priming valves for preventing liquid carryover back to the priming system. Priming valves are located above the pump’s volute so that complete gas removal (priming) can be accomplished. The priming system creates sufficient additional vacuum to further lift the liquid up into the priming valve so that it closes, effectively isolating the priming system from the process. Whether this location is 1 ft or 10 ft above the centrifugal pump, or the priming valve is open or closed, it does not influence or change the pressure at the suction side of the operating pump. As such, the priming system has no affect or influence on the NPSHA of the installation.

The best method to increase the NPSHA is to look over the contributing factors to determine what changes if any that can increase the NPSH are feasible. For example, increasing the elevation of the source liquid level, increasing the suction pipe size, or removing a foot valve used in conjunction with a priming system. Priming systems preclude the need for foot valves, which only increase the pressure drop in the suction piping, and therefore directly reduce the NPSHA.

One additional feature or benefit of a priming system is the removal of some of the entrained air within the pumped fluid. This does not increase the NPSHA, but it does help alleviate some of the negative effects on the pump caused by excessive entrained air. We’ll save this topic for another discussion!

“Reliable through design” LYNN, we understand priming!

Ned Hoag

President, LYNN Engineered Systems

LYNN Automatic Duplex Vacuum Priming System

 
LYNN Reliable through design