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บทความ เกจวัดแรงดัน

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 a superb way to do this.
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 prevent contamination. The BBV can be used to purge the majority of liquid from the DUT. The coalescing filter is really a backup to eliminate any residual liquid that may be present even after the initial purge and will separate entrained liquid droplets from gas flowing in the direction of the controller. The 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 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 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 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. 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 appearing out of the system.
With the block and the bleed valves securely closed, vent the controller.
After pressure gauge 10 bar of the aforementioned procedure it should now be safe to calibrate the DUT. To calibrate, open the block valve and close the bleed valve. เกจวัดแรงดัน will coalesce (join together) any liquid droplets that could still be present in the gas, and gravity will draw the liquid to the bottom of the filter housing. The filter will also remove particles bigger than the specified size. A computerized or manual valve could 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 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 within this system.
3. You will have a pressure drop across the filter in the flowing condition; at steady state pressure there is no pressure drop.

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