# How an Orifice Measures Flow?

An orifice meter is a type of flow meter that uses the Differential Pressure Measurement concept to measure the rate of flow of liquid or gas, particularly steam. It is mostly utilized for heavy-duty applications because of its resilience and low cost.

It comprises an Orifice Plate, which is the instrument’s basic element, as the name implies. When this Orifice Plate is arranged in a line, a pressure difference is created across the Orifice Plate.

This pressure drop is linear and proportional to the rate at which the liquid or gas flows. Because there is a pressure drop, similar to a Turbine Flowmeter, it is utilized where a pressure drop or head loss is acceptable.

A pressure drop is created by an orifice meter, which is both a conduit and a restriction. The flow restriction can be a nozzle, venturi, or a thin sharp-edged orifice.

It is required to empirically calibrate any of these devices before using them for measurement. 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. No further calibration is necessary if the normal mechanics of construction are followed.

## Orifice Flow Meter

In the diagram below, an orifice in a pipeline is illustrated with a manometer for monitoring the pressure drop (differential) as the fluid flows through it. The vena contracta refers to the jet’s smallest cross-sectional area.

## 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 climbs until it reaches a maximum pressure point about 5 to 8 diameters downstream, which is lower than the pressure upstream of the orifice.

The higher velocity of the gas traveling 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 velocity of flow rises, the pressure drop across the orifice (P in Fig.) increases. There is no disparity when there is no flow.

Because differential pressure is proportional to the square of velocity, the difference is proportional to the square of the rate of flow assuming all other variables remain constant.

## Orifice Construction

### Inlet Section

For an incoming flow connection, a linearly extending portion of the same diameter as the inlet pipe is used as an end connection. The fluid/steam/gas input pressure is measured here.

### Orifice Plate

To establish a pressure drop and hence measure the flow, an Orifice Plate is put between the Inlet and Outlet Sections.

### Outlet Section

A part that extends in a straight line, comparable to the Inlet section. For an end connection for an outgoing flow, the diameter is also the same as the exit pipe. The pressure of the media at this discharge is measured here.

A gasket is used to seal the area between the Orifice Plate and the Flange surface, as indicated in the diagram below, to avoid leaking.

An aperture is provided in sections 1 and 2 of the Orifice meter for adding a differential pressure sensor (u-tube manometer, differential pressure indicator).

## Material of construction :

The Orifice plates in the Orifice meter, in general, are made up of stainless steel of varying grades.

### Shape & Size of Orifice meter :

Orifice meters come in a variety of shapes and sizes, depending on the application. The shape, size, and location of holes on the Orifice Plate define the Orifice Meter Specifications as follows:

• Concentric Orifice Plate
• Eccentric Orifice Plate
• Segment Orifice Plate

#### Concentric Orifice Plate

It is made of stainless steel and ranges in thickness from 3.175 to 12.70 mm. Any of the following parameters should not exceed the plate thickness at the orifice edge:

• 1 – D/50 where D = The pipe inside diameter
• 2 – d/8 where d = orifice bore diameter
• 3 – (D-d)/8

*Beta Ratio(β): It is the ratio of orifice bore diameter (d) to the pipe inside diameter (D).

#### Eccentric Orifice Plate

Except for the offset hole, which is drilled tangential to a circle, concentric with the pipe, and of a diameter equal to 98 percent of the pipe’s diameter, it’s comparable to a Concentric Orifice plate. It is commonly used to measure fluids containing

• Media having Solid particles
• Oils containing water
• Wet steam

#### Segment Orifice Plate

It has a hole that is a semicircle or a circle segment. The diameter is usually 98 percent of the pipe’s diameter.

## Operation of Orifice meter

• The fluid flows inside the Inlet section of the Venturi meter having a pressure P1.
• As the fluid proceeds further into the Converging section, its pressure reduces gradually and it finally reaches a value of P2 at the end of the Converging section and enters the Cylindrical section.
• The differential pressure sensor connected between the Inlet and the Cylindrical Throat section of the Venturi meter displays the difference in pressure (P1-P2). This difference in pressure is in direct proportion to the flow rate of the liquid flowing through the Venturi meter.
• Further, the fluid passed through the Diverging recovery cone section and the velocity reduces thereby it regains its pressures. Designing a lesser angle of the Diverging recovery section helps more in regaining the kinetic energy of the liquid.

• In comparison to other types of flow meters, the Orifice meter is quite inexpensive.
• Installing requires less area, making it excellent for applications with limited space.
• The operational response can be meticulously planned.
• Vertical, Horizontal, and Inclined installation directions are all options.

### Limitations of Orifice meter

• Due to contaminants in gas or opaque liquids, it is easily clogged.
Due to limits in the vena-contracta length for an Orifice Plate, the minimal pressure that can be achieved for reading the flow is sometimes difficult to achieve.
In contrast to Venturi meters, downstream pressure in Orifice Meters cannot be recovered. The overall head loss ranges from 40% to 90% of the differential pressure.