LPG Pump Starvation

A rough guideline is that the suction piping to the pump should be at least the size of the pump connection, and probably a size larger, with as few elbows as possible. I'm not sure there should be a strainer, but if there is, make sure it isn't plugged. Also make sure any suction line valves are open all the way while pumping.

You might also look for info on NPSH (net positive suction head) for pumps.

A common problem with pumping propane is that once the pump is dead headed or the flow rate is reduced too far the propane in the pump itself heats up just enough to boil off and cause a vapor lock in the pump.

My transfer pump had problems with that and the only solution I had that would work was to put a low pressure bypass valve on it to return tho outgoing flow all the way back to s second port on my bulk tank. I think my bypass relief valve is set for only around 15 - 20 PSI so it does not take much for it to open up and start dumping the excess flow back to the tank in order to keep a fresh supply of liquid flowing through the pump.

"We have considered putting a by-pass or return pipe that will serve as a spill back to the source."

Consider, 'minimum flow, recirculation valve'. This does the same job of by-passing the required amount of flow which will bot allow starvation.

Your by pass valve and piping return to the tank should handle the full flow from the pump working against a small head pressure, this will allow the pump to do the least amount of work, this is assuming you have an equalising gas return from the vessel you are filling. If not and the pump is working against an increasing pressure the by pass valve pressure will have to be compensated accordingly.

It sounds as if your pump is doing a lot of work with very little flow for most of the time, possibly you could look into some pressure switching control with a timing circuit for a shut down once certain parameters have been satisfied you did not specify what the pump is actually doing or supplying.

Make sure that any spill back return pipework does not return except to the top of the tank. Not near the outlet to the suction going to the pump.

If you have any pressure regulating valves downstream after the pump, and if they return any excess fluid, make sure they do not return to the suction line of the pump.

Also, make sure that you tank is high enough above the pump suction line to keep the supply of Liquid to the pump.

All this in addition to what has been said about too low a flow etc.

A spillback is a good idea, but when we designed a pump system recently we came to the conclusion that it was just as important to keep the suction line cool, especially at start-up.

The following applies in relatively high temperature conditions. If your suction pipe is exposed to high solar radiation or high ambients it applies. Otherwise you may have heat coming in from somewhere else such as the pump.

I assume you are working with a supply tank that has vapour above the liquid so the pressure is the vapour pressure at that temperature. The strange thing is that the actual temperature and pressure are not as critical as the rise in temperature along the suction line. You only need 1 or 2 degrees temperature rise to promote vaporisation as the pressure will be dropping as it travels along the suction pipe - maybe not much, but it will not be increasing so pressure effects do not assist you. If you work it out, the pressure difference between vapour pressure (the point at which vaporisation will occur) and the pressure in the suction line, is the head of LPG between the top of the suction line and the level of LPG in your storage vessel. Work out how this "head" is used up by pipe pressure drop and increases in temperature and you will see that you could start vaporising in the suction line.

If the above applies you might be able to increase the low level setting in the tank (marginal improvement, but it may be enough; Try heat shielding the line from solar radiation - sheet metal will do or any thing to shade the line for a trial; insulating the line may help. Another method of cooling the line is allowing some lpg tovaporise in it by hitting a small valve at the highest elevation and as you fill from the bottom allowing "boil-off" from the top. You should discuss this with your piping and process engineers before you try it out. Good luck.