Instrument Installed

What do you get for readings?

What does the manual say?

What does the instrument engineer say?

What does the janitor say?

Have you looked on the web?

Did you ask Flow?

Yes. Presumably the technology is differential pressure (DP) (coriolis, thermal dispersion, turbine, are all in-line measurements)

Vapor is a gas. Installation for a DP transmitter for gas flow measurement is ABOVE the pipe, so that condensate or liquid in any form can not drop into the impulse tubing, collect there, and exert a head pressure which appears as a measurement error. The error can be positive or negative, depending on which port is affected and how much.

That's not correct, you can place the transmitter lower than the tapping points as long as you hae accounted for potential liquid accumulation and placed a 'catch-pot' or some other item that will ensure the liquid that could potentially form, won't interfere with the pressure/instrument/flow reading.

Yeah, it can mount over the pipe, but top mount is considered "convention" for good reason.

The reason for recommending above-the-line mounting is because many, many process 'vapors' or gases are loaded with something that will condense and drop out of the gas stream. The accumulation of the liquid in the impulse pipe between the main pipe and the transmitter eventually builds a head pressure which the transmitter sees as a measurement error.

Mounting a DP below the line in gas service might work for measuring, say, nitrogen from Air Liquide's storage tank after it's been vaporized from its liquid state, where it's a given that the piping runs through ambient temperatures well above nitrogen's -196°C boiling point so that the gas is not going to re-liquify.

A drip or drop leg that extends below the datum can catch the liquid, but there's always the issues of having to maintain it by draining the liquid and the subsequent disposal of a liquid that might be toxic or hazardous.

There's all sorts of installations, from good to bad, and those who mount it below the line seem to be the same people who use a DP with no vents, no drip leg, or a drip leg with no block valve.

The major vendors who sell the transmitters know the good and the bad and they publish info showing conventional installations, like Rosemount's:

So yeah, whoever 'anonymous' is can have his DP below the line, but chances are as good as not that he's likely to have
- measurement issues with liquid build-up in the impulse lines
- maintenance issue for empting the impulse lines
- disposal issues with liquid
especially when he has to ask the question in the first place.