Pipelines which transfer petroleum products serving bulk liquid terminals, aircraft refueling and petroleum production are often protected by double block & bleed valves. The objective of the valve system is to prevent undue contamination of the product or where it is necessary for the system to continue operating while equipment can be taken offline for repair, maintenance or cleaning.
Today, double block & bleed valves are integral whereas in the past it was necessary to install two gate valves in the line with a manual bleed fitted between them, the bleed was installed in a spool which was in place between the two independent valves. For this to function it was necessary for the operator to close one valve and then the other. Once both valves were closed, the operator could then open the bleed in the spool, once the entire product left in the spool was exhausted, only then was it known that the pipeline was closed in both directions.
Although this double valve and spool arrangement works, it does have flaws. The twin valves and spool take up a considerable space in the line and because the valves are gate valves they have ground seats allowing for a metal to metal seal. If there was any foreign matter in the lines it can easily score the seating surfaces.
Double block and bleed valves which are commonly used today are an integral unit which performs the tasks of three different components; the two isolator valves and the bleed. This arrangement saves a considerable amount of space in the line as well as weight. These valves function by isolating both the upstream and downstream flow and pressure of the material in the line. Rather than two gate valves, glove valves can be used back to back with the bleed in-between them, in the center cavity. It is now a simple matter to drain the cavity which is required when the flow is to be diverted or when maintenance or an integrity check must be carried out.
Double block & bleed valves are available in a number of different configurations but they are all designed for the same purpose, that is to save critical space and extraneous weight.
Energy Products Company is a leading distributor of double block and bleed valves which are used extensively in the oil and gas industry.
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