What is an Orifice Meter?
A pressure drop is created by an orifice meter, which is both a conduit and a restriction. A type of orifice is an hourglass. The flow restriction can be a nozzle, venturi, or a thin sharp-edged orifice.
To utilize any of these instruments for measurement, they must first be empirically calibrated.
To create a standard for measuring other quantities, pass a known volume through the meter and record the reading.
The narrow sharp-edged orifice has been adopted as a standard and substantial calibration work has been done such that it is widely acknowledged as a standard way of measuring fluids due to its ease of duplication and simple construction. Assuming that the standard mechanisms are followed.
The illustration depicts an aperture in a pipeline with a manometer for monitoring the pressure drop (differential) as the fluid travels through the orifice. The “vena-contracta” is the minimal cross-sectional area of the jet.
Orifice Meter
How does it work?
The pressure rises slightly as the fluid approaches the aperture, then decreases abruptly when the orifice is passed. It continues to decline until it reaches the “vena contracta,” then gradually raises until it reaches a maximum pressure point at 5 to 8 diameters downstream.
The higher velocity of the gas travelling through the smaller area of the orifice causes the pressure to drop as the fluid goes through. As the fluid exits the orifice at a lower velocity, the pressure rises and tends to return to its initial level.
Because of friction and turbulence losses in the stream, not all of the pressure loss is recovered.
When the rate of flow rises, the pressure drop across the orifice increases. There is no disparity when there is no flow. Because differential pressure is proportional to the square of velocity, if all other variables remain constant, the differential is proportionate.
BETA RATIO
is the ratio of the orifice plate bore to the pipe diameter The Beta Ratio, or d/D, is the ratio of plate bore to pipe I.D., where d is the plate bore and D is the pipe I.D.
THE ORIFICE PLATE
To handle varied flow measurement duties, the orifice plate bore can be produced in a variety of forms. The flowing circumstances should be examined to determine which arrangement is best for each measuring task.
a. The Thin Plate, Concentric Orifice
To ensure precise and reliable measurement, certain basic principles must be followed in the design and use of orifice plates. The orifice’s upstream edge must be crisp and square. The minimum plate thickness is specified depending on pipe I.D., orifice bore, and other factors.
The plate shall not deviate from flatness by more than 0.01 inch per inch of dam height (D-d)/2 along any diameter. to comply with
b. Eccentric Orifice Plates
The eccentric plate features a round orifice (bore) that is perpendicular to the pipe’s inside wall. Because the opening at the bottom of the pipe allows the solids and liquids to pass through rather than collect at the orifice plate, this type of plate is most typically used to measure fluids that contain a small number of non-abrasive materials or gases with small amounts of liquid
c. Segmental Orifice Plates
A segmental orifice plate’s aperture is similar to a partially opened gate valve. This plate is typically used to measure non-abrasive impurities in liquids or gases, such as light slurries or exceptionally dirty gases.
Both the eccentric and segmental plates have less predictable precision than the concentric plate.
d. Quadrant Edge Plate
For fluids with a high viscosity, a quarter-circle or quadrant aperture is utilized. The orifice has a rounded edge with a certain radius that is a function of the orifice diameter.
e. Conic Edge Plate
A 45° bevel on the conic edge plate faces upstream into the flowing stream. It’s useful for Reynolds numbers even lower than the quadrant boundary.
Meter Tap Location
a. Flange Taps
With a + 1/64 to +1/32 tolerance, these taps are one inch from the orifice plate’s upstream face and one inch from the downstream face.
b. Pipe Taps
c. Vena – Contracta Taps
These taps are one pipe diameter upstream and downstream of the point of minimum pressure (this point is called the vena-contract). However, this threshold fluctuates with the Beta ratio, therefore they are rarely utilized in applications other than plant measurement where flows are relatively stable and plates are not altered. Tables with exact dimensions are provided.
d. Corner Taps
Upstream and downstream, these taps are immediately adjacent to the plate faces. Corner taps are most commonly used in Europe, where they are utilized with custom sharpened flow meter tubes for low flow rates in lines less than 2 inches.
General Installation Recommendations
- Meter manifold pipework should be installed at all times to allow for calibration and to prevent the differential element from being over-range.
- The meter should be installed as close to the orifice fitting as practicable.
- To avoid any high or low points in the manifold lines, always gently slope them from the orifice fitting to the meter.
- To remove evaporation, use condensate chambers or air traps.
When pressurizing or depressurizing differential measuring devices, it is critical to deliver or release pressure uniformly to or from the high and low meter chambers to avoid imposing excessive over range.
Types of Orifice Fittings
- Single Chamber: It is made out of a single chamber that is used to precisely measure the flow rate of a fluid or gas, as the name suggests.
- Dual Chamber: It comes with O-ring seals and is placed in conjunction with downstream and upstream pieces, removing the need to maintain the device’s gasket. Furthermore, for dual chamber orifice fitting, a valve seal is available.
- Double Block and Bleed (DBB): It has two valves for various chambers, ensuring operating safety. It also protects the equipment from harmful liquids and extends its life.
Applications
- It’s used to figure out how fast fluids flow in their pure state (i.e. gaseous state or liquid state).
- It can also be used to determine the flow rate of fluids that are in a mixed condition (both gaseous and liquid), such as wet steam or natural gas with water.