# Restriction Orifice vs Metering Orifice: What's the Difference? > Restriction orifices reduce pressure and dissipate energy. Metering orifices measure flow per ISO 5167-2. They share geometry but differ in purpose, design rules, edge condition, tap requirements, and uncertainty budgets. This guide explains when to use each and how to specify them correctly. - Category: Selection Guide - Published: 2026-07-07 - Reading time: 15 min - Canonical article: https://www.shizhongflow.com/blog/restriction-orifice-vs-metering-orifice A plant engineer at a chemical facility once replaced a worn metering orifice plate with a spare from the warehouse. The spare had the same bore diameter — 52 mm in a DN 100 line — but no pressure taps and a 45° bevel on both faces. The flow transmitter produced a signal that was 30% below the sizing calculation. The engineer had installed a restriction orifice where a metering orifice belonged. The two devices look identical when you hold them in your hand: a thin metal plate with a hole in the middle. But they serve opposite functions. A metering orifice creates a controlled, predictable pressure differential specifically to measure flow, with a sharp upstream edge, standardized tap locations, and a discharge coefficient calculated from ISO 5167-2:2022. A restriction orifice creates an intentional pressure drop to reduce downstream pressure, limit flow rate, prevent cavitation, or dissipate energy — its geometry is designed for pressure loss, not measurement. This guide explains the design differences, the applicable standards, and the decision framework for specifying the right orifice for your application. ## Purpose Defines Design: Pressure Control vs. Flow Measurement A metering orifice is a sensor. Its purpose is to create a differential pressure that can be converted to a flow rate with a known uncertainty. Every geometric feature — the sharp upstream edge, the cylindrical bore, the tap location, the plate flatness — is controlled to within micron-level tolerances because small deviations shift the discharge coefficient. The Cd equation (Reader-Harris/Gallagher) assumes a specific flow pattern: the fluid separates at the sharp edge, forms a vena contracta, and produces a pressure difference that is a predictable function of the flow rate, the fluid density, and the bore-to-pipe diameter ratio β. A restriction orifice is a process device. Its purpose is to reduce the fluid pressure from an upstream value to a desired downstream value, to limit the maximum flow rate in the event of a downstream pipe break, to prevent cavitation in downstream valves by increasing back-pressure, or to dissipate excess energy in high-pressure letdown stations. Its geometry is designed for the pressure-drop characteristic, not for measurement. The bore may be chamfered on both sides to reduce erosion. The edge condition is not controlled to 0.0004d. There are no pressure taps because there is nothing to measure. This functional difference dictates the engineering approach. For a metering orifice, you calculate β from the desired differential pressure at normal flow, verify compliance with ISO 5167-2 limits, and specify the tap type, carrier, and transmitter. For a restriction orifice, you calculate the required bore diameter from the allowable downstream pressure at maximum flow, verify that the downstream velocity does not cause excessive noise or vibration, and specify the plate thickness to withstand the full differential pressure without deflection. ## ISO 5167-2 Does Not Apply to Restriction Orifices ISO 5167-2:2022 explicitly covers only metering orifices installed for flow measurement. A restriction orifice does not comply with the standard. Four specific differences. (1) Edge condition: a metering orifice requires a sharp upstream edge with a maximum radius of 0.0004d. A restriction orifice typically has chamfered or radiused edges on both faces to reduce erosion, lower the discharge coefficient to increase pressure loss, and simplify manufacturing. (2) Beta ratio: a metering orifice β is limited to 0.10–0.75. A restriction orifice may use β from 0.02 to 0.90, selected for the pressure-drop target, not measurement signal quality. (3) Pressure taps: a metering orifice requires corner, flange, or D and D/2 taps machined to specific tolerances. A restriction orifice has no taps, or if taps are provided for monitoring, they are not located to ISO 5167-2 specifications. (4) Thickness: a metering orifice plate is thin (0.005D to 0.05D). A restriction orifice may be substantially thicker — up to 0.25D for single-stage plates — to withstand the full upstream-to-downstream differential pressure without mechanical failure. ## Single-Stage vs. Multi-Stage Restriction Orifices A single-stage restriction orifice uses one plate to achieve the full pressure drop. It is suitable when the pressure ratio (p_downstream/p_upstream) is above approximately 0.55 for gases and steam, or above 0.30 for liquids. Below these ratios, the flow becomes choked (critical flow in gases, flashing in liquids), producing excessive noise, vibration, and erosion. Multi-stage restriction orifices divide the total pressure drop across two or more plates in series, with defined spacing between stages to allow pressure recovery. Each stage reduces the pressure by a fraction of the total, keeping individual pressure ratios above the cavitation or choked-flow thresholds. The spacing between stages matters: 5D minimum, with 8D preferred for high-pressure-drop applications. The bore diameters may be equal or progressively decreasing. Plates should be oriented with identification tabs visible for installation verification, and the assembly should include a drain between stages for liquids that may condense during startup. Multi-stage assemblies are custom-engineered — they are not catalog items and should be specified by an engineer who has modeled the pressure drop, velocity, and noise characteristics. ## When a Restriction Orifice Can Double as a Flow Indicator A restriction orifice with pressure taps installed across it will produce a differential pressure that changes with flow. This can be displayed on a local gauge or transmitter to provide a flow indication — the flow is going up, down, or steady. This is not a measurement. The Cd is unknown, so the relationship between ΔP and flow cannot be quantified without calibration. However, for applications where the operational decision is binary ("is there flow?" or "has the flow changed?"), a restriction orifice with a DP gauge is cost-effective. If quantitative flow measurement is required, install a separate metering orifice per ISO 5167-2 upstream or downstream with sufficient straight run. Alternatively, a metering orifice can serve both functions: size the bore for the required ΔP, verify the permanent pressure loss achieves the required downstream pressure, and confirm ISO 5167-2 compliance. The sizing becomes more constrained because both the measurement signal and the pressure drop must be satisfied simultaneously. ## Selection Decision: Metering Orifice, Restriction Orifice, or Both Step 1 — Identify the primary function. Is the purpose to measure flow to a stated uncertainty, or to reduce pressure to a target value? Step 2 — Metering orifice: calculate β from desired ΔP, verify β=0.10–0.75, ReD above minimum, straight run meets ISO 5167-2 Table 3, edge and flatness tolerances maintained. The pressure drop is the permanent loss (40–90% of ΔP). Step 3 — Restriction orifice: calculate bore diameter from target downstream pressure. Determine stages: single if p₂/p₁ > 0.55 (gas) or > 0.30 (liquid), multi-stage otherwise. Verify downstream velocity < 30 m/s (gas) or < 10 m/s (liquid). Specify plate thickness per ASME B31.3. Step 4 — Combined function: size metering orifice first. Verify permanent loss achieves required downstream pressure. If not, install separate metering and restriction orifices in series with at least 10D separation, metering orifice upstream. ## Common Pitfalls When Specifying Restriction Orifices (1) Specifying a metering orifice for pressure reduction: the sharp edge erodes rapidly and the thin plate may fail under high differential pressure. (2) Installing a DP transmitter on a restriction orifice and calling it a measurement — the reading is an indication, not traceable. (3) Ignoring choked-flow limits: a restriction orifice on high-pressure gas with p₂/p₁=0.15 operates in choked flow; mass flow becomes independent of downstream pressure. (4) Ignoring noise: a restriction orifice with β=0.20 on high-pressure steam can exceed 110 dBA, causing personnel hazards and pipe vibration. (5) Using a single thick plate with stepped bores as a multi-stage orifice — this is not multi-stage; the pressure drop occurs in one axial location with unpredictable erosion patterns. ## RFQ Checklist for Restriction Orifices Fluid: name, phase, composition. Flow rate: maximum and normal. Upstream pressure and temperature. Required downstream pressure or allowable pressure drop. Pipe ID, material, and schedule. Flange rating. Number of stages: single or multi-stage. Allowable noise level at 1 m. Plate material and NACE MR0175 requirement. Pressure taps: none, or local gauge connections. Tag number. Require the supplier to return: bore diameter(s), β for each stage, calculated downstream pressure, permanent loss, maximum velocity, predicted noise (dBA at 1 m), plate thickness calculation per ASME B31.3, and material certificates. ## Summary: Same Shape, Different Engineering A metering orifice measures. A restriction orifice reduces. They share a common shape but diverge in every engineering detail: edge condition, β range, tap requirement, plate thickness, applicable standard, and uncertainty budget. The most common error is confusing the two. The correction is simple: identify the primary function first, then specify the device that serves that function. Need application-specific sizing? Send your process conditions through our Contact page — we will return bore diameter, β, pressure drop, noise estimate, and material recommendation at no cost. ## Engineering support For project-specific selection, send the fluid, minimum/normal/maximum flow, pressure, temperature, pipe size, viscosity, allowable pressure loss and available straight run through https://www.shizhongflow.com/contact.