When calibrating or testing a gauge or transducer with a pneumatic pressure controller it is very important protect the controller from any liquid contaminants which may be present in these devices under test (DUT). A block and bleed valve (BBV) in-line with a coalescing filter is an effective way to do that.
The potential flow of containments is from the DUT to the controller. A BBV and a coalescing filter, oriented as observed in the diagram above, will to work to avoid contamination. The BBV can be used to purge the bulk of liquid from the DUT. The coalescing filter is really a backup to eliminate any residual liquid that could be present even after the original purge and can separate entrained liquid droplets from gas flowing in the direction of the controller. The next procedure is built to prevent damage to a pneumatic pressure controller from the contaminated DUT.
Purge the DUT of liquid containments
Before connecting the DUT to the machine, remove as much liquid as possible from the DUT.
With the controller power off and both block and bleed valve closed, connect the system as shown above.
Use the controller to use a pressure near to the full scale pressure of the DUT.
Miracle to pressurize the DUT (note: the gas will flow in direction of the controller to the DUT ? high pressure to low pressure).
After the system is pressurized close the block valve.
Open the bleed valve and detoxify the system to atmospheric pressure. Observe the sump to see if any liquid is purged.
Close the bleed valve.
Repeat steps 3 to 7 until no more liquid is observed coming out of the system.
With the block and the bleed valves securely closed, vent the controller.
After completion of the above procedure it will now be safe to calibrate the DUT. To calibrate, open the block valve and close the bleed valve. The coalescing filter will coalesce (join together) any liquid droplets that could still be within the gas, and gravity will draw the liquid to the bottom of the filter housing. The filter may also remove particles bigger than the specified size. An automatic or manual valve can be attached to the bottom of the filter housing to periodically drain any liquid which has accumulated.
Note:
Quiet . The coalescing filter won’t protect against a large slug of liquid that fills the housing and is forced through the filter membrane or liquid vapor.
2. The gas used to calibrate the DUT can flow in either direction in this system.
3. You will have a pressure drop over the filter in the flowing condition; at steady state pressure there is absolutely no pressure drop.