You most likely started your pump selection by calculating total flow (gpm) and head (ft-hd) requirements. However, it is also critically important to understand what’s going on at the suction side of the pump. If there is not adequate inlet pressure on the suction side of the pump, the pump will cavitate. Cavitation can lead to severe pump damage and even explosion of the pump itself. That’s where NPSH comes in.
What exactly is NPSH?
NPSH is an acronym for Net Positive Suction Head.
Net Total in the system (available from the system or required by the pump)
Positive Pressure directed toward:
Suction The suction or inlet side of the pump
Head Equivalent water column height
Put another way, NPSH is the pressure of a fluid on the suction side of the pump.
How do I calculate NPSHa?
Certain attributes of a liquid handling system will determine the amount of pressure available (or NPSHa) on the suction side of the pump. These are:
· Atmospheric pressure on the liquid
· Vapor pressure of the liquid
· Friction loss in the suction piping
· Distance from the pump to the liquid level
The total of all these values is the NPSHa of the system. Let’s discuss each individually.
Atmospheric Pressure
Without going into too much detail, atmospheric pressure is the weight of the air above a measurement point. That weight is pushing down on the surface of the liquid you are pumping. That pressure in turn will be experienced at the inlet of the pump. At sea level atmospheric pressure is 14.7 psi or 33.957 ft-hd (Example: 14.7 psi x 2.31 psi/ft-hd = 33.957 ft-hd). If the system is in an area significantly above or below sea level, you should determine the specific atmospheric pressure at that location and account for it in your calculations.
Things get a little more complicated however, if the liquid is heavier than water and the pump suction is above the liquid level. In this case, the atmosphere still pushes down on the surface of the liquid with the same force. However, the additional weight of the liquid means that it does not in turn exert the same corresponding force at the suction side of the pump. If the liquid is heavier than water, the total pressure at the inlet of the pump will be less. If the liquid is lighter than water, the total pressure at the inlet of the pump will be greater. You can quantify this by simply dividing the atmospheric pressure by the specific gravity of the liquid. Say, the liquid has a specific gravity (SP) of 1.23 (Example: 33.957 ft-hd/1.23 SP = 27.61 ft-hd). Remember this number represents pressure at the pump inlet and as a result is a positive number.
Vapor Pressure of the Liquid
Vapor pressure is pressure at which a fluid becomes a vapor. This value can be found in a reference material such as a hydraulic data book. It changes based on the temperature of the liquid. For water, this value at 68 degrees F is 0.34 psi or 0.78 ft-hd (Example 0.339 psi x 2.31 psi/ft-hd = 0.78 ft-hd). As this value is negative (Example: 27.61 ft-hd – 0.78 ft-hd = 26.83 ft-hd).
Friction Loss in the Suction Piping
The friction loss in the suction piping will depend on the length of pipe, pipe material, number of elbows in the pipe, etc. The Hazen-Williams Equation or a reputable online friction loss calculator can be used can be used to calculate this value. Say we accounted for a 12” plastic inlet pipe with one 90-degree elbow and found the total friction loss to be 1.00 ft-hd. This friction loss in the inlet piping is working against the atmospheric pressure. Put another way the friction loss is resistance to flow in the pipe and requires additional energy or pressure to overcome it. As a result, this number is negative (Example: 26.83 ft-hd – 1.00 ft-hd = 25.83 ft-hd).
Distance From the Pump to the Liquid Level
This measurement starts at the centerline of the pump (the center of the eye of the impeller) and extends to the liquid level.
In this case, the liquid level is below the centerline of the pump. As a result, the necessity to lift the liquid to the inlet of the pump is too working against the atmospheric pressure. Put another way, the atmospheric pressure on the liquid must be great enough to push the liquid up the suction pipe and into the suction end of the pump. As a result, this number is too negative. Say, this distance is 8.00 ft (Example: 25.83 ft-hd - 8.00 ft-hd = 17.83 ft-hd).
Our example system has 17.83 ft-hd available or NPSHa = 17.83 ft-hd.
How do I calculate NPSHr?
NPSHr is determined by the characteristics of the pump. This value should be provided by the pump manufacturer as part of the pump data. NPSHr is on a curve and as a result its value changes as the flow rate of the pump changes. Say, we select the point on the NPSHr curve that matches our expected flow rate. Here we discover that the NPSHr of the pump at that point is 11.2 ft-hd.
Conclusion
For the pump not to cavitate, the NPSHa must be greater than the NPSHr. Put another way, the pressure at the inlet of the pump must be great enough so that the pump does not loose suction. (17.83 ft-hd – 11.20 ft-hd = 6.63 ft-hd). Since our NPSHa > NPSHr this pump will not cavitate.
Simply calculating total flow and head requirements of a system is not alone sufficient to properly design a liquid handling system. Understanding NPSH is important to ensure a liquid handling system is safe and functions appropriately. Neglecting to do so can result in pump damage and problems which are costly to correct.
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