Orifice flow meter is a type of differential pressure flow meter. Other type of differential flowmeters includes venture and nozzle. The orifice plate is simply a restriction in the form of plate with an annular hole, which is placed in the flow line. In simple terms, velocity of the fluid passing through the orifice is proportional to the differential pressure across it.
The orifice plate is called the primary element. Impulse lines must be connected from the upstream and downstream pressure tapings, to a secondary device known as a DP or Differential pressure cell to measure the differential pressure. Information is fed from the DP cell to a flow indicator to provide the density compensation.
A hole at the bottom of the pipe is drilled, this prevents the build up of water on the steam lines.
There is considerable scope for variation in the geometry of orifice plate, especially in the positioning of the pressure tapings.
Correct installation is absolutely essential and the standards used are BS 1042 and ISO 5167 respectively.
Advantages:
1. Orifice are small plates and easy to install/remove.
2. Offer very little pressure drop of which 60 to 65 % is recovered.
3. Easy maintenance.
Disadvantages:
1. Turndown is limited to 4 or 5:1
2. A minimum of 20 upstream and 10 downstream straight unobstructed pipe diameters are needed for correct installation. This can sometimes be difficult to achieve in practice where unrestricted space is simply not available.
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