What is a double block bleed valve?
At its simplest it is a method of providing isolation of particular sections within a functioning pipeline or industrial process. It has particular utility where hazardous or pressurized materials are being handled. The double block valve system allows such material to be kept separate from the isolated section, so that essential maintenance, repairs or safety checks can be carried out with reduced risk of accident and injury to personal. In other words the term delimits a level of isolation within an industrial system. The double refers to the two separate points which are isolated from the rest of the system by the two valves. The bleed in the definition refers to the point between the two valves, from which any isolated material is bled.
How do they work?
The basic requirement is that each valve holds back the materials being isolated at the correct temperature and pressure. This should not lead to an inference that no leakage from the system enters the section being isolated, in many industries, particularly those handling high pressure substances this is almost impossible. This means that many double block bleed valve isolation systems are designed to allow a certain amount of acceptable leakage. The only pre-requisite is that each valve that is enabling the isolation has been tested such that it operates effectively within the acceptable rate of leakage for that particular industry. Once this rate has been established the bleed occurs and the pressure differential between the two sections of the system occurs there should be no health and safety issues.
What can go wrong?
1. Actuator (motor) failure: This is the single biggest cause of failure. In essence the seals on the valves which allows the pressure gradient to be maintained fail and pressurized material rushes into the isolated section. This is why it is accepted practice across industries that the pressure seals are replaced after every isolation.
Overall when looking at this technique one should consider the following points:
1. Both barriers have to be tested
2. How rapidly can the bleed from the isolated section occur?
3. How well monitored is the whole system, in particular the impact of pressure fluctuations?
4. How strong are the seals?
5. For how long can a stable isolation be realistically maintained, i.e. within safety limits.
6. How will repeated isolation of different sections of the system affect the structural integrity of the whole system?
Contact energy products for further information on how to use a double block bleed valve. We can also help with your general engineering and valve enquiries.