7 replies to this topic

[robbiejustice]

I am looking at the attached compression system  and I am considering the unmitigated consequences (ignoring all trips/interlocks) of the hot gas bypass (HGB) failing open on either 1^st or 2^nd stage compressor.

 

If the HGB spuriously opens then hot gas will be recycled resulting in higher and higher gas temperatures until there is a mechanical/material failure and loss of containment. The larger the recycle flow the quicker the failure will occur. Is this correct? Could the gas temperature theoretically rise indefinitely if there were no mechanical/material failures?

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[Sathya R]

If the HGB spuriously open, Compressor will not able to develop discharge pressure as it sees only the suction pressure. Hence the Compressor ratio will drop to near 1.0; and hence the discharge temperature will drop. There will not be any gas fresh gas coming compressor suction scrubber.

 

In the event of minor passing the compressor suction and discharge temperature will increase and the speed of the compressor will increase since its delivery is recycled. The suction pressure will increase. Providing TG at the HGB outlet can help to detect the passing.

 

If one of the HGB is passing the other stage will face the shortage of gas and will result surge.

[robbiejustice]

Thanks for getting back to me. So spurious opening of the HGB is not normally considered a safety concern by exceeding the design temperature and loss of containment.

 

Are all HGB valves sized so there is minimal pressure drop at stone wall flow? (I understand their size depends on downstream volumes and compressor run down characteristics.)

 

Say that there is some resistance via the HGB valve (in my system there certainly will be) so that the compression ratio settles at 1.2, would the suction/discharge temperatures rise until there is a mechanical /material failure?

[Sathya R]

Temperature rise can be checked with simulation. However compressor shall not be operated for prolonged period with HGB open. The intent of the HGBV is to provide means of flow during shutdown when the discharge or suction valve is closed.

[robbiejustice]

Yes, I understand the normal purpose of the HGB but I am looking at the unmitigated consequences were the HGB to spuriously fail open and the compressor were to continue running (There is a limit switch on the HGB valve that would shutdown the compressor but I have to assume this does not activate).

 

I am trying to understand if this will result in exceeding the design temperature without doing a dynamic simulation.

[fallah]

robbiejustice,

 

The compressor system you simply described would probably have a HH temperature alarm on the suction side can trip the compressor in the cases such as inadvertent opening of HGBP valve causes the hot gas going the from discharge to the suction of the compressor...

Edited by fallah, 06 March 2015 - 06:28 AM.

[robbiejustice]

Hi Fallah,

 

thanks for having a look.

 

Yes, there is a HH temperature trip BUT I am trying to understand the consequences if there was no protection whatsoever (or the protection failed)...

[fallah]

robbiejustice,

 

Then, considering the temperature limitations there might the compressor seal and labyrinths subject to damage due to the higher temperature than prespecified limit...also there would be an increament in compressor's power consumption...