Nottingham,
The application of double block and bleed valves has usually been client driven in my experience.  I am not aware of any code or standard that requires the double block and bleed.

On my current project we are using two types of single body, double block and bleed valves.  For ANSI class 600 and below we use a twin seal plug valve.  For ANSI class 1500 and above we use an expanding slab gate valve.

A double block & bleed is not a type of valve, but a valve arrangement. It consists of 2 gate valves in series (this gives you your double block) and typically a 3/4" gate valve stubbed in between the two gate valves (this gives you your bleed).

This valve arrangement is meant to give you positive isolation. The 2 gate valves in series are closed and the pipe segment between them is depressured via the 3/4" bleed valve. A pressure gauge can be connected to the bleed valve or it can be connected to flare. If the first block valve passed you can tell so by the pressure gauge or its contents can be routed to flare.

Note: A bouble block and bleed is only consider positive isolation when the pipe segment between the 2 block valves can relieve continuously to a lower pressure than whats being contained by the block valves.

Hope this helps!

Nottingham, whilst Double Block & Bleed is an arrangement it can be acheived using a valve arrangement of twoo isolation valves in series with a bleed isolation valve in between, or using a single bodies double block & bleed valve.
Certainly within certain provinces in Canada, depending on the province their is legislation permitting or not permitting certain DBB single bodied arrangements. As Andrew TX states its usually client driven, based on the companies isolation philosophy and HAZOP studys performed.

For example BP have a preference for Double Expanding Gate valves, then Solid Slab Gate Valves, and thirdly Double Piston effect Ball Valves onyl when space cannot permit the use of the other two. Expanding Gate produces a mechanical seal on both sealing surfaces, where as solid slab relies on seat springs and line pressure to seal.

For smaller instrument connections Integral DBB's (i.e. Oliver, Galperti, Kenmac type) are frequently used to save weight (offshore) and uses the second block valve within the unit as the first instrument isolation (check out EEMUA 182).

double block and bleed is required by code in section I (boiler ) service if there are 2 or more boilers interconnected on a common steam line and the boilers have a manway in the steam drum. This is to avoid cooking the maintenance person that enters the steam drum on the idle boiler .  A similar philosphy would be prudent for any pressure vessel that a person can enter , if the vessel is interconnected to another active system which has dangerous fluids.

For safe working in a confined space such as a vessel, it may be necessary to have such a valve installation. The Statutory Authority responsible for Occupational Health and Safety in your area is a good start. Australian Standard AS2865 titled "Safe working in a confined space" makes reference to double block and bleed arrangements and their use.

A double block and bleed valve is used where positive isolation between two processes is required, but where the piping systems must occasionally be connected. It is almost always a gate valve with discs that can both be positively seated on each seating surface. A good example is a double disc parallel seat gate valve that uses a wedge arrangement (actuated by the downward push of the stem) to push both discs against their respective seats. The bleed is a drain drilled at the bottom of the run so as to detect leakage as well as prevent pressure build-up that might be great enough to push the fluid into the adjacent process.

A flexible wedge valve can be used as a db&b, but only if it has been specifically designed to do so. The force of the stem must not only cause the two discs to conform to the seat plane, but also with sufficient force to seal the seat against leakage and dP.

Double block and bleed arrangements, where there are two conventional isolation valves with a drain between, are required when a boiler is being worked on and is attached to a process steam line supplied by (an)other(s). A single valve cannot be used even if it is designed as a double block and bleed.

I would never want to enter a vessel isolated by a double block and bleed, there have been too many people killed when the double block and bleed didn't do its job because the bleeder was plugged and the double block and bleed was lost.

Sure, it's possible to write a procedure with a double block and bleed that should work.  For my own comfort, I'd want that vessel either blinded or it's physical disconnected so I know nothing is going to get in there when someone is in there.  

My experience is that most facilites now require blinding or physical disconnects, double block and bleeds as isolation for confined spaces are either not acceptable OR if they are allowed, are just about impossible to get buy-off to do.

Actually, there apparently are some plants thata have 2 or more boilers in service, interconnected, that do not have the double block and bleed.

Prior to about 1995, the ASME code sect I required" one stop valve plus one non-return valve". Apparently, some vendors interpreted a non-return valve as a  simple check valve, while other more prudent opinions were that a non-return valve is a stop/check valve combo. In any case, several medium sized boilers were installed in the period 1985-1995 that do not have effective double block and bleed  when in combined/interconnected service.

Since 1996, Section I eliminated the wording "non-retun valve" and now states the need for 2 stop valves plus telltale bleed.

Double Block and bleed Valves can be explained as below.

*. A valve which seals on both upstream and Downstream Seat.
*. both the setas work independently of each other. Thus two different mediums can be separated by the valve.
*. The Fluid trapped in the body cavity could be released through the Bleed connection/Valve.

We use Double Block and Bleed plug valves with two plugs in one body. We can replace a single plug valve with one of these valves to create a double block and bleed without piping modifications because the face to face dimension is the same as a single plug of the same flange rating. They come in most flange ratings. The bleed port can be purchased with 1/2" or 3/4" threaded or flanged. Some manufactures provide valve stems on opposite sides and others with stems on the same side. These valves are called many names, but we prefer to call them Tandem Plug Valve so it will not be confused with a valve arrangement of to two block valves and a bleed valve. Here is a few Websites.
http://www.nordstromaudco.com/products/features.asp?id=56
http://www.bch.dk/

First, I have enjoyed reading all the answers.
I am involved in Instrumentation and Interlock systems.  My experience with double block and bleed valves has been on the fuel gas line on boilers, furnaces, and incinerators.  When the boiler shuts down or trips the two block valves close and the vent opens.

The first time I came accross this was back in 1976.  A package boiler had this setup. The valves were called Maxon valves.  The boiler interlock panel supplied 120 volts to the valves which allowed the operator to latch the block valves open and the vent closed.  When the unit shutdown, the 120 volts were removed from the valves allowing the blocks to close and the vent to open.  Since them I have run into a number of different configurations.
Mark

For Double block and bleed (DB&B), please go to the homepage www.bch.dk, who is the only supplier in the world of this arrangement in one single body (as it has been patented).  You will have DB&B with same face-to-face dimension as for one single valve, thus, saving space.  Furthermore, this DB&B, also called "twin plug valve", is cheaper and lighter that the normal DB&B arrangement.

I agree with most here, double block and bleed is a client driven requirement(in my experience). I first ran into it with custody transfer of crude oil, & recently for safety reason on high pressure steam lines. I too would not enter a confined space with just a double block and bleed valve., here in the oil fields of Calif. we require spec. & pad blinds before entry is permitted. Good Luck! ...Mark