isolation philosophy for drain lines 1

[mohamem]

i ask about if i want to install double block(ball & globe valves ) on drain connection of high pressure service , so what is the correct configuration, ball then globe or the opposite and why?

 

[pleckner]

A globe valve is typically used for throttling control so if you want to throttle the drain, then that is where the globe should be. If you just want to open the line and let it drain then you don't need a globe and just use three balls.

 

[mohamem]

i need globe but i ask it should be after ball or before?

 

[pleckner]

Why globe?

 

[mohamem]

because i need to throttle the drain to low preesure recovery system but i need the correct configuratoin

 

[JoeWong88]

Typically in oil & gas, a drain will consist a block valve (ball or gate subject to service, ball in most cases) and a globe valve.

Block valve for isolation purposes while globe valve for thlottling to avoid excessive flow (gas pushing flashing liquid) to closed drain system. Drain is normally minimum 2" (general practice). Sudden opening of 2" block valve would results excessive flow to close drain system. It subsequently leads to noise, vibration, erosion-corrosion, etc.

If order to minimize excessive flow, normally the drum to be drained would be
- 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

Block valve shall be located minimum distance from isolated equipment and globe valve is normally located downstream of block valve. Reason being throttling process leads to subzero temperature due to JT effect (for certain service only), potential icing form at the block valve stem and unable to close the block valve if required (in case of block valve downstream of globe valve). In addition, 600 minimum distance is maintain between block and globe (in case globe valve is located downstream of block valve).

Above is just one way of doing thing. Different company & industry might do it differently.

Hope this help.

 

[mohamem]

similar question for blow down valve
the manual isolation valve downstream the RO or upstream(my configuration is on/off BDV , manual valve & RO

 

[JoeWong88]

Please read this...Why Restriction Orifice is some distance from Blowdown valve ?

HTH

 

[itdepends]

I'd also put the throttling valve downstream of the ball valve- that way you can use your ball (isolation valve) to isolate the drain line if the throttling valve needs to be replaced (which should happen more often than the block valve will need to be replaced)