Pressure transmitter and pressure gauge tapping

[junah]

Dear member,

I have a question regarding tapping location for pressure sensor. while API, recommended practice often state that PT/PDT tapping for liquid should be in pipe bottom (45 or 90 deg), why does this rule not apply to pressure gauge ? shouldn't gas trap be an issue too in bourdon tube ?

is this obligatory requirement or could we relax from it ? say for crude oil in custody meter where we install PT as compensator. considering small entrained gas in the fluid, could we install it on top ?

thank you for your answer in advance.

 

[IRstuff]

I would guess that gas can't be "trapped" like liquids.

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[danw2]

Trapped air/gas in liquid lines is an issue in nuclear power plants, probably because they encounter some very long runs of impulse tubing where the length of the run precludes 'self-venting', where sloping or slanting liquid service impulse tubing downwards towards the instrument allows any gas or air to travel upwards and vent at the 'top' into the process.

H.M. Hashemian calls air/gas in liquid lines 'voids'. He has a short paper, Pressure-Sensing Line Problems and Solutions, here:

http://www.instrumexpert.com/Documents/1159-Pressure-Sensing-Lines.doc

page 4:
•Voids. Air or gas entrapped in liquid-sensing lines can cause false pressure readings, sluggish response, and extraneous noise as a result of acoustic resonances. For example, in differential-pressure measurements, an air pocket on the low-pressure side can cause the pressure indication to be higher than normal. It can also add a delay in the transmission of the pressure information. Purging air from voids is difficult. Though one would expect air pockets to dissolve in the liquid under the high pressures common in industrial pressure measurements, the problem of voids persists.
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A longer paper with math treatment: Pressure sensing line diagnostics in nuclear power plants here

http://tinyurl.com/pj5nqfy

My experience is that lower pressures, like 5m w.c. head pressure (tank level), or 50"w.c. on a DP transmitter can show a serious error with trapped air. It seems to be a less obvious effect at higher pressures.

'Process head' gauge pressure transmitters can have a vent in the process head (the one on the right in the picture):

But an in-line style gauge pressure transmitter (the one on the left), like a pressure gauge, can get air/gas trapped between the vent on the block-and-bleed valve because there is no vent above the bleed valve on the valve assembly.

Recommended practice is just that: RP.

 

[icewannabe]

what are the advantages you can see installing a PT on top considering a liquid application?