Double Block and Bleed Valves

Block and bleed valves are configured so that flow through the valve is blocked at both the inlet and outlet and a small port is provided to drain or depressurize the space between. This configuration is often required to isolate high-pressure sections of a system to facilitate safe maintenance or rapid depressurization. These valves are also referred to as double block and bleed valves.

Double Block and Bleed:

First of all this "term" (Double Block and Bleed) does not normally refer to a single valve. It is most often an "assembly" of piping components that provide positive shut-off of a system. This assembly is sometimes simplified on the P&IDwith a single block valve symbol and "DB&B" printed under the valve. When translated to real life the piping designer needs to know and understand the true make-up. This assembly is normally made up of a line size/line spec block valve, a short line size spool piece with a small size vent or drain valve then another line size/line spec block valve. The small size vent or drain (bleed) valve is normally a line spec valve but 3/4" or 1".

The purpose of a DB&B is safety. Safety in an operation process plant is first, last and always. The use of DB&B is used on all connected lines to provide a positive isolation of an individual piece of equipment when a person must do work. It is also used on the all of the Battery Limit isolation points to shut down a complete process unit for maintenance.

DB&B valve assemblities would not normally be fitted with a motor operator, however it could be done.

I hope this helps.

Block and bleed and double block and bleed are both designed for safety. Usually used in high pressure or toxic service to isolate some vessel or equipment to make it safe to work on.

A block and bleed arrangement usually consists of a line size "block" valve with a smaller "bleed" valve or port to relieve the pressure either upstream or downstream of the block valve, depending on which needs to be depressured. These can either be assembled from individual components or may be purchassed as a single unit. For larger systems they are usually assembled from individual components and for smaller systems they are usually purchassed as a single unit.

For the single unit variety, the bleed port on the valve may or may not be provided with a separate valve. Often, the bleed port is arranged such that it is open when the valve is closed. This is most typical with a ball type block valve.

In the case of the DB&B, there are two block valves with the bleed between. These may be combined into a single valve as well, as in the case of a double seat ball valve with the bleed being open to the ball cavity when the block valve is in the closed position. These are more common in the smaller pipe sizes. In the larger pipe sizes, the bleed on a ball valve is often connected to a pressure relief valve to prevent pressure buildup in the captured volume in the ball from damaging the seats.

For absolute protection, sometimes there is a flanged and removable section placed between the double block valves, or a blind flange may be installed. It is imperative to relieve the pressure from the system before removing the safety spool or installing a blind flange to prevent pressure release when the flanges are broken to remove the spool or install the blind.

All of these arrangements can be motorized by the application of an actuator to each valve stem or by some mechanical linkage. In the case of a DB&B or a B&B that uses a single valve arrangement with a port rather than a bleed valve, there would only be a single actuator.

What it "looks like" depends on what valve arrangement you have and on what the chosen actuator or actuators looks like.

Different plants have different standards on where and when to use each type, but it should always be applied with safety and good engineering practice in mind. Any of the larger valve manufacturers can help you with the arrangement.

The previous comments have it right for the most part.

Double block and bleed valves are usually used in industrial gaseous fuel supply systems (natural gas / synthetic fuel gas especially) to provide positive isolation of fuel from a combustion process with multiple sources of ignition (multiple burner systems). These are almost always assemblies of three valves and one operator. The operator is normally remote controlled to allow an automatic control (or remote control) to switch from blocking to supplying fuel.

The bleed valve is connected to a vent to exhaust any trapped fuel gas and to vent fuel gas in the case of an upstream block valve failure or seat leakage. If the fuel gas is heavier than air, the vent really needs to be a 'drain' and the exhanusting gases will need to be actively managed to prevent accumulation in low regions.

Steve

Only point that possibly could be added is that the block valves should be a fast closing, normally shut valve where as the bleed (or vent valve) should be a normally open valve.

I have worked with configurations where the vent valve closes first and a PLC looks for any pressure rise in the space locked in between the three valves as an additional safety check.

Normally this is used in Gas trains of Burners. There are 2 nos sets of Main Valves in series (Solenoid/motorized) with normally close connection(to open on power) with a small 1" Solenoid Valve in the middle with normally close connection (to open on power off).

When the Main panel is shut down, obviously the Main Valves too are off the 1" bleed valve remains open to vent any gas leakage . this vent is to be taken out off the roof or the plant.

This is just a safeguard against any accidental gas inside the machine during the night/shutdown period lest, there is a burner backfire due to accumulated gas far above the purge timer/blower capacity. This safeguards against explosion.

In some countries this is standard & compulsary in others this is not.

To further safeguard you must have a gas filter prior the gas train.