1. The isolation of the main stream and of the ancillary equipment on pig traps requires careful choice of valve type and configuration to allow safe operation, maintenance and repair without depressurising or decommissioning the pipeline.
2. A double block and bleed system, consisting of two separate valves in series with a bleed point in between to allow diversion to a safe location of any fluid leaking through either valve, should be used in the following situations:
1. Toxic fluids, any pressure class.
2. Category C fluids, and non-toxic category B and D fluids, ANSI class 600 and above.
