When calibrating or testing a gauge or transducer with a pneumatic pressure controller it is important to protect the controller from any liquid contaminants that could be present in the device under test (DUT). A block and bleed valve (BBV) in-line with a coalescing filter is a wonderful way to do that.
The potential flow of containments is from the DUT to the controller. A BBV and a coalescing filter, oriented as seen 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 a backup to remove any residual liquid which may be present even after the initial purge and will separate entrained liquid droplets from gas flowing in direction of the controller. The next procedure is designed to prevent harm to a pneumatic pressure controller from the contaminated DUT.
Purge The Truth of liquid containments
Before connecting the DUT to the machine, remove just as much liquid as possible from the DUT.
With the controller power off and both the block and bleed valve closed, connect the machine as shown above.
Use the controller to apply a pressure near to the full scale pressure of the DUT.
Open the block valve slowly to pressurize the DUT (note: the gas will flow in the 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 purge the system to atmospheric pressure. Take notice of the sump to see if any liquid is purged.
Close the bleed valve.
Repeat steps 3 to 7 until forget about liquid is observed appearing out of the system.
With the block and the bleed valves securely closed, vent the controller.
After completion of the aforementioned 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 larger than the specified size. A computerized or manual valve can be attached to underneath of the filter housing to periodically drain any liquid which has accumulated.
Note:
1. The coalescing filter will not 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 no pressure drop.