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Constant Pressure System Problem

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Dokuen

Electrical
Jun 12, 2007
2
I have a variable speed constant pressure system with check valves, transducer, and pressure tank after the discharge of the pumps. My problem demand for water has ceased and the pumps turn off my pressure immediately falls 2 PSI which causes the system to come back on again. This results in constant cycling. I have increased my on/off band to prevent the cycling but when the pumps turn off and the pressure is trapped shouldn't it remain constant and not drop 2 PSI?

What could be causing the 2 PSI drop?

I don't think it's a leaky valve because then the pressure would continue to fall more than 2PSI.
 
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I presume the pressure tank has an air cushion or accumulator lung. Increase the accumulator air charge or increase the volume of air in the tank to about 30% when fully charged.

Offshore Engineering&Design
 
To stop cycling put a flow switch in the outlet line (after the pressure tank) and look for a flow AND drop in pressure to start the pump. Control the pump speed from the pressure per normal operation and stop on a no flow from the pressure switch. Non return valves are notorious for leaking and therefore not holding pressure.

Mark Hutton


 
"Non return valves are notorious for leaking and therefore not holding pressure."

This statement is exactly right. The narrow bandwidth between on and off pressure for most of these smaller Variable Speed Pumps, means a small drop in pressure will restart the pump as you have seen. A flow switch working in combination with the pressure switch or transducer means the pressure has to be lower AND a certain amount of flow must be seen to start the pump. When a check valve leaks back, all the water stored in the little tank will drain out slowly. So when you open a faucet, there is no water in the tank to activate the flow switch, and the pump won't start.

What brand of pump do you have? Some use pressure transducers and others use a pressure "switch". Which one you have will determine how to solve your problem.
 
Something is causing the part of the system that contains the trapped volume to increase. This increase causes the pressure to decrease. To chief's point, increase the volume of the pressure tank charge so it can take up the increase in system volume to maintain pressure to less than 2 psi drop.

Ted
 
Those little 2 gallon size pressure tanks they use with the so called "constant pressure pumps" only have ½ of a gallon of draw down with a 40/60 bandwidth. With a 2 PSI pressure bandwidth, you only have about 6 ounces of water stored in that tank. Just normal closing of a check valve, when the pump shuts off, will use up 2 or 3 ounces of the water from the tank. Leak another 2 or 3 ounces and the pump has to come back on.

Constant pressure is a good thing WHILE you are using water. However, a wide pressure bandwidth is needed to STORE water in a pressure tank. With only a 2 PSI bandwidth, the pump must start for every glass of water, every time the ice maker fills, rinsing a toothbrush, or even when the check valve leaks back a couple of ounces. The fact that the pump must start every time a couple of ounces of water is used, also means the check valve must also open and close ever time any water is used. This could happen dozens or even hundreds of times per day, which wears out the check valve and causes the problem to get even worse.
 
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