Pressure loss in water system

[Diogoarv]

Hello world.

I have a pumps system with three verticak pumps in parallel that takes water from a tank and pumps it to a building. It is controlled by pressure. Each pump runs at a time and each has its own check valve at the discharge. there is another check valve after the main discharge header and there is a another discharge valve after this main check valve. When the pump starts, it pressurizes the system at 150 psi. As soon as the pump stops, the pressure before and after the check valves starts to drop rapidly without water consumption and “stabilizes” at 105 psi. I tried to isolate the problem by just leaving 1 pump on, closing the other two pumps discharge valves, pressurizing the system, closing the main discharge valve that is after the main check valve and closing the remaining pump discharge valve and the gauge still drops. All the suction piping is made of carbon steel and also until the main discharge header. It then couples with sch 80 pvc piping. Between the header and the main discharge valve i have like 1 ft of pvc piping.

Does anyone know what could be happening? And what else can I try for the pumps not to run at a no flow condition?

 

[Valveman]

Could just be a bad check valve. All that cycling on and off by the pumps wears out check valves as fast as it does pumps and everything else in a pump system. A picture of the system would help me understand the control scenario.

 

[Diogoarv]

Could just be a bad check valve. All that cycling on and off by the pumps wears out check valves as fast as it does pumps and everything else in a pump system. A picture of the system would help me understand the control scenario.



View attachment 47972View attachment 47973

We also thought about a bad check valve. We installed a gauge at the discharge of the main check valve (4 in). I made a test: closed two pumps discharge valve, let the remaining pump fill the line until it shut off, closed the main discharge valve (on the PVC piping), closed the remaining pump discharge valve, and the new gauge still drops.

We will make additional pressure readings and tests, but we will also remove a check valve that was installed on the pressure tank line (water gets in the tank but doesn't go out). One of our theories is this is happening because of the PVC expansion due to high pressures. Hopefully removing the check valve will help.

I attached some pictures of the system. The two vertical lines are independent supplying to different areas of the buildings, but the setup is the same. If you or someone else has another theory, please let me know. First time i've seen something like this.

Thanks!!!

 

[Valveman]

There are only two kinds of valves ever made; those that leak and those that will leak. Check valves, butterfly valves, or any kind of valve will leak or will eventually leak. The control system just needs to be set up to deal with it, the same as it needs to deal with low or no flow rates.

I do not see a check valve on the line to the tank. But if there is one it should be removed. The tank is what keeps the system pressurized when the pumps are off. If the tank cannot communicate with the system because of a check valve or if the tank is bad the system pressure will drop quickly. There is not much expansion in the piping. But, without the pressure tank it only takes a drop of contraction for the system to loose pressure. If it is losing pressure when the tank is communicating, the tank bladder is probably bad.

I see those are C series Grundfos pumps and I see a pressure transducer, so they are being controlled by VFD or variable speed. While VFD's and variable speed pumps sound really cool, they are actually made to remove more money from the owners pockets than other means of pump control. Pump companies really do push what makes them the most money, which is never a good thing for the pump owner or end user. A Cycle Stop Valve varies the flow rate and provides constant pressure just like a VFD, but without any electronics, varying the pump speed, or all the problems that go with. I just repaired 3 Cycle Stop Valves on a pump system like I pictured above. They and the pumps they control have not been touched in 26 years. Repaired the CSV's and replaced a couple pressure switches and it won't surprise me if they run another 26 year. I probably won't be around to validate that, but maybe my son or granddaughter will.

I would have put the check valves on the suction side of the pump. This makes the pumps starts against pressure, which is much easier on them than starting with no head like when having a check valve on the discharge. I would have then installed 3 Cycle Stop Valves where the check valves are located now. With 3 mechanical pressure switches on the pressure tank and 3 across the line starters for the pumps, the Cycle Stop Valves would make those pumps last 26 years as well. That is why the Cycle Stop Valve is called a "disruptive" product and is not promoted by any pump manufacturer. Simple mechanical controls like the Cycle Stop Valve can last a lifetime, while computerized controls like VFD's will cause problems and need expensive repairs often just as you are dealing with. With CSV's in control when you open a faucet water always comes out. The more bells and whistles like VFD's you add the less likely water is to come out of the faucet when you open it.

There are only two kinds of valves made. The CSV was designed to leak and uses the leak to determine minimum flow, tank fill rate, and give a mechanical soft start and soft stop. It is so simple it is hard to explain, and easy to dismiss. But things like that pump running when there is no flow, or cycling on/off at low flow does not happen with a Cycle Stop Valve.

I am sorry for your problem and would be glad to help. But it sounds like you need to get one of those expensive techs to come out and reprogram everything again. Setting the minimum hertz correctly for the jockey pump should keep it from running when there is no flow. But a building like that rarely gets to zero flow and the pump system needs to deal with it. Most likely the minimum hertz will not be low enough to keep the pump running during low flow rates and the pump will cycle on and off. That maybe what caused the bladder in the tank to go bad and started the problems you are having now. VFD controls are just a way to keep feeding more money into the pump industry.