This month’s column is not about the Queen rock song made famous by David Bowie. Instead it highlights an issue that is common in the pump industry—misunderstanding the concept of vapor pressure. In recent weeks, I have been asked to solve several pump issues that resulted from a misunderstanding of vapor pressure. The first case involved a self-primer pump lift application where the fluid was water at 150 degrees Fahrenheit (F). The second case involved confusion regarding net positive suction head available (NPSHa) on a condensate pump. The pump suction pressure was 134 pounds per square inch (psi). Consequently, the user incorrectly assumed there was no need to calculate the NPSHa.
Self-Primer Pump Application
In the first case of the self-primer pump misapplication, I offer two examples to illustrate this suction lift application. We refer to this as a lift application because the surface level of the fluid supply is below the centerline of the pump impeller. For a self-primer pump in a lift application, understand that the pump does not reach out and pull or suck the fluid into the pump. There must be some external force that causes the fluid to flow to the pump suction.
In most cases with self-priming pumps, it is atmospheric pressure pushing down on the fluid that supplies the energy to push the fluid to the pump impeller. Because the acting force is atmospheric pressure, the maximum amount you can supply depends on the elevation of the pump relative to sea level.
However, there are instances where it is possible to be below sea level, such as Death Valley in California (282 feet below sea level) and the Dead Sea in Israel (1,300 feet below sea level), to name two well-known examples.
If the margin is not sufficient, the pump will cavitate, damage will occur and the pump performance will drop off or fail altogether.
First, let us review the formula for NPSHa and recognize that a key factor in the equation is the head from vapor pressure. Note that vapor pressure will always be a negative value in the NPSHa equation, so it is also a factor that will always work against you. As I tell my pump students, “vapor pressure is never your friend.” A simple formula for NPSHa is shown below, where the units are expressed in absolute feet of head. Since this is a lift condition, the static head component will be a negative value.
Two Versions of a Self-Primer Example
In both versions, the application is identical, except the temperature of the fluid is higher in the second version. The higher temperature means the vapor pressure has increased.
In each version, the suction lift is 10 feet, the friction loss is 3 feet and the site elevation is at sea level. Positioning at sea level means the absolute pressure rounds out to 34 feet of head (14.7 psi absolute atmospheric pressure x 2.31 divided by the specific gravity of 1.0 = 33.96 feet ≈ 34).
In the first version, the fluid is water at 68 F, and in the second version the fluid is water at 150 F. When you look up the vapor pressure of water for each temperature, note it is normally expressed in pounds per square inch absolute (psia), so you must convert that value to feet (head) to have the component value in the proper units needed to do the NPSHa calculation.
To convert psi to head, you must multiply by 2.31 and divide by the specific gravity for that temperature. For the first version of 68 F water, the specific gravity is 1.0 and the vapor pressure is 0.33889 psia. For the second version of 150 F water, the vapor pressure is 3.7184 psia and the specific gravity is 0.9802. You can find the vapor pressure and specific gravity values in several places. I use the “Cameron Hydraulic Data Book,” and there are also several reputable web-based sources.
Once you have the conversions, fill in the values for the formula and do the simple math steps (0.33889 X 2.31 = 0.783 feet. And 0.783 divided by 1 remains = 0.783). Then, fill in the values in the NPSHa formula and complete the steps for the answer. Repeat these steps using the higher temperature values for version 2.