How to fix Water hammer in pump process system

[A Learning Engineer]

I just finished a project at my first internship/co-op that involves making a pump system automated. Basically, the operator will type in the pressure setpoint, and a variety of devices will work together to increase the pump to pressure. The pump I am using is a Haskal 4b-14 pump. I have a 1/2-inch air line running into a 1/4 in air supply pipe. On the 1/4-inch air supply pipe there is a 3/2 solenoid valve, and an Ip current to pressure transducer. Both have 1/4 inch in and out ports, and the transducer controls the pressure that is going into the pump. On the outlet side of the pump there is crossroad, where many different pipe sizes are, first there is a pressure transmitter that is 1/4 inch, the outlet side of the pump is 1/2 inch so a reducer is needed at the crossroad for it. Then there is a 1/2-inch mechanical relief valve that is inline and goes to the part we are testing. At the bottom of the crossroad there is a pressure gauge, and it is lead to a hydraulic solenoid valve and its ports are 3/8 inch in size. The hydraulic solenoid valve is used as a safety valve if the pressure transmitter detects an overpressure. There is also a fluid supply to the bottom of the pump which is 1/2 inch.

I gave you all of these details because I have only one problem with my system. It has water hammer. When the system pumps up to setpoint and starts holding the pressure, the pressure in the IP transducer stays constant. While either holding the pressure and starting testing time, or when pumping up to pressure, whenever the pump cycles it creates a water hammer effect. The pump cycles and the pressure on the gauge jumps to 100psi over the setpoint for 1 second before returning to its original point.

For example, I was testing the system at 200psi, I started the test and the IP transducer slowly increased pressure until setpoint was met. Once it reached that setpoint, the IP transducer held the pressure constant, and the PLC started its test time. After about 10 to 15 seconds the pump cycles and water hammer occur, the pressure on the gauge spikes to about 280psi sometimes 300psi and then comes back down to around 200psi. My pressure transmitter also reads this and sometimes thinks there is an overpressure, I have a setpoint for overpressure, say 100psi over setpoint then open the hydraulic solenoid valve. This water hammer is not only causing my overpressure device not to work, but also gets dangerous and unsafe as the pressure increases. As I increase the setpoint to let's say 800 psi, the water hammer will be even stronger and the pressure wave will be around 400 psi, which is not good for the system and the part we are testing.

My question is how do I fix this water hammer and why is it happening?
I will go and buy a check valve for the inlet side of the pump just to make sure that no air is creating backpressure and then the pump wants to cycle so it forces a bit more air than it needs to.
Why does the manual version of this system not have water hammer? Manual version uses the same pump but instead of electronic controlled valves and transducers everything is manual, so the operator needs to increase pressure on the regulator and hold it constant. When the pump cycles no hammer head happens, the opposite happens, the gauge on the outlet side of the pump drops to 0 psi for a second then comes back to the original pressure.
Let me know what you think the problem is.

 

[Valveman]

I would think the gauge dropping to zero is a negative pressure wave just as much as what you are seeing is a high pressure wave, which are both parts of water hammer. I don't really understand what you are trying to do? When you say the pump cycles, is is cycling on or off?

 

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[A Learning Engineer]

I am building an automated hydrostatic tester. In simple terms, a computer controls the air pressure given to the pump, and there is a pressure transmitter on the water outlet of the pump that works with the controller to pump to a specific setpoint. Everything works as it should considering the electronic parts, the only problem I am having is when the pump reaches its setpoint. Say the pump reaches 200 psi and the air pressure stops (air pressure stays constant, pump outlet stays constant) at 9.5 psi. This pressure must be held for a certain amount of time for a pressure test. So as the pressure is being held and everything is staying constant (9.5 psi constant to the pump inlet) the pump cycles randomly and causes water hammer. I would be holding the pressure, and out of nowhere the pump cycles, and the pressure outlet spikes to 300psi for half a second then drops back down close to 200psi again. I am not sure if the pump is cycling on and off, I am trying to figure out how this pump works exactly when holding pressure. This water hammer is a big problem because if I want to pressure test something at 1000psi, the water hammer will be a violent spike of 500psi. Not sure why the pump is doing this, we have the exact same pump that cycles as it is being held at pressure but there is no water hammer at all.

 

[Valveman]

I am not sure how that pump works either. But water hammer happens when a pump starts and/or stops. If you are holding, the pump should not start or stop, which is what is causing the water hammer. Sounds like a control issue to me as it doesn't happen in manual mode because the pump is not starting when it shouldn't.