Measurement of flow rate and quantity is the oldest of all measurements of process varlables in the field of instrumentation. It is made for determining the proportions and the amount of material flowing in or out of process. For the purpose of cost accounting, such measurement is often required to be made for steam, water and gas services. Without flow measurements, plant material balancing, quality control and even the operation of any continuous process would be almost impossible.

Orifice plates are designed as per BS EN ISO 5187. Orifice plate can be supplies complete with ANSI B16.36 orifice flanges. Orifice plates are the simplest and cheapest form of primary elements and are use more frequently than all other types. An orifice plate is inserted in the line and the differential pressure across it is measured. There are five types of orifice plates which are listed below;

• Concentric orifice plate
• Quadrant edge orifice plate
• Eccentric orifice plate
• Restriction orifice plate
• Segmental orifice plate

It is mostly widely used. These plates have a sharp, square edged hole bored in a flat thin plate with or without bevel at the outlet depending on size & usage. It has a circular hole (orifice) in the middle, and is installed in the pipe line with hole concentric to the pipe.

It is similar to the concentric plate except for the offset hole which is bored tangential to circle, concentric with the pipe and of a diameter equal to 98% of that of the pipe. Location of the bore prevents damming of solid materials of foreign particles and makes it useful for measuring fluids containing solids, oils containing water and wet stream. The eccentric orifice plate is used where liquid fluid contains a relatively high percentage of dissolved gasses, and installed with the bore tangential to upper surface of the pipe when the flowing material is liquid and tangential to the lower surface of the pipe when the fluid is a gas.