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In the last blog of this series we explained why a pump requires a positive inlet pressure and introduced the term NPSHR. As promised, this blog we will investigate the counterpart of NPSHR, Net Positive Suction Head Available (NPSHA).
The pressure available at the pump inlet is a summary of multiple factors. These factors, all expressed in the same units (normally feet of head), either add to the available pressure or detract from it. The five factors are as follows; atmospheric pressure (Ha), static elevation difference between the liquids surface & the centerline of the impeller (Hs), friction losses in the suction piping (Hf), velocity head of the fluid (Hv) and the vapour pressure of the liquid (Hvp).
The formula that is used to calculate NPSHA is:
NPSHA = Ha +/- Hs – Hf + Hv – Hvp
To keep things simple this week we are going to assume that all applications are on water at 33 degrees Fahrenheit (vapour pressure is negligible), and we will ignore velocity head as it is generally a small number. I will discuss Hv (velocity head of the fluid) and Hvp (vapour pressure of the liquid) in future blogs.
Simplifying the full formula for the purpose of today’s discussions leaves us with:
NPSHA = Ha +/- Hs – Hf
The first factor in this equation is atmospheric pressure. As discussed in the last blog, Required Inlet Pressure, it is 33.9 feet of water head at sea level. If the pump was operating at the top of a mountain, then the atmospheric pressure would be lower based on the reduction in the weight of air above the pump. It may just be me but I have not experienced many pumps operating on mountains so for the blogs sake we will move forward with sea level. The column titled “Equiv. Head of Water (Feet)” in the chart below can be used to quantify Ha (atmospheric pressure) at different altitudes.
The next factor in our equation is the elevation difference between the liquids surface and the centerline of the impeller (Hs). If the pump is located above the surface of the liquid (as in the illustration below), then the vertical suction lift above the liquid is subtracted from the atmospheric pressure.
If the pump is located below the surface of the liquid (as in the illustration below), then the vertical distance is added to the atmospheric pressure.
The last factor in our simplified equation is friction losses in the suction piping (Hf). Also referred to as suction losses. Hf is the loss in available head due to friction and any other restriction on the suction side of the pump. In simple installations it may only be pipe friction losses, but if other fittings are present such as valves, elbows, or strainers, all associated losses must be included.
To tie it all together lets look at a straightforward application example. In the illustration below, a pump is operating on cold water at seal level. With the water being very cold we can cancel out vapour pressure again and for simplicity sake we can state the velocity head as zero.
At sea level we know atmospheric pressure (Ha) is 33.9 ft. If the friction in the suction system (Hf) was calculated at 5 feet and the elevation distance (Hs) was 3 feet our formula would be:
NPSHA = Ha +/- Hs – Hf
= 33.9 + 3 – 5
= 31.9 ft
Since most standard centrifugal pumps have an NPSHR well below the available NPSH of 31.9 feet, we could assume this application would function properly.
That’s NPSHA in a nut shell and that, my friends, is it for this blog. Next blog we will address Vapour Pressure, one of the two subjects we skipped over in this blog. See you then!
Until next time,