An engineering director in one of the reputable company answered different questions pertaining to Double Block and bleed valves, noted where they can be found such as this page, the reason for their usage and why they help in improving safety while reducing the size as well as weight of the installations.
Q: What are double-block-and-bleed (DBB) systems, and why are they necessary?
The double block and bleed valve helps in the provision of additional layer of safety whenever you want to shut off a pipeline which is conveying natural gas, oil or other gaseous or liquid material which is highly flammable.
They have two different valves which are operated independently and a bleed valve between them which can be used to empty the chamber between the valves immediately it is closed. A second valve become compulsory for the oil and gas pipeline isolation after the Piper Alpha disaster which occur in 1998, there was an inquiry about the incident and recommendations were made on ways by which the safety of the system can be improved, one of which is the use of DBB valve.
Q: Why was the single-unit DBB valve developed?
The main reason why the single unit of double block and bleed valve were developed is to save weight and space and also to reduce the number of joining which could result to leakages. However, the new valve is widely accepted by everyone in the market.
Q: What are the main advantages of having all three valves incorporated into one casing?
The major advantage of incorporating the three valves into a single casing is to reduce the weight, size as well as the safety standard of the valves.
The normal double block and bleed system would consist of three different valves which have a series connection and would take more space and also weight more when compared to a single unit. The DBB valves which are in the market functions properly and help to attain the desired function when needed.
Also, this technique helps in the total reduction of the number of potential leak paths as well as emission by minimizing the joining.
Q: Where do they use double block and bleed valves frequently?
The DBB valves are used commonly used because of the added safety, convenience as well as reduction in the potential leak points. However, it is crucial to use the valve in positions where you have to consider the weight as well as space.
An example of such positions include a storage and offloading facilities, floating production due to the fact that you have to mount the processing and transportation plant on a vessel which has a strict limit on loading weight and a limited installation space.
In addition to that, the refinery applications which are downstream are other area where the space saving benefits of the single block and bleed valve can be valuable. Also, the fugitive emissions are likewise a major concern in the petrochemical and refinery environment, and these kinds of valve provide more benefits by reducing the leakage points.