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Orifice Flanges
A comprehensive engineering reference for EPC contractors, process instrumentation engineers, flow measurement specialists and procurement teams — covering orifice flange design, pressure tap configurations, Jack Screw function, ASME B16.36 dimensional data, orifice plate beta ratio selection, differential pressure flow measurement principles, material grades and full project documentation.
Orifice Flange Design, Pressure Tap Configurations
& Jack Screw Function
Flange Tap · Corner Tap · Jack Screws · Weld Neck · Slip-On
Definition and Engineering Function
An orifice flange is a special flange type — supplied and installed as a matched pair — specifically designed to house a removable orifice plate (a thin disc with a precisely sized central bore) between the two mating flange faces for differential pressure flow measurement. The orifice plate restricts the flow, creating a pressure drop across the plate. By measuring this differential pressure (ΔP) between two pressure tapping points — one upstream and one downstream of the orifice plate — the volumetric or mass flow rate of the process fluid can be calculated using the ISO 5167 orifice meter equation.
The orifice flange differs from a standard pipe flange by incorporating: (1) radially drilled and tapped pressure tap holes in each flange body, connecting the pipe bore to the flange face where the upstream and downstream pressures are transmitted to the connected differential pressure (DP) transmitter; and (2) Jack Screws — threaded bolts installed radially in the flange body that push against the upstream and downstream flange faces to separate the flanges sufficiently to allow the orifice plate to be slid in and out without breaking the pipe connection.
Jack Screws are radially threaded bolts installed in both flanges of an orifice flange pair. When turned inward (clockwise), the Jack Screw tips bear against the face of the adjacent flange and push the two flanges apart — creating a gap sufficient to slide the orifice plate in or out without removing the pipe bolts, disconnecting the pressure impulse tubing, or breaking the pipe connection. Jack Screws eliminate the need to remove the pipe flanges entirely to access the orifice plate — the plate can be changed with the line in service (after depressurisation). Typically two Jack Screws per flange (four per pair) at 90° to the pressure taps, sized 1/2" or 3/4" UNC per ASME B16.36.
Orifice Flange Anatomy — Four Key Features
Pressure Tap Types — Engineering Descriptions
Flow Measurement Principles, Beta Ratio
& ASME B16.36 Dimensional Data
ASME B16.36 · Class 300–2500 · NPS 1"–24"
Differential Pressure Flow Measurement Principle
The orifice meter operates on the Bernoulli principle: as fluid accelerates through the restriction created by the orifice plate bore, the velocity increases and the static pressure decreases. The differential pressure (ΔP) between the upstream and downstream pressure taps is a function of the flow rate. The flow rate Q is calculated from the ISO 5167 orifice meter equation:
β = d / D // Beta ratio; d = orifice bore [m], D = pipe bore [m]; typical range 0.30–0.75
C_d ≈ 0.603 – 0.624 // Discharge coefficient for flange taps per ISO 5167-2 (function of β and Re_D)
// BETA RATIO SELECTION GUIDE:
// β = 0.30–0.45: High ΔP, low flow range, high permanent pressure loss; small bore meter
// β = 0.45–0.65: Balanced ΔP and accuracy; most common range for process flow measurement
// β = 0.65–0.75: Lower ΔP, wider flow range, lower permanent pressure loss; large bore meter
Beta Ratio (β) Selection and Permanent Pressure Loss
The beta ratio (β = orifice bore / pipe bore) is the primary design parameter for an orifice meter. A lower beta ratio produces a higher differential pressure (better meter sensitivity and turndown) but also a higher permanent pressure loss in the system — energy that is permanently lost to the process as heat due to the turbulence downstream of the orifice plate. A higher beta ratio reduces permanent pressure loss but also reduces the differential pressure signal and meter sensitivity.
The permanent pressure loss as a fraction of the differential pressure is approximately: ΔP_perm / ΔP_total ≈ (1 − β²). For β = 0.5: ΔP_perm ≈ 75% of ΔP_total — a significant energy penalty in high-flow, high-pressure systems. For β = 0.7: ΔP_perm ≈ 51%. The trade-off between meter sensitivity, turndown ratio, accuracy and permanent pressure loss must be evaluated for each application by the flow measurement engineer.
| NPS | Flange OD (mm) | Bolt Circle (mm) | No. Bolts | Bolt Size | Flange Thick. (mm) | Tap Hole Size | Jack Screw Size |
|---|---|---|---|---|---|---|---|
| 2" | 165.1 | 127.0 | 8 | M16 | 28.6 | 1/2" NPT | 1/2" UNC |
| 3" | 209.6 | 168.3 | 8 | M20 | 31.8 | 1/2" NPT | 1/2" UNC |
| 4" | 254.0 | 200.0 | 8 | M20 | 35.1 | 1/2" NPT | 1/2" UNC |
| 6" | 317.5 | 269.9 | 12 | M20 | 38.1 | 3/4" NPT | 3/4" UNC |
| 8" | 381.0 | 330.2 | 12 | M24 | 44.5 | 3/4" NPT | 3/4" UNC |
| 10" | 444.5 | 387.3 | 16 | M24 | 50.8 | 3/4" NPT | 3/4" UNC |
| 12" | 520.7 | 450.8 | 16 | M27 | 57.2 | 3/4" NPT | 3/4" UNC |
ASME B16.36-2015. Weld neck orifice flange dimensions for Class 300. Flange OD and bolt circle are the same as ASME B16.5 Class 300 standard flanges — orifice flanges are bolt-compatible with standard flanges of the same NPS and class. Orifice flanges are typically thicker than standard B16.5 flanges to accommodate the tap and Jack Screw drillings. Orifice flanges are always supplied as matched pairs (one upstream + one downstream); tap hole position and Jack Screw holes are drilled and tapped at the manufacturer before dispatch. Bore specified to match pipe schedule per ASME B36.10M.
| Upstream Fitting | Beta = 0.30 (D upstream) | Beta = 0.50 (D upstream) | Beta = 0.70 (D upstream) | Downstream (all β) |
|---|---|---|---|---|
| Single 90° elbow | 10D | 14D | 22D | 5D minimum |
| Two 90° elbows (same plane) | 16D | 22D | 42D | 5D minimum |
| Two 90° elbows (different planes) | 34D | 44D | 70D | 5D minimum |
| Reducer (2D to D, >0.5D taper) | 5D | 8D | 12D | 5D minimum |
| Expander (0.5D to D, full bore valve) | 16D | 22D | 44D | 5D minimum |
| Globe valve (fully open) | 18D | 24D | 46D | 5D minimum |
| Gate valve (fully open) | 5D | 8D | 12D | 5D minimum |
ISO 5167-2:2022. D = pipe inside diameter. Straight pipe run requirements are the minimum unobstructed straight pipe lengths (no fittings, no valves, no changes in direction) required upstream and downstream of the orifice plate to achieve the uncertainty values stated in the ISO 5167-2 discharge coefficient tables. Insufficient straight run is the most common cause of orifice meter measurement error in installed systems. Always verify available straight run during piping layout — insufficient run requires a flow conditioner upstream.
Material Grades, NACE Compliance
& Bore and Face Finish Requirements
NACE MR0175 · Bore Finish · Tap Hole Cleanliness
| Material | ASTM Grade | Yield (MPa) | Temp Range (°C) | Corrosion | Application |
|---|---|---|---|---|---|
| Carbon Steel | A105 N | ≥250 | −29 to +538 | Low | Standard process piping, gas measurement, utilities |
| LTCS | A350 LF2 Cl.1 | ≥260 | −46 to +343 | Low | Cold service, LNG vapour, ethylene plant |
| 1.25Cr-0.5Mo | A182 F11 Cl.2 | ≥310 | −29 to +593 | Moderate | High-temp refinery flow measurement, H₂ service |
| 2.25Cr-1Mo | A182 F22 Cl.3 | ≥310 | −29 to +621 | Moderate | Hydrocracker, reformer, high-temp/pressure refinery |
| 9Cr-1Mo-V | A182 F91 | ≥585 | −29 to +649 | Moderate | Ultra-supercritical power steam flow measurement |
| SS 304/304L | A182 F304/F304L | ≥205 | −196 to +816 | High | Chemical, pharmaceutical, food, cryogenic measurement |
| SS 316/316L | A182 F316/F316L | ≥205 | −196 to +816 | Very High | Offshore, chloride, chemical, pharma measurement |
| Duplex 2205 | A182 F51 | ≥450 | −50 to +315 | Very High | Offshore sour gas flow measurement, seawater |
| Super Duplex | A182 F53/F55 | ≥550 | −50 to +300 | Extreme | Subsea, severe sour, HPHT flow measurement |
| Inconel 625 | A182 F625 | ≥414 | −196 to +980 | Extreme | High-temp corrosive, acid service measurement |
Critical Bore and Tap Hole Finish Requirements
Unlike standard pipe flanges, orifice flanges have two additional critical finish requirements that directly affect the accuracy of the flow measurement: (1) Pipe bore surface finish — the internal bore of the pipe in the straight run region approaching the orifice plate must have a surface roughness of Ra ≤ 3.2 µm (125 micro-inch) for a length of at least 2D upstream of the orifice flange, per ISO 5167-2. A rougher bore surface causes additional turbulence that shifts the discharge coefficient outside the published ISO 5167 calibrated range, introducing measurement error. The orifice flange bore itself must also be machined to Ra ≤ 3.2 µm; (2) Pressure tap hole cleanliness and finish — the pressure tap holes must be free of burrs, chips, weld spatter and rust on the inside bore and at the opening into the pipe bore. Any protrusion of the tap hole edge into the pipe bore, or any roughness at the opening, creates a local flow disturbance that shifts the measured pressure at that tap and introduces a systematic measurement error. The tap hole opening at the bore must be sharp-edged and flush with the bore surface — neither recessed nor protruding.
The pressure tap hole opening at the pipe bore wall is the most critical machining feature on an orifice flange. The opening must be: (a) sharp-edged — no burr, chamfer, radius or bevel at the bore opening; (b) flush with the bore wall — the tap hole must not protrude into the bore (a protrusion of even 0.1 mm introduces a stagnation zone that reads a different pressure than the true static pressure); (c) perpendicular to the bore wall — the drilling axis must be exactly radial; an angled tap introduces a velocity component into the pressure reading; (d) correct diameter at the opening — ISO 5167-2 limits the tap hole diameter to a maximum of D/8 (one-eighth of the pipe bore diameter) at the bore wall opening. Each of these requirements must be verified by inspection with a dial gauge and optical examination before the orifice flange is dispatched. A non-conforming tap hole cannot be corrected in the field — the flange must be remachined or replaced.
Installation Requirements, Applications
& Quality Control and Documentation
Gas · Oil · Steam · Chemical · Refinery · Power Plant
Installation Requirements
Before finalising the orifice flange location in the piping layout, the upstream and downstream straight pipe run requirements per ISO 5167-2 (Table 2 above) must be confirmed as achievable in the actual piping arrangement. The straight run is the most commonly violated requirement in orifice meter installations — often because the meter location is chosen for convenience without checking the distances to the nearest upstream and downstream fittings. Insufficient straight run is the primary cause of orifice meter measurement uncertainty exceeding the ISO 5167 stated limits. Where adequate straight run cannot be achieved, a flow conditioner (e.g. Gallagher-type, CPA-type or tube bundle) installed upstream can reduce the required straight run length — but the specific conditioner type and its qualifying run must be verified per the conditioner manufacturer's data and the applicable flow measurement standard.
The orifice plate must be installed with the bevelled (upstream) face of the orifice plate facing upstream — the flat face must face downstream. The ISO 5167-2 discharge coefficient tables are calibrated for this specific orientation. Installing the plate reversed (flat face upstream) produces a significantly different discharge coefficient — typically 2–5% different — introducing a systematic measurement error. The orifice plate tab or handle must project outward from the flange gap and must be marked with the flow direction arrow and the plate serial number, beta ratio and bore diameter for identification without removing the plate.
The upstream and downstream pressure taps are connected to a differential pressure (DP) transmitter by impulse tubing. For liquid service, the impulse tubing must be routed downward from the orifice flange taps to the DP transmitter to prevent gas pockets forming in the impulse lines — gas pockets cause the measured pressure to be different from the true process pressure at the tap. For gas service, the impulse tubing must be routed upward to prevent liquid accumulation in the lines. For steam service, condensate pots (steam legs) must be installed in the impulse lines to equalise the liquid column on both sides of the DP transmitter. Incorrect impulse line routing is the second most common cause of orifice meter measurement error in installed systems.
Applications by Industry
Orifice flanges per ASME B16.36 and AGA Report No. 3 (equivalent to ISO 5167-2 for natural gas) are the most widely used primary flow measurement device in natural gas transmission pipelines, gas distribution systems, custody transfer metering stations and gas processing plants. Carbon steel A105 weld neck orifice flanges in Class 300 through Class 1500 are standard for high-pressure gas transmission. Flange tap configuration is standard for all AGA-3 / ISO 5167 custody transfer applications requiring the published uncertainty values.
Orifice flanges are the standard flow measurement device for process flow measurement throughout refineries and petrochemical plants — measuring crude oil feed, product streams, hydrogen flow, steam flow, cooling water, fuel gas, chemical injection rates and utility flows. A105 for standard carbon steel service; A182 F11/F22 for high-temperature hydrogen service; A182 F304L/F316L for corrosive chemical service; duplex 2205 (F51) for sour service with chloride. All refinery orifice flanges require EN 10204 3.1 MTC minimum; NACE sour service flanges require 3.2 with TPI.
Weld neck orifice flanges in A182 F22 (2.25Cr-1Mo) and A182 F91 (9Cr-1Mo-V) are used for main steam, hot reheat and extraction steam flow measurement in power generation plants. High-temperature steam orifice flanges require PWHT after butt welding to the pipe and post-PWHT hardness verification. The bore finish requirement (Ra ≤ 3.2 µm) is particularly important in steam service where condensate on a rough bore surface can cause local flow asymmetry and measurement error during warm-up transients.
Duplex 2205 (A182 F51) and SS 316L (A182 F316L) orifice flanges are standard for flow measurement on offshore platforms — production wellhead flow measurement, separator test separators, gas lift metering, produced water metering and utility cooling water measurement. All offshore orifice flanges require EN 10204 3.2 MTC with TPI, PMI, NACE hardness mapping, Charpy impact testing (LTCS grades), ferrite content measurement (duplex) and bore finish verification reports as standard documentation.
Quality Control — Orifice Flange Specific Requirements
QC for orifice flanges covers all standard weld neck flange requirements (dimensional inspection, bore match, face finish, NDE) plus the following orifice-specific checks: (1) Tap hole position — the centreline of each tap hole must be at exactly 25.4 mm (1") from the upstream and downstream flange faces for flange tap type, measured along the pipe axis; (2) Tap hole opening diameter — must be ≤ D/8 per ISO 5167-2; (3) Tap hole opening condition — sharp-edged, flush, no burr, no chamfer, verified visually and by dial gauge; (4) Bore surface finish — Ra ≤ 3.2 µm measured with a profilometer in the tap hole region; (5) Jack Screw thread engagement — Jack Screws installed and function-tested (full in/out travel verified before dispatch); (6) Matching of paired flanges — upstream and downstream flanges must be tagged as a matched pair with serial numbers cross-referenced in the inspection report.
Export Packaging
- Orifice flange pairs packed together as matched sets — upstream and downstream flanges must not be separated during transit; matched set tag on both flanges cross-referencing each other
- Pressure tap holes plugged with clean plastic thread protector plugs (not metal — metal plugs can produce swarf inside the tap hole during removal); Jack Screw holes also plugged
- Flange faces protected with foam or cardboard face pads; bore sealed with plastic bore caps
- Jack Screws installed finger-tight in the flange holes for transport; additional spare Jack Screws supplied with the matched pair in a tagged poly bag
- MTC (EN 10204 3.1/3.2), dimensional inspection report including tap hole position and opening diameter, bore finish report (Ra), tap hole opening condition report, Jack Screw function test certificate, PMI (alloy grades), NDE reports, NACE hardness (sour service), Charpy impact (LTCS), ferrite content (duplex) — all in waterproof sealed envelope with each matched pair
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 (TPI co-signed) for offshore / NACE / alloy / safety-critical |
| 02 | Dimensional Inspection Report | ASME B16.36 | OD, bolt circle, flange thickness, bore ID (per pipe schedule) — all mandatory |
| 03 | Tap Hole Position Report | ASME B16.36 / ISO 5167-2 | Tap centre at 25.4 mm (1") from face ±0.5 mm confirmed for flange tap type |
| 04 | Tap Hole Opening Condition Report | ISO 5167-2 Clause 5.1.4 | Sharp-edged, flush, no burr; dia. ≤ D/8; dial gauge and visual verification |
| 05 | Bore Surface Finish Report | ISO 5167-2 / B46.1 | Ra ≤ 3.2 µm at tap hole region — profilometer measurement mandatory |
| 06 | Jack Screw Function Test Certificate | ASME B16.36 | Full travel in/out verified; tip condition; thread engagement confirmed |
| 07 | Face Finish Report | ASME B16.5 / B46.1 | 125–250 AARH for RF gasket face; RTJ groove dimensions confirmed |
| 08 | PMI Report (XRF / OES) | Project specification | 100% SS, duplex, Inconel and all alloy grade orifice flanges |
| 09 | Hardness Test Report | ASTM E10 / NACE MR0175 | NACE sour service: ≤22 HRC cross-section; duplex: ≤310 HB |
| 10 | Charpy Impact Test Report | ASTM A370 / EN ISO 148 | Mandatory for A350 LF2/LF3 LTCS; ≥27 J at MDMT |
| 11 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include pressure flange forging and precision bore machining |
| 12 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports orifice flanges as matched pairs in weld neck and slip-on configurations per ASME B16.36, in all pressure classes (300–2500), in A105, A350 LF2, A182 F11/F22/F91, A182 F304L/F316L, A182 F51/F53 and Inconel 625. Flange tap (1/2" and 3/4" NPT) and corner tap configurations; Jack Screws (1/2" and 3/4" UNC) included with each pair. Bore matched to pipe schedule per B36.10M; bore finish Ra ≤ 3.2 µm verified by profilometer. Tap hole position and opening condition inspection reports included. EN 10204 3.1/3.2 MTC, PMI, NACE hardness, Charpy impact, ferrite content (duplex), TPI witness by BV/DNV/Lloyds/SGS/TÜV. 48-hour express dispatch on standard in-stock sizes.