An ultrasonic gas metering skid combines a multipath ultrasonic meter, flow conditioner, straight pipe, pressure and temperature measurement and a flow computer on one skid-mounted frame. The design hinges less on the meter body than on the velocity profile reaching it, nearby ultrasonic noise sources and how the meter's diagnostics will be used. The technology is typically considered for high-flow lines, for applications needing wide rangeability or low pressure loss, and where gas may flow in either direction.
Below: the transit-time principle, skid layout, installation effects and selection inputs. Billing-grade measurement governance and the system uncertainty budget belong on the custody transfer metering page.
Transit-time measurement and why multiple paths matter
A transit-time ultrasonic meter fires acoustic pulses between angled transducer pairs. A pulse traveling with the flow arrives slightly sooner than one traveling against it; the difference gives the average gas velocity along that path, and the same measurement yields the speed of sound. Volumetric flow is calculated from the path velocities and the meter geometry.
A single path samples only one slice of the pipe. Multipath meters place paths on several chords, sampling the velocity profile at different positions and reducing the effect of asymmetric profiles or swirl from upstream elbows and valves. Path count and layout vary by design and directly affect the straight pipe required. The principle is symmetrical, so bidirectional flow can be measured, but flow conditioning and calibration must be planned for each direction.
Where ultrasonic metering fits best
· High flow and large line sizes: Transmission pipelines, power plant inlets and underground storage, where one meter is expected to cover a wide flow range.
· Limited pressure drop: Full-bore meter bodies have no flow restriction, so the permanent pressure loss attributable to metering can be kept low.
· Bidirectional service: Storage facilities that inject and withdraw through the same line.
· Condition-based monitoring: Custody transfer and allocation points where meter health should be tracked continuously without removing the meter.
Ultrasonic gas metering standards assume single-phase flow. Where liquid carryover, wall deposits or a strong nearby noise source are expected, the design must show how those risks are handled before ultrasonic metering is proposed. For small, low-pressure delivery points, turbine, rotary or orifice metering may be a better balance.
What sits on the skid
Item | Role on an ultrasonic skid | Scope note |
Inlet filtration | Limits buildup on transducer faces and the pipe wall | Depends on gas quality; may be a separate filtration skid |
Flow conditioner and upstream straight run | Delivers a developed, swirl-free profile to the meter | Sized for path count and target accuracy class |
Multipath ultrasonic meter | Produces path velocities, speed of sound and diagnostics | Standard |
Pressure and temperature measurement | Inputs for base-condition conversion and theoretical speed of sound | Standard |
Flow computer | Performs volume, energy and audit calculations; collects diagnostics | Standard; sized to the number of meter runs |
Gas chromatograph interface | Composition data for compressibility, heating value and speed-of-sound checks | Optional, for energy measurement |
Installation effects: flow profile and straight pipe
Performance demonstrated at a calibration facility carries over to the field only if the station piping delivers a similar flow profile. Elbow combinations, partly open valves and diameter changes just upstream distort it. OIML R 137 limits the error shift caused by flow disturbances to one third of the maximum permissible error, and manufacturers specify inlet length by path count, accuracy class and use of a flow conditioner. The skid layout should therefore be developed together with the piping drawings from the outset.
Ultrasonic noise from regulators and control valves
At high differential pressure, regulators and control valves generate noise well above the audible range. When it overlaps the meter's operating band, the received pulse is harder to separate from background noise and the signal-to-noise ratio drops. Field and test-facility work shows that a regulator upstream of the meter is the more critical arrangement, and that the noise spectrum varies with regulator type, trim, pressure drop and flow rate.
Typical responses are to place the meter upstream of the regulator where possible, add elbows, distance or a noise attenuator between them, and assess meter and regulator as a pair; what is sufficient depends on the specific combination. Regulator-side noise reduction is covered on the gas pressure regulating skids page.
Using diagnostics for condition monitoring
Ultrasonic meters report on their own health while they measure. Because deposits, flow restrictions or changing operating conditions can affect accuracy, this data should be reviewed routinely. Typical parameters include:
· Path-to-path speed of sound agreement: In a homogeneous gas every path should read a similar speed of sound; a deviation can point to a transducer or timing issue.
· Measured versus calculated speed of sound: The theoretical value is calculated from composition, pressure and temperature using the AGA Report No. 10 method and compared with the measured value, which also provides an indirect check on those inputs.
· Gain and signal-to-noise ratio: Early warning of transducer fouling or interference from noise sources.
· Profile factor, symmetry, crossflow, swirl and turbulence: Reveal changes in the velocity profile, for example a fouled flow conditioner or a partly closed valve.
Diagnostics do not replace verification and recalibration, but they let intervention be based on evidence. Set alarm thresholds at commissioning, after recording the meter's site baseline.
Meter to flow computer interface
The meter passes actual volume to the flow computer via pulse or frequency outputs and a digital serial link that also carries path-level diagnostics. The flow computer uses pressure, temperature and composition to calculate base volume and, where required, mass and energy, keeps audit logs and forwards data to the PLC, DCS or SCADA system. If the meter electronics also calculate corrected volume, define up front which value is used for billing and how the two are compared.
Design inputs that drive selection
Parameter | Why it matters in design | Effect on selection |
Flow range and gas velocity | Resolution suffers at very low velocity; noise and pressure loss dominate at high velocity | Sets meter size and the need for parallel runs |
Flow direction | In bidirectional service both ends act as the inlet section | Requires symmetrical piping, bidirectional calibration and conditioner placement |
Upstream disturbances | Elbows, valves and diameter changes distort the profile | Affects path count, conditioner type and straight length |
Nearby regulator or control valve | Ultrasonic noise degrades signal quality | Drives meter location, spacing, elbows or attenuator |
Gas cleanliness and liquid content | Deposits and liquids affect path readings and the profile | Defines filtration scope and diagnostic alarm limits |
Target accuracy and calibration approach | Flow calibration and the correction curve feed into system uncertainty | Determines whether the meter is calibrated together with its meter run |
How TLY Enerji supports ultrasonic metering projects
TLY Enerji provides engineering support in reviewing flow, pressure and piping data and assessing whether ultrasonic metering suits the application. Depending on project scope, this can include supplying the ultrasonic meter and pressure and temperature instruments, meter installation and commissioning, integration of measurement and diagnostic data into a PLC- or SCADA-based monitoring system, and post-startup technical support. The scope of supply, including who builds the skid, is agreed against the project specification.
Data we need for a review
· Minimum, normal and maximum flow, and whether flow is one-way or bidirectional
· Line size, operating pressure and temperature range
· Piping layout upstream and downstream of the meter, including existing elbows and valves
· Location and pressure drop of any regulators or control valves on the same line
· Gas composition, risk of liquid carryover and existing filtration
· Purpose of the measurement, target accuracy class and calibration expectations
· Flow computer, gas chromatograph and control system interface requirements
Related solutions
· Orifice gas metering skids: The differential pressure alternative, with a comparison table.
· Custody transfer natural gas metering stations: System uncertainty and verification for billing-grade measurement.
· Natural gas metering stations: Meter run count, redundancy and station layout.
· Natural gas quality measurement and analysis: Composition data for speed-of-sound checks and energy.
· Gas pressure regulating skids: Regulator selection and the noise source itself.
Questions about ultrasonic metering skids
How many paths should an ultrasonic gas meter have?
There is no single right number. More paths sample the profile at more positions and generally reduce sensitivity to installation effects, which can shorten the straight pipe needed. The choice depends on target accuracy class, plot space, upstream disturbances and whether an independent second measurement inside the meter body is wanted for checking.
Can an ultrasonic meter be installed downstream of a regulator?
It can, but it is the more demanding arrangement, because high-frequency regulator noise can reduce the meter's signal-to-noise ratio. The regulator type and pressure drop are reviewed, spacing, elbows or an attenuator are designed in, and signal quality is confirmed from diagnostics at commissioning. Where layout allows, the meter goes upstream of the regulator.
Do diagnostics replace calibration?
No. Diagnostics show whether the meter's behavior has changed and flag situations that need attention early. The meter error itself is established by flow calibration or by the verification methods defined in the contract, and recalibration intervals follow national regulations, contract terms and the operator's metering policy.
Can an ultrasonic meter handle gas with small amounts of liquid?
Ultrasonic gas meter standards are written for single-phase gas. Droplets and deposits disturb the acoustic paths and the velocity profile, causing measurement error and diagnostic alarms. Where liquid carryover is possible, provide filtration and liquid separation upstream and orient the meter so liquid cannot collect around the transducers.