Is it possible the pump needs to be mounted lower to reduce the NPSH?
Or raise the pickup slightly (I know this reduces effective fuel capacity)?
I know fuel pumps typically don't publish NPSH, and it would be even harder to calculate with such a short distance, small lines, and inexact measurements.....
SUBJECT : Cavitation
Cavitation means that cavities are forming in the liquid that we are pumping. When these cavities form at the suction of the pump several things happen all at once.
-We experience a loss in capacity.
-We can no longer build the same head (pressure)
-The efficiency drops.
-The cavities or bubbles will collapse when they pass into the higher regions of pressure causing noise, vibration, and damage to many of the components.
The cavities form for five basic reasons and it is common practice to lump all of them into the general classification of cavitation. This is an error because we will learn that to correct each of these conditions we must understand why they occur and how to fix them. Here they are in no particular order :
-Vaporization
-Air ingestion
-Internal recirculation
-Flow turbulence
-The Vane Passing Syndrome
Vaporization .
A fluid vaporizes when its pressure gets too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement is supplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degrees Fahrenheit (20 degrees Centigrade) is the fluid being pumped.
Since there are losses in the piping leading from the source to the suction of the pump we must determine the head after these losses are calculated. Another way to say this is that a Net Positive Suction Head is Required (N.P.S.H.R.) to prevent the fluid from vaporizing.
We take the Net Positive Suction Head Available (N.P.S.H.A.) subtract the Vapor Pressure of the product we are pumping, and this number must be equal to or greater than the Net Positive Suction Head Required.
To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H. Required. We shall look at each possibility:
Increase the suction head
-Raise the liquid level in the tank
-Raise the tank
-Mount the pump lower
-Reduce the piping losses.
These losses occur for a variety of reasons that include :
-The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter.
-Solids have built up on the inside of the pipe.
-A suction strainer is clogged.
-Be sure the tank vent is open and not obstructed.
-Something is stuck in the pipe
-The inside of the pipe, or a fitting has corroded.
-A bigger pump has been installed and the existing system has too much loss for the increased capacity.
-Lower the fluid temperature
-Insulate the piping
-Be careful of discharge recirculation lines, they can heat up the suction fluid.
-Reduce the N.P.S.H. Required
