7 replies to this topic

[AdamMM]

Hi,

 

I am new in the industry and need some advice:

My company is running an offshore production which oil and ascociated gas are treated and compressed by a leased facility, operated by others.

 

During start up and ramping up, we estimate that for the first 3 month we only have a limited amount of ascociated gas.

Gas Compressor type : Centrifugal, 3 stages (2 trainx 50% capacity)

Design gas compression capacity : Total feed gas of 70 MMscfd, 35 MMscfd design capacity/train

Required pressure of gas export header is 100 barg max

 

The plan was to run one of the  compressor train once we see enough gas to run it with full recycle, matching the design capacity:

Feed Gas : 19 MMscfd with recylce of 16 MMscfd (19 +16) ~ 35 MMscfd

 

Our contractor seems reluctant and said that the operations will be troublesome with lots of process trips because of high temperature and vibration on our first meeting explaining this plan. I want to be fully equiped with understanding of the problem first before meeting with them again.

 

The thing is we have a recyle line from the last stage discharge back to the 1st stage, upstream the cooler and scrubber that technicaly adequate to reduce the temperature back to the 1st stage feed gas temperature. Everything seems fine to me, i do not understand where their concern comes. Did I missed something?

 

Thanks,

Mada Analaum

 

 

 

[Bobby Strain]

Best you should ask your contractor to explain. After all, you are paying him for his knowledge as well as his time.

 

Bobby

[PingPong]

Moreover outsiders like us do not have the compressor datasheet, the compressor curves, the P&ID and the proceess control description of the system, so we cannot really advice you properly.

 

The thing is we have a recyle line from the last stage discharge back to the 1st stage, upstream the cooler and scrubber

Is that recycle line the anti surge line? That line is not meant to be used continuously for capacity control, but only to kick in in case of insufficient process gas flow rate to prevent surging of the compressor. It is meant to protect the compressor.

 

If that 19 MMscfd is close to, or more likely, slightly below, the surge limit of the compressor, it could mean that running on that forward capacity makes the anti surge kick in, and out, and in, and out, et cetera .... , leading to a very unstable operation. Maybe that is what your contractor is worried about.

 

I suggest you get acquainted with the theory of compressor surge limit, anti-surge control, and obtain a copy of the compressor curves, which should indicate the surge limit of this particular compressor.

[Rush123]

I agree with asking the contractor to explain their side. However, if you want to arm yourself with information to prepare for that conversation, find out the brand of the compressor (or any centrifugal compressor vendor) and ask them the question directly. That can at least give you another avenue to get info.

 

But also search google for 'compressor surge theory' or something similar. There's a welath of info there. I came across some sometime ago for a Rolls Royce compressor which explained things pretty well. If i can find it, i'll upload it here. Admittedly, I can't remember much about it at the moment.

[curious_cat]

The thing is we have a recyle line from the last stage discharge back to the 1st stage, upstream the cooler and scrubber

Is that recycle line the anti surge line? That line is not meant to be used continuously for capacity control, but only to kick in in case of insufficient process gas flow rate to prevent surging of the compressor. It is meant to protect the compressor.

 

If that 19 MMscfd is close to, or more likely, slightly below, the surge limit of the compressor, it could mean that running on that forward capacity makes the anti surge kick in, and out, and in, and out, et cetera .... , leading to a very unstable operation. Maybe that is what your contractor is worried about.

 

Possibly a naive question but if such a situation arises where  flows are below the surge limit of an oversized compressor what are typical solutions?  Can an additional recycle line (with cooling & a pressure reduction station) be fitted parallel to the anti-surge to boost feed rate to compressor? 

Edited by curious_cat, 14 November 2013 - 01:58 AM.

[curious_cat]

 

The thing is we have a recyle line from the last stage discharge back to the 1st stage, upstream the cooler and scrubber that technicaly adequate to reduce the temperature back to the 1st stage feed gas temperature. 

 

 

If the recycle line is upstream of cooler how will it reduce temperature? Am I confusing something?

[S.AHMAD]

.. get the perormance curve of the compressor ...

[AdamMM]

Thanks all,

 

I did what you all recommend and learn more on compressor curve. The meeting went well.

It turns out that this is just a matter of commitment and convenient.

Their compressor has almost full turndown (90%) that once we have enough fuel gas and a little bit of flow, we could start the compressor.

 But they do not want to promise that the compressor will be reliable enough to run on recycle whenever we have lower flow.

 

Thank you so much for all of your kind guidance