Introduction to Orifice Flange
Orifice Flanges are widely used in conjunction with orifice meters for measuring the volumetric flow rate of liquids and gases through a pipe. Two orifice flanges are called an orifice flange union. Each flange comes with a pipe tap for measuring the pressure drop of the flow through an orifice plate. This makes separate orifice carrier or tapping in the pipe wall unnecessary. Orifice plates do not come with the flanges and are sized based on the requirements of the process. One / Two jack screws are used to spread the flanges apart in order to change the orifice plate.
Orifice flanges are basically the same as standard weld neck, slip-on and screwed flanges except for the provision of radial, tapped holes in the flange ring for meter connections and additional bolts to act as jack screws to facilitate separating the flanges for inspection or replacement of the orifice plate.
- For more on weld neck, slip-on and screwed flanges, check out Introduction to Flanges
In choosing the type of orifice flange, the considerations affecting the choice of weld neck, slip-on and screwed standard flanges apply with equal force. They are made in pressure classes 300#, 600#, 900#, 1500# and 2500#; in carbon steel, stainless steel and alloy materials; and raised faced or ring joint.
They are not available in 150# sizes because the thickness of 150# flanges are not enough to allow for drilling pressure taps.
Orifice flanges are installed in a straight run of pipe in order to avoid turbulence at the plate. As a rule of thumb, an orifice flange has ten diameters of straight pipe upstream and five diameters of pipe downstream. There are guidelines set out in (American Gas Association) AGA Report #3 which is the Orifice Metering of Natural gas.
When installing the orifice plate flange the pressure taps need to be at the same elevation to each other. The assumption when making the calculation for the orifice plate is that static head of the tubes is the same on both sides of the orifice plate. Pressure taps should not be installed facing down as the taps could become plugged with debris in the line.
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