Orifice  flow meter

An orifice in a pipeline is shown in below figure with a manometer for measuring the drop in pressure (differential) as the fluid passes thru the orifice. The minimum cross sectional area of the jet is known as the “vena contracta.”

How does it work?

As the fluid approaches the orifice the pressure increases slightly and then drops suddenly as the orifice is passed. It continues to drop until the “vena-contracta” is reached and then gradually increases until at approximately 5 to 8 diameters downstream a maximum pressure point is reached that will be lower than the pressure upstream of the orifice.

The decrease in pressure as the fluid passes thru the orifice is a result of the increased velocity of the gas passing thru the reduced area of the orifice.

When the velocity decreases as the fluid leaves the orifice the pressure increases and tends to return to its original level. All of the pressure loss is not recovered because of friction and turbulence losses in the stream.

The pressure drop across the orifice ( ΔP in Fig.) increases when the rate of flow increases. When there is no flow there is no differential.

The differential pressure is proportional to the square of the velocity, it therefore follows that if all other factors remain constant, then the differential is proportional to the square of the rate of flow.

Orifice Construction

Inlet Section

A linearly extending section of the same diameter as the inlet pipe for an end connection for an incoming flow connection. Here we measure the inlet pressure of the fluid / steam / gas.

Orifice Plate

An Orifice Plate is inserted in between the Inlet and Outlet Sections to create a pressure drop and thus measure the flow.

Outlet Section

A linearly extending section similar to the Inlet section. Here also the diameter is the same as that of the outlet pipe for an end connection for an outgoing flow. Here we measure the Pressure of the media at this discharge.

As shown in the adjacent diagram, a gasket is used to seal the space between the Orifice Plate and the Flange surface, prevent leakage.

Sections 1 & 2 of the Orifice meter, are provided with an opening for attaching 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 are built in different forms depending upon the application specific requirement, The shape, size and location of holes on the Orifice Plate describes the Orifice Meter Specifications as per the following

Orifice Plates

Orifice plates are one of the oldest DP flow meter technologies and were first documented in Roman times. The first U.S. patent for the orifice plate was awarded to T.R. Weymouth in 1916. Orifice plates have remained popular because of their simplicity and the inherent scalability and repeatability of the flow through a sharp-edged orifice bore. Orifice plate pressure-based flow elements in existence, the most common is the orifice plate. This is simply a metal plate with a hole in the middle for fluid to flow through. Orifice plates are typically sandwiched between two flanges of a pipe joint, allowing for easy installation and removal

Theory of Head Meters

Head-type flow measurement derives from Bernoulli’s theorem, which states that, in a flowing stream, the sum of the pressure head, the velocity head, and the elevation head at one point is equal to their sum at another point in the direction off low plus the loss due to friction between the two points. Velocity head is defined as the vertical distance through which a liquid would fall to attain a given velocity. Pressure head is the vertical distance that a column of the flowing liquid would rise in an open-ended tube as a result of the static pressure. This principle is applied to flow measurement by altering the velocity of the flowing stream in a predetermined manner, usually by a change in the cross-sectional area of the stream. Typically, the velocity at the throat of an orifice is increased relative to the velocity in the pipe. There is a corresponding increase in velocity head. Neglecting friction and change of elevation head, there is an equal decrease in pressure head. This difference between the pressure in the pipe just upstream of the restriction and the pressure at the throat is measured. Velocity is determined from the ratio of the cross-sectional areas of pipe and flow nozzle, and the difference of velocity heads given by differential pressure measurements. Flow rate derives from velocity and area.

The point where the fluid flow profile constricts to a minimum cross-sectional area after flowing through the orifice is called the vena contracta, and it is the area of minimum fluid pressure. The vena contracta corresponds to the narrow throat of a venturi tube. The precise location of the vena contracta for an orifice plate installation will vary with flow rate, and also with the beta ratio (β) of the orifice plate, defined as the ratio of bore diameter (d) to inside pipe diameter (D):

beta ratio (β) = Bore diameter (d) /  Inside pipe diameter (D)

Types of Orifice Plates

There are 4 types of Orifice Plates that we will discuss

• Concentric Orifice Plate (Hole is in the center)
• Eccentric Orifice Plate (Hole is off center)
• Segmental Orifice Plate
• Others – such as Conical and Quadrant, and Edged Entrance types.

Concentric Orifice Plate

This is generally the most commonly used type of orifice plate with the orifice bore positioned along the pipe centreline. When designing and configuring orifice plates, there are several factors which must be followed to ensure accurate and reliable measurement. The orifice upstream edge should be sharp and square. The minimum plate thickness is standardized based on orifice bore, inside pipe diameter etc. The beta ratio (d/D) should be within recommended limits to ensure conformance with recommended practices. The plate surface flatness should also be within the acceptable tolerance limits.

It is made up of SS and its thickness varies from 3.175 to 12.70 mm. The plate thickness at the orifice edge should not be exceeded by any of following parameters

• 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.

The eccentric orifice plate is similar to the concentric device in that it also exhibits a circular opening/bore. The bore position however is offset from the centreline of the pipe. When the bore is offset and oriented downwards of the pipe centreline, this enables flow of impurities in the measured fluid through the orifice without forming deposits and sediments in front of the aperture.

Eccentric orifice plates are often used in pipelines transporting heterogeneous mixtures, fluids carrying small amounts of non-abrasive solids or gases with small amounts of liquid or where there is the potential for formation of sediment, tar deposits etc.

It is similar to Concentric Orifice plate other than the offset hole which is bored tangential to a circle, concentric with the pipe and of a diameter equal to 98% of that of the pipe. It is generally employed for measuring fluids containing

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

Segmental Orifice Plate.

It has a hole which is a semi circle or a segment of circle. The diameter is customarily 98% of the diameter of the pipe.

This device is essentially a thin plate with an opening in the shape of a circle segment which is comparable to a partially opened gate valve. This device is generally used for measuring liquids and gases carrying non-abrasive impurities including light slurries or exceptionally dirty gases.

The predictable accuracy of both segmental and eccentric orifice plates is not as high or reliable as the concentric orifice plate device.

Quadrant Edged

This type of orifice plate is used for flow such as crude oil, high viscosity syrups or slurries etc.

The inlet edge of the bore of this orifice plate is rounded to a quarter circle. This orifice plate is usually used for viscous fluids & Reynolds number between 2000 to 10000. or above or in between to 3,000 to 5,000 with a accuracy coefficient of roughly 0.5%.

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 enter the Cylindrical section.
• The differential pressure sensor connected between the Inlet and the 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.

Advantages of Orifice meter

• The Orifice meter is very cheap as compared to other types of flow meters.
• Less space is required to Install and hence ideal for space constrained applications
• Operational response can be designed with perfection.
• Installation direction possibilities: Vertical / Horizontal / Inclined.

Limitations of Orifice meter

• Easily gets clogged due to impurities in gas or in unclear liquids
• The minimum pressure that can be achieved for reading the flow is sometimes difficult to achieve due to limitations in the vena-contracta length for an Orifice Plate.
• Unlike Venturi meter, downstream pressure cannot be recovered in Orifice Meters. Overall head loss is around 40% to 90% of the differential pressure .
• Flow straighteners are required at the inlet and the outlet to attain streamline flow thereby increasing the cost and space for installation.
• Orifice Plate can get easily corroded with time thereby entails an error.
• Discharge Co-efficient obtained is low.

Applications of Orifice meter

• Natural Gas
• Water Treatment Plants
• Oil Filtration Plants
• Petrochemicals and Refineries