Double block and bleed - Is a vent required?
Double block and bleed - Is a vent required?
(OP)
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?





RE: Double block and bleed - Is a vent required?
RE: Double block and bleed - Is a vent required?
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
RE: Double block and bleed - Is a vent required?
"Nobody expects the Spanish Inquisition! "
RE: Double block and bleed - Is a vent required?
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
RE: Double block and bleed - Is a vent required?
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.