Double block and bleed valve is a single valve which comprises of two seating surfaces. It can provide a seal against pressure from both valve ends when the valve is closed. The basic operating feature of this valve is that it bleeds or vents the cavity between the seating surfaces. The double block and bleed valve was designed so that the need of bolting down individual valves can be avoided. A single valve can give the dual isolation that reduces the cost by over 60%.
There is a bleed valve along with two block valves that act as a manifold or a uni. This assembly is installed to provide a positive isolation. The double block and bleed valves are used for hazardous or toxic fluid processes or for the high- pressure systems and many more critical processes. These valves can bear a pressure range of usually 10,000 psig and a temperature range of generally -70.6°F to +1,002°F.
The double block and bleed valve finds applications in chemical injection connection, gauge isolation, instrument drain and chemical seal isolation. The double block and bleed valves need much lesser weight and space but can do the job of three different valves i.e. two isolation valves and one drain valve.
Some readers might be confused about when are the double block and bleed valves required. So, basically, this valve is used when a vent valve is required in between two isolation methods.
The double block and bleed valves are used to attain a positive isolation while you the maintenance activities are being performed in live process plant. The positive isolation can be performed during the shutdown, maintenance or repairing activities. At first, both block valves are closed. Afterwards, the shutdown side is vented to prepare for maintenance. The process fluid trapped in the space between two block valves is relieved by the bleed valve assembly. In this way, the shutdown side is separated from process fluids through the isolation valves.
There are different types of double block and bleed valve and they are mentioned as follows:
In this type of double block and bleed valve, the role of isolation valves or the block valves is played by ball valves whereas needle or ball valves play the role of drain valves or the bleed valves. This way, the block and bleed assembly are modified. This assembly is designed to use in the downstream, midstream or the upstream applications of oil and gas.
A single block and bleed valve is used while the maintenance activities are going on in a live process plant. It helps to attain positive isolation. Basically, one bleed valve and one block valve is made in the form of a single assembly or it can be called as a manifold. Now, this single block and bleed valve manifold is able to get readily installed for the isolation purpose.
The integral double block and bleed valves are employed for the primary isolation duties for gaseous or liquid service. These valves are manufactured because of the space and weight saving features of these valves. Moreover, they are available in a variety of sizes and materials. You can easily get the head unit combinations too in order to meet all your requirements. The outlet is NPT threaded while the inlet is flanged. You will get full range of sizes in both threaded outlet and flanged inlet. The connection options of this valve are threaded by threaded, flange by threaded or flange by flange.
This is another type of double block and bleed valve which is designed for the instrument applications. This valve is suitable for almost all the types of fluids which includes methanol as well. This valve is absolutely fire safe as well. The material used to manufacture this valve comprises of stainless steel, exotic alloys, carbon steel and low temperature carbon steel.
The Double block and bleed valve allows you to get additional advantage over the traditional hook up. The cavity present in between the two balls is very small which means that an operator can evacuate the space quite efficiently and hence, establishing an environment which is absolutely safe to work.