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 a wonderful way to do this.
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 is used to purge the bulk of liquid from the DUT. The coalescing filter is really a backup to get rid of any residual liquid that could be present even after the initial purge and will separate entrained liquid droplets from gas flowing in direction of the controller. The following procedure is built to prevent damage to a pneumatic pressure controller from a contaminated DUT.
Purge the DUT of liquid containments
Before connecting the DUT to the system, remove just as much liquid as you possibly can from the DUT.
With the controller power off and both the block and bleed valve closed, connect the system as shown above.
Use the controller to apply a pressure close 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 detox the system to atmospheric pressure. Take Humility 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 should 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 which 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 underneath of the filter housing to periodically drain any liquid that has accumulated.
1. The coalescing filter won’t protect against a big 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 see a pressure drop across the filter in the flowing condition; at steady state pressure there is absolutely no pressure drop.