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Chemical Process Technology

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Saturday, October 18, 2008

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Control valve is widely used in Oil & gas, refinery, Petrochemical and chemical plant for control purposes of operating parameters i.e temperature, pressure, level, etc. Several articles as compiled in "Useful Documents Related to Control Valve" are pretty useful to many process and chemical engineers.

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.

Anti-surge control valve (ASCV) is one of required equipment protective function for centrifugal compressor. It is used to protect centrifugal compressor from running in surge region, a phenomenon where centrifugal compressor discharge pressure high enough to results reverse flow follow by severe vibration in the compressor chamber. The Anti-surge control valve is recycling discharge gas back to suction to minimize differential pressure across the centrifugal compressor.

Capacity Control valve (CCV) is normally used to maintain a process parameter of a system by recycling excess gas back to compressor suction. Typical process parameters of a system are fix suction pressure, fix discharge pressure and fix differential pressure.

One of the common question raised is :
Should we provide dedicated control valve for anti-surge and capacity control purpose or combine function control valve ?"

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There are several factors lead to separate control valve for dedicate function.

Control Valve Characteristic
This is one of the most important factor to determine if separate control valve is required. For an anti-surge control purpose, the best control valve characteristic is quick opening. However, for capacity control purpose, an equal percentage is mostly used. Thus, this could easy lead to separate control valve with different characteristics used for dedicated function. However, in many occasion, the valve with equal percentage may also serve the purpose of anti-surge control. This is very much subject to compressor characteristic, system operating pressure, system volume, etc. Only way to prove if a single control serve two purpose is Dynamic simulation.

Safety
In many event, the Anti-surge control system together with anti-surge control valve are within a commercial package of compressor. The purpose is to have single point coordination and responsibility and to avoid unnecessary interference. However, using control valve dedicated to Anti-surge control purpose for capacity control purpose would lead to interference and additional signal managing the control valve. Some company safety principle do not allow an protection function used as control function as well. Thus, a separate / dedicate control valve is used. In many recent serious improvement in technology and working method, one control valve serving dual purposes are widely implemented recently.

Wear & Tear
For a control valve serving dual purposes, the control valve may continues in service and this promote wear and tear of the control valve. This would increase the downtime and availability of this control valve. Doubling the control valves will recover the availability again.

Cost
Dedicated control valve for ASCV and CCV is obviously require higher capital investment.

Used of dedicated control valve for anti-surge and capacity control purpose or combine function control valve is subject to case-by-case basis. No one key fit all locks...

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posted by Webworm, 11:55 PM

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