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Isolation Methods

July 22, 2014
Double-Block-Bleed-1

Double Block and Bleed Valves

The material in this post is extracted from Chapter 3 of the book Plant Design and Operations.

Positive isolation methods are those which remain effective even if there is equipment failure or operator error. These techniques apply not only to vessels, piping and tanks but also to pneumatic and hydraulic equipment.

Figure 3.2 shows some of the various isolation techniques that can be used to protect workers in the process industries. The process containing toxic or flammable chemicals under pressure is on the left; the open system, where the workers are present, is on the right. The order is from the least to the most secure.

Figure 3.2
Isolation Methods

Isolation-Methods-1

Isolation Methods

Level 1 — Closed Valve

The use of a single closed valve is rarely acceptable as a means of isolation in the process industries — except in the most benign services — for two reasons. First, it is very easy for someone else to inadvertently open the valve. Second, the closed valve may leak.

One company places the following conditions on the use of a single block valve to provide isolation.

  • The isolation block valve closes tight and does not leak;
  • It is locked closed and tagged;
  • The job is continuous and uninterrupted;
  • The work is conducted during daylight hours and;
  • No confined space entry or hot work is involved.

If a valve is to be used for isolation, it should generally be a gate, ball, plug or needle valve. In some instances, butterfly valves are allowed in non-hazardous services. Where actuated valves are used, the actuator mechanism must be isolated from all possible supply sources. Check valves, control valves and relief valves are not acceptable as isolation devices.

Types of valve that can be used for single-valve isolation include: gate, ball, plug, and needle. Butterfly valves may be allowed in non-hazardous services. Valves specified for control or throttling service (choke or control valves) should not be used for isolation. Check valves and relief valves are not acceptable for designing means of isolation. In all cases, the valves chosen must be designed to provide a positive shutoff seal for the inventories and pressures involved.

If actuated valves are used then the actuator mechanism must be isolated from all possible supply sources before work commences, and before the valve can be considered secure.

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