text: Isolation valves can be in the normally open position (NO) or normally closed (NC). Normally open valves are located between pressure vessels, pumps, compressors, tanks, pressure sensors, liquid level measurement instrumentation and other components and allow fluids to flow between components, or to be connected to sensors. The controlled closure of open valves enables the isolation of plant components for testing or maintenance of equipment, or allows flow of fluid to specific flow paths. Normally closed valves are used to connect fluids and process components to other systems only when required. Vent and drain valves are examples of normally closed valves which are only opened when required to depressurise (vent) or drain fluids from a system.
  Isolation valves must effectively stop the passage of fluids. Gate valves, ball valves and plug valves are generally considered to provide tight and effective shut-off. Globe valves and Butterfly valves may not be tight shut-off due to wear on the plug or the seat, or due to their design, and may not be appropriate to provide effective isolation.
  Some valves are in a safety critical service and are secured, or otherwise locked, in an open or closed position. Plant shutdown instrumentation must be effectively connected to the plant at all times, therefore the isolation valves associated with such equipment must be secured in the open position to prevent inadvertent movement or closure. Securing mechanisms include car-seals, chain and padlocks and proprietary securing devices. Isolation valves in a flare, relief or vent system must ensure that a flow path is always available to the flare or vent. These valves are secured in the open position (LO). Drain valves that connect a high pressure system to a low pressure drain system are locked in the closed position (LC) to prevent potential over-pressurisation of the drain system. Removal of locks from secured valves is only undertaken in specified and controlled conditions such as under a ‘permit to work’ system. Some relief or pressure relief valves are ‘paired’ to provide a duty and a standby valve, the associated isolation valves are interlock<nowiki/>ed such that at least one relief valve is connected to the system being protected at all times.
  A single valve may provide effective isolation between the live plant and the system being maintained. However, for hazardous systems a more effective means of isolation is required. This may comprise a ‘double block’ consisting of two valves in series. Still more effective is a ‘double block and bleed’ comprising two isolation valves in series plus a bleed valve between them. The bleed valve enables the integrity of the valve on the hazardous side to be monitored.