problem with an hermetic pump

[luissandoval]

HI Guys:

I have a problem with an hermetic pump, this pump works in a ammonia recirculation system with a vertical separator vessel, the pump usually works good but when the system starts its cycle of defrost ( hot gas defrosting ) after the for minutes the differential pressure of the pump goes down and the differential pressure switch turns off the pump. Someone could explain me what is happen?

[NH3LVR]

We have little information to go on here. We do not know how many evaps you have, exactly where in the cycle this occurs, what is happening to the suction pressure etc.
The most likely cause, without knowing all the details, is a sudden drop in suction pressure during the cycle. This can cause bubbles to form, which will make the pump cavitate.
A good explanation of this can be found here.
http://en.wikipedia.org/wiki/Cavitation
Usually this can be adjusted out with unloaders, or by other means.

[US Iceman]

NH3LVR has the right idea I think. Any time the pump does not create differential pressure while running it either has no liquid in the pump or the pump is cavitating.

Cavitation occurs when the pump suction pressure drops below the vapor pressure of the liquid ammonia. Essentially, this means the liquid refrigerant is boiling because the pressure of the liquid is lower than what the pressure should be at the saturation temperature.

What you are describing seems common to other situations I have seen. When the evaporator(s) start to come out of defrost, the pressure in the vessel starts to rise. This causes the compressors to load up, which drops the suction pressure. If this happens to cause the pressure to drop just enough while the pump is running it can cause cavitation.

[smpsmp45]

I had attached a paper from ARI last time, which had given the basic connections of Hot gas return to avoid this situation. But since the Info is not very clear, it is difficult to see if that is the point

[Grizzly]

May I just add to the excellent advise above.
Try looking at the working liquid level within the separator.
Is there room to raise it slightly?
Has it changed recently?
Meaning is this a new problem or historic?
Have you changed the usage of any of the chambers it supply's?
Basically as has been already said you need to prevent the sudden fluctuations of liquid/ pressure within the vessel.
Cheers Grizzly