SUBJECT : Calculating net positive suction head (NPSH) in non-metric units 11-12.

The definition of NPSHA is simple: Static head + surface pressure head - the vapor pressure of your product - the friction losses in the piping, valves and fittings.

But to really understand it, you first have to understand a couple of other concepts:

Lets look at each of these concepts in a little more detail :

To calculate the net positive suction head (NPSH) of your pump and determine if you are going to have a cavitation problem, you will need access to several additional pieces of information:

To convert surface pressure to feet of liquid; use one of the following formulas:

There are different ways to think about net positive suction head (NPSH) but they all have two terms in common.

NPSHR (net positive suction head required) is defined as the NPSH at which the pump total head (first stage head in multi stage pumps) has decreased by three percent (3%) due to low suction head and resultant cavitation within the pump. This number is shown on your pump curve, but it is going to be too low if you are pumping hydrocarbon liquids or hot water.

Cavitation begins as small harmless bubbles before you get any indication of loss of head or capacity. This is called the point of incipient cavitation. Testing has shown that it takes from two to twenty times the NPSHR (net positive suction head required) to fully suppress incipient cavitation, depending on the impeller shape (specific speed number) and operating conditions.

To stop a product from vaporizing or boiling at the low pressure side of the pump the NPSHA (net positive suction head available) must be equal to or greater than the NPSHR (net positive suction head required).

As I mentioned at the beginning, NPSHA is defined as static head + surface pressure head - the vapor pressure of your product - loss in the piping, valves and fittings .

In the following paragraphs you will be using the above formulas to determine if you have a problem with NPSHA. Here is where you locate the numbers to put into the formula:

In the following examples we will be looking only at the suction side of the pump. If we were calculating the pump's total head we would look at both the suction and discharge sides.

Let's go through the first example and see if our pump is going to cavitate:

Given:

Now for the calculations:

NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings

NPSHA (net positive suction head available) = 34 + 5 + 0 - 0.62 - 2.34 = 36.04 feet

The pump required 9 feet of head at 100 gpm. And we have 36.04 feet so we have plenty to spare.

Example number 2 . This time we are going to be pumping from a tank under vacuum.

Given:

Now for the calculations:

NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings

 

NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 0.62 - 2.34 = 13.34 feet. This is enough to stop cavitation also.

For the third example we will keep everything the same except that we will be pumping 180° F. hot condensate from the vacuum tank.

The vapor pressure of 180°F condensate is 7 psi according to the chart. We get the specific gravity from another chart and find that it is 0.97 sg. for 180° F. Fresh water.

Putting this into the pressure conversion formula we get:

NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings

NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 16.7 - 2.34 = -2.74 feet.

We need 9 feet, so the pump is going to cavitate for sure.

A few notes about this last example:

If you are given the absolute and vapor pressures in psia, and you forgot how to convet to feet of head; you can use the following formula, providing you know the specific weight of the liquid you are pumping :

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