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Question for Pressure Transmitter Installation Location in Liquid Service

Question for Pressure Transmitter Installation Location in Liquid Service

Question for Pressure Transmitter Installation Location in Liquid Service


I would like to get opinions about pressure transmitter installation location in liquid service.
When measuring liquid pressure in pipeline, the pressure transmitter generally is installed below pipeline with a side pressure tap on pipeline.
This purpose is to vent freely entrapped gas/vapor(it can cause the measuring error) in liquid fluid through impulse line back to pipeline. Moreover, it can prevent the slurry or sediment in liquid fluid from entering impulse line(it can block the impulse line).
This is general practice as i know.

However, some owner or licensor recommend always installing a pressure transmitter above pipeline regardless the fluid status like vapor/gas, condensate, liquid, steam), and they tell it is for preventing measuring error caused by static head pressure.

I agree with that, but it is hard to understand easily.
So, i tried understanding with the fluid basis and Bernoulli's equation and the below is my conclusion.

If the transmitter is installed at the below of pipeline with side pressure tap, the hydrostatic pressure generates due to an effect of elevation “h” which is impacted to gravity.
This makes the arbitrary datum plane “z” change from pipeline’s elevation(z) as the original reference level to transmitter’s elevation ground(h).

Vice versa, if the transmitter is installed above pipeline, the transmitter can measure only static pressure in pipeline since there is no impact of the hydrostatic pressure.

Is it right? please give your help for my understanding.

RE: Question for Pressure Transmitter Installation Location in Liquid Service

What is the height of impulse tubing? The liquid column does add to pressure. If the liquid column height is 1 meter and is water then the extra pressure will be 0.1 bar. If it is differential pressure measurement, for ex. flow, then you can ignore the error.

RE: Question for Pressure Transmitter Installation Location in Liquid Service

If your impulse line is at an angel upwards then yes, I suppose you could argue that the side orientation prevents gas being trapped there.

However trapped gas doesn't impact the pressure reading, but would affect transmission of pulses of pressure.

In practice though most above ground pipes run on low level sleepers or supports and having your instrument at ground level is difficult to access and protect.

A lot of pressure transmitters are just direct mounted or run off the top of the pipe ( your small tapping is more protected and doesn't cause an obstruction) and then the TX is placed on a pole close to the tapping point.

Unless the TX is located many metres different in height, the Z value is often ignored in reality unless you're dealing with very low pressure fluids.

Your diagram is providing the difference between a tapping point on the outside of a pipe versus a pitot tube pointing opposite to the direction of flow. That's something completely different. Again in reality unless you've got a very high velocity low pressure liquid, the dynamic pressure is often ignored as it's very small in comparison to static pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Question for Pressure Transmitter Installation Location in Liquid Service

Perhaps there are some authoritative sources which recommend locating the pressure tap on the side of the pipe, but I haven't seen them. And even if they exist, I'm not buying it. A vertically oriented tap minimizes accumulation of debris, and it doesn't cause any error at all in the pressure reading. The presence of a gas pocket in the instrument tubing simply dampens the pressure impulses but it doesn't change the pressure value. It seems to me that a gas pocket is actually beneficial because pressure readings in an incompressible system can be noisy.

Also, side mounted pressure taps will unnecessarily obstruct space in the piperack.

RE: Question for Pressure Transmitter Installation Location in Liquid Service

This issue is usually only relevant to flow meters or level sensors which are measuring low differential pressures (<100 in H2O). In these cases if both lines are kept full of liquid or gas, then the error cancels out. So impulse lines are often purged to insure this, and to keep out dirt. In most other cases the error due to impulse lines can be ignored. However, every situation has to be evaluated individually.

RE: Question for Pressure Transmitter Installation Location in Liquid Service

Thanks for all your replies.
I get to know that the gas entrapped doesn't give any impact to reading and I agree with the side tap orientation tends to make obstruction in the pipe rack.

However, unlike your replies, API RP 551 and PIP which are international standards, as well as most of the transmitter manufacturers such like Emerson/Yokogawa/ABB/E+H etc, have been recommending the preferred tap position as 45 degrees below horizontal plane or the side for liquid measurement.(of course, these documents recommend using a short impulse line as possible.)

Could you give any explanation to me why international standards and manufacturers still prefer and recommend this design against your reply as a top of the pipe?

RE: Question for Pressure Transmitter Installation Location in Liquid Service

IMO, I won't argue the orientation of the tap either at top or horizontal since both are all real and existed in the operation. For the side mounted tap, be sure there is no safety issue or any impact to the future usage of the space. For the top mounted tap, be sure there is all liquid packed pipe. If any doubt, may consider the tap located on the vertical pipe, so there is no orientation issue as on the horizontal pipe.

RE: Question for Pressure Transmitter Installation Location in Liquid Service

If you need to understand more about pressure transmitters, I would recommend to check out a couple of videos, articles, etc.
I think other's experience may make it easier for you to understand than theory (Bernoulli's equation)

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