5 replies to this topic

[iyer]

In a 3 stage reciprocating compressor on site, the discharge temperatures are 106/107/116 deg C for the first, second and 3rd stages. The trip values are at 140/140 and 145 deg C. The Compression ratio is slightly ascending with the third stage having the highest. I do understand that a higher compression ratio would lead to a greater discharge temperature. The suction temperatures are the same as needed for a case as close to isothermal.
My question is what standards do we follow is setting the trip setting for the stage discharge temperature

[ankur2061]

Iyer,

If you are looking for a standard for setting discharge temperature settings (alarm/trip) than there are very clear guidelines in Section 2.3.2 of API STD 618 for them.

Alternatively Art Montemayor our adminisatrator could give you some advice because of his vast experience in dealing with reciprocating compressors.

Regards,
Ankur.

[Zauberberg]

GPSA also offers guidelines for maximum practical discharge temperature limits:

To reduce carbonization of the oil and the danger of fires, a safe operating limit may be considered to be approximately 300°F. Where no oxygen is present in the gas stream, temperatures of 350°F may be considered as the maximum, even though mechanical or process requirements usually dictate a lower figure.
Packing life may be significantly shortened by the dual requirement to seal both high pressure and high temperature gases. For this reason, at higher discharge pressures, a temperature closer to 250°F or 275°F may be the practical limit.
In summary, and for most field applications, the use of 300°F maximum would be a good average. Recognition of the above variables is, however, still useful.

API 618 covers low-speed compressors. API 11P covers packaged high-speed separable compressors with speeds from 900–1800 rpm.

Edited by Zauberberg, 22 April 2010 - 01:31 AM.

[Art Montemayor]

iyer:

Both Ankur and Zauber have cited the one, central source of guidelines on this subject: API 618.

However, one important fact has not been mentioned: is your recip an oil lubricated or non-lube unit with respect to the cylinder design? This is very important because, as you will note in API 618, the conventional guidelines that set the maximum operating discharge temperature in a reciprocating compressor is the kind and type of (oil) lubrication you employ. What is even more important is the existance of NON-LUBRICATED (or mini-lube) Cylinders and valves.

The use of non-lubricated features means you are resorting to elastomer materials to do the effective sealing in the cylinders and valves - and this means that you must abide by the temperature limits on these organic materials. While an oil-lubricated cylinder is kept at about 250 to 300 ^oF as a maximum (due to oil degradation and sludge/carbon formation), the Teflon, PEEK, etc. materials used on non-lubricated service are often limited to a lower temperature limit - which also limits the compression ratio of each stage (or cylinder).

I have always demanded that reciprocating compressor compression ratios be designed for a discharge temperature of 250 ^oF and I have set my trip-out point as 325 to 350 oF. I am glad to see that you are following those same guidelines I learned and established over 40 years ago. You will not regret following these guidelines - especially if what you have is an oil-lubricated, air compressor. Any gas that is an oxidizer (air, carbon monoxide, etc.) should be treated with caution when compressing it in a lubricated, reciprocating compressor. I learned this lesson a long time ago when people were getting killed due to explosion of oil-lubricated air compressors with high compression ratios within one stage.

In summary, I cannot offer direct, specific recommendations because you have failed to give us specific basic data and background information. But I believe that you understand the principles and hazards involved in compressing a gas and can appreciate that although there are no so-called "standards" or codes involving the recommended discharge temperatures to be reached in a reciprocating compressor, you can understand that common, good engineering judgment and horse sense applies in this case. Your existing temperatures indicate you are in a conventional, safe operating mode - although it would certainly have helped if you told us the whole story: what gas, type of compressor, type of lubrication, application, etc., etc.

[iyer]

Art.
Thank you for your response.The cylinder is lubricated, but I need to check if the valves are (From what I remember they are and should be). Do you need any more information. Hydrogen comes from the PSA.
It is a 3 stage compressor, double acting. Compression ratios of 1.95 1.99 and 2.09 for first second and third stages respectively. The first stage has two cylinders. Flow rate is about 50KNm3/hr at 100% loading.
Any more info needed?

[Art Montemayor]

iyer:

As long as your compressed gas is hydrogen (presumably 99%+ vol.) you should have no problem with your discharge temperatures set where you state they are at.

If you are lubricating your cylinders with a conventional forced-feed, drop-wise lubricator then you are automatically lubricating your cylinder valves as well. The valves are kept in a film-wise lubricated state by the wiping action of the reciprocating piston that splashes minute oil droplets on both the suction and discharge valves at the end/beginning of each stroke. All cylinder oil-lubricated reciprocating compressors employ this feature to keep the valves in lubricated state.

Your compressor seems to be well designed for the hydrogen service. The compression ratios are to be admired - and kept at that level by judicious maintenance of the lubrication and mechanical state of the piston rings and cylinder bore. A double-acting cylinder should be monitored and inspected as often as possible to detect piston ring and cylinder wear. Hydrogen, being so small a molecule, gives problems in by-pass leakage through the piston rings and causes RECOMPRESSION of gas. This deficiency (or inefficiency) leads to progressively higher and higher discharge temperatures in the affected cylinder. This is why it is so important to ALWAYS MONITOR AND REGISTER CYLINDER DISCHARGE TEMPERATURES. And THAT is the topic of this thread and the subject at hand. It is a very important topic when applying reciprocating compressors and merits all the understanding and study you can justify. I presume that you are compressing hydrogen in order to put it through an adsorption purification system (PSA - Pressure Swing Adsorption with Molecular Sieves). If so, the compressor's role in this operation is a critical one and you should be monitoring the discharge temperatures 100% of the time. When I managed operating gas plants I had my supervisors always walk through our compressor installations and physically place their hands (or fingers) on the discharge valve covers of our reciprocating compressors - at least once a shift. This is an old-fashioned (but effective) way to detect the onset of leaking piston rings on double acting cylinders or of leaking discharge valves. With practice, a person can detect excessive temperatures by touch. I also had our compressor cylinders (especially the valve covers) painted with a special aluminum paint that started to blister and decompose at temperatures in excess of 400 ^oF. I could visually spot an over-heated cylinder by looking at the discharge valve cover and its paint.

I hope this experience helps you out.