A flow metering skid is designed by first defining the duty – custody transfer or check metering, liquid or gas, flow range and required uncertainty – and then selecting the meter technology, the supporting equipment (filtration, flow conditioning, valves, proving, flow computer, sampling) and the testing scope around it. Most design decisions are driven by the fluid properties and by the contractual and regulatory requirements of the measurement. This guide from Akan Industrial and Automation outlines the main steps for liquid and gas metering skids.
What should be defined before designing a metering skid?
A metering skid can only be designed from complete process and project data. Typical inputs:
- Fluid type and composition, density, viscosity and, for gases, composition for property calculation
- Minimum, normal and maximum flow rates, and expected future flows
- Operating and design pressure and temperature
- Required measurement uncertainty and the reference standards to be applied
- Duty: custody transfer, fiscal, allocation or check metering
- Number of meter runs and redundancy philosophy
- Proving or verification method
- Area classification, available plot space and access
- Interfaces to the control system, SCADA and the counterparty
Ambiguities in these inputs are the main source of late changes, so they should be resolved in the basis of design.
Custody transfer or check metering?
Custody transfer metering measures quantity where ownership changes hands, so the measurement directly affects invoicing. Its design is governed by the contract between the parties and, where applicable, by legal metrology requirements. It typically involves stricter uncertainty targets, redundancy, proving or meter verification, sealing and audit trails.
Check metering verifies another meter or provides data for allocation, process control or loss monitoring. Requirements are set by the operator and are generally less stringent.
The duty determines much of the skid: the number of runs, whether a prover or master meter is permanently installed, the flow computer specification and the documentation required. See custody transfer metering systems for Akan’s scope.
How do you choose the flow meter technology?
| Technology | Typical fluids | Strengths | Points to consider |
|---|---|---|---|
| Ultrasonic (multi-path) | Gas and liquid | No moving parts, very low pressure loss, wide turndown, diagnostics | Sensitive to flow profile; straight runs or flow conditioning needed; deposits and noise from control valves can affect performance |
| Turbine | Low-viscosity liquids, clean gas | Well-established, good repeatability | Moving parts subject to wear; performance changes with viscosity; filtration needed |
| Coriolis | Liquids and gas | Direct mass flow and density; largely independent of flow profile | Pressure drop and size limits at large line sizes; installation stresses and two-phase flow can affect reading |
| Orifice (differential pressure) | Gas, steam, liquids | Simple, no moving parts, dimensions-based, widely standardised | Limited turndown, permanent pressure loss, plate edge condition and upstream straight lengths are critical |
| Positive displacement (PD) | Viscous liquids | Good accuracy at higher viscosity, low flows | Moving parts; sensitive to solids; pressure drop |
Selection usually narrows quickly once fluid, viscosity, line size and allowable pressure drop are known. Achievable uncertainty and turndown depend on the specific meter model and installation, so compare manufacturer data and the applicable standard rather than generic values.
What equipment goes on a metering skid?
| Component | Purpose |
|---|---|
| Inlet and outlet headers | Distribute flow between meter runs and allow runs to be isolated |
| Filters, strainers and separators | Protect meters, especially turbine and PD meters, from solids and liquids in gas |
| Flow conditioners and straight lengths | Provide the flow profile required by ultrasonic, turbine and orifice meters |
| Isolation and double block-and-bleed valves | Isolate runs and verify that valves in the measurement path do not leak |
| Control and motorized valves | Flow or pressure control and automatic run switching |
| Pressure and temperature transmitters | Correction of volume to base conditions and density calculation |
| Densitometer or gas chromatograph (where needed) | Density or composition for mass and energy calculations |
| Prover or master meter | In-situ verification and meter factor determination |
| Flow computer | Calculations to the applicable standards, totalisation, audit trail and reporting |
| Sampling system | Representative samples for quality and composition, for example for liquid hydrocarbons |
| Skid frame, piping and instrument tubing | Structure, process connections and access for maintenance |
| Junction boxes and E&I | Field wiring interface to the control and metering system |
Not every skid needs every item; the list is set by the duty, the fluid and the standards adopted for the project.
Which standards are relevant?
The applicable standards are set by the contract, the regulator and the project specification. Commonly referenced families include:
- API Manual of Petroleum Measurement Standards (MPMS) – chapters covering metering, proving, temperature and pressure correction, sampling and calculation for petroleum liquids and gas.
- OIML R117 – international recommendation for dynamic measuring systems for liquids other than water, often the basis of legal metrology approval.
- ISO 5167 – measurement of fluid flow by differential pressure devices such as orifice plates.
- AGA reports – American Gas Association reports for gas metering, including orifice, turbine and ultrasonic meters and gas property calculation.
National legal metrology requirements and the counterparty’s specifications may add further obligations. The exact editions and clauses should be fixed in the project’s basis of design and confirmed with the parties involved.
How are metering skids tested?
Testing normally has two stages:
- Factory acceptance test (FAT) – carried out at the fabrication shop before dispatch. It typically covers dimensional and visual inspection, documentation review, hydrostatic or pressure testing, loop checks of instruments, functional tests of valves and the flow computer, and simulation of calculations. Meter calibration certificates are reviewed; where required, meters are calibrated at an accredited facility.
- Site acceptance test (SAT) – after installation, confirming that the skid works with the plant’s control system and the actual process, including communication, alarms and, where applicable, proving under operating conditions.
Test procedures, acceptance criteria and witness points should be agreed with the client and, for custody transfer, with the counterparty or authority before the FAT.
How does Akan deliver metering skids?
Akan Industrial and Automation designs and integrates flow metering skids with ultrasonic, turbine, Coriolis, orifice or PD meters combined with master meters, sized from the customer’s process data. The scope covers:
- Flow meter sizing and selection
- Selection of control valves, motorized valves, strainers, piping, pressure and temperature instruments and electrical instruments
- Mechanical design and calculations
- P&IDs and electrical and instrumentation (E&I) design
- Fabrication with carbon steel, stainless steel and special alloys, mechanical and electrical pre-assembly
- Inspection and testing according to applicable codes, standards and customer specifications before dispatch
- Installation, commissioning and start-up
The same capabilities are used for fuel gas skids and process skid packages.
How Akan can help
Akan’s engineers can review your process data and duty, propose a meter technology and skid configuration, and deliver the design, fabrication and commissioning as one package. See our instrumentation and controls engineering, engineering and design and procurement services and the oil, gas and petrochemical industry page, or contact us with your process data to discuss a metering skid.
