4 replies to this topic

[ASH25320]

We have got an extraction cum condensing turbine which is prime mover for compressor. Recently in our plant due to a problem in downstream section plant has to be shut down for 5 hours. during which period this turbine was kept running; with compressor on venting. this is a 9 MW turbine.
Can this turbine be kept on running at MGS(Minimum Governing Speed) for a period of say 4 hours, for the purpose of energy saving?
Generally during start up we keep this turbine on MGS for not more than 10 minutes.

Any comments are most welcome...

[Art Montemayor]

ASH:

First of all, I believe this is a steam turbine you are describing as the main driver for a centrifugal compressor. Is that correct?

What, specifically, do you call “energy saving”? I have to start assuming again that you mean the “bottom line” to the Plant (or profit center) under which this steam turbine is being depreciated. Am I correct? What I mean by “bottom line” is the net profit or loss results. If this is also correct, then I don’t have any qualms or problems with the turbine operating for as long as it takes – as long as that maneuver is in the best economic interests of the profit center.

Any decent steam turbine – especially the “big Mamas” – is designed to operate constantly under design load for 18 months (sometimes more) without stopping. Any machine that is properly designed will operate at design (sometimes, even above design) rates for very long or extended periods when it is given the proper design feeds, fuel, lubrication, cooling, care, attention, and maintenance. This applies to compressors or pumps. There is nothing wrong with this operating philosophy. It falls correctly into the scope of a recognized machine fabricator. What is terribly wrong is when the machine is abused, mis-used, and punished with dirty lubrication oil, no lubrication, bad fuel, little or no maintenance, flawed and varying power supply, and over-all general abuse by its operators. If this is the case, then the machine is prone to fail either under partial load or design load.

If a well-maintained and cared-for steam turbine is found to be economically better off operating a partial load for an indeterminate amount of time – while properly lubricated, cooled, and maintained – then, the common sense and good engineering judgmental decision should be to do it. This type of operation doesn’t damage or harm the machine nor does it represent a hazard for the operators.

It may present a questionable situation vis-à-vis the environmental condtions around the machine, but that is another issue that should be addressed before making the first decision to keep the machine running. I have operated air separation plants pumping pure Oxygen and Nitrogen to the atmosphere for hours on end – just as if I wanted to throw money away. But that was not the case. The real cause of the venting and continous running of the heavy compressors at partial load was to keep the cold box operating normally and in cryogenic conditions until my plant could start to fill storage tanks again. The bottom line was that the economics running the units to vent was much less expensive than shutting down the unit and having to startup the unit later and go through an expensive and laborious cool-down. I’m sure that something analogous to this is what is causing your profit center to kept the steam turbines operating with a partial load and venting to atmosphere.

Have I succeeded not only in guessing correctly as to what is happening and in explaining it as well?

[ASH25320]

ART,
You are pretty close to what all process engineers believe & practice in process plants that is continueous & uninturupted running. And I fully agree with you regarding your final points of your post.

My question was of only academic interest as to whether exhaust hood temperature of turbine
will rise while keeping the turbine in unloaded condition? If yes then what could be the reason?
The only intention here is to save extraction steam venting to atmosphereduring plant shut down...

Regards for your reply.

[Art Montemayor]

ASH:

You said the turbine was loaded - to the vent. The work load of the turbine may be reduced, but it is still loaded. The exhaust hood temperature on the turbine should not rise any more than the normal while the turbine is working like this. Is there something else going on that you haven't told us?

If this is a steam turbine, then the steam is still expanding isentropically - with a reduction in the exhaust steam to the condenser. Please explain what it is that concerns you.

[ASH25320]

ART,
I did not explain it properly.
Case I- turbine / compressor running at operating conditions & connected to system.
turbine steam consumption=72 MT/hr
Extraction=55 MT/Hr, Condensate=17 Mt/hr.

CaseII- turbine / compresoor running at operating conditions & compressor on venting.
Steam Consumption as above. Extraction steam is vented as no consumption possible downstream(plant under shut down).

CaseIII-Turbine running on MGS, compressor on venting.
It will consume minimum steam, no major extraction venting. no thermal shocks of stopping.
Will exhaust temperature is affected in any manner?
Thanks Again...