A shutdown of an individual piece of equipment, such as a compressor, may progress through up to three stages, as follows:
• Stage 1. Shutting down the driver and permitting the machinery to stop, while it is still pressurized with process fluid.
• Stage 2. Automatically blocking in the equipment with shutdown valves, in addition to (1) above.
• Stage 3. (1) and (2) above with an automatic bleed valve to depressurize the equipment.
The first stage might be implemented in the event of a process shutdown elsewhere in the facility or in the event of an equipment failure that does not pose a hazard. The second stage could be implemented upon detection of a potentially unsafe condition that does not present an immediate hazard and for which a complete depressuring would cause an extended shutdown of the facility. The third stage, which taken in conjunction with the first two is often referred to as “block and bleed,” would be resorted to in the event of a fire, major release, or any other event of similar magnitude. Blocking and bleeding protects the equipment by isolating it from hydrocarbon sources and by relieving hydrocarbons contained within that system, thus significantly reducing the amount of fuel that can be released or that can burn.
Higher levels of shutdown utilize the same principles. A process train or process system shutdown would block in that facility, or portion of a facility, shutting down all equipment which does not need to be kept running. Typically, cooling water systems, air coolers, and other equipment which act to reduce the threat of possibly dangerous plant operation should be kept running. Similarly, any equipment critical to the safe shutdown of the facility, such as the fuel system feeding the flare, should be kept operating. The plant may or may not be depressured, depending upon the amount of potential fuel within the block valves and the threat of fire.
In an automatic emergency shutdown, the facility would be blocked in, all equipment would be shut down except that necessary for a safe shutdown (usually a flare). In most cases the entire plant or predetermined sections of the plant would be depressured in a controlled fashion. (As a minimum the flare system must be designed for the maximum emergency depressuring rate.)