# Managing Lost and Unaccounted for Gas Volumes

Editor’s note: The content of this article represents a summary of Part 1 of a series of papers prepared by Bradley Bean and presented at various venues.

By Bradley B Bean, PE Managing Member, B3PE

(P&GJ) — Lost and unaccounted for (LAUF) gas is a calculated value that compares the amount of gas coming into a system (purchased) to the amount of gas leaving that system (sold or otherwise accounted for). Although LAUF is often equated to leaking pipes and equipment, leaks generally only represent a small portion of the value.

LAUF is more often largely the result of poor accounting and improper adjustment and correction of measured values. This article will focus on managing this portion of the LAUF value. For the remainder of this article, the term LAUF will be re-branded to unaccounted for gas (UFG). This is a much more definitive description.

This article uses nomenclature, terms, and values associated with the distribution segment of the natural gas industry in the United States (U.S.). However, the concepts and issues presented are applicable regardless of the segment or location of the operation.

The Cubic Foot

In the U.S., the basis for a quantity of gas is commonly the cubic foot. This means that an amount of gas is quantified by how many molecules fit into a physical space of one cubic foot, the “cube.”

Natural gas expands and contracts depending on pressure and temperature. To determine how much gas is in the cube, we need to know the pressure and temperature of the gas contained in the cube. This is where the concept of a “standard” cubic foot becomes important. A standard cubic foot of gas is the amount of gas contained in the cube at a given pressure and temperature, referred to as the base pressure and base temperature.

These base conditions vary based on jurisdiction and contract. In 1963, the American Gas Association (AGA) endorsed the values of 14.73 psia for Base Pressure and 60 Fahrenheit for base temperature. Although common, these values are not consistently used throughout the U.S., nor are they valid for every location.

Using the wrong or inappropriate base pressure and base temperature is a common error in gas accounting.

Atmospheric Pressure

When referring to pressure, realize that there are several different values of pressure. Gauge pressure is the value commonly referred to when discussing delivery pressure. Barometric pressure refers to the hydrostatic pressure of the atmosphere above the measurement point. Absolute pressure is the sum of the gauge and barometric pressures.

When determining the quantity of gas actually measured by a meter, the absolute pressure within the meter must be known. The most accurate measurement would result from using the local barometric pressure to calculate the absolute pressure in the meter. This is not necessarily practical for the millions of small capacity meters used by local distribution companies.

Barometric pressure changes constantly. The value is affected by many factors, including temperature and humidity. In the natural gas industry, it is common practice to calculate an average barometric pressure for a service area using a formula based on geographic elevation. This value is known as atmospheric pressure.

There is no industry standard for calculating atmospheric pressure. Using the AGA recommended calculation method yields an Atmospheric Pressure value at an elevation of mean sea level of 14.73 psia, a familiar number. This value should not be confused with the AGA recommended Base Pressure value. Even though the two numbers are the same value, they represent completely different parameters.

Using a meter pressure of 0.25 psig, the commonly used base pressure value of 14.73 psia is only valid at locations where the local barometric pressure is 14.48 psia. Using the AGA calculation method for atmospheric pressure, this value would only be valid for locations where the elevation above mean sea level is about 475 feet.

At a 0.25 psig delivery pressure, using an Atmospheric Pressure value that is 1.0 Psia in error would result in about a 5% misreporting of the measured volume.

Base Pressure

In the past, local distribution systems in the United States commonly established a base pressure using the local atmospheric pressure plus the delivery pressure at the meter. This combination closely equates to the absolute pressure in the meter. Currently it is becoming more common to find a base pressure of 14.73 psia being used regardless of location. This practice is not necessarily a problem if appropriate adjustments are applied.

A common form of gas measurement is by diaphragm meter. With this type of meter, a chamber of fixed size is continually filled and emptied to measure the gas flow. The quantity of gas contained in the chamber is related to the absolute pressure present in the chamber. The base pressure value is commonly intended to represent this value.

Representing a volume measured at local conditions for a non-local Base Pressure requires applying an adjustment factor to the quantity measured by the uncorrected meter index. This factor is defined in Equation 1.

Equation 1

A 0.25 Psi error in the Base Pressure value results in approximately a 2% error in measurement.

Base Temperature

The actual quantity of gas passing through a meter is affected by the temperature of the gas. The cooler the gas in the meter, the greater the quantity passing through. The warmer the gas in the meter, the lesser the quantity of gas passing through the meter.

In the U.S., a base temperature value of 60 Fahrenheit is commonly used throughout the industry. This is not necessarily representative of the flowing temperature of the gas moving through the meter in many cases. Most meters are installed downstream of a pressure-reducing regulator.

The gas temperature upstream of the regulator is approximately the same as the ground temperature at the burial depth of the upstream piping. As the gas passes through the regulator, the temperature is reduced. It is not practical to measure this value in the millions of local sales meters. Therefore, it is desirable to have the meter compensate for the actual flowing temperature. Most modern meters can provide intrinsic temperature compensation for the actual flowing gas temperature.

If the meter is not temperature compensated, and the Flowing temperature is known, the measured (indexed) value can be corrected for flowing temperature using Equation 2:

Equation 2

A 5-degree Fahrenheit difference in Flowing Temperature compared to Base Temperature will result in an approximate error of 1% in volume measurement.

Compressibility

Most gas volume calculations are derived from the Ideal Gas Law. Natural gas is a “real” gas that does not follow “ideal” gas behavior. This deviation from ideal behavior is known as compressibility. It varies with changes in pressure, temperature, and gas composition.

The factor used to compensate for compressibility is known as the compressibility factor. A compressibility factor value of 1.0 represents ideal gas behavior. At low pressure, natural gas closely follows ideal gas behavior. As pressure increases, deviation from the ideal gas law is more exaggerated.

There are many methods for calculating the compressibility factor. Most electronic correction instruments have simplified versions of these methods programmed in their firmware and can estimate the compressibility during measurement. For non-corrected metering applications, where the meter pressure is reasonably constant, the Compressibility factor can be periodically calculated for the anticipated conditions and applied using Equation 3.

Equation 3

Often compressibility is ignored in low-volume, low-pressure measurements from distribution systems. Compressibility should be considered for high-pressure delivery and for large-volume applications at any pressure.

Meter Pressure

The quantity of gas measured by a meter is directly affected by the absolute pressure within the meter “chamber”, the meter pressure. Realize that this value is not necessarily the same as the “delivery pressure”, which commonly refers to the set pressure of the service regulator upstream of the meter.

There is always some pressure loss between the service regulator and the meter chamber. If the absolute meter pressure is different from the established base pressure, and is not corrected, the reported volume will be misreported.

Measurement at a meter pressure different than the base pressure can be accounted for by the application of an appropriate pressure adjustment factor, often known as the fixed pressure meter factor (FPMF). The FPMF can be calculated using the following equation:

Equation 4

It is prudent to periodically review and audit the delivery pressure to meters in your system to ensure that the Meter Pressure is as intended, that the FPMF is correct, and that the FPMF is appropriately entered and applied in the accounting/billing system.

Gas Laws

All of the previously mentioned volume corrections or adjustments were derived from the basic gas laws, which include Boyle’s Law, Charles’ Law, and their combination known as the Ideal Gas Law. With the addition of compressibility, the general volume correction Equation 5.

Equation 5

Applying the above equation, with the appropriate Compressibility Factors, to an uncorrected measured volume will provide the most accurate accounting of the gas passing through the meter.

Volume Comparisons

When comparing measured volumes, ensure that both volumes are with respect to the same base pressure and base temperature (compressibility is generally ignored). If the values are expressed at different base conditions, one value can be expressed in terms of the other using Equation 6.

Equation 6

A comparison of two volumes at different base conditions is not a valid comparison. This is particularly important to consider when completing required regulatory reports where the required reporting base conditions may be different than your base conditions.

Heating Value

It is common practice to buy and sell gas based on energy content. Unfortunately, there is no practical energy meter for gas flow. Instead, the gas flow is continuously measured on a volumetric basis, and the energy content is periodically determined through a separate process. The accumulated energy bought or sold is then calculated from the determined energy value and the measured gas volume using Equation 7.

Equation 7

Equation 7

With respect to UFG, an error often occurs when energy-based values are being used to compare bought and sold volume values. For this comparison, the energy values must be converted back to volume values. Energy content varies over time. For a distribution system supplied from a transmission system, it is common for the gas being supplied into the system (bought) to be corrected for energy at close intervals.

The gas being supplied to the distribution customers (sold) is only corrected for energy at the end of the billing cycle, which is most often monthly. These mismatched correction intervals make it difficult to accurately calculate the original volume values from the accumulated energy values.

When comparing volumes, it is best to use the actual measured volume values. Using volumes calculated from energy values most always results in error.

What It All Means

If you are not applying the appropriate adjustments and corrections to your volume measurements, you are not providing the most accurate accounting of your sales or receipt volumes. Furthermore, if your correctors and instruments are not set up using the appropriate base pressure, base temperature, and atmospheric pressure, they are reporting inaccurate values.

If your annual LAUF/UFG value is 3% or higher, or your LAUF/UFG value indicates that you are selling more gas than you are buying, you probably have room for improvement. In this case, you should consider auditing your current correction and adjustment methods, including implementing the corrections mentioned in this article.

Most gas accounting/billing software can accommodate the entry of the required values, making implementation of these adjustments and corrections quite simple.

Variable Definitions

EnergyB = Energy content of accumulated volume at specified base pressure, base temperature, and corrected for compressibility.

FPMF = Fixed pressure meter factor

HVB = Heating value (energy content) per unit volume at specified base pressure, base temperature, and corrected for compressibility.

PB = Base pressure

PMeter = Gas pressure in the meter (gauge).

TB = Base temperature

TMeter = Gas temperature in the meter (absolute).

VPB1,TB1 = Measured (first) volume at the specified first base conditions.

VPB2,TB2 = Equivalent first volume at the specified second base conditions.

VB = Corrected volume at specified base pressure, base temperature, and corrected for Compressibility.

VIndex = Measured volume as indicated by the meter index.

VPB = Corrected volume at specified base pressure.

VTB = Corrected volume at specified base temperature.

ZB = Compressibility factor calculated at the associated base pressure and base temperature.

ZMeter = Compressibility factor calculated at the flowing pressure and temperature in the meter.

Note: When used in the listed equations and formulas, each parameter must be in compatible dimensional units.

Author: Bradley Bean is the senior partner of B3PE, which has provided engineering software and services to the natural gas Industry since 1992. Bean has been involved in the industry since 1982 and has been involved with several UFG audit projects. Bean can be reached at bbb@b3pe.com.