Centrifugal Compressor Seal Balance versus gas balance

[Sawsan311]

Dear All,

I would like to inquire on the gas balance within the same compressor section and which involves an external piping from the high pressure stages to the low pressure stages.. does this balance line contributes in the leakages across the labyrinth within the compressor stage and hence for multistage compressor located within the same casing.. we would have a combined settle-out pressure. Additionally, does this seal balance line serves as an additional measure for balancing the thrust of the machine and hence reduces the thrust load on the balance piston in (compound arrangements) or division walls in back to back arrangements?

My other question is that for measuring the differential pressure between the seal gas port at the driven and non driven end of the compressors, will the other end of the measurement be attached to the above balance line?

Last question, for a two stage compressor located within two separate casing each, I saw on the P&ID additional line similar to the gas balance only in the 2nd stage .. but is labelled as seal balance.. do you think is an external seal stream where the source of process gas from intermediate crossover is taken to seal the casing of the 2nd stage?

Thanks

regards,

 

[rotw]

Par. 1/ It contributes as the external seal gas re-circulation line goes from discharge back to suction and count toward total flow compressed through impellers arrangement. It is the purpose - not a result, of the external seal gas line is to balance the axial load and thus reduce thrust bearing size at the cost of some extra absorbed power (except for very high pressure machines) plus generally limited temperature increase at compressor inlet. Bigger thrust bearing implies extra lube oil requirements, has adverse rotor-dynamic effects it increases bearing span when mounted onboard or constitutes overhung load when outboard.

Par. 2/ Depends on seal gas configuration plan being applied. Reference pressure can be taken otherwise at other location for low pressure control application, e.g. flare control. Refer to API 614 for various seal gas schemes ; vendors have their methods that might have some peculiarities too.

Part.3/ Question need to be re-framed and/or a P&ID posted.

 

[AnhsirkT]

it is taken from downstream of balancing piston after labyrinth and goes to suction side to balance the axile thrust forces.
ans for seal gas supply pressure they have taken as reference from this balance line .

 

[rotw]

ans for seal gas supply pressure they have taken as reference from this balance line .
No. Not always.
High suction pressure system, yes seal gas injection is controlled by differential (constant delta) between seal inlet pressure and balancing chamber pressure.
Low suction pressure, for example flare control, primary vent pressure is used as reference. You need positive pressure to avoid pressure reversal on seal rings.

If you plan an escape, you must succeed as if you fail, you will be punished for trying. Never say or write down your plan. Heart is the only place where secrecy is granted.

 

[AnhsirkT]

Yes agree with you
@rotw
But let's say reference taken from balance line and compressor is on settled out condition (pressure higher than normal like 7bar)
Then what would be the value for reference pressure ?
It would be more than 7bar?
What pressure we required for seal ? It should be higher tahn reference.

 

[AnhsirkT]

Is tehre any chance of two different line one for balance line one reference line (other than flare ) in casing?

And for turbine also I have 3 balnce line in turbine casing what is purpose of 3?
One is not sufficient

 

[rotw]

A variant of this is by signal selection - one controller maintains constant deltaP over flare back pressure while another one maintains minimum deltaP above suction pressure.

If you plan an escape, you must succeed as if you fail, you will be punished for trying. Never say or write down your plan. Heart is the only place where secrecy is granted.

 

[Sawsan311]

Thank you Mr.Rotw and Anihsirk, I actually have the configuration of seal gas pressure control by measuring the pressure of the supply seal line and the reference seal port on the balance line. Failure of the seal gas system can be detected by either high differential pressure across the primary seal gas vent, high pressure , or low pressure on the seal gas vent on either the driven end or the non driven end of the compressor sides.

Thanks

Regards,

 

[Sawsan311]

Dear All,

For a two stage compressor where each stage is located in two separate casing sections, dor the 2nd stage would seal gas supply be from:

- discharge of 2nd stage
- crossover line between 1st stage seal and second stage.

The two stages are run by the same shaft, so although each casing is provided with its own seal system, however, the driven end and non driven ends would be common for both compressor casing sections. Would there be an overall crossover balance line from downstream the balance piston to the other end of the compressor at the non driven end. And hence this line can serve as a source of seal gas supply to the 2nd higher pressure stage casing.

Thanks

regards,

 

[AnhsirkT]

Its totally depends on your compressor design
Like if it is back to back (means suction from both end of casing) than no need of balance line
By reference line only you can check for seal gas supply
If you have high pressure at one end of casing than need of balance line
And there also a pdi b/w balnce line and reference line.
For a common balance line not looking good bcz you are equilizing pressure from hp to lp
But for another side of casing using it for reference may harmful (this may have higher pressure than balance line

Do not think twice

 

[AnhsirkT]

For two casing singal balnce line can't used as reference for all side

Do not think twice

 

[Sawsan311]

thanks Mr.AnhsirkT, can you please elaborate on the reference line? what do you mean by this line and how it is different from the balance line?

and if I want to seal the higher pressure stage, what is the common source of seal gas supply other than the discharge of the 2nd stage itself?

thanks

regards,

 

[AnhsirkT]

You can used some external supply from any booster or small compressor to supply the seal gas at higher pressure.
Reference line is tapping from casing end(after the impeller)after librinth used to supply the seal more than reference line pressure.
Balance line is used to balance the axile thrust .
If compressor is able to balance the thrust then no need of this balance line like back to back compressor ( suction from both end and discharge in mid)
Its tapping after the piston to suction side
Most of the time balance line and reference line pressure are same.
Also a pdi is provided b/w reference and balance line to know about the flow through balance line (if more flow chance of large clearance at piston)


Do not think twice

 

[Sawsan311]

Thank you Mr.AnhsirkT..