Calibration / contamination control with a block and bleed valve and coalescing filter

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 could 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 prevent contamination. The BBV is used to purge the bulk of liquid from the DUT. The coalescing filter is a backup to get rid of any residual liquid that may be present even after the original purge and can separate entrained liquid droplets from gas flowing in direction of the controller. The next procedure is made to prevent harm 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 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 Colossal to 7 until no more 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 may still be present in 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. An automatic or manual valve can be attached to the bottom of the filter housing to periodically drain any liquid that has accumulated.
Note:
1. 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. There will be a pressure drop over the filter in the flowing condition; at steady state pressure there is no pressure drop.

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