Gas Metering Skid

Overview

A gas metering skid is the measurement point where natural gas changes ownership, at a wellhead, a gathering inlet, a processing plant tailgate or a pipeline interconnect. Unlike a liquid, gas is compressible and its energy content varies with composition, so a gas skid measures three things at once: how much gas flows, the conditions it flows under, and what it is made of. From those it computes a corrected volume at standard conditions and an energy total in million BTU, which is what the contract settles on. A skid handles anywhere from 50 MSCFD at a small lease to 500 MMSCFD at a major interconnect.

The skid mounts everything on a single Skid Base and Piping with parallel meter tubes fed from a common header, so individual runs can be taken out of service for plate changes or proving while the others keep flowing. Two metering technologies dominate. Orifice metering, governed by AGA Report 3, infers flow from the differential pressure across a precisely bored Orifice Plate. Ultrasonic metering, governed by AGA Report 9, times acoustic pulses across the pipe to measure gas velocity directly. Both feed a Fiscal Flow Computer that applies the corrections.

Inlet conditioning

Gas measurement is sensitive to anything riding in the stream. Liquid slugs, compressor oil and pipe scale all bias the reading and can damage the primary element, so the Inlet Conditioning section cleans the gas first. The Filter Separator drops out liquid in a two-stage vessel, coalescing fine mist on its elements and shedding free liquid through an Automatic Liquid Drain. Downstream, a Flow Conditioner Plate removes swirl and flattens the velocity profile.

Flow conditioning matters because both orifice and ultrasonic meters assume a fully developed, symmetric flow profile. A bend, a valve or a tee upstream creates swirl and asymmetry that the meter reads as extra or missing flow. The standards specify minimum straight lengths of pipe upstream and downstream of the primary element, and a flow conditioner plate lets those lengths be shortened while still delivering a clean profile.

Primary metering

The Primary Meter Tube is a precision-bored straight run carrying the primary element. In an orifice run, the gas passes through the bore of the orifice plate, creating a differential pressure that a Multivariable Transmitter reads along with static pressure and temperature. The senior Senior Orifice Fitting is a dual-chamber device that lets the plate be removed and inspected under full line pressure, a routine task because plate wear and edge damage are the main causes of orifice drift.

In an ultrasonic run, the Ultrasonic Meter Spool carries several pairs of Ultrasonic Transducer elements set at angles across the bore. Each pair times a pulse traveling with the flow and against it; the difference in transit time gives the gas velocity along that path, and multiple paths build a profile that the Ultrasonic Meter Electronics integrates into a flow rate. Ultrasonic meters have no pressure drop and no moving parts, which suits high-volume pipeline service.

Composition and energy

Gas is bought and sold by energy, not volume, and energy depends on composition. The Gas Analyzer System runs an on-line Gas Chromatograph that separates the gas on a packed column and quantifies methane, ethane, propane, the heavier hydrocarbons, nitrogen and carbon dioxide. From that composition the flow computer calculates the heating value and the compressibility factor.

Compressibility is the link between actual and standard conditions. Natural gas does not behave as an ideal gas at line pressure, so the Fiscal Flow Computer applies the AGA 8 equation of state, using the measured composition, pressure and temperature, to convert metered conditions to a standard 14.73 psia and 60 degrees F. The Sample Conditioning System system regulates and dries the sample so the chromatograph sees clean gas, and a Carrier Gas Bottle supplies the helium carrier.

Flow computation and control

Each run has its own fiscal Fiscal Flow Computer so a fault on one run does not corrupt another. The computer reads the primary element, the Multivariable Transmitter or ultrasonic electronics, the RTD Temperature Probe and the composition, applies the AGA flow and compressibility equations, and totalizes corrected volume and energy. It logs hourly and daily records to a Data Logger and serves them to the operator through an Remote Terminal Unit.

The Pressure Regulation section holds the metering pressure stable with a working Pressure Regulator and a Monitor Regulator standing by, and protects the skid with a Pressure Relief Valve and a Slam-Shut Valve that trips on overpressure. The Valve Manifold routes flow between runs and isolates a run for maintenance, with Check Valve protection against backflow.

Materials, power and standards

Process piping follows ASME B31.8 for gas transmission, with weld-neck flanges and carbon-steel spools, while wetted instrument parts and the demister use stainless steel against trace hydrogen sulfide and water. All field electrical equipment is rated for Class I hazardous areas, and the analyzer lives in a climate-controlled Analyzer Shelter. Remote sites often run on a Solar Power System system with battery backup, since many metering points sit far from grid power.

The whole skid is built and operated to a stack of standards: AGA 3 and AGA 9 for the primary measurement, AGA 8 for compressibility, API MPMS Chapter 14 for sampling and Chapter 21 for the electronic flow measurement and audit trail. Together they make the skid's numbers defensible in a custody dispute, which is the entire point of fiscal metering.