Emergency Block (or shutdown) Valves (EBV), either isolate or divert flammable or toxic material, or they depressurize a piece of equipment. They are installed in situations that are potentially hazardous to humans, the environment, or major process equipment.
You should not use a modulating process control valve as an EBV. An actuated quarter-turn or rotary process valve is the best choice. The EBV control components and logic should be separated from process control logic. Of the five levels of protection defined in Section 1331, only Level 5 — Emergency Shutdown — requires automated valves to be EBVs. For Levels 2-4, the guidelines for EBV’s may be followed in whole or part.
Decide if there is a need for EBV’s by conducting a process hazards analysis (PHA) during the initial design period and by reviewing the need during subsequent PHA’s as defined in API RP-750. A hazard and operability study (HAZOPS) is an example of a PHA. Be sure to establish a means of obtaining and maintaining accurate drawings, specifications, and other documentation for each EBV and its associated actuation system. Basically, EBV’s perform three functions:
• Emergency isolation
• Emergency depressurization
• Emergency diversion