Measurement Accuracy.............................................................................................................................1
Volume..................................................................................................................................................1
Pressure................................................................................................................................................1
Temperature..........................................................................................................................................2
Heating Content....................................................................................................................................2
Correction Factors.....................................................................................................................................2
Pressure Correction..............................................................................................................................2
Fixed Factor Billing................................................................................................................................3
Average Pressure Factor Billing............................................................................................................3
Compensating Indexes and Correcting Instruments.............................................................................3
Meter Reading...........................................................................................................................................4
Meter Designs............................................................................................................................................5
Diaphragm Meters.................................................................................................................................6
Rotary Meters........................................................................................................................................7
Turbine Meters......................................................................................................................................9
Orifice / Differential Pressure Meters..................................................................................................10
Ultrasonic Meters................................................................................................................................12
Mass Flow Meters...............................................................................................................................14
Additional Meter Types.......................................................................................................................15
Meter Standards......................................................................................................................................16
Diaphragm Meter Standards...............................................................................................................16
Rotary Meter Standards......................................................................................................................17
Inferential Meter Standards (Turbine, Ultrasonic, and Orifice Meters)................................................17
Meter Sampling and Testing Requirements............................................................................................17
Meter Installation Requirements..............................................................................................................18
Meter Handling....................................................................................................................................18
Location (CFR 192.351)......................................................................................................................18
Inside Locations..................................................................................................................................18
Outside Locations...............................................................................................................................19
Additional Installation Requirements...................................................................................................19
Meter Selection........................................................................................................................................21
Gas Theft.................................................................................................................................................23
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LIST OF TABLES
Page
Table 10-1: Heating Content Expressions................................................................................................2
Table 10-2: Classes of Small Diaphragm Meters...................................................................................16
Table 10-3: Classes of Large-Volume Diaphragm Meters......................................................................16
Table 10-4: Meter Capacity....................................................................................................................22
LIST OF FIGURES
Page
Figure 10-1: Residential Regulator and Meter..........................................................................................1
Figure 10-2: Daniel Flow Computer.........................................................................................................4
Figure 10-3: Hand-Held Meter Reading Device.......................................................................................5
Figure 10-4: Sized Meter.........................................................................................................................5
Figure 10-5: Diaphragm Meter.................................................................................................................6
Figure 10-6: Diaphragm Meter Accuracy Chart.......................................................................................7
Figure 10-7: Rotary Meter Design...........................................................................................................8
Figure 10-8: Rotary Meter........................................................................................................................8
Figure 10-9: Rotary Meter Accuracy Curve.............................................................................................9
Figure 10-10: Turbine Meter.....................................................................................................................9
Figure 10-11: Turbine Meter...................................................................................................................10
Figure 10-12: Turbine Meter Accuracy Curve.........................................................................................10
Figure 10-13: Orifice Meter.....................................................................................................................11
Figure 10-14: Orifice Plate......................................................................................................................11
Figure 10-15: Daniel Orifice Meter..........................................................................................................12
Figure 10-16: Small City Gate Station....................................................................................................12
Figure 10-17: The Sensus Sonix 6 Ultrasonic Meter..............................................................................13
Figure 10-18: Ultrasonic Meter...............................................................................................................13
Figure 10-19: Ultrasonic Meter Accuracy Chart......................................................................................14
Figure 10-20: Daniel Coriolis Meter........................................................................................................14
Figure 10-21: Coriolis Meter Accuracy Chart..........................................................................................15
Figure 10-22: Axial Gas Flow Meter.......................................................................................................15
Figure 10-23: Regulators Installed Inside...............................................................................................19
Figure 10-24: Guard Post Protecting Outside Meters.............................................................................19
Figure 10-25: By-pass Line.....................................................................................................................20
Figure 10-26: Low- and Medium-Pressure Outside Meter Installations..................................................21
CHAPTER 10: VOLUME MEASUREMENT
Introduction
A meter is a device that records a measured amount of specified material. Within the gas industry, meters are used to account for large volumes of gas delivered at gate stations and to record smaller amounts supplied to individual customers for billing purposes.
Figure 10-1: Residential Regulator and Meter
Measurement Accuracy
Metering accuracy is affected by several variables. These include Volume, Pressure, Temperature, and Heating content. A discussion of their impact follows:
Volume
Because natural gas is a compressible material, its volume is related to absolute pressure and absolute temperature. This relationship is expressed mathematically as the Basic Gas Law: PV/T= constant.
The standard unit of volume recommended by the American Gas Association is one cubic foot of gas at a base temperature of 60° Fahrenheit and base pressure of 14.73 psia. Volume is measured in cubic feet (CF), hundred cubic feet (CCF), or thousand cubic feet (MCF). Metered volume is affected by temperature, heating content, and pressure.
Pressure
A gas consists of widely separated molecules in rapid motion. Pressure is generated as gas molecules strike the walls of a container. As pressure increases, molecules are forced closer together, resulting in a greater number of molecules per cubic foot.
Pressure is measured in pounds per square inch (psig) or inches water column (w.c.). Low-pressure distribution systems are normally operated at around 7 inches w.c. pressure. A pressure of 1 psig is approximately equal to 27 inches w.c. Gas pressure will remain relatively constant at a meter because of
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upstream pressure. However, an average atmospheric pressure must also be specified for billing purposes.
Temperature
As gas temperature increases, the molecules move faster and farther apart. Assuming the gas pressure remains constant, warm gas will have fewer molecules per cubic foot than colder gas. Metered volumes are normally adjusted to a base temperature of 60° Fahrenheit.
Heating Content
Natural gas is actually a mixture of different gases. As a result, the amount of heat (Btu’s) in one cubic foot (cf) of gas will depend upon its chemical composition.
Table 10-1: Heating Content Expressions
Btu
Amount of energy, or quantity of heat, required to raise the temperature of 1 pound of water 1° Fahrenheit.
1,000 Btu’s
Approximate energy content of natural gas per cubic foot at atmospheric pressure
MMBTU
1 million Btu’s
Dth Dekatherm
Amount of gas containing a heating value of 1 million Btu’s
Gas is normally purchased by distribution companies at gate stations on the basis of energy content. Energy content⎯or heating value⎯is measured in Btu’s. Gas chromatographs and instruments are used to measure the quantity of energy bought or sold. Gas is normally sold to individual customers on the basis of volume and is delivered to customers at a controlled pressure. As a result, pressure correction is normally unnecessary. However, outdoor meters in a variable climate will require temperature compensation to provide accurate measurement.
Metered volume will change by approximately 1% for each of the following variations:
• 5° Fahrenheit change in flowing gas temperature from the 60° Fahrenheit base temperature
• 4-inch w.c. change in gas pressure or equivalent change in atmospheric pressure
Metered volume may be corrected or uncorrected for gas temperature, pressure, and heating content depending on the amount of variation from standard conditions. Due to the impact measurement variables have on metering, correction factors exist.
Correction Factors
Various correction factors and instruments are used to correct for pressures and temperatures that vary from standard conditions.
Pressure Correction
If gas flows through a meter at a pressure other than its calibrated test pressure, a pressure compensating index, or correction factor, should be used to convert metered volume to standard conditions. Higher altitudes may also require pressure correction of metered volume. Higher metering pressures require a super compressibility factor to be applied. If super compressibility factors are
Chapter 10 ⎯ VOLUME MEASUREMENT 10-3
ignored at moderately elevated pressures, gas volumes ranging from 0.25% at 15 psig and up to 1.71% at 100 psig will be lost.
Metered volume is displayed on an index or counter. Compensating meter indexes can automatically correct the index reading to adjust for higher pressures. The index will read in standard volume units for billing purposes. Several methods are used to apply correction factors for customers that are metered at elevated pressures and include:
• Fixed factor billing
• Average pressure factor billing
• Compensating indexes
• Correcting instruments.
The metered volume is multiplied by a pressure correction factor to adjust for non-standard conditions.
Fixed Factor Billing
Fixed factor billing can be used when a constant delivery pressure is supplied to a meter.
The amount of metering error will be directly proportional to the variation from meter calibration pressure. In other words, if the delivery pressure is 1% above or below the calibration pressure, you will have a metering error of 1% as well.
A recent development within some distribution companies has been the use of 2-psig metering for domestic customers. This allows the use of small, flexible tubing for the house fuel lines. The correction of the meter readings may be done by a fixed factor that incorporates the change from standard pressure conditions, or by using a meter that incorporates the correction in the index reading.
Average Pressure Factor Billing
Average pressure factor billing is used when pressure at the meter is not held constant. The average pressure factor billing method requires the use of a volume-pressure (VP), or volume-pressure-time (VPT) chart, or electronic corrector. The varying inlet pressure must be averaged and applied against a standard meter volume.
If the average pressure must be taken from a recording chart, good judgment and skill are required. The chart must be broken down into increments of volume versus pressure and the pressure factors eliminated for any position of the chart where a "no-flow" condition appears.
Compensating Indexes and Correcting Instruments
Volume and Temperature correction can also be accomplished using compensating meter indexes and various correcting instruments.
Instruments can convert uncorrected metered volumes to standard volume units for billing. These instruments can correct for pressure only, temperature only, or for both pressure and temperature. Electronic correctors can perform both volume correction and real-time super compressibility calculations.
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Instrumentation and compensating meter indexes minimize the need for volume calculations, chart changing, and billing preparations. All of this helps reduce overall costs. However, keep in mind the initial meter cost is higher and routine calibration is required.
Newer instruments have the capability of providing output directly to a computer and can alert the company to unusual conditions. Such conditions include high or low pressures and high or low flow rates. Flow computers are able to control sophisticated flow measurement systems.
Figure 10-2: Daniel Flow Computer
Source: Daniel Measurement and Control, Inc.
Meter Reading
A common meter reading frequency is six times a year. Billing occurs every month with six reads and six estimates. Larger meters are often read more frequently. Meters in buildings can be read quickly if the meter can be read from outside. To permit outside reading, the meter index can be faced outward opposite a glass block in the wall or the reading may be mechanically transmitted to an index mounted on an outside wall. Outside meter sets eliminate the problem of building entry, and they can be temperature-compensated.
Automatic Meter Reading (AMR) systems have been developed to allow faster, easier, and more economical gathering of meter data. As employees either walk or drive through a neighborhood, these systems transmit information to an electronic recorder.
Chapter 10 ⎯ VOLUME MEASUREMENT 10-5
Figure 10-3: Hand-Held Meter Reading Device
Fully automatic AMR devices store meter reading information and transmit it to the billing computer through a communications link. Fixed-base AMR systems can be mounted on utility poles or high in buildings and connect to the utility's host computer over some type of telephone system or wireless network.
Deregulation has resulted in AMR use for point-of-sale delivery in the commercial and industrial markets. AMR systems are also being developed to monitor appliance usage, provide real-time pricing, and transmit electronic utility messages directly to individual customers.
Meter Designs
In order to select an appropriate meter, you must determine the gas load characteristics and metering requirements. A meter that is undersized will overspeed and cause premature wear and failure of meter components. If a meter is oversized, low flows could be registered incorrectly or not be metered at all.
Figure 10-4: Sized Meter
The capacity of a specific meter depends on the allowable pressure loss across it. In low-pressure systems, this pressure loss is limited by the maximum pressure loss that can be tolerated. Many companies have standardized a value of 0.5 in. w.c.
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The 4 primary meter designs used in distribution systems are ⎯
1) Diaphragm meters
2) Rotary meters
3) Turbine meters
4) Orifice meters.
These meter designs are classified as either positive displacement or inferential meters.
Positive Displacement: Positive displacement meters have separate measurement compartments that alternately fill and empty as the meter rotates. Diaphragm and rotary meters are positive displacement meter designs. Inferential: Turbine and orifice meters have no measurement compartments to trap and release the gas. These meters are classified as inferential meters because the volume of gas that passes through them is inferred by measurement of a physical property. Each meter design has its own range of measurement accuracy and must also satisfy load requirements.
Diaphragm Meters
Diaphragm meters are the most common meter type for residential applications. Larger sizes are also available for small industrial or commercial use.
Diaphragm meters are typically used for loads from 100 to 500 cubic feet per hour (cfh). However, these meters have also been manufactured for loads as large as 11,000 scfh at elevated pressures.
A diaphragm gas meter measures gas by alternately filling and emptying two flexible diaphragms. As downstream gas demand occurs, a pressure differential causes the diaphragms to expand and contract. Valves and a mechanical linkage control the flow of gas and connect with an index to register the amount of gas measured.
Figure 10-5: Diaphragm Meter
Diaphragm meters are accurate over a flow range of 100:1 and they minimize gas loss at low, pilot flow rates that other meters allow. Modern design techniques and materials have improved the accuracy and
Chapter 10 ⎯ VOLUME MEASUREMENT 10-7
reliability of diaphragm meters over time. Diaphragm meters are low in cost, but they are restricted to lower-pressure applications. The physical size of large-capacity diaphragm meters also limits their use.
Diaphragm meters were originally built for low-pressure distribution (inches w.c.) and their housings were constructed of tinned steel with soldered joints. Meters of this type are still in service, but are no longer manufactured. Tin case meters were replaced with cast iron and later with aluminum “hard case” meters. The working pressures for residential aluminum case meters are 5 to 10 psig. Large-volume aluminum meters supply gas at pressures up to 100 psig. Meters with cast iron bodies can operate at pressures ranging from 2 psig up to 1000 psig. Diaphragm meter accuracy is closely maintained over the entire flow range. A compact residential gas meter has been developed with a capacity of 250 cu ft/hr that is 50% smaller and weighs just 7 pounds.
One manufacturer has come up with a more aesthetically pleasing residential meter and regulator set by combining the meter with a compact residential regulator positioned behind the meter to minimize visible piping.
Figure
