According to the standards, pressure transmitter should be installed below the pressure tapping point for the water process line.

what if pipe line is installed on the ground level, so transmitter is installed above the tapping point? Is it possible to solve this problem with offset elevation setting? What is key reason of installing of pressure transmitter below the tapping point?

Thanks.

 

Offsetting for elevation accounts for the difference in hydrostatic pressure (head) of the water in the impulse line between tap and element. When mounted above the process pipe, the water in the impulse line may drain out if the process pipe is ever drained, replacing water with 'air'. That changes the head of the impulse line and thus the calibrated-in elevation offset.

However, if the process operating pressure is high enough that the impulse line head pressure is negligible, mounting above isn't a problem IMO. Others opinion may contradict

 

1) A water liquid pressure measurement is 'low' by whatever elevation the meter/gauge is mounted above the tap/port. The error varies by the density of the fluid. It's called hydrostatic error.

A 28" elevation reduces the reading about 1 psi (water).
0.5m elevation reduces the reading about 5kPa.

If your measurement is an electronic signal, you can 'add' the error back in by scaling the receiver device's input.
Some mechanical pressure gauges can be adjusted/calibrated for the hydrostatic offset error.

2) A meter/gauge mounted above a liquid line can fill with air over time. At very low pressures, that can produce a measurement error.

Meters/gauges for gas service are mounted 'above' to provide drainage of condensate/liquids. The error elevation for gas is minimal.

 

>However, if the process operating pressure is high enough
>that the impulse line head pressure is negligible, mounting
>above isn't a problem IMO. Others opinion may contradict

I'm with Smart_S on this, if your transmitter range is 0 - 100 psig a couple of feet is nothing to get concerned about.

For a buried line you often have no choice.

If you are measuring low pressures for example from an orifice plate you really need to take care to keep air out but you can still mount it above with thought put into tubing design. (the same applies to mounting a transmitter below a gas line with entrained liquid)

Is there any possibility of freezing?
If the takeoff point is at the top and there's any entrained air chances are the impulse lines will soon be full of air also protecting it from a freeze up.

For dirty liquids (entrained solids) it's better above

The only reason really to mount the transmitter above the line is if it has a display and you want the operator to be able to read it easily otherwise why not mount the transmitter at pipe level.

 

Dear dragonmsi,

The most important thing is how the tapping point is installed on the pipe.

For example: you have horizontal pipe with the liquid. The tapping point HAS TO BE installed 45 DEG below the horizontal line of the pipe. Then there are three ways how to install PT:

1) On the same level with the tapping point (the best way if it is possible)

2) Below the tapping point (if pipe is installed too high). Then you have to consider to set up the negative offset value. It will be calculated as a column of the liquid between the tapping point and the PT.

3) Above the tapping point (if pipe is installed too high). Then you have to consider to set up the positive offset value. It will be calculated as a column of the liquid between the tapping point and the PT.

All three ways are correct and never will bring any air to the instrument pipe because the TAPPING POINT HAS INSTALLED CORRECTLY.
Of course, the instrument pipe has to be mounted with the required slopes (in respect to the PT installation).

If the tapping point has been installed wrong then it does not matter where will be installed your PT.

If for example, the tapping point for liquid was installed above the horizontal line of the pipe then it will allows to small air bubbles to gather in tapping point and then during long operation it will create an air block there which bring the measurement error.

 

We can adjust the zero by off-set. The reason for mounting below the line is avoid vapor or air trap, which will affect the zero elevation done during the calibration.

K.V.Kumar