Double Block & Bleed Wedge Gate Valve

Typically double block and bleed valving is justified by the need to insure that neither valve trim is leaking. The test is an opening into the downstream piping that would leak fluid if the either valve leaked.

In hydrocarbon custody transfer sites especially Liquid Automatic Custody Transfer [LACT] API recommended practices [RP] require this valve
configuration in a certain position.

When a volumetric prover is lined up in series with a 'LACT' meter [PD high accuracy meter] the downstream normal flow line is diverted into
the prover system. API RP's call for a DB&Bld valve here to insure that no possible leakage is considered in this measurement test. The
down stream [normal] flow piping must be completely diverted into the prover by such a valve configration. This ["proof of block"] bleed
must be opened as part of the test sequence.

DB&Bld valves can be plug, wedge/gate, or 'semi-ball' trim designs. Each single trim valve features the downstream chamber to bleed or
drain and the double trim or dual valve body type has the bleed in the chamber between the two trim sets.

These valves may be any pair of single trim valves or one body with dual trim or seals. This is at least one instance of their use.
Double block and Bleed (DBB) Valves are used in applications where it is necessary to confirm online that there is absolutely no leakage getting past the valve. Effectively the valve body is sealed from both the upstream and downstream process and the body cavity can be tested to confirm this. The granddaddy of DBB valves is the General Twin Seal. These have two seating slips mounted independently on a tapered plug by dovetails. During closing, the wedging action of the plug forces the sealing slips against the valve body. During opening, the dovetails pull the slips away from the body. The plug is guided by upper and lower trunnions. Resilient seals are permanently bonded into the slips.

In the pipeline industry for example, DBB valves are commonly used in custody transfer flow meter calibration circuits to ensure that all flow is diverted through the online meter prover (an accurately calibrated volume). The bleed port on the DBB valve body is checked (typically by a PDS between the body cavity and the inlet or outlet side) to see that the valve seal is tight
(indicated by a positive pressure within the body).

Ian Milne