The basics of natural gas metering systems
KROHNE Oil & Gas
We manufacture instrumentation, metering systems and software for the Oil & Gas industry - from wellhead to refinery
Introduction
Natural gas prices have been fluctuating significantly over recent years. Furthermore, the industry has some challenges ahead, such as increased LNG trading, natural gas consumers paying for CO2 emissions and possible injection of hydrogen in the gas grid. These developments make accurate measurement of natural gas more important than ever. In this article we explain the basics of a natural gas metering system.
Metering system design
A metering system usually consist of multiple meter runs. Using multiple smaller meters means very low flows can still be measured accurately by sending all flow through one meter run. In addition, a meter run can be isolated for repairs or inspections while flow measurement continues through the other runs.
In many cases one of the meter runs is assigned as master meter run. The goal is to use the master meter only for periodical verification of the duty meters, so possible contamination or wear in the duty meters will cause a difference with the master meter that is only used periodically. Below overview shows, on the top part of the picture three duty runs are used, on the bottom part one of the duty runs is verified with the master meter.
Flowmeters
The heart of the metering system is formed by the flowmeters. Custody Transfer ultrasonic flowmeters are mostly used for high accuracy measurement of natural gas today , but also Coriolis or turbine meters. It is important that the meters are installed with sufficient inlet piping to eliminate measurement errors due to installation effects. If only limited space is available, flow conditioners could be considered to optimise the flow profile.
Analyser house and flow computer cabinets
Larger metering systems are usually equipped with an analyser house that contains instrumentation such as Gas Chromatographs, Dew Point, Sulfur, Mercury, Density, and Trace Oxygen Analysers. For the analysis it is important that the gas in the sample lines is representative to the gas in the main pipeline. The location and design of the take-off probe, sample line routing and sample conditioning system are hence crucial, to minimise lagtimes and maintain the sample properties.
A flowmeter calculates the exact volume and calorific value that has been supplied. Usually, the flowcomputer needs to be certified by a 3rd party, such as NMi or PTB, to guarantee that the calculations are correct. It is typically integrated in the flow computer cabinet, together with the supervisory computer, UPS and PLCs, allowing the operator to control the metering system from the supervisory screen in the control room.
Further considerations
An increasing number of natural gas metering systems are nowadays designed in a master-duty lay-out, offering the best possible measuring in a very dynamic natural gas market. Ultrasonic flowmeters are often used as they offer substantial diagnostic possibilities, including an AGA10 based speed of sound verification. Designing, construction and testing of a metering system requires specific knowledge of instrumentation and local metrology requirements. In case of questions, do not hesitate to contact your local KROHNE office.
Author: KROHNE – Hilko den Hollander ([email protected])
(RETIRED) Member at Wantima Country Club
2yA very good summary. The only bit missing is how gas system owners manage this data from the measurement systems. At this point it is not understood the benefits of automatic stream selection, cumulative totalisation of measurement during maintenance, predictive production benefits. As a retired metering engineer it is important to have these concepts embedded early in the design. So often gas contracts are let and design engineers get straight to work on the field equipment and they can save on minimum equipment, without looking at the bigger picture.