3 replies to this topic

[Megalith]

Hi Everyone,
I searched thru but wasn't able to find an answer or help to my problem. I'm hoping that you can help me.

There were some issues with maintaining start permissive temperature for Flash Gas and Export Gas Compressors. This was primarily because of the heat losses on the lube oil tank and the buffer gas supplied to the drive and non drive end seals of the compressors. In order to reduce heat losses lagging has been done on lube oil tanks of machine and seal heating oil supply and return lines. But the problem didn't seem to go away, it still takes fair amount of time to be able to start compressor.

I would appreciate if you can share your experience/knowledge on this subject

Regards,
Megalith

P.S. Happy New Year!

[Root]

Hi,

we face this problem with our Recovery Plus Compressor but we solve this problem with running both lube oil pump until stratup permissive got satisfied.
my suggestion for your compressor also same, Run both Lube oil pump then you can achieve your required temp, actually your heater looks inefficient.

Good Luck

[Art Montemayor]

Mega:

Your query is full of holes that are left undefined or simply not explained. You state that you have "issues with maintaining start permissive temperature for Flash Gas and Export Gas Compressors", but you fail to tell us the TYPE of compressors, the application, the temperatures, and the gas you are compressing. All of these details are needed in order to address your query. Toor thinks he/she faces this same problem - but all he/she is doing is GUESSING. It is not possible to form any type of firm or serious idea about what you are writing about without getting all the details. All we can do is GUESS. Why do you put us through this mental dilemma? Why don't you simply put as much effort into your problem as we would put into putting together a positive response to the query?

Tell us:

if your application is recovering associated natural gas from a production field - on land or offshore;
if your compressor is a centrifugal, and what kind of driver and drive train you have; how many stages, inlet pressure & discharge pressure; normal suction temperature & actual suction temperature;
what kind of seals does your compressor have? are they lubricated or pressurized? how big is the lube tank? what is the ambient temperature? what is the heater element?
the quality and/or dewpoint of your (natural?) gas;
the temperatures that you are confronting - design operating temperature, actual startup temperature, and minimum startup temperature; what does your operating procedure call for? who designed and fabricated this compressor module and what does the startup procedure call for?

There is a lot of more questions probably waiting after the above facts and details are furnished. It would also help tremendously if you revealed where this installation is located at, its capacity and rated horsepower.

Await your reply.

[Megalith]

Mayor,
Sorry for the lack of information and thus for wasting your time with guesses.
This is an offshore oil platform, the worst weather conditions observed so far when compressors failed to start is wind : 35 knots and temp: 5 degC.

System Overview

Flash Gas Compressors: The Flash Gas Compression system is designed to take gas from the outlet of the LP Separators -at around 7 barg- trains 1 and 2 and compress the gas (26 barg) to enable it to be mixed with gas from the HP Separators for export/further processing. The seal start permissive for the flash gas machines is set in the compressor UCP at 55oC. Due to temperature losses through the start sequence this means that the seal housing needs to be above 62 oC before starting the sequence in order to still be above 55oC by the time the compressor is ready to start.

Export Gas Compressors: The two 50% trains of Gas Export Compression are each designed to compress from 24.9 barg to 112.7 barg. The compressor can be run in a wet gas mode or a dry gas mode. The seal start permissive in the UCP is set up for the dry gas mode (the least conservative case).

If dry gas can be guaranteed then the seal housing temperature should be at a minimum of 42deg C prior to commencing the start sequence such that the start up permissive of 35deg C can be met after the purge. If the compressor is to be operated in wet gas mode then the seals must be at 47oC or more immediately prior to a start which translates to a temperature of 54oC prior to starting the sequence.

Gas Compressor Dry Gas Seal System
Flash and Export Gas Compressor uses a dry gas seal system to minimise loss of hydrocarbons via the shaft seals. Process gas is drawn from a dedicated tapping on the compressor casing and is passed through a filter (one on stream and one on standby) which removes liquids and particulates from the gas. The dry seal gas then flows to the inboard mounted compressor seals. Gas that leaks via the seals flows to the LP Flare via a backpressure control valve, which maintains a pressure of 4 barg in the chamber between inboard and outboard mounted seals. High pressure in the chamber vent to flare lines (indicating seal failure) will cause shutdown of the compressor.

The outboard mounted seals use nitrogen as seal gas. Any gas leakage via the outboard seals flows to atmosphere via a vent line, the nitrogen is supplied from the platform Inert Gas distribution system. High pressure in the chamber outboard of the outboard seals (indicating seal failure) will cause compressor shutdown.



Compressor Lube Oil and Seal Heating Oil Systems
A shaft driven lube oil pump which supplies lube oil to the various compressor bearings pumping the oil from a tank located in the compressor base frame via a cooling medium cooled exchanger and a pair of filters (one on stream one on standby). Prior to compressor start and during compressor shutdown, lube oil is supplied from an electric motor driven pump.

The lube oil tank contains an electric heater to maintain the lube oil temperature ready at about 62 C degree for compressor start, during periods when the compressor is shutdown

The lube oil tank also contains a separate but communicating reservoir of lube oil with its own electric heater and pump, which is used to circulate warm oil around the compressor dry gas seals to prevent liquid condensation within the seals during periods when the compressor is shutdown. This is the seal heating oil system.

The inner seals, which face the process side, are supplied with cleaned gas from compressors seals as gas. The outer seals and the bearing housings are supplied with dry and clean nitrogen produced by the nitrogen generator as buffer gas. Thus no secondary process gas leakage from the gas seals to the atmosphere can occur. The buffer gas flow both the suction and discharge side of the GS is measured, indicated on the related control system and alarmed for high limits. The buffer gas injected between the inner and outer seal and buffer pressure is kept constant 4 barg by pressure control valve. For maintaining this pressure nitrogen, flow rate is limited by high alarm level of 29 kg/h and low alarm level 12 kg/h.

Hope this is more informative. Thanks again for your help.

Regards,
Megalith