Gas Metering
Coriolis vs Ultrasonic: Selecting a Flow Meter by Service
Orifice, ultrasonic and Coriolis technologies have different installation, accuracy and maintenance requirements. Sound selection starts with the service and the process conditions, not with design habit.
Selection starts with the service, not the technology
In a custody transfer metering system, the choice of primary element shows its effect earlier than any other decision: the skid layout, the amount of flow conditioning, the run lengths and even the proving branch all follow the meter. Meter selection is not a procurement detail; it is the first decision.
Two questions settle most of it. Is the service gas or liquid, and does the contract settle on volume at reference conditions or on mass? In gas service, a multipath ultrasonic meter or an orifice plate is the conventional choice. In liquid service, where mass is the determining quantity, Coriolis is selected.
Flow profile sensitivity is the main difference
Ultrasonic meters and orifice plates in gas service are calibrated against a fully developed, swirl-free profile. That means the meter needs filtration, a flow conditioner and specified upstream and downstream straight lengths in order to see the same flow it was calibrated for.
Here is what makes this expensive in practice: if the profile is wrong, the meter does not fail and does not become erratic. It reads stably, and wrong. An error of this kind is invisible in the output and usually surfaces only at proving, or in a dispute with the counterparty.
A Coriolis meter is not sensitive to profile in the same way and needs far less upstream conditioning. That is not unconditional freedom: in liquid service a strainer and gas elimination are still required.
Pressure and temperature correction
Flow is measured at line conditions but settled at reference conditions, so whatever the correction needs must be measured too. On a gas line, pressure and temperature are taken at the meter, and the flow computer calculates corrected volume and mass to AGA, API MPMS and ISO.
A Coriolis meter reports mass and density directly and needs no pressure correction to determine quantity. Temperature still matters, and wherever the contract asks for volume at reference conditions, a densitometer is added.
What changes once the meter is chosen
The meter determines the layout. Metering runs are arranged in parallel so that one can be taken out of service for proving or maintenance while the others carry the load, and the control valve sits downstream so that its disturbance is never upstream of the measurement.
None of these technologies is simply “more accurate” than the others. The accuracy achieved in practice comes from matching the meter to the service, respecting its installation requirements, and running a proving programme that keeps the number valid over time.