It is essential for everyone to know how double block and bleed valve works because this is a kind of valve which is provided whenever there is need for two isolation methods which has a vent valve placed between them.
According to the definition of valves manufacturers, this kind of valve is said to have two seating surface and helps in releasing pressure https://www.kp-lok.com/product/double-block-and-bleed-valves from the system. It is also noted in this definition that whenever one side is under pressure, a positive double isolation will not be provided by the valves.
It is also essential to have positive isolation for some kinds of work such as confined space entry and hot work which is done in a confined space. There are various methods which can be used for providing the energy required; they include disconnecting, DBB, blinds or misaligning piping. There are some applications which do not consider the use of DBB as an effective method in the industry, but it is acceptable in most cases.
What double block and bleed valve does majorly is to close tow block valves,you can then open the vent valve which is placed between them. This implies that any leakage which passes via the upstream valve will proceed to the vent rather than leaking via the downstream valve. There are many companies that sell double block and bleed valve which comprises of two different valves in the same body of valve and it also include a void space which has a vent valve on it. The double block and bleed valve is very suitable for different kinds of applications, but may be expensive; they likewise result to a reduction in the pressure and are not piggable.
Most of the ball valves which are manufactured have two seating surface which are independent of one another with a seats which is loaded with spring as well as the vent in the body cavity. The challenges which most people have with the double block and bleed valve is that the body cavity can relieve itself, this implies that if there is a buildup of pressure in the cavity, then the downstream piping will be relieved, this scenario can occur if the small ports in the body bleed plug up. However, there are some governments that have removed Trunnion Ball Valves from the list of DBB device while others do not make any decision on the issue. Some companies likewise exclude Trunnion Ball valve from the list of DBB device while other companies consider it as a DBB devices and others sit on the fence about the issue. If the company you are working with is silent about the issue and the applicable government agencies are also silent, then the engineer that is concerned can then make the appropriate decision on the use of the valve and whether it make adequate protection to the staff and also ensure a positive energy isolation scenario. I have also considered the risk side of it because the double block and bleed valve has never injured anyone but some other people consider that as luck.