The stem is covered with a rubber washer and metal cap similar in appearance to a gland nut.
Because they lack a stem seal they will leak unless fully closed and installed in the correct direction or fully open, causing the disk to compress the top washer against the stem.
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.
[4] Normally closed valves are used to connect fluids and process components to other systems only when required.
[3] Some valves are in a safety critical service and are secured, or otherwise locked, in an open or closed position.
[3] Removal of locks from secured valves is only undertaken in specified and controlled conditions such as under a ‘permit to work’ system.
[5] A single valve may provide effective isolation between the live plant and the system being maintained.