Pressure Surge from High Service Pump Help
Pressure Surge from High Service Pump Help
(OP)
At the local WTP they are having a surge problem from one of the high service pumps. The pump is a 1600 GPM pump with a swing check and soft start drive. The other two pumps are 2400 GPM pumps with soft starts and electric check valves. The problem seems to be that the pump ramps to full speed before the check even opens, and when it does the pressure spikes from ~110 psi to +160 psi. The drive is an Allen-Bradley SMC flex and according to the tech support folks it doesnt matter what the ramp time is set to, the drive will ramp the motor to full speed as soon it can, which should be easy to do when the pump isnt moving water. We tried to current limit the motor from the drive. When it was set at 200% FLA the motor quickly ramped up to partial speed but when the contacts closed at the end of the set ramp time it jumped to full speed. I changed the current limit to 300% and it ramped up as it always did - with the surge pressure when the check opened. I think ideally what the drive needs to do is quickly ramp to a speed maybe 70%-80% of full speed and slowly ramp up to 100%, but the tech support folks say it wont do that.
Does anyone have any suggustions as to what we can do to fix the surge problem without replacing the drive or installing an electric check valve?
Also, there is a surge relif on the high service line leaving the WTP, if it's working shouldnt this prevent surges from entering the system due to a quick pump start?
Thanks,
Mike
Does anyone have any suggustions as to what we can do to fix the surge problem without replacing the drive or installing an electric check valve?
Also, there is a surge relif on the high service line leaving the WTP, if it's working shouldnt this prevent surges from entering the system due to a quick pump start?
Thanks,
Mike





RE: Pressure Surge from High Service Pump Help
RE: Pressure Surge from High Service Pump Help
RE: Pressure Surge from High Service Pump Help
We installed a pressure relief valve between the discharge and the suction to solve our problem. It would open and cut the top out of our spike.
There are a lot of variables involved in your situation as you have 3 pumps of varying flows. I don't know if your problem occurs with the pump when it is alone, when it is with 1 other pump, or when it is with 2 other pumps.
Either way, you need to make sure your pressure relief valve is operating. Then the tricky part is to find the correct setting. If you set it for a pressure with the 1600 gpm pump, your 2400 gpm pump will probably cause it to remain open when that pump is operating or when multiple pumps are operating.
You say the problem is with 1 pump. What about the other 2? Does the same thing occur? If not, what is different about them?
Some other thoughts. Is the swing check sticking? Is it spring and lever or weight and lever? If so, loosen it up, take weight or tension off.
RE: Pressure Surge from High Service Pump Help
Thanks,
Mike
RE: Pressure Surge from High Service Pump Help
This eliminates the water hammer and pressure spikes caused by entrapped air in the piping system.
The jockey pump can discharge to a pressure vesssel with either a precharged bladder or air cushion.
This allows the jockey pump to operate on a differential pressure switch.
Safeguard the pressure vessel safety valve by fitting an orifice in the line to the main system. The main pump flow rate is too high for the jockey pump pressure vessel safety valve,should there be a reverse flow .
Fit a high point auto air vent.
Main pump can be configured to start on a pressure drop below the jockey pump differential settings.
Offshore Engineering&Design
RE: Pressure Surge from High Service Pump Help
If you have hammer on startup you may also have hammer on shutdown that can do even more damage but is hidden until something major breaks.
Hydrae
RE: Pressure Surge from High Service Pump Help
Well, you could always have the operator standby and open the discharge valve by hand.
With that size of pump motor, you should look into the use of a VFD.
RE: Pressure Surge from High Service Pump Help
regarding your existing surge relief valve - these things require tuning to operate properly with every system. If you are not experienced with these, call out the manufacturer to inspect and adjust it.
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RE: Pressure Surge from High Service Pump Help
The pump will not produce any flow until it reaches a specific speed. From that point, the flow will increase as will the pressure.
If the check valve is sticking or hard to open (too much weight, too much spring tension) it will require even more flow to open. This then causes the valve to spring open which does affect the hammer.
Eliminating this won't solve the problem; but, it could help.
If you don't want a pump control valve or VFD, you will probably have to resort to a pressure relief valve. Note that you will have to set it for the larger pumps in order for it not to blow off when they are operational. This will then cause a small spike when the smaller pump turns on.
RE: Pressure Surge from High Service Pump Help
RE: Pressure Surge from High Service Pump Help
Surge on startup would have to been fixed using a VFD or mechanical device such as a pump control valve:
http://www.controlvalves.com/series/125/125.html
RE: Pressure Surge from High Service Pump Help
In this latter case prof ARD Thorley in Fluid Transients in pipelines recommends an air valve with a pressure control discharge be fitted before the check valve.
Check valves for non slam application can be found at www.noreva.de. The concept of a check valve for non slam is to have low mass of moving parts and short travel distance. thats why a swing check valve with counterweight is not a good solution. What type of check valve do you have?
I suggest you get a specialist transient analsyst invovled to model the system before outlaying your clients money.
A VFD does nothing for pump trip on power loss in mitigating transients.
Look at www.pipingdesign.com and search for surges. I have some papaers on their on mitigation of pump transients.
Another reference text you may consider if Pressure transients in water engineering by Ellis.
If one could just say what to do to mitigate the waterhammer I would not have to invested tens of thousands of dollars in this software! On top of that spending countless hours modelling different scenarios to cover all the bases on different systems.
An analyst will cost you less than what you pay a plumber for fix a pipe and heaps less than the lawyer you might need if you kill someone.