Air Flow Instrument Location

[althodr]

I know that typically air flow dP cells are mounted above the flow element. What do I do if I have to mount the dP cell below the flow element?

 

[btrueblood]

You have to worry about condensation entering the cell and causing corrosion.

You could try putting in a "drip loop" (essentially just a U-shaped loop of tubing hanging below the level of the cell on each tube run). Checking the loops for condensation and draining them periodically (e.g. thru a purposely installed valve or drain fitting) will help; some people put a very small pinhole in the bottom of the loop tube to drain away condensation, but this causes some concern over accuracy at low speeds, and you have to make sure the water drips don't go somewhere you don't want them to.

 

[waross]

Condensation in a u-tube may also cause issues ranging from inaccurate measurements to total measurement failure.
Not only do DP cells work best above the flow elements but the connections should not have any "traps".
respectfully

 

[willip]

Althodr....... if you take the dp tapping points off the top of the line and then sweep (like a swan neck) the impulse lines down to the measuring device then this will eliminate (or at least significnatly reduce) any liquid carry over to the dp cell. By bringing the cell down below the line you increase the maintenance accessibility...... I'm a C&I Engineer by discipline..... if you want more detailed info let me know.........

 

[Scramode]

A Dessicant dryer or a refrigerated air dryer rated to system pressure might help. Or look into gauge isolaters

 

[waross]

The problems develop when the air flow is close to the dew point and the environment surrounding the DP cell may drop below the condensation point. Most of my air flow DP experience was under these conditions. We finally abandoned DP cells and proved air flow by monitoring rotation of the fan shaft. In a friendlier environment I am sure the other posts are valid.
respectfully