Compressor damage due to liquid carryover

Hello everybody, I am newbie at both the forum and refrigeration world. However I am starting the hard way, and need to explain serious damage to a screw compressor. The compressor has run for about 2 years until it finally got to this situation.

This is a NH3 VSD water chiller with a single compressor and plate type condenser/evaporator. The compressor was badly damaged and need to be scraped completely. Explanation by our contractors was liquid carryover, however they are reluctant to give a specific reason because (I guess) they provided the equipment (I think they were involved with the design) and start up.

Evidently something failed in regards to liquid separation / shutoff protection.

I would like to know from you guys what are the normal causes of liquid carryover into the compressor and what protections should be in place to either shut off/remove the liquid.

Thanks in advance

Can you explain your system a little more, liquid carry over isnt considered normal but some causes could be system not designed properly,vessels not sized properly, high and low level cutouts bypassed or not even installed.

How long has this system had problems? Since install or just recent?

More information could be helpfull.

Just to add some more info.
Equipment designer was York International, Model PAC/USCA 128 HR. Main panel is Sabroe UNISAB II.
The evaporator is flooded and plate type heat exchanger(evaporator is under separator). Separator does not look very big (0.395 m3).
Cooling capacity of the unit is 509.6 KW.
Outlet from the separator to the compressor starts horizontally close to the top of the vessel in a 4" line + insulation, then down to the compressor. On the side of the highest section of this line there is a manual angle valve and an uninsulated 1/2" tubing that goes down to a smaller vessel that I guess pumps back the liquid to the system.
I guess the tubing is the last liquid colector before entering the compressor, but shouldn't it collect the liquid from the bottom side of the line?.

Sorry for too many guesses. Any help would be appreciated.

Separator only has a sightglass, no level switches.

Equipment has started up 2 years ago. There were vibrations issues upon start up that were supposedly tracked to the base. And supposedly solved. Unit runs 24 hrs a day with almost no supervision.

Attached a shematics PID from the equipment.

Quote:

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Originally Posted by mariano-arg

Equipment has started up 2 years ago. There were vibrations issues upon start up that were supposedly tracked to the base. And supposedly solved. Unit runs 24 hrs a day with almost no supervision.

Attached a shematics PID from the equipment.

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Mariano.
Basically your drawing is flawed as it does not show the liquid level control.
Sited on the side of the seperator which controls the working level of the liquid Ammonia.
Coupled with these are high and low level switches / alarms.
Any surplus is accomodated in the liquid reciever.

Something HAS failed and it may well of caused Liquid carrry over.
Rersulting in catastophic compressor damage.
What scares me is the statement that
"Unit runs 24 hrs a day with almost no supervision. "
When is it serviced?
I am a big Fan of Sabroe Pac's but no matter how good they are, even they require so "tender love and attention" occasionally!

Any vibration is monitored as part of good maintenance practice.
"Which i assume you carry out?"
Do you suspect Vibration as an issue?
Because one of the neat things about the unisab/rotatune software is that you can "tune out" any vibration Harmonics.
I must admit that I too an baffled as to how liquid
could of carried over?
But if there are so many inbuilt messures to stop it occuring.
As there is within the Unisab controller it must happen from time to time.
Grizzly

Thanks Grizzly. Equipment is patrolled every day for abnormal noise, vibration and anything else that is evident. (Not at the level of recording pressures, temperatures).
Equipment is serviced when tripping off or alarming, or other issues such as leaks, oil changes, amonia refills, etc.
Vibrations checks are carried out at a 3 month frequency.
Attached is a document another schematic with every instrument. It does not have level switches in the separator. Is that the standard for small chillers ?
Another issue I was thinking about, since this circuit is cooling a chemical process, ocassionally water temperature (normally at 9 to 15°) can run up to 40-50°C when the units trip off because process keeps running. Operators start these chillers with water running at those temperatures and try to stabilize the circuit. Some more experienced replace the whole water loop. I guess evaporation is wild when trying to start the chiller with 40-50°C water in the circuit. I doubt there are enough controls to prevent liquid carryover under these circumstances.

Quote:

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Originally Posted by mariano-arg

...ocassionally water temperature (normally at 9 to 15°) can run up to 40-50°C when the units trip off because process keeps running.

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This type of situation is sufficient to cause the occasional flooding problem. Does the flooding occur after the system is re-started after the water temperature goes to 40-50°C?

The refrigerant liquid boils very rapidly due to the higher temperatures in addition to the compressor rapidly trying to decrease the suction pressure. The system is normally designed for what we call stead-state. This means the cooling loads are relatively constant and their normal operating pressures.

To prevent this from occurring after the system is re-started, only allow the compressor suction pressure to reduce slowly (you may have to do this manually by partially closing the compressor suction service valve. Then as the water temperature begins to approach the normal temperatures, the service valve may be fully opened.

Looking at the diagram it would seem that the sperator is just a header tank for the evaporator and the control is hp float and that the only way you could get liquid carry over is through overcharge?

This is a very typical York Pac chiller. I agree, the only way to crash this compressor is over charging. Even though you sometimes have to start up under high load conditions this is not uncommon for these units.

Thanks US Iceman, Keepitcool and HallsEngineer. Yes there is a float valve at the condenser to return NH3 to the tank driven by condenser pressure.
However a lot more evaporation than steady state would occurr if chilled circuit is at high temperature and chiller trying to reduce it working at high load. I wonder if the liquid in the tank mixed with all these bubbles would be enough to raise level and cause that much carryover to destroy the compressor. Also, why the controller does not limit capacity to prevent a dangerous situation ?

I don't know when the flooding occurred, we just noticed the damage through vib-monitoring and after overhauling.

Quote:

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Originally Posted by mariano-arg

I wonder if the liquid in the tank mixed with all these bubbles would be enough to raise level and cause that much carryover to destroy the compressor.

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Yes, it can. :eek:

If the compressor achieves full capacity at the higher suction pressure, the volume of gas flow is very high. This can pull liquid right back to the compressor. Very quickly!!

Hi Guys just come back to the thread after a day at a 150ton cheese per day processing plant.
Also sabroe and on ammonia (approx 7 ton).
Anyway thanks for the support Iceman.
I totally agree the plant above behaves the same way and it is not uncommon for it to trip out on high level many times.
Until someone actually loads it up manually (slowly) and once the levels, pressures etc have balanced out.
The plant can be reset to remote.
Mariano.
Thanks for the detailed input, I think you realise that the usage of this plant is a big contributer (as i first thought).
In fairness the make is slightly irrelevant, cause this would occur on most plants of this style.
The simplest answer for the future.
Would be wire up the high level no run permit contact in the unisab.
Secondly teach your selected operatives how to manually load back up the plant after a failure!
Otherwise you will inevitable get more of the same.
Cheers Grizzly

Hello, Mariano, you must stabilize water temperature, not 40-50 ° C, ammonia many cooks, high evaporation, high intake temperature, you have to give the exact content of ammonia.
Otherwise, you will need to observe the compressor and manually regulate how US Iceman wrote.
Namyslím compressor that bad for 2 years can run 17,520 hours, in terms of what you prefer is a lot.

Thanks guys for all the help.
I know need to explain the issue to process engineers (that want supportive data). I have historic chilled water inlet and outlet temperature and flow is a DCS system. I have the dates and times that both temperatures go up so would explain the failure events. That's fine. I tried to calculate the KW cooled, however sometimes this value is over the equipment name plate capacity. Are these moments of overload potentially dangerous too ?
I also need to recommend new panel settings to avoid damage for liquid carryover. What would be the best readings to rely on?. For what I read so far it would be high suction pressure and maybe low suction temperature also. This panel does not have "pressure ratio" capability and a level switch is not installed. I may consider the latter for a long term protection.
Thanks again.

Thanks for the Liq. Carry over insight. Our engine room has been plagued by high level shut downs recently.
( Ice Cream Plant, 2 stage Nh3 System, intermediate side
Shuts down High Level) Milk storage silo's may be the culprit during CIP (Clean in place) cycles. Going to check the nh3 interlocks. Would a pre wash pumpdown be needed? Happy Holidays to all.............:cool:

Me2Mike I would think a prewash pumpdown would probably help the high level problem.However I would need more information on the Nh3 flows and overall plant operating procedures before I could give more help. C.D.

mariano-arg We all were newbies once. Ive been smelling NH3 since 1981. I still run into odd situations like yours. It can be very humbling when your the expert and you have no idea whats wrong and the boss, owner are wanting the problem fixed immediately.The only thing i would add to the good advice in this thread is to drain oil from the system regularly.I have seen oil cause nothing but trouble with level controllers and float valves Good Luck C.D.

Hi,

Been reading the threads. This is a critically charged plant. Charged to superheat at compressor inlet (suction). Definately fit a high level safety switch. Also I suggest fit a "fully mechanical" crankcase pressure regulator in the dry suction. Check with Hansen/Danfoss PM1 or such with screwed in module. If the the drum pressure is high after trip. The pressure regulator will only let through the massflow of gas set on the module. Stable running, wont overload motor, less boiling in drum... + you protected from liquid carry over with high level switch. Pressure regulator will only fully open when set point on module reached. Does the same as training someone up to close off suction isolation valve and manually open as pressure in drum drops.

Automatic and safe!!!

Grant D.

mariano-arg. Is it possible to train the process operators to restrict the load going back to evaporator, can this not be done automatically by way of a diverting valve?