Double block and bleed - Is a vent required?

[Kajtek08]

If you have a piping configuration where you have two gate valves a spool in between and a drain, and the spool needs to be drained when the valves are closed, would you also need a vent for draining to prevent a vacuum? Does the spool length determine if you need a vent?

 

[cvg]

depends - (fluid, viscosity, size of drain hole, size of pipe) but all things being equal an air vent will speed things up

 

[LittleInch]

Yes you would need a vent unless your drain was an open drain and >30% of the header diameter otherwise the air won't glug back inot the pipe so it's reall about D of pipe vs D of drain. The longer the spool is the longer it will take to drain this way wthout a vent and you may end up with some left in it. Without a drain you won't get a vacuum as you won't have enough of a head to pull a vacuum unless you've got a vertical spool of several metres.

The vent can be quite small compared to the drain, say 50% or less of the diam or even a screwed plug like people use for draining and venting hydrotest water, but a proper valve is better.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[davefitz]

Usually a "block and bleed" is configured so that the user can view the exhaust of the bleed, so as to confirm there is no pressurization of the downstream system that is being maintained. If the fluid is a dangerous fluid, then a PI may be added in lieu of a viewable exhuast stream.

"Nobody expects the Spanish Inquisition! "

 

[LittleInch]

The title says block and bleed, where the bleed is a drain of liquid when pressure is present, especially an ongoing passing of one of the valve, which as davefitz righlty says needs a view of the drain to establish wheter flow is present or not. Generally a complete draining of the spool in this instance is non preferred as it needs filling again on re-start of flow and results in loss of product to the drain system. Therefore can you advise why you think that a full draining of the spool is required and on what sort of frequency?

For maintenence however a full draining of the spool via a connected drain is a safe and sensible thing to provide for and then a vent is required, but may only be opened every few years, if at all.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

System looks like this, right?

Tank > ===X (valve 1) ============X (valve 2) =====>.... (system)
...............................................|
...............................................X.. (drain
0
...............................................|

So, if you're going to work on the system downstream of valve 2, how will you drain and vent that part of the system downstream of valve 2?
Now, Close valve 1 to isolate the tank. Can you still drain and vent the system, AND drain and vent the portion of the pipe between valve 1 and valve 2 if you crack open valve 2? Remember, block and bleed requirements are to protect you (your workers) and the system from the pressure (fluids) inside the tank, NOT protect the tank from the system.