Monday, September 12, 2016

Trunion Mount Ball Valves in Double Block and Bleed Assembly

Double block and bleed valve assembly
Double block and bleed valve assembly
Valbart - Flowserve

The nature of some processes, as well as applicable industry standards, can call for a specific valve arrangement that provides isolation of a piping section with a safety factor not inherent in a standard stop valve.

Combined into a single compact unit, a double block and bleed valve consists of two trunnion mounted ball valves and a third smaller valve. While it is possible to employ other valve types in place of the ball valves, the trunnion mounted ball valve is well suited for the heavy duty applications involved in the oil and gas industry.

Each of the ball valves serve to block the process flow and the third valve serves as the bleed or drain valve for the space between the two block valves. Combining both ball valves into a single unit reduces leakage paths, weight, and installation complexity.

In addition to serving as a positive isolation valve set, a double block and bleed is also applied in meter calibration. Calibration of the meters used for custody transfer must be accomplished with a confirmed zero flow rate. Any valve leakage has the potential to impart an offset of the zero value on the meter, resulting in continuously incorrect readings. Closing off the flow with a double block and bleed valve, draining the inter-valve space with the bleed valve, then monitoring the bleed port for any continued flow can confirm zero flow in the metered line.

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