View Full Version : propane compressor suction drum

30-12-2005, 06:10 AM
dear collegues,
i have a problem in propane compressor which is the suction drum get propane liquid level which lead to compressor trip.
i think it is related to the suction pressure need to be increased a little, but i need the basics on what increasing the pressure like the dew point of propane in different pressure and if there any way to can calculate it by my self to use it with other refrigerants
i hope anybody could help

US Iceman
30-12-2005, 02:37 PM
Hello Ahmed,

The suction drum can have high liquid levels that occur for many reasons.

Liquid can accumulate in the suction drum due to a sudden decrease in the suction pressure. If the upstream process (kettle or other heat exchanger) undergoes a sudden decrease in suction pressure, the lower suction pressure causes any liquid refrigerant to suddenly flash off.

This can result in periodic liquid volumes returning to the suction drum. This can occur over many hours, or a short time.

The best way to reduce this is to have the compressor loading occur more slowly. This reduces the amount of pressure change upstream of the suction drum and lowers the potential for liquid slugging.

A second way this can happen is by condensation of vapor in the dry suction line. During normal operation, the suction line from the process can be colder. During periods of lower capacity, the suction pressure may rise slightly.

When this occurs, the suction vapor will condense on the inside of the suction line. The suction line is colder than the saturation temperature of the propane at the higher pressure.

A third example might be if the suction drum has a pipe coil inside of the drum. Sometimes, the drum manufacturer will provide a pipe coil inside the drum to subcool the liquid propane, and to also boil off any liquid accumulation in the drum. I have seen these pipe coils start to leak and flood out the drums.

A fourth example is dependent on the addition of extra heat exchangers upstream of the suction drum. If additional heat exchangers have been installed, the suction vapor flow will be increased.

The resulting higher suction volume flow can increase the velocity of the liquid/vapor through the suction drum. The higher vapor velocity carries liquid out of the suction drum, to the compressor.

It is more probable that your situation is being caused by one of the first three mentioned above.

If you can provide me with the units you use (SI or Imperial) I can generate a vapor pressure curve for propane for you.

30-12-2005, 08:34 PM
thx, usiceman,
i appreciate your respond, by hte way the accumulation of liquid is during normal operation and we overcome it by opening the recycle valve a little bit manually, but this ofcourse performing more load on the compressor.
and iam using si units (bat , celsius degrees,....)

US Iceman
30-12-2005, 09:21 PM

Attached you will find the vapor pressure curve for R-290 (propane). This is for refrigerant grade propane and does not consider any other gas that could be mixed in.

I have seen some propane systems that had some ethane and other gases mixed in. This can change the relationship shown in the curve slightly.

The pressure shown in the y-axis is listed in Bar (absolute). The x-axis is of course in degrees C. This shows the saturation curve for the refrigerant. This would be similar to any pressure temperature chart you may have for other refrigerants.

Is the recycle valve for hot gas? I have not worked in a LNG facility so I am not familiar with the exact operation.

31-12-2005, 09:36 AM
thx us ice man for valuable help
and yes the recycle valve of the compressor is of hot gas because it is coming directly from the compressor discharge before condensing.

US Iceman
31-12-2005, 06:42 PM

Does the piping downstream of the recycle valve go into the suction drum as a sparger? Is so, the hot gas is injected into the liquid, which desuperheats the hot gas and boils off the liquid in the suction drum.

This would certainly increase the compressor load.

If the liquid build-up in the suction drum occurs during normal operation, the problem would seem to be the upstream process to the suction drum.

It sounds like some heat exchanger or process vessel is spilling over liquid propane into the suction line returning to the suction drum.

If this is true, you should investigate the liquid level control valves on the process vessels or heat exchangers. The liquid level may be controlled at too high of a level, which results in liquid carryover when heat loads are applied, or the suction pressure is decreased too rapidly.

It is hard to say much more without additional details from you.

US Iceman
31-12-2005, 10:50 PM

Here is a link for additional information about spargers.


If your system does not have one of these, does your discharge temperature on the compressor increase when the recycle valve is open?

01-01-2006, 08:01 AM
for the piping downstream recycle valve .. i have no idea
the opening of recycle vlave is controlled, and normally there is no level in suction drum.. when it starts to form level we open the valve a little bit ... although it is a small opening but it increase the load a little.