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Thursday, January 17, 2008

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This post is just to capture what has been discussed recently on "Why a globe valve is located downstream of manual block valve on drain line ?" in Eng-Tips forum.



A maintenance drain line is typically consists of a block valve (ball or gate subject to service) and a globe valve. Block valve is provided for isolation purposes while globe valve for throttling to avoid excessive flow to closed drain system.

Drain minimum line size is typically 2" (typical but may be different elsewhere). Sudden opening of 2" block valve would results excessive flow to close drain system and potentially leads to high noise level, acoustically induced vibration, erosion-corrosion, etc. A globe valve is provided on the drain line to limit sudden large flow into closed drain line.

Other than above mentioned consequences, pressure drop across globe valve will results low temperature (lower than zero degC) due to JT effect. Manual block valve located downstream of globe valve would experience similar temperature and leads to ice form at valve body, stem, shaft, etc. This would disable re-closure of the manual block valve. Thus, it is always advisable to locate manual block valve upstream of globe valve.

In earlier post (Why Restriction Orifice is some distance from Blowdown valve ?), discussed about the Blowdown valve and restriction orifice. Similarly the cold temperature may extends from globe valve to block valve which located upstream. Thus, it is always advisable to provide minimum 600 mm between block valve and globe valve.

As good operation practices, it is always advisable to minimize excessive flow through proper draining procedures. Normally the draining activities would be carried out in the following sequences :
  • transfer liquid to next drum until drum to be drained is at minimum level
  • depressurized pressure to about 2-3 barg
  • then only drain the inventory
  • to accelerate draining activity, nitrogen would be used to maintain a good positive pressure (2-3 barg) in the drum
Above is only typical arrangement for oil & gas plant. However, it may not be adopted in other chemical and industry plant.

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posted by Webworm, 11:36 AM

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