How big is the pipe, and how much differential would you expect to see from the top to the bottom? Another issue might be, where does debris tend to collect, and would it clog the gauge?

What pressure do you want to measure? There is your answer. Top of pipe may be your vapor pressure, but likely not. A vapor pressure measurement would normally be taken from the head space of a tank.

Hi

The pipe is 1.5 inch. Location of gauge will be after a pressure control valve , to read downstream pressure. Pressure is from 3-4 bar. Not sure abt debris

Thank you

Side tap:



Root valve needed? block and bleed? drip leg?

Hi danw2,

Thank you , realy helpful. Can you please share the isa standard number?

Thank you

I would if I knew. The image has the appearance of a training slide. If it's from a standard I don't know which one it is. ISA runs training gigs and I suspect the slide comes from a training gig.

API 551's figure 24 is labeled wrong; gas service is labeled as liquid and vice versa.
The diagram is wrong, but API's description is correct:

c. Placement of taps on the bottom of the line should be
avoided because of the possible presence of sediment or
scale.
d. Transmitters in liquid or condensable-vapor service such
as steam should be self-venting (that is, mounted below the
process connection, with all lines sloping toward the instrument)
to prevent gas from being trapped in the instrument.
e. Transmitters in gas service should be self-draining (that
is, mounted above the process connection, with all lines
sloping toward the process connection) to prevent liquid
from being trapped in the instrument.