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

Reboilers

Question:

            Should we de-superheat our steam to our amine regeneration reboiler?

Answer:

            Yes. Use of saturated steam will retard the thermal degradation of the amine, and reduce the rate of reboiler tube failure. It’s a standard industry practice, especially critical if you are using MEA.

Reboilers

Question:

            Our steam supply is contaminated with non-condensibles (maybe CO₂?). We opened the vent on the top of the channel head to purge out the non-condensibles which we think are reducing steam condensation rates and our reboiler duty. This did not help. What do you suggest?

Answer:

            The non-condensibles will not accumulate in the top of the channel. They accumulate below the bottom pass partition baffle in the channel head. That is, the portion of the channel head that must be vented. Opening the vent on top of the channel head will vent out steam, not the non-condensible CO₂.

Reboilers

Question:

            I retubed a butane splitter (iso vs. normal) reboiler recently. Twenty percent of the tubes were plugged. Yet I lost reboiler capacity with the retubed bundle. Why did we lose capacity, rather than increase capacity by twenty percent?

Answer:

            The problem is smooth tubes that lack nucleate boiling sites. This happens all the time. The old tubes were rough and pitted. Increasing the condensing pressure of the reboiler steam will only make the problem worse. One possibility is to sand blast the tubes to roughen their surface. Alternately, you can use “Linde Hi-flux,” sintered metal coating. Whichever you find most cost effective.

Reboilers

Question:

            Does superheating steam aid or retard heat transfer?

Answer:

            I had a reboiler on a debutanizer reboiler in Aruba, supplied with 400 psig, 650°F superheated steam. There was a de-superheating station (i.e., a condensate injection point). As I reduced the steam supply temperature (at a constant pressure) with the condensate injection, my reboiler duty did not change. I believe the conventional answer is that superheat retards heat transfer. But, this was not the case in my only field experiment.

Reboilers

Question:

            Sometimes, when I OPEN the channel head condensate drain valve, my reboiler duty increases. But, sometimes when I partly CLOSE off the same valve, my reboiler duty decreases. What’s happening?

Answer:

            There are two problems with draining condensate from the channel head:

1.      Blowing the condensate seal.

2.      Condensate back-up.

If you open the drain valve too much, steam and water will blow-out together. The high velocity steam, blowing though the tubes, will reduce the condensing heat transfer coefficient. Hence, the observed loss in heat transfer and reboiler duty.

If you close the drain valve too much, water will back-up and submerge the lower few rows of tubes, which will reduce the surface area exposed to the condensing steam, which also diminishes the rate of heat transfer.

Without some method of observing the steam condensate level in the channel head, the optimum position of your condensate drain valve can only be found by trial and error.