Ammonia Pump Tripping

[ROCKY123]

Hello,

I would like to wish everyone in this group a happy new year.

We've encountered a challenge in one of our recent ammonia pumped refrigeration projects, where the pump keeps tripping. The system in question has a blast freezer only and operates smoothly for the first 6 hours of the total 8-hour cooling cycle, but after that, the pump starts tripping. One theory that emerged during our brainstorming session points to bubbles as the potential culprit and the argument goes as follows.

Towards the end of the cooling cycle, with minimal load on the blast freezer, more liquid returns to the separator, creating a splashing effect. These bubbles formed during the process might be carried to the pumps, causing the tripping issue.

Has anyone else faced similar issues, and if you don't mind, could you share your experiences?

[frank]

What is the running current of the pump and what is your pump overload set at

[NH3LVR]

Just to be clear-is the pump tripping on motor overload? Or is it a safety that cuts the pump off if it loses pressure for a period of time?

[Tycho]

Hello,


Towards the end of the cooling cycle, with minimal load on the blast freezer, more liquid returns to the separator, creating a splashing effect. These bubbles formed during the process might be carried to the pumps, causing the tripping issue.

Has anyone else faced similar issues, and if you don't mind, could you share your experiences?

I would lean more towards low suction head in the separator, as you said more liquid has migrated to the evaporators.

[RANGER1]

Rocky123,
What is suction pressure normally & at point load drops off ?
Pump pressure?
Pump trip pressure & time delays?
If compressor has a set point & gets below it, it may need to unload.
When pump cavitates, unload compressor a bit to see what happens.

[ROCKY123]

Hi,

The pump is tripping in low DP from the DP switch. There is almost 1.8 meters static height to the bottom of the vessel. The pump downcomer is sized for 0.25 m/s velocity.

The compressor is unloading when the vessel gets below the setpoint and also the downcomer is large to avoid head loss during the pull down phase due to boiling liquid. The pump is acting completely normal during the startup and pull down phase with the same settings.

[seanf]

Is the pump pressure stable throughout the first 6 hours, or is there a slow drop in performance?

Is the vessel liquid level stable throughout the full cycle? And are the level readings being taken from a level sensor, or from looking at a liquid level column glass?

[NH3LVR]

This sounds like a case where the suction pressure is dropping too rapidly at the end of the cycle. This can be a real issue in a system with only one set of evaporators. Make sure that the compressors are unloading properly or you will form bubbles in the pump vessel which will cause the pumps to stop pumping. Try manually unloading when this starts to occur. This will point you in the right direction.
I did work on a system once where the designer had included a solenoid and thermostat in the liquid feed to the evaporator. Caused the engine room to ramp up and down constantly.

[RANGER1]

Do any liquid line solenoids close?
Does pump have a minimum flow line?

[GrantD]

The pump is tripping in low DP from the DP switch. There is almost 1.8 meters static height to the bottom of the vessel. The pump downcomer is sized for 0.25 m/s velocity.

The compressor is unloading when the vessel gets below the setpoint and also the downcomer is large to avoid head loss during the pull down phase due to boiling liquid. The pump is acting completely normal during the startup and pull down phase with the same settings.

Hi Rocky,

Is this a "critically charged" plant and possibly short of refrigerant charge? I am leaning towards the liquid level in the low stage vessel falling as there would be less boiling in the evaporator towards the end of the blast freeze process. The evaporator will hold more liquid and the wet return line will full up. Is there a level switch on the vessel which cuts out the pump when the minimum head NPSH level drops? What is the DP switch set at and is this electromechanical (say DANFOSS: RT260A) or pressure transducer? I presume this is a hermetic pump (possibly of HERMETQUE manufacture) - their manual calls for a low level switch in addition to the differential pressure (no flow) safety. How long does your controls allow the pump to run below the DP set point? Do you allow a few auto reset trips (say 3) before you "lock out" the pump? If so what is the time between the auto reset attempts? If the issue is no due to liquid level, consider the capacity control of the compressor - possibly not unloading quick enough. Is there a minimum flow line and is it installed + set up correctly?

Additional Notes:

1.) The 0.25m/s velocity is good but have you based this on the pump max. duty or the design flow? I say this as I assume there is a Q.max orifice fitted and the pump is not running off the curve.

2.) Is there a suction strainer and have you checked that it is clean? Possibly after initial commissioning process you can remove the basket / insert.

[NH3LVR]

Sis you have any luck resolving the issue?

[mbc]

Hi

To solve the problem
You adjust the minimum flow valve in NH3 PUMP

[Grizzly]

Hi Rocky.
All the above are good and valid points, all of us to a degree guessing as we are not there to diagnose.
I would like to add an option which is an old favorite of mine.
raise the working level of the separator and see what effect that has?
Grizzly

[mbc]

Hi

when we go to low Temperature , we need less NH3 liquid to pump the liquid pipe

In this times we have to adjust our Minimum flow in NH3 pumps

[sterl]

At low evaporator loads, evap coils, suction risers and return piping all tend to fill up with liquid and especially where risers are substantial a quick increase in load as per a fan restart can cause a high rate of return liquid flow and a rise in the LPR level. I have seen that cause issues of false liquid level readings and even separation issues but not loss of NPSH at the pump.

How is liquid makeup to this vessel? From an intermediate temperature or the condenser as saturated or ??? Is it introduced to the inside of the vessel or the return pipe? What I have seen on large vessels with minimum load: The makeup solenoid shuts off, the compressors are already at minimum load but still spinning, the vessel pressure drops quickly and the entirety of the contained liquid becomes froth...and the pump won't make pressure pumping saturated froth.

[Grizzly]

Sound advise from Sterl.