Air compressor problem

[arbainmahmood]

Dear all,

1. We (Ethylene plant) have 3 unit of MHI compressors (centrifugal)designed each at 3100nm3/hr & 8.6 barg disch pressure, 2 unit are motor driven & 1 steam turbine manufactured by Shin Nippon.

2. When the plant was commissioned in ~1995 not much of air demand only 1 compr is running. When plant expansion was completed in 2005, demand had increased & 2 units were made to run all the time. Since then problem started with several shutdown due to 'surge'!, hi temp on 3rd stage air cooler. With 2 compressor running at 100% IGV open, the compr still unable to meet normal discharge 8.1~8.4 barg pressure requirement. Each compr put out 7.8 & 7.2 respectively. Total flow for 2 compr running is 3390 nm3/hr ( A is at IGV 100% open, B is also > 90% IGV open), still like not giving the expected flow.

3. A quick review on the compr operation, found below:
a) water traps for all 3 stage all compressors are malfunction, therefore the drain valves are intentionally left slight open (source of leakages),
b) the air receiver drain valve is also left very slight open - intentionally to remove water.
c) No load sharing control installed, instead individually control with Blow off valve to prevent going into surge.
d) 3rd compr unableb to run due to IGV, Blow off stuck & controller problem
e) external water is poured continuously onto 2nd stg cooler in order to prevent compr-B shutdown on hi temp??
f) suspect internal compressor fouling since all compressor filter houses are in very bad condition. We will replace compr A filter house with replacement filter is made of Carbon steel. (Our plant is near to sea)

4. We are checking with other user on Load sharing control experience (existing or upgrade.

5. Question: why the compressors unable to meet plant air demand?

Any feedback or advice, thank you very much.

 

[gerhardl]

I believe you have answered your own question.

I think perhaps the real question is: how much malfunctioning components, water and dirt can the units have and still not fall below specification.

 

[dcasto]

1.) are they running the correct speed?

2.) The compressor doesn't "creat" pressure. The piping system creates pressure that the compressor meets.

 

[StoneCold]

Take a shutdown. Clean up all the problems you have identified and start them back up. You might also want to add flow meters and figure out where all the air you are compressing is going.

Good luck

StoneCold

 

[Sam654]

It sounds like your machines need a good dose of maintenance.

1. Your interstage drains being open can rob downstream stages of volume so don't open them too far.

2. As you mentioned, your intercoolers sound as if they need cleaning. Running water on the shell is most likely just cooling the thermocouple. Improper intercooling increases the specific volume of the air that can starve the next stage leading to surge. Centrifugal compressor capacity drops significantly with increasing temperature.

3. I don't know your compressor design but if you have the capability, check your clearance between you impellar and volute. Excess clearances can lead to recirculation, over heating and reduced capacity.

4. Check the condition of your impellars. If the filter boxes are in the condition you say they are in, the leading edges may be eroded. Also, check the leaving edges. Erosion there can also deplete capacity. If that were the case, you may (or may not)see increased vibration.

5. You didn't mention if you had any system metering. You should install a system flow meter to understand your demand before investing in controls.

6. Check for distribution system leaks. At a previous employer, we were able to shutdown a 450 HP compressor after an extensive leak detection program. Make employees aware of the high cost of compressed air.