All about NPSH 15-10
I get a lot of e-mail, and an occasional telephone call, from someone that is confused by the term NPSH. I have published several papers on the subject, but evidentially I haven't done the job very well, so let me try again:
We do not want bubbles in our process fluid for a lot of reasons:
Bubbles or cavities form in a liquid when the fluid temperature gets too high, or the fluid pressure gets too low. This is called vaporization, or sometimes boiling. I do not like the word boiling because we associate boiling with hot, and we all know that if you throw dry ice into cold water it will bubble and vaporize, and no one is going to call that hot! We'll stick with the term "vaporize" and further state that a fluid will vaporize any time the pressure falls below its vaporization point.
Since temperature is a variable with different fluids, there are charts that will give you the vapor pressure for any fluid at its various temperatures. Take a look at the following chart and you will note that the vapor pressure for 60-degree Fahrenheit chlorine is 80 psi, and the vapor pressure for 68-degree F. fresh water is about 0.3 psi. We will need numbers like this to calculate our NPSH available.
You should know that you can lower a fluid's pressure several ways:
Heating of he incoming fluid is not usually a problem, but it can occur several ways:
The next step we have to learn is that the word "pressure" is going to disappear from our vocabulary whenever we discuss centrifugal pumps. We are going to substitute the word "head "instead. We do not know how much pressure a centrifugal pump will develop, but we do know the head it can produce. The head is a function of the shaft speed and the impeller diameter. The faster the speed,
The larger the diameter, the bigger the head To determine the pressure we have to know the weight or "specific gravity" of the fluid we are pumping, and since any given centrifugal pump can move a lot of different fluids, with different specific gravities, it is simpler to discuss the pump's head and forget about the pressure.
Here are the formulas you can use to convert from one to the other:
In the above formula:
The pump manufacturer has decided how much head his pump needs to prevent cold water from vaporizing at different capacities. He publishes these numbers on his pump curve. He got these numbers by testing the pump at different capacities, created by throttling the suction side and waiting for the first signs of cavitation. He then noted the pressure, converted it to head, and transferred this information to his pump curve.
He calls this observed number the "net positive suction head required (NPSHR) or sometimes shortens it to the NPSH. Take a look at the following curve and you can see these numbers. On the chart they are located at the bottom of the dotted lines and they run from 2 to 16. According to this graph a 13-inch impeller, running at its best efficiency point (60+%), would need a NPSH required of 9 feet. An 11-inch impeller running at its best efficiency point would need 7 feet of NPSH required. Remember this requirement is for cold water (68F) only.
Be sure to keep in mind that any discussion of NPSH or cavitation is only concerned about the suction side of the pump. There is almost always plenty of pressure on the discharge side of the pump to prevent the fluid from vaporizing.
Now that we know what head is required, we can calculate the head we have available, and remember we are only interested in the suction side of the pump. It turns out you will be looking at three kinds of head:
In summary, NPSH Available is defined as:
NPSHA = Atmospheric pressure + static head + pressure head - the vapor pressure of your product - loss in the piping, valves and fittings.
You can learn about the actual mechanism of cavitation by clicking here.
If you would like to learn how to make the calculations for NPSH available, click here
If you want to see the charts that will help you calculate the friction loss in the piping, valves and fittings, click here.
For information about my CD with over 600 Seal & Pump Subjects explained, click here
Link to Mc Nally home page www.mcnallyinstitute.com