FPD350 Torbar
Averaging pitot tubes
4 Connecting a Differential Pressure Measuring Instrument
OI/FPD350–EN Rev. H
29
4 Connecting a Differential Pressure Measuring Instrument
4.1 General
Before connecting a differential pressure measuring instrument,
read the manufacturers’s installation instructions.
The Torbar may or may not be supplied with isolating valves
fitted to the two output connections. Check whether or not they
are required.
Connect the Torbar's high-pressure connection (marked H) of
the Torbar (see Fig. 2.1, page 3) to the high pressure side of the
differential pressure instrument and the low pressure connection
(marked L) to the low pressure side of the differential pressure
instrument.
4.2 Impulse Tubing
Use minimum 6 mm (0.25 in) ID piping. For steam flow
measurement, use 13 mm (0.5 in) ID tubing.
Keep tubing as short as possible but ensure the differential
pressure measuring instrument can operate within its
specified temperature limits.
Support tubing over its entire length and isolate it from
sources of vibration or damage.
Route the tubing from the high and low pressure
connections as close together as possible to maintain
equal temperatures.
Do not route the tubing in areas where the ambient
temperature may fluctuate.
Liquid and steam applications
– mount the differential
pressure measuring instrument
below
the level of the
Torbar head, ensuring the tubing slopes
up
towards the
Torbar connections at an incline of at least 25 mm per 300
mm (1 in per 12 in).
Gas applications
– mount the differential pressure
measuring instrument
above
the level of the Torbar head,
ensuring the tubing slopes
down
towards the Torbar
connections at an incline of at least 25 mm per 300 mm (1
in per 12 in).
Liquid applications
– ensure the tubing from the high and
low pressure connections are mounted at the same level
in order to balance the head of liquid in both tubes and
vent any gas build-up.
4.3 Purging
Torbars can be used successfully for measuring the flow of dirty
fluids without purging.
If purging is necessary, periodic purging is the simplest method.
To purge the Torbar, refer to the specific operating instructions
and drawings supplied with the purge unit and:
1. Isolate the measuring instrument from the purge lines.
2. Connect a compatible pressurized medium to the Torbar
output and blow it through the Torbar to force out
contamination.
Torbar Purge Systems are available for intermittent air purging.
4.4 Steam Applications
Ensure both high- and low-pressure Torbar connections are at
the same vertical height to maintain equal condensate levels
above the secondary instrumentation – see Fig. 4.1, page 30.
Mount the filling tees or condensate pots at the same level and
as close as possible to the Torbar head to ensure no
condensate forms in the lagged sections.
Mount the secondary instrumentation below the level of the
Torbar and connect 13 mm (0.5 in) ID tubing (rated for
continuous use at the steam pressure) to the bottom of the filling
tee or condensate pot. Ensure the tubing has a minimum slope
of 25 mm per 300 mm (1 in per 12 in) and ensure the two lines
are close together to maintain equal temperature. Use a 3- or
5-valve manifold to interface the connections between the
secondary instrumentation and the Torbar fitting. Ensure the
condensate temperature does not exceed the temperature limit
of the secondary instrumentation.
For information on start-up and operation procedures, refer to
the manufacturer's instructions for the secondary
instrumentation.
