Reciprocating Gas Compressor

Has the temperature instrument been correctly calibrated?

What is the basis for a theoretical temperature to two decimal places?

What is the temperature of gas at entry of 1st stage?

What is the compression ratio of 1st stage?

Are the 2nd and 3rd stage discharge temperature within reasonable limits?

Check for inlet screens between inlet volume bottle and cylinders and suction scrubber. If its a new installation they could be plugged off with Derbies. Also make sure the VVP is set correctly.

In order to get a sensible answer you need to supply the information requested by others. Firstly the design information from the machine data sheet and then the actual data recorded in the field. Also check the power consumption against calculated or design. Other things to check also mentioned by others, inlet filters if fitted, blockage of de-misters or knockout vessels, blockage of coolers to second stage, recycle valves leaking, trace heating of suction lines etc etc.

One very good thing is that you have first calculated the theoretical discharge temperature, and then looked at the most common source of high discharge temperatures, the discharge valves. They are the culprit most times.

Just some comments on higher than expected discharge temperatures, they can be caused by:

• Increase in suction temperature. Td rises by the same amount as Ts. Your Ethene (Ethylene) will boil off at around 169 K, but at what temperature are you getting the gas at the suction of your machine.
• Leaking discharge valves. Working on the same gas over and over.
• Poorly adjusted unloaders/capacity control if fitted.
• Change in specific heat ratio of the gas - not likely for boil off compressor
• Change in pressure ratio, eg. high discharge pressure or low suction pressure.
• etc..(sure I must have missed some)
• Have fun....

Looks like we have a high temperature problem. Replacing valve and piston rings can actually increase your compressor compression pressure and hence increase your working load if the frictional stress is high. How about the cooling system? Does this compressor runs with a coolant circulation at above of the head cylinder?Might be a clogged in the coolant line flow, I guess.

rajindran

The valves do not "increase your compressor compression", the system does that for you. Leaking discharge valves mean that you leak hot high pressure gas back into your cylinder on the return stroke and then re-compress this hot gas thereby increasing its temperature further. The same occurs if hot gas is bypassing the rings and entering the cylinder on the other side of the double acting piston during its "suction" stroke. Installing thermocouples close to the discharge valves is a fairly effective way of monitoring valve condition.

Cylinder cooling in this type of process compressor has more to do with the surface temperature of the cylinder liner wall than it has to do with cooling the gas. Think of the dwell time that the gas has in the cylinder and how much cooling effect the liner jacket cooling could have on the process stream.

Dear All,

I am really so happy to see your interest...really glad to join this forum...i have tried this forum first time and got the amazing replies and suggestions....thank you so much for that...you guys are great.....

Now coming back to my problem...I am specifying the basic operating parameters for the compressor:

Suction Pressure : 0.06 KG/CM²G

Suction Temperature : -52°C

Discharge Pressure : 3.01 KG/CM²G

Discharge Temperature Theoretically : 14.38°C

Discharge Temperature Actually : 52°C

I have already check the instruments's calibration...it's just perfect...Compressor ration seems to be okay i guess this should not make problem....as the inlet temperature is so low there need not to be cooling required as per API specification....

Due to high discharge temperature of first stage...second and third stage also deliver the higher discharge temperature...as it is low temperature application there is no need of inter cooler and after cooler....

If it is problem of leakage due to valves or piston rings, the temperature should be in rising manner as per my knowledge but actually it is stable around at 52 °C.

I will again check with the unloader and volume bottle assembly if i find out something there.....thanks for your suggestions....

Regards,

Nims

nimesh1401,

It looks to me as if you have used a ratio of specific heats, Cp/Cv, of about 1.25 for your calculation to get your approximately 15^o C discharge temperature. Using k rather than n is always an approximation but is good enough for your needs in this case. However, k varies with temperature and pressure and you have used k at 15^o C. I do not have anything with me to find out k at -52^o C, but I would guess it is around 1.3 to 1.32 or so. Using 1.31 you get a theoretical isentropic (adiabatic) discharge temperature of around 30^oC.

This does not solve your quandary but the deviation is less.

Check instruments again, look at unloaders, clearance volume adjusters if fitted etc. These are easier to check than the valves. By the way if you take discharge valve cover surface temperatures are they the same on both sides of the cylinder? If they are much different then you know which valve to go and look at.

The difference that you are seeing is not so big that I would panic yet, but any increase needs to be looked into. What happens with a leaking discharge valve is that it allows a fairly constant backflow into the cylinder, thus effectively raising the suction temperature to a certain value and this remains constant. Only if this backflow increases to a point where you are not getting any further cooling from the incoming gas will you see the temperature continuously rising.

A reciprocating compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.