An orifice gas metering skid calculates flow from the differential pressure created by a plate with a precisely machined bore installed in the line. The skid brings together the orifice plate and its holder, a dimensionally controlled meter tube, a flow conditioner, differential pressure, static pressure and temperature transmitters, and a flow computer that runs the calculation. Accuracy is not set by any single device: plate geometry and condition, beta ratio, upstream piping, the range over which differential pressure is measured and how well gas density is known all play a part.
How differential pressure metering works
Gas accelerates through the restriction and its pressure falls. The difference between pressure taps on either side of the plate is measured, and flow is calculated with standard equations from that differential pressure, the pipe and bore diameters, the discharge coefficient, the expansibility factor and the gas density at flowing conditions. Flow is proportional to the square root of differential pressure, so halving the flow cuts the differential pressure to roughly one quarter.
Geometry, installation and calculation rules are defined by the ISO 5167 series and by AGA Report No. 3 / API MPMS Chapter 14.3. They apply to subsonic, single-phase, non-pulsating flow in a pipe running full. ISO 5167-2 also sets application limits on pipe diameter and Reynolds number; an installation outside those limits cannot be treated as a measurement calculated from the standard equations.
Skid components
· Orifice plate: Concentric, square-edged plate. Taps may be flange, corner or D and D/2; AGA 3 / API 14.3 is based on flange taps.
· Plate holder: Orifice flange union, single-chamber fitting or dual-chamber fitting. A dual-chamber fitting lets the plate be withdrawn and inspected without isolating or depressurizing the line.
· Meter tube and flow conditioner: Dimensionally controlled straight pipe upstream and downstream of the plate, with a flow conditioner where needed to remove swirl and correct the profile.
· Transmitters: Differential pressure, static pressure and temperature. More than one differential pressure transmitter may be used to cover a wide flow range.
· Flow computer: Applies the standard equations, compressibility calculation and base-condition conversion; stores plate data and change records.
As meter-run manufacturers point out, system accuracy is the combined result of plate manufacturing precision and the dimensional accuracy of the meter tube. The meter tube and plate holder are therefore designed and documented as one assembly.
What drives orifice measurement accuracy
Factor | Why it matters | How it is managed |
Plate geometry and condition | Edge sharpness, flatness and bore diameter directly affect the discharge coefficient | Periodic plate inspection, correct orientation, plate records |
Beta ratio | Bore-to-pipe diameter ratio affects calculation uncertainty and pressure loss | Plate sizing within the range recommended by the standard |
Upstream layout | Elbows, valves and swirl distort the pressure profile and cause bias | Adequate straight pipe and a suitable flow conditioner |
Differential pressure range | At low flow the differential becomes very small and transmitter error grows proportionally | Stacked transmitters, parallel runs or plate change |
Gas density and composition | The flow calculation depends on flowing density | Accurate pressure and temperature, current composition data |
Pulsation and liquids | The standard equations are not valid under these conditions | Remove the source; filtration and liquid separation |
Beta ratio and the turndown limit
AGA 3 / API 14.3 Part 2 covers beta ratios of roughly 0.10 to 0.75 and identifies about 0.2 to 0.6 as the range with the lowest discharge coefficient uncertainty. A larger beta gives lower differential pressure and lower pressure loss at a given flow; a smaller beta raises the differential but increases permanent pressure loss.
Because of the square-root relationship, the usable flow range of an orifice meter is limited: a tenfold change in flow means a hundredfold change in differential pressure. Where flow swings widely by season or time of day, typical answers are two differential pressure transmitters set to different ranges, parallel meter runs brought on line as flow rises, or a plate change for each operating season. The 2022 edition of ISO 5167-1 includes annexes on turndown and permanent pressure loss.
Plate inspection and maintenance
The strength of orifice metering is that the element that sets accuracy can be physically removed and examined; the weakness is that if inspection is neglected, the resulting error does not show up in the flow computer by itself. API 14.3 Part 2 likewise notes that the lowest uncertainty depends on correct application and maintenance of the metering system, not only on design. Inspections look for edge damage, bowing, deposits and correct orientation, and the plate identification and bore are checked against the flow computer configuration.
In a dual-chamber fitting the plate can be moved into a separate chamber and changed while the line stays under pressure. Flange-type holders require the run to be isolated, so a spare run or a metered bypass should be planned. Every plate change and parameter update is logged to maintain the auditability expected of electronic gas measurement. The latest edition of ISO 5167-1 also addresses diagnostics and condition monitoring for differential pressure meters.
Orifice versus ultrasonic metering
Criterion | Orifice skid | Ultrasonic skid |
Measuring principle | Differential pressure and standard equations | Transit-time difference of acoustic pulses |
Usable flow range | Narrow because of the square-root law; stacked transmitters or parallel runs may be needed | Generally wide |
Permanent pressure loss | Noticeable, depending on beta ratio | Low in a full-bore body |
Installation sensitivity | Needs straight pipe and flow conditioning | Needs straight pipe and flow conditioning; also sensitive to noise sources |
Verification approach | Plate and tube dimensions can be checked in the field | Flow calibration and continuous diagnostics |
Flow direction | Usually designed for one direction | Bidirectional measurement possible |
An orifice skid remains a strong option for operators with existing orifice infrastructure and maintenance teams familiar with it, for points with a narrow flow range, and for applications where the ability to verify the primary element geometrically is valued.
TLY Enerji scope
TLY Enerji's product portfolio includes orifice metering systems and meter runs as well as pressure and differential pressure transmitters. Depending on project scope, support can cover engineering for beta ratio and transmitter range selection, equipment supply, flow computer and control system integration, site installation and commissioning, and technical support during maintenance. The scope of supply is defined together with the project specification.
Information to share for an orifice metering inquiry
· Lowest and highest flow over the year and how long each persists
· Line size, pressure class, operating pressure and temperature
· For existing orifice installations: fitting type, plate size and transmitter ranges
· Whether plates must be changed with the line under pressure
· Acceptable permanent pressure loss
· Gas composition and any sources of liquid or pulsation
· Calculation standard and base conditions to be used in the flow computer
Related pages
· Ultrasonic gas metering skids: The alternative when wide rangeability and diagnostics are needed.
· Natural gas metering stations: Number of meter runs, redundancy and station layout.
· Custody transfer gas metering: Uncertainty and record-keeping duties for billing-grade orifice measurement.
· Gas filtration and metering skids: Filtration that protects the plate from deposits and liquids.
Common questions on orifice metering
What does a dual-chamber orifice fitting offer?
A dual-chamber fitting allows the plate to be moved from the main chamber into a separate chamber and withdrawn while the line remains under pressure. Plate inspection or a change to a different bore therefore does not require the line to be isolated and blown down. It suits points with little tolerance for interruption and frequent plate changes; simpler flange-type holders call for a spare run instead.
Does an orifice meter run need flow calibration?
When the geometry and installation requirements of the standards are met, flow can be calculated from the discharge coefficient equations without calibration, which is a large part of why orifice metering is so widespread. The 2022 edition of ISO 5167-2 also defines calibration as an alternative to the standard equations. Which route applies depends on the standard's limits, the target uncertainty and contractual terms.
Why use two differential pressure transmitters?
Because flow varies with the square root of differential pressure, the differential becomes very small at low flow and the relative error of a single transmitter grows. With one transmitter ranged low and one ranged high, the flow computer selects the appropriate signal and the usable range widens. Parallel meter runs or plate changes are the alternatives.
How often should an orifice plate be inspected?
There is no single interval. It depends on gas cleanliness, the risk of liquids or deposits, the commercial importance of the measurement, contractual terms and national regulations. Lines carrying dirty gas need more frequent checks than those with clean, filtered gas. Recording plate condition and dimensions at each inspection allows the next interval to be set from that history.