What is Orifice Meter and How does it work?

Orifice Meter

An orifice meter is a type of flow meter that measures the rate of flow through a pipe by inserting an orifice plate between two pipe flanges and setting up a proper pressure connection. The Venturi meter and the Orifice meter both operate on the same principles. 

What is the principle of Orifice meter?

The orifice plate allows the liquid or gas whose flow rate has to be measured to pass through. The consequence is a pressure differential between the outlet and inlet segments due to a pressure drop through the orifice plate that varies with the flow rate.

Orifice meter components 

A standard orifice meter consists of an orifice plate, a manometer, and a flange or carrier ring, pipe, flow dissipator.

1.   Orifice Plate – Orifice plates are metal plates with a hole punched through them that have the same diameter as the pipeline and reduce the volumetric flow rate to lower pressure.

2.   Manometer – A manometer is a device used to gauge the pressure within an orifice’s input and exit sections. 

3.   Flange/Carrier Ring –  used with orifice meters to hold the orifice plate in place.

4. Inlet – this is where the fluid enters from the pipeline and the change in velocity and pressure happens here

5. Overflow pipe – It would prevent the overflow of the glass cylinder

6. Flow dissipator– this is the place that we can see orifice plate, flow dissipation is done by the taps and thus pressure changes

7. Outlet– It is the place where the fluid exits the device

8. Pitot tube – it measures the pressure drop

Beta Ratio

         The orifice plate to be utilized, its design, and manufacture all affect how well orifice meters work. Typically, mild steel, stainless steel, and phosphor bronze are used to create orifices. The Beta Ratio is one of the general factors taken into account while developing an orifice plate. 

         The ratio of the orifice diameter to the pipe diameter is known as the beta ratio. It is discovered that the flow coefficient is steady between beta ratios of 0.2 and 0.7.

Beta Ratio

A higher beta ratio will result in a modest pressure difference that is difficult to monitor and will necessitate a longer pipe length. 

A lower beta ratio will result in more pressure loss, which may demand a larger pump. Frictional loss will also increase the cost of the system.

Operation of an Orifice Meter

            In the fluid’s upstream direction, the orifice plate’s aperture is approached by a fluid with a uniform cross section of flow. Prior to the orifice plate, this region of pressure is thought to be at its peak. 

         The fluid’s cross section is lowest and uniform when it exits the orifice plate’s opening, and after traveling a certain distance, it begins to diverge in the downstream direction.    

The minimum cross-sectional area of the fluid obtained at downstream from the orifice edge is known as vena-contracta. When the fluid exits the orifice opening, its pressure is minimal, and this minimal pressure remains constant in the minimum cross section area of fluid flow at the downstream.

Types of Orifice Plate

         In general, the types of Orifice meter are classified as concentric, segmental, eccentric, quadrant edge.

Design of Orifice Plate

Concentric square edged

         The main applications for this kind of orifice meter are the measurement of clean liquid, gas, and low-velocity vapour fluxes. It has a thin, flat plate with a sharp, square-edged hole punched into it. This kind is employed to lower pressure and restrict flow. High accuracy can be reached with this sort of meter if the installation is done correctly.

Concentric – Bore & Bevel 

The flow opening in an eccentric orifice is not located in the middle of the plate. Typically, fluids carrying a tiny amount of non-abrasive solids are measured using this type. The majority of chemical industry employs this type. 

High viscosity fluids are typically employed with this type of opening. The plate thickness will be at least the inlet’s radius because the inlet is a quarter of a circle.

Concentric – Bore without Bevel 

         For some combinations of line size and plate thickness, a bevel is not needed to meet edge thickness requirements. In this case, the hole is drilled all the way through the plate’s thickness. This hole is also used for measuring in both directions and for restricting flow.

Bore & Counterbore

The bore and counterbore is a unique way to limit the thickness of the plate edge. Instead of the usual 45-degree angle, the edge of the plate is counterbored to the thickness that is needed.

Segmental Bore 

         Segmental bore orifice plates are offered to permit the passage of solids, liquids, and bubbles. It is manufactured so that the circular section of the bore is tangent to 98% of the pipe ID. This type is typically employed in locations with significant entrained air or water as well as locations where fluid suspension occurs.  Thus, the accumulation in front of the orifice plate would be prevented. The sewage treatment, steel, chemical, water conditioning, paper, and petrochemical industries are just a few that utilize this kind of bore.

Eccentric Bore 

         The flow opening in an eccentric orifice is not located in the middle of the plate. Typically, fluids carrying a tiny amount of non-abrasive solids are measured using this type. The majority of chemical industry employs this type. 

            High viscosity fluids are typically employed with this type of opening.  steel, paper, atomic, and petrochemical industries all employ eccentric orifice plates. The plate thickness will be at least the inlet’s radius because the inlet is a quarter of a circle.

Quarter Round Bore (Quadrant Edge) 

• The quarter round or quadrant bore offers a rounded inlet edge. The radius of the quarter round bore is a function of the orifice to pipe ratio with thickness at the throat equal to radius. 
• Overall plate thickness is frequently greater than the thickness is frequently greater than the thickness of standard plates. This bore is specifically designed for fluids of high viscosity, including heavy crudes, syrups.

Types of Pressure tap used in Orifice Meter 

Corner Taps

• Openings for the pressure taps are situated in the upstream and downstream flanges holding the orifice plate in this configuration. 
• The orifice plate should be as near as feasible to these apertures. They are, prone to dirt freezing and hydration blockage. 
• Compared to flange or vena contracta taps, they are typically less reliable and more susceptible to upstream disturbances.

Radius Taps

• The upstream tap is far enough upstream to be unaffected by distortion of the flow in the immediate area of the orifice, and the downstream pressure tap is situated at roughly the mean location of the vena contracta, making this design the most advantageous from a practical standpoint. 
• The upstream tap can actually be placed up to two pipe diameters away from the plate without impacting the outcome.

Pipe Taps

• In this, the orifice plate is 2½pipe diameters upstream and 8 pipe diameters downstream of the holes for the pressure taps. Both taps are situated at the area where the flow has fully grown. 
• So they are helpful for calculating the overall pressure losses in the pipeline since they provide the whole pressure loss caused by the orifice.

Flange Taps

• One pressure tap hole is located one inch upstream and the other one inch downstream from the orifice plate in this configuration.
• They can be inspected because of their accessibility and near proximity to the flange’s face. 
• They may be used to measure flow in either direction because of their symmetry. Because the downstream tap is situated in a highly unstable pressure area, they should not be used in pipe sizes smaller than 2 inches where the ratio is high.

 Vena Contracta Taps

• The downstream tap is situated at the point of minimum pressure, the vena contracta, while the upstream pressure tap is 12 to 2 pipe diameters from     the orifice plate. 
• The pressure at these taps should be the lowest and the pressure drop for fluid going through the orifice should be the greatest, according to theory. 
• The appropriateness of these sorts of taps is, however, constrained by a practical issue, such as the vena contracta’s tendency to shift location with flow rate.

Orifice Selection Guidelines

• The CPI handles a wide variety of fluid streams, including clean liquids, wet steam, and liquids with solid particles. The applicability of the various types of orifices depends on the stream’s characteristics. 
• The best way to gauge the flow of pure liquids, gases, and low velocity vapour’s is via concentric orifices. 
• Install square-edged orifices for Reynolds number under 10,000. However, the discharge coefficient of square edged orifices changes significantly with either flow rate or viscosity at Reynolds number lower than 10,000. Because of this, either quadrant orifices with conical edges are favored
• Orifices with quadrant edges are comparatively resistant to the effects of corrosion, erosion, and the deposition of materials on the orifice’s surface.
• Concentric orifices are typically not used with granular grains in flowing fluids, condensate in steam, or vapour or gas in a liquid because the projecting rims of the orifice form a dam and these foreign elements pile up in the approach pipe at the plate, changing the distribution of flow.
• Install an eccentric or segmental orifice plate with the hole at the bottom of the horizontal pipe to allow the free flow of granular solids and condensate. 
• However, the concentric orifice is chosen because of its greater discharge coefficient accuracy if the orifice can be situated in a vertical run with the flow going downward. 
• Use a segmental orifice with the opening at the top of the pipe when measuring liquids that contain gas or vapour in a horizontal pipe. 
• Concentric orifices are preferred for these fluids, albeit, for greater precision, provided suitable places are available in vertical pipes with the flow going upward
• Measurements of liquids containing sickly or liquid dense particulates should not be performed using segmented orifice plates. 
• Similar to a concentric orifice, this design is impacted by deposits on the face or edge of the orifice. 
• Annular orifices are advised when fluid streams comprise both heavy materials and gas or vapour.

 Application of Orifice Meter

1. The concentric orifice plate, which has been standardized, is used to monitor the flow rates of pure fluids. Concentric orifice plates have beta ratios between 0.25 to 0.75. 
2. The flow rates of fluids containing suspended components, such as particles, oil mixed with water, and wet steam, are measured using eccentric and segmental orifice plates. 
3. Eccentric orifice plates are often utilized for contaminated gases and liquids. 
4. Segmental orifice plates are preferred over eccentric orifice plates when dealing with heavy fluids because they allow for better drainage all the way around the pipe.

Advantages of Orifice Meter

1. The method to monitor flow rate is very affordable and simple. 
2. It takes up less room and has predictable properties. 
3. It is possible to use it to gauge the flow rates in big pipes.
   

Disadvantages of Orifice Meter

1. The orifice plate’s sharpness and the pipe’s inner wall roughness affect the vena-length contracta’s. Due to the aforementioned reason, it can occasionally be difficult to tap the minimal pressure. 
2. The downstream pressure recovery is below average.
3. When the flow of the suspended fluids, it becomes obstructed. 
4. The orifice plate corrodes, and eventually this leads to inaccuracy.

Calculations on Orifice Meter

         The assumption that the pipe is horizontal and the disregarding of friction are necessary steps in the process of deriving the equation for the orifice meter.

Taking into account the continuity equation for the constant p,

 we will first make a substitution, then we will multiply a factor by the result (friction loss factor in orifice),

Therefore the equation for the orifice,

where,

Installation of Orifice Plate

How to install orifice plate in pipeline?

The transmitter must be connected to the process using the connectors present in the pipeline in order to measure the differential pressure. Impulse lines and differential pressure sockets are the technical names for these connection lines. These elements designs must be in line with the major element type and the kinds of connections that were employed in the computation. The orifice plate with flange sockets is typically utilized. Two valves known as “root valves” are inserted into the orifice flange sockets, allowing the impulse lines and the transmitter to be isolated from the primary process line.

Horizontal Installation 

         To allow trapped vapours to escape from the connection lines and to stop silt from entering these lines, pressure connections for horizontal lines should be established at the side of the line. In this manner, both the pressure taps and the instrument will only measure the differential pressure that corresponds to the flow rate, which is always full and balanced.

Horizontal Installation for

• Clean Fluids
• Dirty or corrosive Fluids using a seal 
• Clean Non-condensable gas 
• Vapor or dirty or condensable gases

Vertical Installation

The impulse pipework should be configured depending on the direction the orifice plate is installed.The impulse pipework needs to be set up as stated below if the fluid is ascending. The pressure transmitter’s compensation setting can be used to account for variations in static head pressure.

Vertical installation for

• clean liquids
• Dirty or corrosive liquids
• Clean non-condensable gases                                 
• vapor condensable gases or dirty gases

Orifice Meter installation for Vapor condensable gases or dirty gases

How Venturi meter is different from Orifice meter?

• The main difference between a venturi meter and an orifice meter can be that the orifice plate in the orifice meter can easily alter as per the various flow rates while the venturimeter is rigid with the change in flow rate.
• Consequently, this represents a key distinction between the two instruments.
• Unlike an orifice meter, which is inappropriate for measuring higher flow rates, a venturi meter can measure higher flow rates. Because the losses in a venturimeter are so low, the coefficient of discharge is higher, whereas the losses in an orifice meter are significant and cannot support a high coefficient of discharge.
•  When compared to an orifice meter, the installation and repair costs of a venture meter may be higher. In compared to an orifice meter, the venturi meter may also require more room.