Ammonia pump pressure drops

[l2embo]

Dear frineds
We are dealing with commissioning aa ammonia refrigeration system. There are two stages with piston gea compressors.
The problem is when we start the ammonia pumps of -40 stage to feed the salons the pressure of pump decreases and finally the pressure switch stop the pump. We tried disabling q min of the pump is not working. Filters are clean. Any suggestions?
Please write it to me. Thanks

[NH3LVR]

Can go into detail more after I have had my morning coffee.
This is not a unusual problem at all.
In an older plant it is often a oil problem.
Assuming this is the startup it points to one of two problems.
Design-Inadequate head.Low level of liquid over pump causing Cavitation
Cavitation caused by bubbles in the vessel that form as you pull down the pressure in the vessel or allow bubbles to form as the pressure drops in the pump inlet.
Try this. It often solves the problem.

Pull the pump vessel down to your operating pressure. Unload the first stage and allow the pressure to rise a few pounds. This will effectively subcool the liquid in the vessel. Now start the pump. All should be good then. Used this trick many times.

Another thing you can do is to restrict the outlet of the pump a bit. Less flow will decrease cavitation.

[NH3LVR]

Please note I edited this post. I said second stage when I should have said first.

[Grizzly]

Hi l2embo.
NH3LVR has nailed your likely issues.
Giving very wise advise.
I would like to add from my experiences that I to have seen pump cavitation, resulting in the Liquid pump stalling.
Where the working level in the surge drum above the pumps has been set to high (or the level control system has malfunctioned).
Which can result in the compressor suction acting upon the reduced vapour space in the surge drum. Causing the liquid Ammonia within the drum to be held up ( for want of a better description). This then stalls the pump and circuit flow fails.

As NH3LVR states the working level of the surge drum has a huge impact upon the pump head and its function.
Both high and low level have the ability to create similar problems.

We even had the situation at a Ammonia spiral freezer plant where we overcame a similar problem. By not turning on the evaporator fans until the system was down to its required relative suction Pressure temp.
The plant would then run happily.
Out of interest what type of pumps are you using?
Good luck and please keep us all informed as to how you get on?
Grizzly

[l2embo]

Dear NH3LVR
Thanks a lot for your explanations. our problem was cavitation and it solved with sub cooling the refrigerant and limiting the outlet of pumps by hand regulating valves.
Best Regards