The economic impact of a spurious or nuisance trip of an ESD system can be disastrous. An ESD system is an important layer of protection to prevent and prevent hazardous situations from occurring. So, it is needless to mention that the ESD system must be extremely reliable and function on demand. During an emergency, it must put the process in a safe state in orderly fashion. Also ESD systems design shall be based on a fail safe independent system, that is, ESD systems are such that during a failure of a component the process reverts to a condition considered “safe” and not a vulnerable serious hazardous event. Reliability and availability are major parameters for ESD system performance. Reliability is a function of system failure rate (its reciprocal) and mean time between failures. Spurious trip conditions may initiate a so-called fail safe incident that may result in accidental shutdown of equipment or processes. However, undetected process design errors or operations may initiate dangerous incidents that may disable the safety interlock and may even cause accidental process
fluid release/equipment damage/fire/explosions. Reliable and fail safe design of the ESD system:
The specification of fail safe for final elements can be designed as fail close, fail open, or stay put/fail lock, that is, the last position before failure depending on application. Usually, ESD valves are the fail close type for isolations (in some cases it may be fail open). In this connection Fig. IX/6.0-1 may be referred to. Also, just as an example, Clause 10.4.2 of NORSOK standard S-001:2008 may be referred to (normally used in offshore facilities). Some of the characteristics of ESD valves especially for offshore facilities (NORSOK standard) are: