flow and level measurement - Omega Engineering

the installation and on the nature of
the upstream components in the
pipeline. For example, when a single
90° elbow precedes an orifice plate, the
straight-pipe requirement ranges from
6 to 20 pipe diameters as the diameter
ratio is increased from 0.2 to 0.8.
In order to reduce the straight run
requirement, flow straighteners
(Figure 2-2) such as tube bundles,
perforated plates, or internal tabs
can be installed upstream of the primary
element.
The size and orientation of the
pressure taps are a function of both
the pipe size and the type of process
fluid. The recommended maximum
diameter of pressure tap holes
through the pipe or flange is G" for
pipes under 2" in diameter, K" for 2"
and 3" pipes, H" for 4 to 8" and I" for
larger pipes. Both taps should be of
the same diameter, and, where the
hole breaks through the inside pipe
surface, it should be square with no
roughness, burrs, or wire edges.
Connections to pressure holes
should be made by nipples, couplings,
or adaptors welded to the
outside surface of the pipe.
On services where the process
fluid can plug the pressure taps or
might gel or freeze in the lead lines,
chemical seal protectors can be
used. Connection sizes are usually
larger (seal elements can also be
provided with diaphragm extensions),
and, because of the space
requirement, they are usually
installed at “radius tap” or “pipe
tap” locations, as shown in Figure 2-
3. When chemical seals are used, it
is important that the two connecting
capillaries, as they are routed to
the d/p cell, experience the same
temperature and are kept shielded
from sunlight.
The d/p transmitter should be
located as close to the primary element
as possible. Lead lines should
be as short as possible and of the
same diameter. In clean liquid service,
the minimum diameter is G",
while in condensable vapor service,
the minimum diameter is 0.4". In
steam service, the horizontal lead
lines should be kept as short as possible
and be tilted (with a minimum
gradient of 1 in/ft with respect to
the piping) towards the tap, so that
condensate can drain back into the
pipe. Again, both lead lines should be
exposed to the same ambient conditions
and be shielded from sunlight.
In clean liquid or gas service, the lead
lines can be purged through the d/p
Flow
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2
cell vent or drain connections, and
they should be flushed for several
minutes to remove all air from the
lines. Entrapped air can offset the
zero calibration.
Seal pots are on the wet leg in d/p
cell installations with small ranges
(under 10 in H 2 O) in order to minimize
the level variation in the legs. In
steam applications, filling tees are
recommended to ensure equal
height condensate legs on both sides
of the d/p cell. If for some reason
the two legs are not of equal height,
the d/p cell can be biased to zero
Pipe Taps
Flange Taps
1 in. 1 in.
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Figure 2-3: Differential Pressure Tap Location Alternatives
2 Differential Pressure Flowmeters
out the difference, as long as that
difference does not change.
If the process temperature exceeds
the maximum temperature limitation
of the d/p cell, either chemical seals
have to be used or the lead lines need
to be long enough to cool the fluid. If
a large temperature drop is required, a
coiled section of tubing (pigtail) can
be installed in the lead lines to cool
the process fluids.
The frequency of inspection or
replacement of a primary element
depends on the erosive and corrosive
nature of the process and on the
overall accuracy required. If there is
no previous experience, the orifice
plate can be removed for inspection
Corner Taps
during the first three, six, and 12
months of its operation. Based on
visual inspection of the plate, a reasonable
maintenance cycle can be
extrapolated from the findings.
Orifices used for material balance
calculations should be on the same
maintenance cycle.
• Sizing the Orifice Plate
The orifice plate is commonly used
in clean liquid, gas, and steam service.
It is available for all pipe sizes,
and if the pressure drop it requires is
free, it is very cost-effective for
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