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Chemical Process Technology

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Tuesday, October 21, 2008

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Centrifugal compressor is widely used to increase process fluid pressure head to meet process requirement. All centrifugal compressors are equipped with Anti-surge control valve (ASCV) for equipment protective purpose. In many events, This valve may also serve as capacity control valve in order to maintain a specific process parameter.

One of the question being discussed :
Should we locate compressor Anti-surge control (ASC) or Capacity Control (CC) Valve in vertical upward run ?

Good engineering practice is to install anti-surge control valve in horizontal run and no low pocket along the inlet and outlet of the control valve to avoid any possible liquid (or solid) accumulation which possibly lead to issues like corrosion, liquid slug, liquid freezing, solid plugging, etc.

Liquid Condensation & Accumulation Causing Corrosion
When ASCV / CCV in close position, vapor with mist liquid may diffuse along the inlet and outlet of ASCV/CCV. Due to heat loss to ambient and/or mist coalescence, mist is possible condensed and accumulated in low pocket and/or downstream of ASCV/CCV downstream on vertical upward run. If the fluid is wet and contains corrosive compounds i.e. Hydrogen Sulfide (H2S), Carbon Dioxide (CO2), low pocket and/or upward run where liquid is accumulated will experience acid corrosion. Pitting and crevice corrosion may be experienced.

Hydrate formation and/or Water Freezing
A fluid with hydrate former and wet, when there is liquid accumulated due to external ambient cooling, there is potential risk of hydrate formation and water freezing. This potential partially or totally clog the recycle line.

Fluid Possibly Crystallization and Solidify
A fluid contain compound possible crystallize and solidify when it is cooled by ambient, when the fluid is cooled by ambient in the stagnant section in recycle line, the fluid is possibly accumulated at low pocket and/or vertical upward run, downstream of ASCV/CCV and crystallized or solidified and plugged the recycle line

Liquid Slugging flow & Induced Vibration
ASCV/CCV is normally closed and liquid is accumulated along the recycle line due to external cooling. In the event of ASCV / CCV open to recycle vapor at compressor discharge back to suction, vapor will push the liquid column flow along the recycle line. This liquid column will be knocking at the ASCV/CCV, bend and tee. Severe slugging and piping vibration may occur and potential damage ASCV/CCV and line.

Thus, it is always recommended to install ASCV / CCV in horizontal run and no low pocket along the inlet and outlet of the control valve that potentially promote corrosion, slugging flow, liquid freezing and solid plugging.

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posted by Webworm, 1:19 PM


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