Liquid pump pressure

[Markridgway]

we have two compressors with unisab controls for chilling a tunnel down to -47. When we start the compressor after about five minutes we lose liquid pump pressure. The only way to regain the pump pressure in to fully unload the compressor and wait until pressure is regained and then switch compressor to automatic.
We have tried everything to try to overcome this but to no avail. Is there anything you can suggest that would contribute to this.

[Grizzly]

try a controlled run down to say -30 without any fans running in the tunnel and then turn on the fans.
you may well be sucking so hard on the surge drum during start -up.
That you are cavitating the Pump suction by having such a low pressure within the surge drum.

We have a spiral freezer which does the same. But once diagnosed the start up can be tuned to eliminate the problem.
Basically I suspect it's a system pressure in-balance issue on start up.
Suck it and see.
Working out how to coax the system into life will eliminate most of the issues, incidentally what is the surge drum and ammonia liquid levels like?
Any changes from the norm?
Grizzly

[RANGER1]

Agree with Grizzly, very slow controlled pull down.
Allow for this before production.
Also drain oil from all points
In plants like this liquid pump overhaul recommended every 1-2years to assess due to cavitation.

[NH3LVR]

I have seen this many times. When you start the refrigerant is boiling in the vessel. Pull down the suction pressure without the pump running. Then unload the compressor and start the pump, which is pretty much what you have been doing. If the suction is dropping while you are starting the pump it may well cavitate every time.
Some times just pulling the suction down without the pump is enough. The operational details very according to the particular system. This is where a experienced operator comes in,

[Magoo]

Totally agree with all the above posts. Minimum 1 hour pre production starting. Slow and easy loading on compressor to see 100% fully loaded.

[Markridgway]

Well i think there may lie the answer . We have always started the compressor and with in 3 or 4 minutes its at 100% load. I will try your method of starting the compressor and I will let you know thanks a lot guys.

[Segei]

There are two approaches. First, pull down pressure and start up the pump. It takes time and level in low pressure receiver will jump up and down. I prefer second approach. Start up pump and blast or spiral or tunnel. This operation create significant refrigeration load. Than compressor start up and it will gradually pull down the pressure. However, you should not have too much compressor capacity. This approach work very well with blast freezers, because we have immediate load from the product. For the tunnel, it takes time to put product and create load. To pull down temperature, run the pump but reduce compressor capacity. Try use 1 compressor to pull down pressure and start up second compressor when load increase.

[sterl]

All of the above is good stuff.
The only method we have employed occasionally to limit rate of pull down is to base a "second load control" on the compressor based on motor current....if the compressor pulls 100 amps unloaded at start up type temperatures, set the set point for about 80 and you will come down real slow. Once the production air temp is reached, swithc the machine back to normal pressure control. You will have to fool around some to establish what the system tolerate....We also have adopted a strategy for spiral freezers and similar where the standby liquid level is considerably higher than the operating, this tends to prevent the and reduce the flash gas load and the pump cavitation to some extent.

[mbc]

I my own experience with a lot of this kind works , I start compressor 20 minutes with out starting pump
and after the compressor pull down suction pressure and cut of automatically then i start pump it more safe for running pump without any over pressure head .

[Markridgway]

I have tried what you suggested but after an hour the temperature did not move from 13.0. I did notice when Liquid pump was turned on the pressure was 0. When i unloaded the compressor and waited for pump pressure to build it was fine after that.
But i still need to run the compressors with a 1 button start and not having to unload the compressor.

[Grizzly]

Hi Mark.
Why so long to get back to us?
What I think you are saying is the comp loaded pulls to hard and cavitates the pump.
Which is effectively, what we all have been talking about.
Unisabs can be set-up to limit the loading rate and therefore eliminate the problem you have.
All you need is someone to set it up!
It's called suction limiting. Some refer to it as ramp limiting other systems call it rate control.
Whatever!
You basically control the rate at which the comp loads up.
Pump cavitation happens on a lot of flooded systems from time to time. Its just a case of controlling it!
Grizzly