Centrifugal Compressors

Mr. Norman P. Lieberman, Process Improvement Engineering, Metairie, Louisiana, USA. This is Ashok from India. Explain please why compressors surge.

--S.R. Ashok, Process Engineer

 

Ashok – Read my book, Process Equipment Malfunctions (McGraw Hill Publisher) for my best explanation. Process Factors promoting surge are:

n  Low molecular weight

n  Rotor fouling

n  Higher temperature gas

n  Low suction pressure

n  High discharge pressure

n  Low flow

n  Closing spill-back

n  Suction throttling

--Regards, Norman Lieberman

Centrifugal Compressors

Norm – A quick question. A relatively minor surging incident has severely damaged our compressor. We have experienced much worse and longer periods of surging without noticeable damage. Why this extreme damage now?

--Harry Hambin

P.S. Regards to Liz

 

Dear Henry – Hope you had a good holiday. It could be that the main bearing lube pump is driven off of the compressor shaft itself. Then, if the auxiliary lube oil pump failed to come-on automatically, with the resulting low lube oil pressure (assuming the compressor itself tripped-off due to surging and was then restarted by the operators), this could have damaged the radically support bearings. If the thrust bearing is not particularly damaged, this would support my theory. Hope this helps you.

--Norm

Centrifugal Compressors

To Mr. Norman Lieberman – In your book, Process Engineering for a Small Planet, you write about damage to a compressor because of failure of the bearing lubrication pump. The same failure occurred in our plant. How could this have been prevented?

--Oscar Harrod

 

Oscar – Are you checking that the back-up pump will start automatically? Bleed-off the pressure to the auto-start switch for the pump and see if it will start by itself and that it develops the required lube oil pressure as stated by the manufacturers.

--Norm

Centrifugal Compressors

Dear Norman – Our vacuum tower is now working fine. Thanks for your help. But I have another question. I have a combustion air blower which surges during hot weather. The surges do not damage the compressor (blower). But when we have surge, there is a loud banging sound. That banging is coming from the check valve on the compressor discharge. Should we consider removing this check valve?

--Chuck Hendrics

 

Chuck – No. The check valve dampens down the force of the surge and helps protect the stationary elements in the compressor case from damage.

--Norm Lieberman

Centrifugal Compressors

Dear Mr. Norman Lieberman – Thank you and Mrs. Lieberman for writing Working Guide to Process Equipment. A question I have is when our compressor begins to surge, the surging seems to get louder. Then when I lower the discharge pressure to stop the surge, it does not help, even though it’s the same discharge pressure we had before the surging stared. Please explain to me.

--Kia Vala

 

Mr. Vala – Once surging starts, the gas inside the compressor case goes forward, then back, then forward,  and then back, etc. This causes the gas to become hotter and less dense. The reduced density reduces delta P developed by the compressor, which promotes gas flow reversal.

--Regards, Norm Lieberman

Centrifugal Compressors

I attended your seminar in Sasol, Secunda in 2008. Our problem is that when we open our compressor spill-back, which we thought would stop surge, it sometimes makes the compressor surge. What is our problem?

--H.K. Masa

 

Your problem is that the spill-back flow is hot. This reduces vapor density. Which reduces compressor delta P. Which reduces compressor discharge pressure. Which allows gas flow to stop and reverse. I had this exact problem in 1974 on my alky unit in Texas City. I desuperheated compressor suction by injecting a spray of liquid isobutane into the suction. This would quickly bring the compressor out of surge. Hope this helps.

--Norm