View Full Version : Propane compressor suction drum level issues

24-05-2008, 08:01 PM
Please advice,

We have a problem similar to that as described by Achmed; Propane compressor where the suction drum builds up a propane liquid level which could lead to a compressor trip.

Although we don't have any issues during spring/winter months we have issues when the ambient temperature increases above the 35C (during the Southern Hemisphere summer months the ambient temperature can reach up to 50C)

Any suggestions?


US Iceman
25-05-2008, 01:49 AM
Welcome to the RE forums.

The most likely possibility is that the liquid is accumulating in the suction knock-out drum from periodic liquid carryover from the evaporators.

This could be caused by process upsets where the compressors might load up quickly and cause the liquid to boil violently in the evaporator. this could cause the liquid carryover.

Another case might be the process load increases quickly for some reason, which could cause the above to happen also.

In either case, the liquid carryover may accumulate in the knock-out drum until the liquid level trips the high level shut down float switch.

26-05-2008, 08:43 AM
Hi US Iceman

Thanks for the response, unfortunately none of the suggested upset conditions occur. The increase in level in the suction drum level will happen only during extreme hot ambient temperatures, the level increases over a period of 10 hrs from 2% to 15%. When the temperature goes down late in the afternoon usually around 17.00hrs the level slowly decreases.
I'm thinking more in SG changes but I'm not sure how to explain this phenomenon.

26-05-2008, 11:43 AM
Hi Inu.
sounds like expansion control devises are overfeeding at high ambient. More condensing required during heat of the day

26-05-2008, 12:16 PM
Hi again Inuendo. To explain further expansion valve are rated on press differential across valve, increase the hih side and the flow rate increase to evaporator, the trick is to install an accumulator to trap over feed from entering compressor. A rule of thumb is to be able hold 80% of system charge and a safety margin.

US Iceman
26-05-2008, 05:46 PM
I'm thinking more in SG changes but I'm not sure how to explain this phenomenon.

If this is indeed a suction knock-out drum then specific gravity shouldn't have anything to do with it. The pressure in the vessel should be relatively constant so the liquid density on the suction side should not be affected.

Barring any process upsets then I would have to say your problem is related to some control valve(s) overfeeding during this 10 hour period. It would seem from your descriptions that the overfeeding occurs as the pressure differential increases due to high discharge pressures (i.e., related to an increase in warm ambient temperatures).

Without seeing any P&ID's I can only guess what the general issue might be. However, somewhere in your system it would appear there is a control that is being fed by high-pressure liquid propane. As the pressure differential increases (suction is relatively fixed, while the discharge pressure continues to increase through the day) the flow capacity of a control valve increases. If the controller does not force the valve to throttle down, it will simply flow to much.

The fact that the liquid level goes down in the cooler periods would tend to reinforce this as this discharge pressure should be decreasing during this time. I'm assuming you are using air-cooled exchangers.

By any chance does the knock-out drum have a liquid feed valve upstream to provide desuperheating of the suction gas before the gas enters the vessel?

27-05-2008, 12:30 AM
I have to go along the lines with US Iceman. The ris something basically wrong with the design of the evaporator if you are getting liquid over into the suction drum on a regular basis.

Can you describe the evaporator and tell us how much vertical disengaging space you have. It would really help to see the design drawings and data sheets on the evaporator.

A section drum (accumulator) should be dry except in the case of a process upset.

Who designed the evaporator and suction drum? If it was one of the process design companies, they generally do not understand the requirements of a refrigeraiton evaporator and suction drum. I have yet to see a non-refrigeration company properly design an evaporator and suction drum. They will always be short on disengaging space and run the separation velocities too high which will lead to a continual liquid carry over from the evaporator which will over fill the suction drum.

I like to design a suction drum with a "boil-out coil" to insure that there is a reasonable way to disapate any liquid carried to the suction drum.

If you can get some drawings and data sheets to me or US Iceman, we can look them over.


31-05-2008, 08:14 AM
Thank you all for responding to my question, I will get you the details as requested. When I have returned back to work I will gather this info.

Kind regards

02-06-2008, 09:19 AM
question 1
what is temperature of suction gas before suction drum
is suction drum colder then the gas condensation will occur.
assuming rest of installation is oke
we always assume suction is warmer as the drum
but it does not have to be like in your case