centrifugal compressor primary vent

[rQuestionEngineering]

Hi, I am curious to know why at centrifugal compressor primary vents, there are sometimes orifice and sometimes needle valves designed ?
During commissioning, these orifice had to be adjusted to get the values obtained during the factory acceptance test.

Hope somebody can clear my confusion as many so called experts told me its for suppliers to make money , save production cost etc ...
Surely these are design to a certain logical guideline or standard.

Thanks in advance for contributors.

 

[rotw]

Needle valve is used when application have suction pressure which is "low" and the control of DGS supply pressure is based on flare control (sensing primary vent) ; during commissioning it is necessary to adjust the delta P over the valve so to avoid reverse pressure problems and potential damages to seal. For application where suction pressure is "high", orifice is commonly installed combined with backpressure control valve.

 

[georgeverghese]

The second arrangment is much better than the first - the second maintains constant flow by action of dPIC on dPCV even while compressor suction pressure varies, while the first cannot do this, since there is only a fixed needle valve.

The 2nd arrangement is the typical standard arrangement.

In many compressors, suction pressure will vary somewhat , and will go even higher on compressor shutdown and settleout. Seal gas flow should be maintained constant for all these variations.

 

[rQuestionEngineering]

Hi ROTW, i rechecked the P&ID and found that your explanation is very accurate. The needle valve is for a low pressure application.