Orifice plate aSHVANI Shukla

C&Ireliance

Orifice Plate

An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow (in the latter two cases it is often called a restriction plate). Either a volumetric or mass flow rate may be determined, depending on the calculation associated with the orifice plate. With an orifice plate, the fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict.

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Orifice Plate is the heart of the Orifice Meter. It restricts the flow and develops the Differential Pressure which is proportional to the square of the flow rate. The flow measuring accuracy entirely depends upon the quality of Orifice plate, its installation and maintains.

When measuring wet gas or saturated steam a weep hole is drilled in a concentrically bored orifice plate. This is a small hole drilled on the orifice plate such that its location is exactly on ID of the main pipe.

The Orifice plates are manufactured as per ISA / AGA/ API / ANSI standards and in various materials

such as SS304 /SS316 / SS316L /Hestoly C / Monel / PTFE coated.

Various bores are used for various applications. Orifice Plate is categories in two types :- Paddle Type & Universal Palate.

Paddle Type Orifice Plate This plate is sandwiched between two Orifice Flanges. Tag Plate of orifice plate

projects out from Orifice flanges and it indicates the existence of Orifice plate. Details such as Tag NO /Orifice ID / Pipe ID / Plate Material are stamped on one side of the tag plate which faces upstream side of the pipe line. Outside diameter of the orifice plate equals to PCD-1 Bolt Dia. This ensures the concentricity with the main pipe line. The other method to maintain the concentricity is by using sleeves on the bolts or by providing dowel pins on the Orifice Flanges.

Universal Orifice Plate This is a circular plate designed to fit in the Orifice fittings / Plate holders /

carrier rings / Ring Type Joints(RTJ).

Technical Specification

1.Size for Integral Design : 15, 20, 25, 40 mm 2.Size for Flanged Design : 25, 40, 50, 65, 80, 100, 150 ...250 mm 3.Material- Flanges & Carrier Ring : A105 / SS304/ SS316 / SS316L / CS & Other materials on request. 4.Orifice Plate : SS304, SS316, SS316L, Hast C, Monel, PP, PVC,PTFE, Coated or Clad with PP / HDPE / PTFE. 5.Gasket : CAF / SS Spiral Wound + CAF / PTFE / PVC / Rubber, Other materials as per special request. 6.Stud / Nut : ASTM A193 Gr B/ASTM A194 CI 2H A193 B16/A194 C14 7.Standards Applicable : Design - ISA RP 3.2 / DN 1952 / BS 1042 - 1981-84 8.Bore Calculation : ISO 5167 / BS 1042 / RW Miller / L. K. SPIN / AGE - 3.7 9.Flanges : ANSI B-16-36 / or Equivalent 10.Types : Square edge concentric, Quadrant edges, Conical entrance, Eccentric. 11.Pressure Toppings : For 1" to 16" - Flange Taps / Corner Taps. Above 16" - D x D/2

Type Orifice Plate

Paddle Type Orifice Plate

Concentric Beveled BoreApplication : This Most Common Bore Used In The Industries. This Is The Only Type Generally Accepted For Use In Custody Transfer Measurement, Since Adequate Data Is Not Available For Other Bores. Used Primarily For Clean Homogeneous Liquids, Gases, Non Viscous Fluids. The Bevel Is Matched At 45° Angle To The Desired Throat Thickness.

Application : This Type Is Not Used For Flow Measurement But For Dropping The Pressure Considerably And Reducing The Flow Accordingly. The Bore Is Not Beveled But Kept Straight. The Beta Ratio Has No Limit As Accuracy Is Not The Goal

Restriction Bore

Eccentric Bore

Application : Used For Measurement Of Flow For Fluids Containing Solids And Slurries. It Is Also Used For Vapors And Gases Where Condensation Is Present. The Eccentric Bore Is Offset To Where The Bore Edge Is Inscribed In A Circle That Is 98% The Line Id.

Segmenta Bore

Application : The Segmental Bore Is

Located In The Same Way That The Eccentric Bore Is. This Type Is Used Primarily For Slurries Or Extremely Dirty Gases Where The Flow May Contain Impurities Heavier Than The Fluid.

Quadrant Bore

Application : Used For High Viscous Fluids Such As Heavy Crude, Syrups  And Slurries. It Is Always Recommended For Flow Where Reynolds Number Is Less Than 10,000.The Inlet Is Quarter Of A Circle And The Plate Thickness Must Be At Least Radius Of The Inlet.

Ring Type Joint –Integral

Application : These Are

Available In Oval Or Octal Shapes. Orifice Plate Is A Part Of RTJ Gasket.

Ring Type Joint- Separate

Application : These Are Available In Oval Or Octal Shapes. The  Orifice Plate Is Universal Type And Snap Fitted On The RTJ Gasket By Screws.

Universal Orifice Plates

Application : This Is A Circular Plate Designed To Fit In The Orifice Fittings / Plate Holders / Carrier Rings / Ring Type Joints(RTJ).

Various Orifice Assemblies

WNRF - Flange Taps WNRF - Corner Taps

Orifice working principle

Working:§The orifice plate, being fixed at a section of the pipe, creates an obstruction to the flow by providing an opening in the form of an orifice to the flow passage.

When an orifice plate is placed in a pipe carrying the fluid whose rate of flow is to be measured, the orifice plate causes a pressure drop which varies with the flow rate. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a measure flow rate. The flow rate is given by.

Where, Qa = flow rateCd = Discharge coefficientA1 = Cross sectional area of pipeA2 = Cross sectional area of orificeP1, P2 = Static Pressures

•The main parts of an orifice flow meter are as follows:

A stainless steel orifice plate which is held between flanges of a pipe carrying the fluid whose flow rate is being measured.•It should be noted that for a certain distance before and after the orifice plate fitted between the flanges, the pipe carrying the fliud should be straight in order to maintain laminar flow conditions.•Openings are provided at two places 1 and 2 for attaching a differential pressure sensor (U-tube manometer, differential pressure gauge etc) as shown in the diagram.

Operation of Orifice Meter

The detail of the fluid movement inside the pipe and orifice plate has to be understood. The fluid having uniform cross section of flow converges into the orifice plate’s opening

in its upstream. When the fluid comes out of the orifice plate’s opening, its cross section is minimum and uniform for a particular distance and then the cross section of the fluid starts diverging in the down stream.

At the upstream of the orifice, before the converging of the fluid takes place, the pressure of he fluid (P1) is maximum. As the fluid starts converging, to enter the orifice opening its pressure drops. When the fluid comes out of the orifice opening, its pressure is minimum (p2) and this minimum pressure remains constant in the minimum cross section area of fluid flow at the downstream.

This minimum cross sectional area of the fluid obtained at downstream from the orifice edge is called VENA-CONTRACTA.

The differential pressure sensor attached between points 1 and 2 records the pressure difference (P1 – P2) between these two points which becomes an indication of the flow rate of the fluid through the pipe when calibrated.

Applications of Orifice Meter

The concentric orifice plate is used to measure flow rates of pure fluids and has a wide applicability as it has been standardized.

The eccentric and segmental orifice plates are used to measure flow rates of fluids containing suspended materials such as solids, oil mixed with water and wet steam.

Advantages of Orifice Meter

It is very cheap and easy method to measure flow rate. It has predictable characteristics and occupies less space. Can be use to measure flow rates in large pipes.

Limitations of Orifice Meter

The vena-contracta length depends on the roughness of the inner wall of the pipe and sharpness of the orifice plate. In certain cases it becomes difficult to tap the minimum pressure (P2) due to the above factor.

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. Gets clogged when the suspended fluids flow. The orifice plate gets corroded and due to this after sometime, inaccuracy occurs. Moreover the orifice plate has

low physical strength. The coefficient of discharge is low.