Daniel Bernoulli introduced the concept of the conservation of energy of fluid flows. Bernoulli determined that an increase in the velocity of a flowing fluid increases its kinetic energy while decreases its static energy. It is for this reason that a flow restriction causes an increase in the flowing velocity and also causes a drop in the static pressure of the flowing fluid. When flow is turbulent the pressure drop across a restriction is proportional to the square of the flow rate. Therefore flow can be measured by taking the square root of a differential pressure cell output. When the flow is laminar a linear relationship exists between flow and pressure drop. A differential Pressure flow meter basically consists of two parts:
1. Primary Elements
2. Secondary Elements
1. PRIMARY ELEMENTS : Differential pressure are known as primary elements; they are
- Orifice plate
- Venturi Tube
- Flow nozzle
- Dahl tubes
- Pitot tubes
2. SECONDARY ELEMENT :Secondary elements are those which measure the differential pressure produced by the primary elements and convert them to signals. Various secondary elements are,
- Bellow/ Diaphragm meter
- Transmitters (mechanical/Electrical/Electronic/pneumatic)
Here I only discuss about orifice plate.
ORIFICE PLATE :
The simplest and the most common pipeline restriction used in DP method of flow measurement is the Orifice plate, which is a thin, circular metal plate with a hole in it. It is hold in the pipeline between two flanges called orifice flanges. It is the easiest to install and to replace.There are four type of orifice plate:
- Concentric Orifice Plate
- Eccentric Orifice plate
- Segmental Orifice plate
- Quadrant Edge Orifice plate
CONCENTRIC ORIFICE PLATE :
It is most widely used. It is usually made of stainless steel and it thickness varies from 3.175 to 12.70mm (1/8 to ½ inch) depending on pipe line size and flow velocity. It has a circular hole (orifice) in the middle, and is installed in the pipe line with the hole concentric to the pipe.
ECCENTRIC ORIFICE PLATE : It is similar to the concentric plate except for the offset. It is useful for measuring fluids containing solids, oils containing water and steam. The eccentric orifice plate is used where liquid fluids contain a relatively high percentage of dissolved gasses.
SEGMENTAL ORIFICE PLATE : This Orifice plate is used for the same type of the service as the eccentric orifice plate. It has a hole which is a segment of a circle.
QUADRANT EDGE ORIFICE PLATE :
This type of orifice plate is used for flows such as heavy crudes, syrups, and viscous flows. It is constructed in such a way that the edge is rounded to form a quarter - circle.