Operation: In normal operation the isolate valves are open while the vent valve and plug are closed. This provides unobstructed flow to the instrument or other equipment. To isolate the instrument for maintenance and or removal, first close the primary upstream valve and then open the vent valve to relieve the trapped pressure. If required, use this vent plug to drain off excess fluid. Now close the secondary isolate downstream valve which acts as a safety back up and then close the vent valve and plug. The instrument or other equipment can now be safely removed and maintenance carried out. Note that both flow paths to the exterior of the system through the manifold have been double blocked for safety 

Instrument Calibration:  It is possible to connect a test gauge set up to the vent plug connection in order to calibrate the instrument without removing it from the system. With use of the vent valve and the plug it may be possible to carry out system bleed, sampling, and injection operations.