When calibrating or testing a gauge or transducer with a pneumatic pressure controller it is very important protect the controller from any liquid contaminants that may be present in these devices under test (DUT). A block and bleed valve (BBV) in-line with a coalescing filter is a good 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 is used to purge the bulk of liquid from the DUT. The coalescing filter is really a backup to remove any residual liquid that may be present even after the initial purge and can separate entrained liquid droplets from gas flowing in the direction of the controller. Crushing following procedure is made 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 just as much liquid as possible 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 use 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 ? ruthless to low pressure).
Following 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 forget about 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 underneath 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 that has accumulated.
Note:
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. Enthusiastic used to calibrate the DUT can flow in either direction in this system.
3. There will be a pressure drop over the filter in the flowing condition; at steady state pressure there is absolutely no pressure drop.