Flow Measurement Under Vacuum

The fact that the line is at a pressure less than atmospheric pressure does not prohibit the use of DP technology to make a flow measurement.

The DP transmitter does not know that the each of the pressures piped to its ports is at a pressure less than atmospheric. It just subtracts the low side pressure from the high side pressure and produces a difference value.

A DP measurement at 'vacuum' conditions will be as good or as bad as a DP measurement at 'positive gauge' pressure conditions.

Thanks for clearing my doubts..one more thing that I want to know is can I use orifice assembly with dp transmitter along with impulse piping instead of using of integral type orifice with dp. If I use impulse piping then where is podition of my dp transmitter??? Will it be up from the orifice tapping or at down from the orifice tapping ..my line fluid is of air+ vapoure of acids .

And the line is at under vaccum..

The decision to mount the dp transmitter either above the taps or below the taps depends primarily on whether or not there will be unwanted substances accumulating in those impulse lines. In gas and vapor service, the concerns is liquid accumulating unevenly in impulse lines (with dp transmitter below taps) which will cause dp errors, causing flow measurement errors. This is why gas service installations always position the dp transmitter above the orifice taps. In liquid service, the concern is vapor bubbles accumulating in the impulse lines, which is why liquid service installations always keep the impulse tubes flooded by mounting the dp transmitter at or below the orifice taps.

Given that your process contains acid vapors, I'm going to assume those vapors can cool and condense into liquid acid inside the impulse lines. If you mount the dp transmitter below the orifice taps, it means you must pre-fill the lines with acid (or with some non-reacting fluid denser than the acid) in order to avoid errors. If you mount the dp transmitter above the orifice taps, it means the impulse lines must be large enough and straight enough and short enough that any liquid droplets consensing inside the lines will not form "plugs" of liquid to cause dp measurement errors.

Here is an interesting paper from Emerson about mounting dp transmitters above orifice taps in steam service, where the vapors in question most definitely condense into liquid inside the impulse lines:

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCQQFjAA&url=http%3A%2F%2Fwww2.emersonprocess.com%2Fsiteadmincenter%2FPM%2520Rosemount%2520Documents%2F00870-0200-4809.pdf&ei=myFBT4f0A5LJiQKNzcjBAQ&usg=AFQjCNF7_3U-k94ayBdLD9R_aMg74bfrPgAlthough

Although your process is not steam, some of the same principles ought to apply. One of the concerns they discuss at length in this paper is temperature. If your process is cool enough, this will not be a concern.

Typical commercial integral orfices are wetted parts and as such are subject to materials compatability with the acid component in the flow stream. If there is corrosion, there can be clogging, leaking or serious orifice degradation.

The failure of any part of an integral orifice meter would need an exact part replacement of whatever material you select, which might very well not be a common stainless alloy, but something like monel.

If the materialof construction is not standard (like 316 stainless, but special, like monel) I'd probably lean toward separate components: an orifice plate, flange unions and impulse tubing, tube fittings, any of which can be obtained from supply channels more readily than a specific monel (? or whatever) sensor body part for a specific brand transmitter.

Tony pretty well spelled out the considerations for mounting.

I would still look at vortex-shedding flowmeters, Guvnor.