chemical & process technology_ combine anti-surge control (asc) & capacity control (cc)...

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Combine Anti-surge control (ASC) & Capacity Control(CC) Functions ?

Saturday, October 18, 2008

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

Centrifugal compressor is widely used to increase process fluidpressure head to meet process requirement. All centrifugalcompressors are equipped with Anti-surge control valve (ASCV) forequipment protective purpose. In many events, This valve may alsoserve as capacity control valve in order to maintain a specific

process parameter.

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

Capacity Control valve (CCV) is normally used to maintain a process parameter of asystem by recycling excess gas back to compressor suction. Typical process parameters ofa 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 orcombine function control valve ?"

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There are several factors lead to separate control valve fordedicate function.

Control Valve CharacteristicThis is one of the most important factor to determine if separatecontrol valve is required. For an anti-surge control purpose, thebest control valve characteristic is quick opening. However, forcapacity control purpose, an equal percentage is mostly used.Thus, this could easy lead to separate control valve with differentcharacteristics used for dedicated function. However, in manyoccasion, the valve with equal percentage may also serve thepurpose of anti-surge control. This is very much subject tocompressor characteristic, system operating pressure, system volume, etc. Only way toprove if a single control serve two purpose is Dynamic simulation.

SafetyIn many event, the Anti-surge control system together with anti-surge control valve are withina commercial package of compressor. The purpose is to have single point coordination andresponsibility and to avoid unnecessary interference. However, using control valve

► Capacity ► Gas Flow ► Compressor ► PressureUseful Documents Related to Pressure Relief Valve(PRV) - Part 1Pressure Relief Valve (PRV) has been widely use inoverpressure protection of pressure containmentequipment. Regardless of vessel/column/drum/tankdesigned in accordance to any pressure vessel codee.g. ASME, JIS, BS, GB, etc,...

Chemical & Process Technology: Combine Anti-surge control (ASC) &... http://webwormcpt.blogspot.com/2008/10/combine-anti-surge-control-a...

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dedicated to Anti-surge control purpose for capacity control purpose would lead tointerference and additional signal managing the control valve. Some company safetyprinciple do not allow an protection function used as control function as well. Thus, aseparate / dedicate control valve is used. In many recent serious improvement in technologyand working method, one control valve serving dual purposes are widely implementedrecently.

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

CostDedicated 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 combinefunction control valve is subject to case-by-case basis. No one key fit all locks...

Related TopicFAQ Related to Control ValvesUseful Documents Related to Control ValveFREE & Reliable Control Valve Sizing SoftwareUnderstand Droop Effect in Self-regulated Control ValveWELKER Jet Contorl Valve HandbookHow to apply valve equation in HYSYS Depressuring ?Potential Problem associate with Double NRV in Series within a Line12 Features required for Shutdown Valve (SDV)

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