In summary, all the parameters except temperature (vapor pressure) are the same, and simply changing the temperature of the fluid from 68 F to 150 F reduced the available NPSH by almost 8 feet. This may not seem like a big deal, until you realize that the pump requires 13 feet of NPSH at the condition point and 12.24 feet is all that is available. At this NPSHa value, the pump will not operate correctly and will be in a constant state of cavitation.
Anytime you have a self-primer pump (or any pump) application, you must calculate the NPSHa, which also means you need to know how to calculate the negative head from vapor pressure. Most manufacturers do not recommend using self-primer pumps on lift applications above 145 F for this very reason. The workable solution for higher temperatures will likely involve a submersible or a vertical sump pump.
High Suction Pressure — Condensate Pump
The second case evolves from the misconception, “If you have high pressure on the pump suction, then there is no need to calculate the NPSH.” Again, vapor pressure is not your friend, and as fluid temperature rises, the corresponding rise in vapor pressure will erode any benefit from the high suction pressures. I have unfortunately witnessed this mistake countless times over my career.
A common steam system supplies saturated steam at working pressure and temperature for the processes in the facility. After the steam does its work as part of the system process, it is condensed to liquid (condensate) and later reheated to repeat the steam cycle. The condensate is normally collected in a tank or “hot well” that can be at a pressure above ambient or in a vacuum.
The example we will look at involves deaerator tanks at some pressure above ambient. Deaerator tanks are primarily used on commercial and industrial steam systems as a method to remove oxygen, carbon dioxide and other noncondensable gases from the system.
They serve many other purposes, but oxygen removal is the primary goal so that system corrosion is mitigated. To force the gases from solution, the fluid is heated to an equilibrium condition. This means the water will be at its saturation point (the fluid is at its boiling point), which also means the vapor pressure will be equal to the pressure in the deaerator tank. Looking at the NPSHa formula above, note that the head (pressure) from the absolute pressure will be equally countered by the head (pressure) of the vapor pressure component. In short, they will negate each other.
In this example the pump suction pressure is at approximately 134 psia, and the fluid is water at 350 F.
This is a common situation with 150-pound steam systems. The error that occurs is when the pump system owner looks at this pressure and incorrectly thinks they have so much excess suction pressure there is surely no need to calculate the NPSHa.
The problem as in the previous example is that the feet of head from the vapor pressure will negate every foot of head from the absolute pressure.
Again, using the NPSHa equation, fill in the components for this example: The static head will be 10 feet, and the friction value will be 3 feet. The absolute suction pressure is 134 psia (converted to head becomes 350 feet). Note: I am rounding, so numbers will be off slightly. The vapor pressure for 350 F water is also 134 psia, so it cancels any gain from the suction pressure (head). Since this is a flooded situation where the fluid level is above the pump, the static head is a positive value.
Vapor Pressure Explained
Due to the confines of this article, we will only discuss vapor pressures for liquids that are not derived from hydrocarbons or mixtures such as gasoline. You should consult a reliable technical reference for a more thorough explanation of vapor pressure.