Temperature high, the compression can not reach the design point pressure

[muksidin taslim]

Hello ...
In our plant, our compressor machine can not reach the design point pressure, our team has performed to evaluate it. The IGV opening, pressure & temperature suction seem normal as usually. But the difference conditions are the temperature after 1st stage, 2nd stage and 3rd stage relative more high than previous operation. In this case, whether if the temperature discharge on each after cooler relative more high, the discharge pressure can not reach?
See the picture at below for actual condition

 

[JJPellin]

Additional details on the machine and the process would be helpful. But, given the information provided, a few possibilities occurred to me. Since you are only monitoring and commenting on the gas temperature after each cooler and the fact that all three coolers share a common supply of coolant (I assume cooling water), you could have a problem with the coolant system. This could be a drop in the differential pressure from coolant supply to return, an increase in the temperature of the coolant supply or a fouling of all coolers caused by a contamination of the coolant system. In order to rule this out, you would need to know the temperature into and out of each cooler to determine if you have lost cooling efficiency.

You could have a general loss of efficiency in all three stages of the compressor. This could be a result of an increase in internal recirculation based on increased clearances in interstage seals. If there was some event such as high vibration because of extended operation at critical speed, operation in surge, rotor bow associated with poor start-up or shut down practices, abrasive process contamination, etc. Depending on the configuration of your compressor, it is possible for a single event to damage the efficiency of all three stages at the same time.

Any further speculation would require details about the compressor and the process.


Johnny Pellin

 

[rotw]

I would assume that based on the 100% opening IGV indication on the picture there is no room for further IGV adjustment (?). Appears a bit strange because it is good OEM practice to leave some margin at the design point for varying the IGV angle in the direction toward higher head (e.g. < 5% to 10%).

 

[JJPellin]

I am also noticing a spill-back control valve that is depicted as closed. This spill-back loop shows no cooler in the circuit. If that spill-back control valve was leaking by, flow would be diverted back to suction. This would cause an increase in process temperature and a drop in discharge pressure. I would close a block valve in that spill-back line to prove that it was not leaking by.

Johnny Pellin

 

[muksidin taslim]

Thank for the answers,
So far we check the opening of bypass valve is okay same as on DCS screen.
This compressor is Recycle Nitrogen Compressor (RNC) to provide High Pressure Nitrogen to Turbo Expander. The source Nitrogen in suction is divided into 3 lines, from discharge turbo expander, N2 from lower column and in part high pressure N2 gas from discharge RNC

 

[georgeverghese]

Stage cooler exit temps look quite normal here (38 to 42degC - presume these are water cooled units)- what were the previous values?
If the current final Pd of 2040kpag is below what you had previously, then one reason could be some limitation in RNC driver power - is this a variable speed steam turbine driver or variable speed electric motor? What is the limiting parameter here that is stopping you from going to higher speed?

 

[rotw]

Is this an integrally geared type of compressor / packaged unit with gas after coolers integrated to main skid?

What about the driver? if this is an eMotor, is the compressor drawing unusually high power vs. design/rated case?
And is the motor capable to provide the extra power demand then? is the RPM decreasing due to high load ?

All in all, if your original design is over optimized or stretched (e.g. cost effectiveness reason or otherwise) or/and if buyer specifications did not ask for the adequate provisions, ultimately this could result in poor contingencies in terms of IGV angle margin, motor power margin and cooler rating (to min. water supply temperature, assuming coolant is water) whereby a marginal decrease of mechanical performance (wear and tear, internal components deterioration such as seals, more internal leakages, etc. and possible maintenance/operation mishandling) will not be copped with easily by the machine.

 

[JJPellin]

I think rotw hit on the key. It did not occur to me (since the OP didn't bother to provide the information) that we were looking at a three stage, integrally geared, high speed compressor package. In my experience, these are notorious for problems with the inter-stage coolers. Ours have a tendency to fouling. Gaskets degrade that allow the cooling water to short circuit the coolers. I would be fairly confident that the coolers are the problem. I would pull the coolers for cleaning and inspection.

Johnny Pellin

 

[JJPellin]

In one of our similar units, we found that the gaskets that direct the cooling water path into and out of the inter-stage cooler had degraded and moved out of position. This allowed the cooling water to short circuit the cooler and resulted in second stage gas temperatures increasing from a normal 78 F (26 C) up to 150 F (66 C). I have attached a picture of the issue with the cooler gaskets.

Johnny Pellin

 

[zdas04]

muksidin taslim,
I hope you looked at the responses you've gotten from JJPellin and ROTW and realized two things: (1) That there is some of the world's best engineering support available on eng-tips.com; and (2) the better the information you provide up front, the better people can help you. Too often we give up on your problem when we have to pull one piece of information at a time.

I'm sure that both of the engineers that have helped with this problem had to copy your screen shot into some other program to be able to blow it up enough to read it, that is a step farther that most of us are willing to go--if you can't be bothered to provide information in a way that facilitates analysis, then why should we bother to expend our limited time and resources to do it for you?

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Muksidin,

you say the temps are "more than the previous operation"

By how much?
what changed?

By how much are you short of your discharge pressure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.