NPSH (USCU units) N011
We do not want bubbles in our process fluid for at least three reasons:
- Bubbles take up space, causing our pumping capacity to diminish.
- Air is a lousy heat transfer medium, meaning that the fluid we are pumping will get hotter and there is no advantage in heating up the process fluid.
- A bubble is a hole in the liquid. In English we call that hole a cavity, and it is those cavities that are going to cause a cavitation problem that nobody needs.
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:
- Put the fluid in a container, and then pull a vacuum on the container. This happens in the hot well of condensers. Later on we will refer to this as a loss of “pressure head”
- Lift the liquid out of a hole. This will diminish the position of the liquid level in respect to the pump centerline. Later on we will call this a loss of “static head”
- Accelerate the fluid. As its velocity increases its pressure will decrease. This is referred to as “velocity head”
- As the fluid moves through piping, fittings, restrictions and valving, some friction losses occur that will drop the fluid pressure. We will talk about that as an increase in friction head, resulting in some loss of “positive suction head.”
Heating of the incoming fluid is not usually a problem, but it can occur several ways:
- Internal recirculation in the pump because of worn wear rings or failure to make an impeller adjustment.
- Piping, exposed to the elements, can heat up the liquid on hot and sunny days.
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:
- Head is measured in feet (ft.)
- Pressure is measured in pounds per square inch (psi.)
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.
- If we go back to our formula and put the 0.3 psi vapor pressure for 68 degree water into the numbers, it come out to 0.7, or less than 1 foot of head is required to stop the water from vaporizing and forming cavities. So why does the NPSH required increase as the capacity is increasing? It’s because the velocity of the liquid is increasing, and as we learned, anytime the velocity of a liquid goes up, the pressure or head comes down.
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:
- The static head measured from the liquid level to the centerline of the pump. If the liquid level is above the pump centerline you will have a positive number. If the level is below the centerline you will have a negative number.
- The pressure head. Here we will be using only absolute numbers. In other words atmospheric pressure is 14.7 psi at sea level so you will add that number (converted to feet, using our formula) to the static head if you have an open tank. If the fluid is under vacuum we will convert the absolute pressure reading to head and use that number, instead of atmospheric pressure. Vacuum is often read in inches of mercury so you will need a formula to convert it to head. Here is the formula:
- The friction loss in the piping will be a minus number. You get the number from charts showing pipes size vs flow, and flow through fittings and valves.
- The next thing we have to do subtract the vapor pressure of our fluid (converted to feet of liquid) using the first formula I gave you. All of the above, added together is the NPSH available. If this number is equal to, or more than the NPSH required by the pump manufacturer, the liquid will not form bubbles or cavities on the suction side, and the pump will not cavitate.
You can learn about the actual mechanism of cavitation by clicking here.
If you want to see the charts that will help you calculate the friction loss in the piping, valves and fittings, click here.