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Chemical Process Technology

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Wednesday, December 26, 2007

A few phenomena in Restriction orifice (RO) has been discussed earlier in "A refresh to Process Engineer on few phenomena in restriction orifice". Even though RO is just to limit the flow passing across but it has been used in many application in different manners.

Minimize Gas/Liquid blowby Rate to Downstream Equipment
This common apply in High pressure and low pressure interface e.g. liquid drain from high pressure vessel to low pressure vessel. A liquid control valve (LCV) is maintaining the level. A RO is provided downstream of the LCV. During normal operation, the flow passing the the LCV and RO is small and RO has minimum impact to operation of LCV. In the event, level controller failure caused LCV FULL open, large amount of liquid follow by gas blowby will pass through the LCV and RO will limit to flow rate and pressure relief load of downstream system.

Avoid large amount liquid overload downstream vessel
Similar to above condition but the context here is to limit liquid flow to downstream equipment and to buy time for operator intervention.

Minimize Reboiling Load And Relieving Load
One of the application is to limit the heating medium flow into reboiler in the event of heating medium control valve failed on full open position. RO possibly limiting vapor generation rate and relief load of PRV on the associated column.

Centrifugal Pump Minimum Flow Protection
Centrifugal pump will experience cavitation at flow lower than allowance minimum flow of the pump. To avoid pump cavitation, a permanent recycle line with a restriction orifice with opening size for minimum flow of the pump will be installed at pump discharge.

Reduce Vent and Flare capacity
The inventory of an Oil and gas facilities shall be evacuated to a safe level within a limited time (i,e 15 minutes per APi Std 521). Thus, automatic blowdown system is provided to depressure the system. A restriction orifice provided downstream of blowdown valve (BDV) will limit the flow in to header and flare system but within the allowable time limit. The limited flow rate will avoid choked flow in the flare subheader/header and this will significantly reduce vent and flare load and ultimately CAPEX of waste disposal system.

Vent and Flare Purging
Vent and Flare header shall be kept at positive pressure to avoid air ingress into the vent and flare system. Inert gas or fuel gas purging is used to purge and maintain possitive pressure. A RO will be used to provided constant flow.

Equipment Protection via Controlled Blowdown
This application is quite similar to pump minimum flow protection but it normally apply in vapor system like compressor, Mole sieve bed, etc. Similar to automatic emergency blowdown system, compressor and mole sieve shall be depressured within safe time limit. Howver, large blowdown rate potentially damage the compressor seal and fluidization of mole sieve bed. Thus, the blowdown rate shall be limited to avoid occurrence of above events. RO is commonly apply.

Pump Seal Cooling
Pump seal shall be cooled either by the process itself or external cooling medium such as demineralizeds water, service water, etc. A Ro is common provided at the feeding line to ensure constant cooling medium supply for pump seal cooling

Above are simple listing of application of restriction orifice in different manner. There may be more applications in different plant. Why not share yours with us here ?

Update : May 09, 2009

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posted by Webworm, 9:38 PM


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