Answer / sam

Gas(non condensing like hydro carbon gasses and air) -
Impulse line comes from pressure taps installed on top of
the flanges or pipe. The DPT is also installed on top of the
pipe so that any liquid that form will run back into the
main line via the impulse lines. It is best to work from a
process zero if the line pressure is above 5Bar.

Liquid - The DPT is installed below the pipe from impulse
lines installed on the sides of the flanges or pipe and
sloping down at a soft angle to the DPT. It is preferred not
to have any sharp bends in these liquid impulse lines. The
idea is to keep these impulse lines filled all the time and
that pressure differences to be transferred as directly as
possible. Keep in mind that in flow, the pressure
differences are sometimes only a few hundred pascals.
You also need to make sure these impulse lines are filled
before you do a zero calibration. The best zero you can do
is when the lines is completely filled but there is no flow
at all, but most of the time you will only be able to do a
zero by isolating the impulse lines from the pipe and then
do the zero.
It is best to work from a process zero if the line pressure
is above 5Bar. Keep the impulse lines filled during the
process zero. (See below)

Steam - The DPT is installed below the pipe from impulse
lines coming from the top of the flanges or pipe. There are
condensation pots (or T-pieces with a needle valve pointing
up)installed at the highest point and they should be at
exactly the same height.
From the bottom of these pots (or T-Pieces) the impulse
lines run down to past and below the DPT and ends with a
small valve at each end for draining purposes. The DPT is
connected with T-Pieces to these impulse lines.
The idea is to keep the condensation pots filled to the
overflow level all the time with hot water so that if any
condensation takes place the pot will just overflow back
into the main line and the different pressures on the DPT
will stay constant. In other words the zero stays constant.
It is best to work from a process zero if the line pressure
is above 5Bar. Keep the impulse lines filled during the
process zero. (See below)
Note:
Impulse line are never installed at the bottom of the
flanges or pipe since any flowing material will normally
deposit some stuff at the bottom inside of the pipe and this
could run into the impulse lines and block it or cause
density changes.

The flow process zero:
It all cases is best to do a process zero if the normal
flowing line pressure is above 5Bar. To do this on line
isolate both the impulse lines and remove the upstream
impulse line from the pressure tapping on the flange or
pipe. Install a flexible tubing with a T-Piece in it, to the
upstream pressure tapping. Connect this flexible tubing to
both sides of the DPT and close the equalization valve on
the manifold.
Open the isolation valve on the pressure tapping so that the
upstream line pressure is now on both sides of the DPT.
Most of the trapped air will flow up the flexible tubing
back into the main line but to be sure you can vent the DPT
at the manifold as well.
Do a zero trim and set the span value.
To make a more permanent installation, install two T-Pieces
in each impulse line just before the manifold and install a
needle valve between the two impulse lines.
Good luck

Answer / samarjit dey

what is purging of DPT in a flow line and what is actual
activity of equaliser and isolation valve at the time of
off line calibration?

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