4 replies to this topic

[roland.dw]

Dear Sir/ Madam

 

I have reciprocating compressor Dresser-Rand

- 2 stages

- Double acting

- Gas: atmospheric air

- Design press ratio 1st stage 1 : 3.4

- Design press ratio 2nd stage 3 : 9.2

 

A months ago discharge temperature at 1st stage is 131 C (measure with thermogun in discharge lane). It's happen for years. There is a spacer  that is added between cylinder and outer head to increase volume so and decrease compression ratio, but it seems give no effect.

The suspect of high discharge temperature at 1st is:

1. Fouling at jacket cooling water

2. High compression ratio

I want to know, What possibly the cause of high compression ratio?

 

Last month there is noise in 1st stage. So, 1 suction valve is removed (without  blank), and the noise is gone for a while. Currently, 1st stage running as single acting mode but the 2nd stage run as double acting mode. The flow may be become one-half but it'is no problem. And because of this operating condition, the high discharge temperature problem is gone, the discharge temperature at 1st stage is about 82 C now.

 

My concern is, What will possibly happen in long term to the recip comp (to the parts, driver, etc) that operated in that operating condition (with 1 suction valve removed)? 

 

Thanks and regards 

Roland

[Technical Bard]

It sounds to me like you have a problem with the #1 cylinder - perhaps the suction or discharge valve is damaged, or material is fouling the piston head or cylinder (increasing the compression ratio).  

[Bobby Strain]

You have increased the rod load on the second stage due to the higher compression ratio. Also, you should fix the first stage to operate double action. You should expect the first stage discharge temperature to be lower because of the lower compression ratio.

 

Bobby

[Art Montemayor]

Roland:

 

Having specified, installed, operated, converted, and modified reciprocating compressors for over 15 years I have more questions and comments than you imagine about your description and explanations regarding your recip:

• It is not enough to state you have a 2-stage machine.  Is it in tandem?  Is it balanced-opposed?  Or is it in parallel, with a common crank shaft?  All these factors can impact or affect, as Bobby points out, the rod loadings.
• Most importantly, you should give all the important details: the size, capacity, model and horsepower of the machine, the design and actual suction and discharge pressures and temperatures on each of the 2 stages.
• Is it a PHE, ESH, or even an HSE model?
• What kind of piston rings do you have?  Is the compressor lubricated or non-lube in the cylinders?  This is important to know with respect to a double-acting cylinder.
• What do you mean by “Design press ratio 1^st stage 1:3.4”?  Do you mean the stage sucks in 1 kg/cm²gauge pressure of air while discharging it at 3.4 kg/cm²gauge pressure?
• The compression ratio in a reciprocating compressor is defined as the absolute discharge pressure divided by the absolute intake pressure.  That’s why it is important to have the actual pressures in each stage.
• Forget about the cooling expected from your cooling jackets.  Cooling jackets on a recip can never give you sufficient heat transfer.  Additionally, the compression step is carried out in an adiabatic process in which there ideally is no heat transferred.  Actual field experiments have shown that within practical limits, this is true.  I - and many other engineers in the field in the past - have operated recips continuously (24 hrs/day, for months) with cooling jackets simply filled with hydraulic oil and no increase in the discharge temperatures.
• The only way to accurately measure the discharge temperature in a compressor is to install a dial thermometer directly in the outlet pipe from the stage being monitored.  You can try a thermocouple, but inherent vibrations always give problems with it.  A “thermogun” is not accurate enough - nor reliable.
• You state you install a “spacer between cylinder and outer head to increase volume” and reduce the compression ratio.  But you fail to state WHICH stage you are alluding to.  By doing this, you are in effect creating an over-sized cylinder space - but on which side of the cylinder?  If you are decreasing the volumetric efficiency of one side of the 2^nd stage, you are also increasing the imbalance in the cylinder’s action.
• Do you have the support of Dresser-Rand to install the “spacer”?  I would not modify any part of the machine that may affect the rod and crankshaft loadings.  In a double-acting machine, you are affecting one side of the piston more than its opposite side.  This puts an unequal load on that cylinder’s reciprocating action and promotes unbalance and vibrations.
• The compression ratio of the second stage is subject to the discharge pressure that you impose on it with whatever is downstream of this air compressor.  This will also affect your 1st stage discharge - albeit, slightly.  You haven’t told us what load you have downstream of the machine and if it is constant.
• You state you removed a suction valve (from the 1^st stage?) and the 1^st stage is presently running in single-acting mode.  This is another different load on the rod, pin, and crank that may or may not be approved by Dresser-Rand.  I would never do this without the manufacturer’s permission.  In my opinion, you are asking for trouble.
• If your machine is operating with atmospheric suction (1 kg/cm² gauge), 3.4 kg/cm² gauge discharge on 1^st stage, 3 kg/cm² gauge suction on 2^nd stage with a final discharge of 9.2 kg/cm², you have a 1^st stage compression ratio of 2.2 and a 2^nd stage compression ratio of 2.6.  These are perfectly normal and conventional compression ratios.  Also normal are discharge temperatures between 250 ^oF to 300 ^oF - depending on the compression ratios.  You state you “want to know, What possibly the cause of high compression ratio”.  What is the high compression ratio you refer to?
• You are correct in worrying about what can happen in the long term to the machine if you continue to operate it in an unbalanced manner.  You should consult with Dresser-Rand about any mechanical modifications you impose on the machine.

I believe your concern should be with how your machine is being modified in the field with or without Dresser-Rand’s support.  This is a very simple application and you shouldn't be having any difficulties or problems with the high quality and experience that Dresser-Rand puts into their machines.

[roland.dw]

 

 

Thanks for your reply, Mr Montemayor.

 

This is compressor performance (summer case - normal).

 

Please see the attachment.

• Compressor configuration is balance-opposed 2 cylinder, Dresser-Rand 9-HSE;
• Piston Rings are Non-lubricated;
• The spacer is installed in the 1^st stage.  It's installed to decrease the compression ratio so the discharge temperature will be lower.  Nope, I have not consulted with Dresser-Rand when installing the spacer.
• At the downstream use, the compressor service is for supply of plant air.
• Yes, 1 suction valve from the first stage is removed, to eliminate the noise.

Yes, I understand that what I am doing is reckless.  In the near future we will do an overhaul (replace some spare parts like liner, valves, and piston rings) and return the compressor to its normal operation (double acting first stage and no spacer).

 

Let's we forget  about the some "changes" that i did to the compressor recently.  This is the data from 2015 when there were no "changes" done to the compressor:

 

See the attachment.

 

At that time the compressor had a higher discharge pressure and temperature than its design.
Based on the compressor Introductions books in the Troubleshooting Section, I did a check and came to the conclusion that the probable cause of high discharge temperature at stage 1 is:

1.  Excessive ratio across cylinder.
2.  Fouled cylinder water jackets.

Now, I am not sure what really is the root cause of "excessive ratio across cylinder"?

and what I am worried about is, after the overhaul the compressor will still have the same problem high discharge temperature at stage 1 as before.

 

Thanks before

Attached Files

•  data.png   16.48KB   4 downloads
•  Actual cond.png   4.24KB   4 downloads

Edited by Art Montemayor, 03 September 2018 - 12:33 PM.
spelling, tense, format