*Be very careful on self-primer lift applications because the liquid temperature*

directly affects its vapor pressure and that…

directly affects its vapor pressure and that…

…**changes the Net Positive Suction Head Available (NPSH _{A}).**

**Example: Self-Primer – two temperatures… two outcomes:**

For the example we will use two versions of the otherwise same application. The applications are identical in both versions except the temperature of the fluid is higher in the second version. The higher temperature signifies the vapor pressure has increased.

**Note: 14.7 PSI absolute atmospheric pressure x 2.31 divided by the Specific Gravity of 1 = 33.96 feet ≈ 34).
Any increase in elevation will reduce your absolute pressure (head) and consequently the NPSH _{A}.*

**In Example A the fluid is water at 68ºF.**

**In Example B the fluid is water at 150ºF.**

*You can find these vapor pressure and specific gravity values in several places;
I use the Cameron Hydraulic Data Book or Cranes Technical Publication 410
(There are also several reputable web based sources).*

Once you have the conversions then fill in the values for the formula and do the simple math steps (0.33889 X 2.31 = 0.783 ft. and 0.783 divided by 1 is 0.783). Then fill in the values in the NPSH_{A} formula and complete the steps for the answer. Repeat these steps using the different values for Example B.

**Note**: The * difference* between the two versions for the value of NPSH

_{A}

**= 7.98 feet which is approximately 8 feet.**

**Summary**: With all the parameters except temperature (vapor pressure) the same and simply

**changing the temperature of the fluid from 68 to 150 degrees we reduced the NPSH**This may not seem like a big deal until you realize that the pump requires 13 feet of NPSH

_{A}by 8 feet._{R}at the condition point and now 12.24 feet is all that is available. The pump will not operate correctly and will be in a constant state of cavitation.

**Corollary**: Most pump manufacturers do not recommend using self–primer pumps on lift applications above 145°F

^{ }for this reason. The solution for fluid temperatures above 145ºF will likely involve a vertical sump pump or a submersible pump.

-Jim Elsey