When an orifice plate is placed in a pipeline carrying fluid, the orifice plate is installed perpendicular to the flow of the fluid to be measured. As all DP flowmeters, Orifice plate makes a restriction in the flow. This restriction causes a pressure drop across the plate. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a measure of flow rate.
This minimum cross-sectional area of the fluid obtained at downstream from the orifice edge is called VENA-CONTRACTA.
A differential pressure is generated across the orifice plate. The differential pressure sensor attached to upstream and downstream records the pressure difference between these two points which becomes an indication of the flow rate of the fluid through the pipe when calibrated.
A simple orifice meter attached to manometer is shown above. The differential pressure is transmitted to DP transmitters and the pressure difference is converted to flow rate. The flow rate is square root function of the pressure difference, Square root extractors are used to measure the flow rate.
Options are available for unclean material or flows that contain liquid and gas of varying densities. When measuring liquids the bore can be positioned at the top of the pipeline to allow the passage of gases. The same applies when allowing suspended solids to pass, by positioning the bore at the bottom and gaining a more accurate liquid flow measurement. Half-circle bores are often used with light slurries or dirty gases.
The pressure removal holes or slits are located in the aperture and mounting rings. The location of these pressure taps depends on the type of tap being used. However, the taps are usually located in the adjacent flanges, or one diameter upstream and one-half diameter downstream.
Types of Orifice plate:
There are two main types of orifices for various applications:
- Concentric, square edged
- Concentric, quadrant edged
- Eccentric or segmental square edged
Concentric, square edged:
This is the most common and basic type of orifice meter. This device is typically a
thin concentric sharp-edged orifice plate. Because of the simplicity, it is inexpensive
to manufacture to very close tolerances. This also simplifies the ease in which it can
be installed and replaced.
Concentric, quadrant edged:
This type of orifice plate is used to give increased stability in flow, and is about 10
times that for conventional plates
Eccentric or segmental square edged:
These are generally used when the process material contains foreign matter that may
block the orifice in the case of a concentric configuration
- Simple construction.
- It has predictable characteristics and occupies less space.
- Easily fitted between flanges.
- No moving parts.
- Large range of sizes and opening ratios.
- Suitable for most gases and liquids.
- Well understood and proven.
- Price does not increase dramatically with size.
- Pressure recovery at downstream is poor, that is, overall loss varies from 40% to 90% of the differential pressure.
- In the upstream straightening vanes are a must to obtain laminar flow conditions
- Inaccuracy, typically 1%.
- The pipeline must be full (typically for liquids).
- Low Rangeability, typically 4:1.
- Accuracy is affected by density, pressure and viscosity fluctuations
- Viscosity limits measuring range
- Gets clogged when the suspended fluids flow.
- The coefficient of discharge is low.