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6.3.7 Orifice plate
6.3.7.1 General information
The orifice plate is a square-edged, concentric bore type, manufactured in stainless steel. It is
designed to be clamped between two flanges, within the PCD of the flange bolts. Working pressure
is up to 24 bar and working temperature is up to 290°C. Width is 3 mm on this vessels line size. In
such cases where a bevel is machined on the bore of the plate, the side with the bevel on should be
facing downstream when installed.
6.3.7.2 Installation of flow sensor
The single most important consideration, when installing an orifice plate type flowmeter is the
amount of straight pipe available on each side of the orifice plate. It is appreciated that long lengths
of straight pipe are not common in the average pump room, but it is essential that the flow through
the orifice plate is not turbulent. The only way to achieve this is, to have a long smooth approach to
the orifice plate. The minimum recommended length of straight pipe upstream of the plate is 10 x
pipe diameters and downstream of the plate, is 5 x pipe diameters. Within these distances there
should be no bends or obstructions such as valves. The effect of reducing these distances is a
considerable reduction of the accuracy of the flowmetering system.
The accuracy of the flowmeter is limited to ±10% and this is easily achieved at high flow rates.
However, at low flow rates, an orifice plate type flowmeter rapidly becomes inaccurate such that the
accuracy limit of ±10% is reached at about 1/10
th
of the maximum flow. If the orifice plate is not
installed with adequate lengths of straight pipe, the effect is a reduction of the effective measuring
range of the flowmeter and a widely fluctuating signal.
The orifice plate should be mounted in such a position so as to ensure a full pipe at all times, which
is usually best achieved in a vertical pipe. The pipeline downstream of the sensor must be arranged
to ensure that no siphoning effect is possible. To prevent siphoning in horizontal installations, the
discharge line must rise by at least 1 x pipe diameter above horizontal centre line. The rise should
be so arranged that it is located at least 5 x pipe diameters after the orifice plate.
It must be stressed that if above installation requirements cannot be met, VAF Instruments should
be consulted for advice. VAF Instruments cannot held be responsible for any flowmetering system
installed incorrectly or without due consultation.
The orifice plate is installed between the flanges using jointing material of 1.5 mm thickness.
Two pressure tapping holes are required, one on each side of the orifice plate. For pipelines larger
than 25 mm diameter, it is advisable to weld a threaded boss to the pipeline. For lines less than 25
mm diameter the pipe itself can be drilled and tapped. In both cases, the hole must be threaded for
¼” BSP. The boss upstream of the orifice plate must be sited 1 x pipe diameter from the centre-line
of the plate. The boss downstream of the orifice plate must be sited ½ x pipe diameter from the
centre line of the plate. In the case of a vertically rising line, the bosses can be sited at any position
on the circumference of the pipe. In a horizontal line, the bosses must be sited on the horizontal
centre line of the pipe.
Reference drawings
Dimensional drawing ball valve flowmeter kit Ballast Monitor 0806-1041
Installation flowmeter Ballast Monitor Mark 5/6 0806-8016
Schematic installation diagram Oilcon Monitor System with 1x dP/I
transmitter Mark 6 0806-8035
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