Orifice Plates and Orifice Plate Assemblies
INTRODUCTION
Orifice Plate Assembly (with Flanges) are widely used in conjunction with orifice meters for measuring the volumetric flow rate of liquids and gases through a pipe. Two orifice flanges are called an orifice flange union. Each flange comes with tapping for measuring the pressure drop of the flow through an orifice plate.
Orifice plates do not come with the flanges and are sized based on the requirements of the process. Jack screws are used to spread the flanges apart in order to change the orifice plate.
Orifice Plate Assembly (with Flanges) are basically the same as standard weld neck and slip-on flanges except for the provision of radial, tapped holes in the flange ring for meter connections and additional bolts to act as jack screws to facilitate separating the flanges for inspection or replacement of the orifice plate.
In choosing the type of orifice flange, the considerations affecting the choice of weld neck and slip-on flanges apply with equal force. They are made in pressure classes 300#, 600#, 900#, 1500# and 2500#; in carbon steel, stainless steel and alloy materials; and raised faced or ring joint. Orifice plates flanges are not recommended for 150# rating because the thickness of 150# flanges are not sufficient for providing flange tapings.
Orifice flanges are installed in a straight run of pipe in order to avoid turbulence at the plate.
When installing the orifice plate flange the pressure taps need to be at the same elevation to each other. The assumption when making the calculation for the orifice plate is that static head of the tubes is the same on both sides of the orifice plate. Pressure taps should not be installed facing down as the taps could become plugged with debris in the line.
BASIC PRINCIPLE OF FLOW MEASUREMENT
Flow is measured based on the principal of DP measurement. Reduction in area causes the pressure drop & increase in velocity. Flow is unaffected by reduction of area. The reduction in pressure is measured at certain points called as tapping points. Difference in pressure measured at tapping points is called as DP. Flow is calculated with the help of DP measured using the relation of Flow and DP (the flow is directly proportional to square root of DP).
The reduced pressure then starts recovering after some distance. The point at which the pressure recovery starts is called as “vena contracta point”. This is also called as the point of maximum pressure loss point. It is located approximately at a distance of “d-pipe id”. But the pressure never recovers fully. The difference between the upstream pressure & the fully recovered pressure is called as “permanent pressure loss (PPL)”. The pressure loss is because of the friction & turbulence in pipe.
For all DP based flow elements, the rangeability is 3:1 i.e., Orifice plate can give correct readings up to 1/3rd of the designed flow. (As DP varies as a square function of flow, any small error in DP measurement will lead to square of the error, therefore at smaller ranges of DP, error in reading is exaggerated).
PURPOSE OF ORIFICE PLATES:
Orifice Plate Assemblies are used for two purposes
For Measurement of Flow (Called measuring Orifice plate or simply Orifice Plate)
For Pressure Drop (Called Restriction Orifice Plate)
ORIFICE PLATES
Bore is decided based on Calculation. Bevel is provided on downstream side for the recovery of pressure. Vent is provided for release of entrapped air for liquid service while Drain is provided for release of moisture for air/steam service. Tab Plate is provided to show plate details along with marking of inlet/outlet direction. Hole is provided in Tab plate to lift the plate during dismantling / replacement of orifice plate.
TYPE OF MEASURING ORIFICE PLATES
SQAUARE EDGE CONCENTRIC TYPE
Most commonly used for flow measurement. Bore is provided at the center of the plate. These plates are used for clean liquids, gases and steam flow. Recommended for Reynold number 5000 onwards.
QUADRANT EDGE TYPE
The Inlet edge of the bore of this orifice plate is rounded to a quarter circle. This plate is used for viscous fluids. Recommended for Reynold number 1500 to 9000.
ECCENTRIC TYPE
Used for liquids containing solid particles that are likely to sediment or for vapours likely to deposit water condensate, this orifice plate is used with its eccentric bore bottom flush with the bottom of the piping inside surface so that the sedimentation of such inclusions are avoided. Likewise, for gases or vapours, it may be installed with its eccentric bore top flush with the id of the piping to avoid stay of gas of vapour in its vicinity. Recommended for Reynold number 3000 to 12000.
SEGMENTAL TYPE
Most useful where there are substantial entrained water or air and also if there are suspension in the fluids. This avoid build up in front of the orifice plate. The orifice hole is placed at the bottom for gas service and top for liquid service. Recommended for Reynold number 5000 to 20000.
SEGMENTAL TYPE
This plate is used for viscous Service. Recommended for very low Reynold number ranging 80 to 1500. High viscosity measuring capacity leading to ruling off application which require accuracy at lowest Reynolds, thus effectively rid off application of magnetic and vortex.
TYPE OF ORIFICE PLATE ASSEMBLIES
INTEGRAL ORIFICE PLATE ASSEMBLY
These are another type of orifice plates used for low flow rates. They are basically a small tube that you can mount directly to the differential pressure transmitter. An option to purchase on some transmitters, and there is an orifice inside the tube that connects the high side cavity to the low side cavity.
ORIFICE PLATE ASSEMBLY WITH RTJ HOLDER
These assemblies are used for higher pressure applications. The plate holder is combination of plate holder and an orifice plate designed for Ring Tongue Joint (RTJ) flanges. Plate holder acts as a holder of the orifice plate and also acts as a gasket to prevent leakage of the process fluid. The plate holder has a oval or octagonal ring for mounting between RTJ flanges. Plate can either be screwed to the plate holder. The plate holder along with orifice plate can also be machined from one piece.
RTJ holder material is selected so that the RTJ holder hardness is less than that of flange hardness.
RESTRICTION ORIFICE PLATES
SINGLE STAGE RESTRICTION ORIFICE PLATES
Bore is decided based on calculation. As requirement is to kill the pressure, no bevel is provided. No tapping is required in Restriction Orifice Plates as it is not used for measurement purpose.
MULTISTAGE RESTRICTION ORIFICE PLATES
Multistage restriction orifice plates are used when require pressure drop is more than 50%. Also used to avoid noise / vibration in Gas service, to avoid cavitation in liquid service. Multistage orifice plate assembly consists of number of orifice plates welded in between pipes. In Multistage Restriction Orifice, eccentric plates are used.
VARIOUS TYPE OF ORIFICE PLATE ASSEMBLIES
Orifice Plate with Weld Neck Flanges
Orifice Plate with Slip On Flanges
Orifice Plate with Carrier Ring
Integral Orifice plate Assembly
TYPE OF PRESSURE TAPS
Flange Taps
Corner Taps
D-D/2 Taps
WELD NECK FLANGE WITH FLANGE TAPS
WELD NECK FLANGE WITH CORNER TAPS
D - D/2 TAPS
CARRIER RING WITH FLANGE TAPS
CARRIER RING WITH CORNER TAPS
INTEGRAL ORIFICE ASSEMBLY
SPECIFICATIONS
Design
Types
Plate Material
Orifice Plate
Tab Plate
Vent / Drain
Flange Union
Pressure Taps
Gasket
Studs / Nuts
Jack Screw
Conforms to ISA RP 3.2, DIN 1952, BS 1042, ISO-5167
Square edge concentric, Quadrant edged, Conical entrance, Eccentric, Segmental
SS304, SS316, SS316L (as standard). Hastelloy-C, Monel, PP, PVC, PTFE coated, etc. can be given on request.
In accordance with ISO-5167, BS-1042, ASME MFC 3M, R.W. Miller, AGA-3
In the same material as plate & is welded to orifice plate. Integral tab plate to the Orifice Plate (i.e., without welding)
can also be offered as a special case.
Vent or Drain holes are provided as per customer’s requirement. Not drilled for the orifice bores smaller than 25.4 mm
Weld neck, slip on, Threaded, Socket welded with RF or RTJ facing Orifice flanges are in accordance with ANSI B16.36
with minimum flange rating of 300# for sizes up to 8” or male – female flanges in accordance with ANSI B16.5.
Corner tapings are recommended for sizes up to 1 1⁄2”, Flange taps from 2″ to 16″, D – D/2 taps for higher sizes.
CAF, PTFE, SS spiral wound + CAF, SS spiral wound + Grafoil, SS spiral wound + PTFE are normally supplied as per process
requirement. Other materials available on request. For RTJ flanges, the plate is fixed on the plate holder. The plate holder is
in Soft Iron material & acts as a gasket.
ASTM A193 Gr. B7/A-194 Gr. 2H as standard, other material on request.
ASTM A193 Gr. B7
DATA REQUIRED FOR MEASURING ORIFICE BORE CALCULATION
Name of the Fluid & State
Operating Temperature / Ambient Temperature
Operating Pressure (abs)
Viscosity (Cp)
Max Flow / Normal Flow
Differential Range
Base Sp. Gravity / Density (Only for Gas)
Operating Sp. Gravity / Density (for Gas & Liquid))
Specific Heat Ratio (cp / cv) only for Gas
Pipe Size & Schedule
Pipe Material
Tap Type
Element Material
Vent / Drain
DATA REQUIRED FOR RESTRICTION ORIFICE BORE CALCULATION
Name of the Fluid & State
Operating Temperature / Ambient Temperature
Inlet Pressure (abs)
Outlet Pressure (abs)
Viscosity (cp)
Max Flow / Normal Flow
Base Sp. Gravity / Density (Only for Gas)
Operating Sp. Gravity / Density (for Gas & Liquid))
Specific Heat Ratio (cp / cv) only for Gas
Pipe Size & Schedule
Pipe Material
Element Material
