The increased activity in the offshore sector of the energy industry has led to additional factors that have to be taken into consideration when designing piping systems. Space in these modern locations is always at a premium and the design of piping systems and their associated components must therefore be more compact. There are structural constraints that are also very important, such as keeping the structure as light as possible, and there are obvious benefits from making components smaller and lighter. Construction site work is also very expensive and any reduction in installation manpower is also beneficial.
The above situation has led to the modification of the patterns of valve components to incorporate savings in space, weight, and labor costs where possible, while still retaining the original function of the valve. The greatest savings are to be seen in the reduction of leak to atmosphere because the potential leak points are minimized compared to the conventional design. Process isolation philosophy has become more complex as safety issues have to be addressed and the requirement for double block and bleed isolation has become more commonly used. Double Block and Bleed isolation requires two in-line isolation valves and a bleed valve, used to drain or vent trapped fluid between the two closure elements.
Double block and bleed valves replace existing traditional techniques employed by pipeline engineers to generate a double block and bleed configuration in the pipeline. Conventionally, in order to achieve a double block and bleed system, engineers would install two standard isolation valves and a separate facility for bleeding the cavity in between.
Space is usually at a premium in the refinery environment, and this approach requiring an additional valve unit to be installed as well as a “T”-junction to allow the cavity to be bled more than doubles the space required compared with a single valve system. This increase in size can often make the installation unfeasible, especially where multiple valves along the line are to be upgraded.

DOUBLE BLOCK AND BLEED MAIN FUNCTIONS

Instrument Double Block and bleed for chemical injection & sampling Double Block and Bleed valves for chemical injection & sampling require an additional straight tubing onto the bore of the valve inlet flange face. This quill enters into the process flow and chemicals can be introduced through the valve.
Incorporated in the valve design an additional check valve is required for this application in order to prevent reverse flow and process fluid contaminating the chemical source. For sampling the reverse is applied.
Instrument Double Block and bleed for pressure connections Double Block and Bleed valves for pressure connections are generally used to isolate instrumentation such as pressure indicators and lever gauges.
In-line Double Block and bleed for process isolation Double Block and Bleed valves for process isolation are located in the primary process stream.

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