scope:

Purpose of Gas Measurement and the AGA Gas Measurement Manual

Natural gas is measured for custody transfer and other purposes such as system control. Getting reliable results from these measurements (i.e., accurate volume and energy quantities on a scheduled basis) is a process that entails:

• Determination of needed measurements

• Agreement on applicable measurement standards between buyer and seller (usually stated in a gas purchase contract or tariff)

• Selecting instrumentation appropriate to the task

• Collecting, auditing and validating data from these instruments

• Converting those measurements into accounting units (as in ccf, mcf, btu or other), to generate billing statements

The American Gas Association (AGA) has published technical reports detailing various measurement methods tailored to the specific needs of the US gas utility and transmission pipeline industries. The methods described in these reports form a commonly agreed design and application basis for the process of custody transfer gas measurement.

This Section 1 of the AGA Gas Measurement Manual provides a basic guide to gas measurement, referencing requirements, practices and techniques that underlie accounting for gas transfers from selling/transporting parties (i.e., production, processing, transportation and distribution) to end-use customers for both custody transfer and operational requirements. Since this is a broad-scope document, readers are directed to consult the many source documents referenced in the body and Bibliography of this publication for relevant detail on topics of interest.

Natural Gas & the Grid

Natural Gas is a hydrocarbon mixture occurring in subsurface geologic formations as a product of decomposing flora, fauna and bio-organisms. It is "produced" with a variety of drilling and stimulation techniques from varying subsurface formations once it is found in commercial quantities through geologic surveys. Gas can also be manufactured from coke or produced from landfills as bio-gas.

Natural gas has value as a fuel because it is possible to produce it for sale and deliver to a customer's burner tip where it is purchased on demand, at a reasonable price for the consumer, and at a profit for the seller. Customers are billed in units of volume and/or energy, and these bills are based on data generated by a Gas Measurement System (billing in units of energy is rapidly replacing billing by volume).

Natural gas, like any gas, is compressible, meaning it can be pressurized by "squeezing" it into a smaller volume. High pressure compression of gas is a critical feature in its economical transportation because it permits reducing the physical size of gas quantities so it is more portable, but most particularly because pressurized gas contains stored energy that seeks a lower pressure state: this makes transmission of gas in pipes possible as gas flows from high point to low pressure points. To demonstrate this, take the example of a tank of high-pressure gas connected to a pipe by a valve. If the valve is opened, gas flows into the pipe seeking the lower pressure state: gas will flow until pressure equilibrium of the new volume (the tank volume plus the pipe volume) is reached. This means we can move large quantities of gas by compressing it and discharging it into a pipe of lower pressure, and repeating the process along the pipe as pressure is "spent" by adding compression to periodically boost the pressure and continue flow in the pipe.

Because natural gas is compressible, its volumetric determination must be made at operating or line conditions, and then that volume corrected to a commonly agreed pressure and temperature base.

Gas quantities can also be measured in mass units using Coriolis meters, among other developing technologies. Direct mass measurement techniques still require that gas quality be determined so that total energy can be calculated for billing and accounting, and in case a particular operator wishes to account for product on a volumetric basis. Natural gas is not homogenous, that is, it is not uniform in its energy content since it is produced from many different geologic sources. Therefore, unit heating value varies between production fields depending on the nature and quantity of the particular hydrocarbon components that contribute to heating value, and the diluents that reduce heating value by displacing burnable hydrocarbons. As a result, it is necessary to measure how much energy is in the gas based on its composition at each point of measurement, including:

• As produced at the well-head

• Pre- and post-processing (dehydration, stripping of heavy hydrocarbons, etc.)

• Establish that it is of acceptable sales quality after processing

• Delivered in the appropriate quantity of energy demanded by the end-use customer to generate accurate billing statements at each point of custody transfer

Hydrocarbon gas mixtures are so named because they are composed of carbon and hydrogen atoms. Methane's chemical formula is, CH₄, represents 1 carbon atom and 4 hydrogen atoms; Ethane (C₂H₆), has 2 carbon and 6 hydrogen atoms, and so on through ~C10 (Decane). Heavier hydrocarbon components do exist, but often cross phase boundaries at higher pressures and condense into liquid. Liquid measurement is beyond AGA scope and the reader is directed to API & GPA documents for information on that subject. Hydrocarbon components with more carbon atoms than another are referred to as being "heavier" than the others because the molar mass of the molecule with more carbon atoms is greater. Heavier hydrocarbons have more heating value per unit volume than lighter hydrocarbons.

Because natural gas is a non-homogeneous hydrocarbon mixture of molecules and diluents, each with different heating value, these components must be identified, quantified on a unit volume basis, and total heating value per unit volume calculated from the weighted contributions of each.

The combination of production facilities, processing plants, transport pipelines and the gas distribution companies, or LDCs ("Local Distribution Company"), are collectively referred to as the gas grid. Various companies own and operate different sections of this grid which are interconnected with one another. Each time gas moves across interconnects on the grid, it changes custody and is metered and sampled. Daily energy reports are combined for an accounting period, usually 1 calendar month, so Sellers can generate an invoice for payment by Purchasers.

Gas Measurement

Measurement is the process of using instruments to gather data that characterizes a substance, product, material, etc. that one desires to describe objectively. These measurements include size, weight or other quantities or material qualities from which an unbiased assignment of value can be made based on the measured substance's agreed unit price.

All measurements are taken relative to a specified standard. In the MKS system (meter, kilogram, second) used globally, government metrological authorities maintain reference standards against which all others are calibrated. NIST (National Institute for Standards & Technology) is the US government body that maintains these standards. A traceability chain is established between NIST standards and the certified test instruments used by industry to calibrate and/or prove the accuracy of field instruments. This permits persons and industries to buy, sell and trade goods on a reliable and fair basis. Commerce is made possible by accurate measurement because it eliminates bias in sales transactions.

Since natural gas is a non-homogeneous (i.e., of varying composition), compressible fluid, there are very specific challenges in measuring its quality (heating value, for example) and quantity (volume).

First, ambient conditions generally don't affect the weight of solids (an exception being elevation), however since natural gas is compressible we must observe the pressure and temperature conditions at which the gas is measured, and apply corrections per Boyle's and Charles' Laws to correct the measured volume to agreed base conditions (refer to Section 2 of this document). Therefore, precise measurements of the gas' pressure and temperature are needed, in addition to instruments that can measure gas volume at pipeline conditions. Often times, metrology of compressible fluids is referred to as measuring at "actual","flowing", or "line" conditions as distinct from volumes that have been corrected to "base" or "contract" conditions. Going forward, the terms "flowing" and "base" shall be used and in formulae, the subscripts "f" and "b" to denote "flowing" and "base" conditions respectively.

Secondly, non-homogeneous natural gas' quality varies depending on where it is produced, and measurements of its composition are needed to calculate component contributions to the gas' heating value since gas from one source may have more ethane, for example, than another which causes their respective heating values to differ.

We must also consider that most items are weighed as static quantities. However due to the physical nature of gas, measurement of each unit volume (e.g. cubic foot) would be so cumbersome as to be prohibitive, so a wide variety of gas meter types have been designed specifically to measure gas at flowing conditions. Taking repeatable measurements of such dynamic quantities presents its own set of challenges which explains why so many meter types have been designed. The challenge for industry is to apply these many measurement instrument types for custody transfer use in a correct and uniform manner: AGA and other trade groups have a positive impact on Industry in this regard.

Organization of the AGA Gas Measurement Manual

The Manual is separated into topic-specific sections which are available for purchase by the general public as individual documents or in total. Separating the Manual into individual topics permits a more detailed focus, and permits AGA to make editorial updates to them independently. The Manual is organized into the following sections:

Part 1 General

Part 2 Displacement Metering

Part 3 Gas Orifice Meters

Part 4 Gas Turbine Metering

Part 5 Other Measurement Methods

Part 6 Auxiliary Devices

Part 7 Measurement Calculations and Data Gathering

Part 8 Electronic Flow Computers and Transducers

Part 9 Design of Metering & Regulating Stations

Part 10 Pressure and Volume Control

Part 11 Measurement of Gas Properties

Part 13 Distribution Meter Data

Part 14 Meter Repair and Selection

Part 15 Electronic Correctors

Part 16 Multipath Gas Ultrasonic Meters (publication pending)

Part 17 Coriolis Metering (publication pending)

Part 23 Meter Proving

AGA also publishes Technical Reports that are specific to measurement instrumentation applied for check, allocation and custody purposes. Reference to specific AGA Reports is made in this document's Section 3, "Principals of Gas Measurement and Metering".