Petroleum Software esmerWGM Wet Gas Meter

The esmerWGM is Petroleum Software Ltd's meter for wet gas, meaning gas lines that carry a small liquid fraction. It pairs a standard venturi skid with real-time hydrodynamic and thermodynamic software that corrects the over-reading a dry gas meter shows when liquid is present.

Wet gas correction method

The software first calculates an apparent gas mass flow rate from the Bernoulli differential pressure equation as if the gas were dry. That figure over-predicts the true rate, so it is multiplied by an over-reading correction factor based on the de Leeuw correlation, which uses the Lockhart-Martinelli parameter and the gas Froude number. Because the correction needs the liquid fraction, esmerWGM finds it by modelling the fluid composition with an equation of state and running a flash calculation at line pressure and temperature. PSL also uses pressure recovery analysis for the liquid fraction.

Technical specifications

Calibration and validation

Units are calibrated in third party wet gas loops, and the NEL flow loop in the UK is commonly used. The brochure cites NEL Report 2013/432 (August 2013) and NEL Report 2006/213 (July 2006), each an evaluation of a 6 inch esmer meter in wet gas, and says NEL provides an independent performance report on request. After installation a meter can be validated and recalibrated against a gravity separator following API 2566 guidelines. Factory acceptance tests are inspected by third parties, and PSL names TUV, Bureau Veritas and Lloyds as organisations it works with on testing and inspection standards.

Sensitivity to pressure and liquid fraction

The brochure charts show that the liquid fraction error has a modest effect: at 30 bar and GVF 0.98, raising the liquid fraction error from 20% to 100% doubles total measurement error from 3% to 6%. Pressure matters more, since dropping from 60 bar to 15 bar under the same conditions raises the error by a factor of three. Plan low pressure duties with that in mind.

Selection and enquiry

Each unit is sized and built for the stated process and fluid. Send line size, pressure range, temperature, rates and the fluid composition. The cone version is more compact where space is limited. See the flowmeters category for related items.

Hardware and software layout

The field unit connects to the pipeline for straight through flow. Models with a cone element are more compact and suit limited space, while the venturi version follows ISO 5167 design practice. A multivariable transmitter measures differential pressure, absolute pressure and temperature, and the meter shares the esmerDigital connectivity platform, electronics and software with the other esmer products. Wet gas algorithms in the package change with the process conditions.

In the hazardous area the computer is an embedded PC in a Class II 2G EEx d IIB T6 enclosure; in a safe area it is an industrial PC in an IP55 rack mount enclosure. The esmerDigital console shows real-time strip charts and diagnostics, stores measurements in a database, and sends them out in analog or digital form over Ethernet and serial ports under several protocols. As with the other models, a small Exe enclosure holds the 24 V supply and converters, and one power cable and one RS485 cable link the field unit to it.

Typical uses

The meter fits gas and condensate production lines, wellhead and gathering lines and offshore platform lines, where the liquid fraction is small but must still be accounted for. The brochure includes a photograph of a venturi esmerWGM on an offshore platform. Material selection receives particular attention to protect against corrosion, and the build can follow customer-specified materials.

Commissioning and verification

General engineering note (not from the brochure): the over-reading correction depends on knowing the liquid fraction, and the liquid fraction depends on the composition used in the equation of state. Before start-up, check that the gas composition entered in the software comes from a recent representative sample, and review it again when the field produces more condensate or water. A comparison against a test separator, or against fiscal export metering where available, gives a direct check of the corrected gas rate.

Because the flow element is a venturi to ISO 5167 or a cone, the differential pressure, absolute pressure and temperature transmitters can be included in the normal site calibration plan. Checking impulse lines for liquid hold-up is particularly important in wet gas service, where trapped liquid can shift the differential pressure reading and therefore the gas rate.

Recording the composition, transmitter calibration dates and comparison results in one metering file makes later audits faster and shows how the correction factor has behaved as the field matured over its production life.

Related Petroleum Software products: Petroleum Software esmerGL Gas-Liquid Flow Meter, Petroleum Software esmerGSE Multiphase Flow Meter and Petroleum Software esmerMPFM Multiphase Flow Meter.

Documents and quotation

The manufacturer brochure can be downloaded here. Seven Star LLC, an independent supplier in Muscat, prepares quotations on request; no prices are published.