Remote Meter Reading Instruments
The measurement of natural gas presents some special challenges. Like any gas,as the pressure and temperature changes, the actual volume of gas changes. Gas can also vary in BTU content according to gas quality and compensation. BTU correction is only done for the largest of industrial loads; all other customers purchase gas on a volumetric basis.
Gas meters are either of the "positive displacement" or "flow" type. All diaphragm and rotary meters are positive displacement - a given sized container is filled and emptied with the number of cycles counted to produce the base measurement. Turbine and orifice meters operate on flow velocity principles with extremely precise machining and instrumentation required to calculate gas flow according to properties of physics.
Originally gas meters did not "know" the temperature of the gas flowing through them, so it was required to keep them in a consistent temperature range to keep the gas measurement consistent/accurate. Therefore, all meters were installed inside. When temperature compensating meters were developed, it became possible to locate meters outside; now, nearly 100% of gas meters are installed outside. Larger industrial meters require add-on devices to "compensate" or measure and adjust for temperature variations, where all smaller meters have built-in temperature compensation.
Temperature affects gas such that the colder it is, the more gas that will fit and flow through a given sized space. All positive displacement meters are built assuming about 60F standard temperature. If internal compensation, cams move to make the linkage "speed-up" a little to account for the increased gas in the given space at colder temperatures. At temperatures above about 60F, the meter "slows down".
Temperature is a relatively small adjustment, affecting flow by usually plus or minus 5% of volume from summer to winter. However, for large industrial users, this is a huge sum of money.
Note that we are talking about the temperature of the GAS not the ambient air temperature. Gas is typically colder than the air temperature in the summer and warmer than the air temperature during the winter. This minimize the temperature compensation range.
To see a Temperature Compensation Chart
All positive displacement meters are designed assuming a standard base pressure of sea level atmospheric pressure (14.73 psia), and a PLUS 0.25 psi (7") delivery pressure through the meter. If the actual gas pressure through the meter is higher, additional compensation must be done for proper measurement / billing. There are two ways to do this.
Fixed Factor metering is a billing / administrative function that applies a "Fixed" multiplier to the meter readings to increase the billed usage by a known pressure factor.
For example, the fixed factor for 10 psi is 1.666. If a meter measuring at 10 psi reads "100 CCF" usage according to its index. Without correction, the customer would be billed for 100 CCF, when they actually used 100 x 1.666 = 166.6 CCF. It is clear to see why correct Pressure Compensation is so important.
To see a Pressure Compensation Chart
There are a variety of Metering Instruments used to correct for temperature and pressure. Old devices are mechanical, using a series of gears and cams to make adjustment. New devices are solid state with electronic transducers sensing pressure and temperature and sending a signal to a computer that makes correction. Some devices also store usage information and even contain modems for dial-up communications.
It is critical to understand the hardware of each individual meter in the event that a meter adjustment is required.
Application
Diaphragm Meters are used on all residential and small commercial loads. They are the least expensive, temperature compensated and very reliable. Sizes range from about 175 CFH to about 5,000 CFH. They are very accurate for fluctuating loads, because they are a positive displacement meter type (see below). Smaller meters, about 450 CFH and below, are only rated for 5 psi, and therefore cannot be used in high pressure applications. The larger diaphragm meters are rated for up to 100 psi, but are typically never used in higher pressure applications - most often because the flows are too high.
The model number usually gives a clue to flow rating. EXAMPLE: an American AL-250 is good for 250 CFH (250,000 BTUs/Hr) with a 1/2" pressure differential; an AL-2300 is good for 2,300 CFH (2,300,000 BTUs/Hr). Allowing a 2" pressure drop through the meter, doubles its throughput capacity. This may be acceptable when delivering pressure above 14", but at standard delivery pressure of 7" (0.25 psi) a 2" loss through the meter may mean that gas equipment may not have enough gas pressure to operate, especially when additional gas pipe pressure losses are added in.
American MeterAmerican Meter products provide accurate, reliable and cost-effective measurement of gas flow in every conceivable application. For Industrial applications, there are products suitable for measuring the gas consumption of a complete plant, a particular department or a single piece of equipment, that can provide management with information needed to operate more efficiently.
Diaphragm meters measure gas with compartments and alternately fill and empty. A slight pressure drop across the meter causes the meter and its measurement compartments to rotate. Each stroke of the diaphragm meter traps a small volume of gas, delivering it to the meter outlet.
METERS - AL-800 AL-1000 AL-1400 AL-2300 AL-5000
Nominal capacities from 800 to 5,000 SCFH (22.7 to 141.6 SCMH). For maximum line pressure up to 100 psi (6.9 Bar), AL-800 also available in 20 psi (1.4 Bar) model; AL-1000 in 25 psi (1.7 Bar) model. Models AL-1400, AL-2300 and AL-5000 available with threaded or flanged connections.
AMERICAN METER
300 Welsh Road
Building One
Horsham, PA 19044-2234 U.S.A.
Phone: 215/830-1800
Fax: 215/830-1890
Web site www.americanmeter.com
View the American Meter Company electronic catalog in PDF Format (Excluded from Demo)
Application
Rotary meters are used for most all industrial applications, and when connected loads exceed 1,000 CFH (1,000,000 BTUS) with a high load factor (load is used on most of the time). Rotary meters must be sized carefully (do NOT over-size), as there is some "slippage factor" where a small amount of gas can pass through the meter without registration under very small load conditions. Older and larger rotary meters are not temperature compensated, so require an external device to correct actual gas flow for temperature. Newer rotary meters up to about 16M are available with temperature compensation. Note that the model number of a rotary meter will indicate its capacity. A "16M" is rated for 16,000 CFH or 16,000,000 BTUs at 0.25 psi. Increasing the pressure through the meter will increase its capacity along the same factor as standard pressure compensations factors.
RootsUse and Limitations
The ROOTS Meter is a positive displacement, rotary type gas meter designed for continuously measuring and indicating the accurate measurement of gas flow in a pipeline.
ROOTS Meters are suitable for handling most types of clean, dry, common gases at either constant or varying flow rates. The meter is not suitable for handling liquids. Measurement accuracy and life expectancy can be impeded by excessive deposits of dirt or other types of foreign material present in the gas stream.
Operating Principle
As shown in Figure 1, two contra-rotating impellers of tow-lobe or "figure 8" contour are encased within a rigid measuring chamber, with inlet and outlet connections on opposite sides. Precision machined timing gears keep the impellers in correct relative position. Optimal operating clearances between the impellers, cylinders and headplates provide a continuous, non-contacting seal.
Because of their unique design, the gas at the meter inlet is always effectively isolated from the gas at the outlet. Consequently, a very small pressure drop across the meter will cause the impellers to rotate. During impeller rotation, the precisely machined measuring chamber traps a known volume between the impeller and the adjacent cylinder wall. With one complete revolution of both impellers, the meter will measure and pass four equal gas volumes. The sum total of these volumes is the Displacement of the meter per revolution. The displaced volume of gas is indicated in Engineering units represent in cubic feet (or cubic meters).
Volumetric accuracy of the ROOTS Meter is permanent and non-adjustable. Measuring characteristics are established by the dimension and precision machined contours of non-wearing fixed and rotating parts.
Temperature Compensation Version
Temperature compensation is accomplished by a mechanical computer with a spiral bi-metallic thermocouple (probe) located in a sealed temperature well at the meter inlet. This system allows Series 3 TC Units to provide corrected gas volume readings in SCF between line gas temperature of -20F and +120F (-29C and +49C)
Source: ROOTS Installation, Operation Maintenance Manual IOM:B3. (Excluded from Demo)
Specifications sheet for Roots Rotary Meters, 8C175 through 56M175 (Excluded from Demo)
Turbine meters contain no measurement compartments. Instead,
a rotor with multiple blades is placed directly in the gas stream. gas
flows through the meter, passing through the rotor blades, causing the
rotor to turn. The rotor rotational speed is proportional to the gas flow
rate, which is directly related to the gas velocity at the rotor. This
is why turbine meters are sometimes referred to as
velocity meters.
Turbine meter rotor revolutions are converted to measured volume by means of internal gearing, resulting in meter output shaft revolutions in either cubic feet or cubic meters. In addition to a mechanical output, turbine meters may be also equipped with high frequency pulsers, where each pulse represents a small increment of volume, defined during meter calibration as a "K" factor.
Since turbine meters measure gas by sensing gas velocity, proper installation is
essential, to make certain the gas is uniform as it approaches the measurement rotor.
AMERICAN METER
300 Welsh Road
Building One
Horsham, PA 19044-2234 U.S.A.
Phone: 215/830-1800
Fax: 215/830-1890
Orifice Meters are used primarily on very large metering applications, such as utility purchase points from interstate pipelines and large power plants.
Application: Metretek is used mostly on industrial/large volume meters to track gas use according to time of use. Used mostly on transportation accounts.
Metretek, Inc. designs and manufactures Automatic Meter Reading Systems (AMR) for commercial, industrial, and "hard to read" residential locations. Working with gas, electric, and water utilities, Metretek's customers are located throughout North America, South America, Europe and Asia. In addition to automatic remote meter reading, Metretek also designs electronic volume correctors, pressure and temperature recorders, data collection and processing systems (DC2000), and load survey with both graphic and report output. Metretek, Inc. offers these products as stand alone systems that can be operated and controlled by the customer.
Metretek, Incorporated
300 North Drive
Melbourne, FL 32934
Telephone: 321-259-9700
Toll-Free: 800-327-8559
Fax: 321-259-2900
E-mail: info@metretekfl.com
Web site: http://www.metretekfl.com
Web site: http://www.powerspring.com
Web site: www.metretekfl.com/home.html
Application: ITron is used by utility companies for remote meter reading programs, most common on residential and commercial meters. Options include radio and telephone reading.
Itron data collection technology is the leading choice of natural gas providers. Our natural gas utility clients use the knowledge Itron systems deliver to improve competitive positioning and increase operational efficiency throughout their meter reading, billing and customer service operations. Our natural gas utility clients are also able to extend the value of the information Itron systems provide to deliver new rates and services, improve distribution system planning and maintenance, and to improve the accuracy of forecasting and nominations to more precisely manage supply and demand in today's increasingly volatile natural gas market.
Itron's solution portfolio for natural gas utilities features handheld electronic meter reading; radio-based handheld, mobile and network AMR systems; as well as telephone-based AMR solutions to serve residential, commercial and industrial gas customers.
Itron Corporate Headquarters:
2818 N. Sullivan Rd.
Spokane, WA 99216
(509) 924-9900
Web site: www.itron.com
Source: Text Bob Fegan 2001; Manufacturer's data from their web sites and literature referenced above, 2001; rev 7/2003