How are they different from other valves ?
Test valves are typically needle valves but I have seen some occasional usage of 1/4 to 1/2 inch globe valves and ball valves used as test valves for sampling. Pressure gauge installations may have test valves installed just below them. The valves are used to isolate the gauge from the system pressure, a second valve is used to vent the trapped pressure so the gauge could return to zero. The same vent valve could then be used to attach a calibrated pressure source to the pressure gauge so as to check (qualify) the gauge reading against the standard. This was done on select gauges where a quick in place qualification could be done without the need to physically remove the gauge from the gauge board. I have also seen test valve setups in systems using a three way valve in a setup to scavenge the sensing line with a clean fluid. This is done when dealing with a slurry which could block the sensing line suach as in a quick lime system. Clean water would flush the line before and after a reading so as to not trap material in the sensing line which could build up and block or cause a false reading. Test valve location also gives a handy mean to obtain a sample of the process fluid.
