basicsofgasmetering-johnmichalak

7/28/2019 BasicsofGasMetering-JohnMichalak

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BASICS OF GAS METERING

Presentedby

John Michalak

NATURAL GAS WHAT IS IT?

Natural gas is a vaporous mixture of lighthydrocarbons; that is materials which aremade up of hydrogen and carbon. One ofthe main components of natural gas ismethane which consists of 1 carbon atomand 4 hydrogen atoms. Natural gas isodorless, tasteless and coloureless. It is

naturally occurring in undergroundreservoirs and also commercially producedwith newly developed technologies.Various grades of gas require refinementand processing. It is then shipped bypipelines to gas utilities (LDCs) fordistribution or directly to commercial plantsfor use.

STORAGE:

Natural gas is commonly stored in depletedreservoirs and salt caverns underground. It

may also be cooled to -260F where itbecomes a liquid and occupies 600 timesless space than as a gas. In this state, it isstored above ground in insulated tanks.

WHY METER GAS?

Gas has been a commercial fuel since theearly 1800s. In 1816 the first gas companyin the U.S. was founded. The LightCompany of Baltimore, Maryland, operatedon a flat-rate basis as it struggledfinancially with little or no growth. The New

York Gas Light Company built their systemincorporating gas meters to measure andbill the gas to their customers, resulting ingrowth and prosperity. Their successeswere noted and copied by the other utilitiesand since this time meters and controldevices are an integral part in every phaseof the industries operation.

METER TYPE OVERVIEW:

Two metering principals commonly used inthe natural gas industry are: positivedisplacement and inferential. The positivedisplacement meter uses a design/methodof trapping a known volume of gas andthrough a rotating or recipicating actiondisplaces the volume downstream. This

repetitive motion transfers each movementeither mechanically or electronically to acounting device or transmitter. The mostcommon P.D. meters used in the gasindustry are diaphragms and rotaries. Aninferential type meter derives or infersvolumetric measurement using the conceptof velocity. In the case of the turbine, thespeed of the rotor is directly proportional tothe rate of gas flow. Other designs utilizedifferential pressure sensing devices and/orvibrating tube sensors to detect phaseshift, which is proportional to flow/mass.

Types include; turbine, orifice, ultrasonic,fluidic oscillation/vortex and coriolis massflow measurement.

COMMON METERING TERMS ANDDEFINITIONS:

Accuracy/Repeatability: The ability toconsistently measure gas at a specified flowrate and error.

Rangeability: A ratio comparing theminimum flow performance to the maximum,i.e.: 50:1 (the higher the number, the moreaccurate the meter registers gas at lowflows).

Differential pressure: The pressure loss ordrop as gas flows through the meter.

Seal period: A term as specified byMeasurement Canada allowing a device tobe used for billing purposes.


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Qmax: Specifies the maximum actual flowability of a meter.

Qmin: Specifies the minimum actual flowability of a meter.

Actual volume/flow: A non-corrected

volume of gas as measured by a meter.

Standard volume/flow (SCFH, SCMH): Acorrected volume of gas which has beencalculated to allow for pressure andtemperature effects and variables.

HISTORY:

Rotary gas meters have been in use for oversixty years in the natural gas distributionindustry. Over the years the constructionhas switched from heavy cast iron bodies to

lighter, high strength aluminum. Advancesin manufacturing techniques such as CNCmachining centers have enhanced themeasurement performance of the rotarymeter.

Traditionally rotary meters are installed onapplications requiring a flow capacity of1,000 to 38,000CFH or 30 to 1100m/h.

PRINCIPLE OF OPERATION:

Rotary gas meters measure the gas bypositively displacing or capturing the gas in

a measurement chamber. Gas entering theinlet of the rotary meter pressure bodyproduces a differential pressure across themeter, which causes the two impellers torotate. The timing gears synchronize theimpellers to turn in opposing directions.Each rotation of the impellers measures fourdisplaced volumes of the gas.

The meter module converts the rotation ofthe impellers into standard non-compensated (NC) units of volume that,depending on the module type, may alsoconvert the NC volume to standard basetemperature and/or pressure conditions.

This converted or compensated volume iswhat is used for the measurement of thecustody transfer or billing of the gas inCanada. The related specifications for themeasurement of the custody transfer of gasare regulated and enforced by MeasurementCanada.

TYPES OF ROTARY METERS:

Rotary gas meters are available in severalpressure ratings and with a variety ofmodule types to meet the diverserequirements of the gas industry. Thispaper will only address the most commonlyinstalled rotary gas distribution system.These meters typically have a 175 psig or12 bar MAOP and an ANSI 125 flangeconnection.

Temperature Compensated Counter (TC)

Since the temperature of the natural gasvaries substantially in Canada from the basetemperature (60F or 15C), themeasurement of the custody transfer ofnatural gas is converted to a volumetricequivalent of this base temperaturecondition. The TC module measures thegas temperature with a bi-metallic probe andconverts the non-compensated (NC) volumemechanically through a combination of atemperature cam and striker lever assembly.

Electronically Compensated Module(ECM2)

The electronically compensated moduleemploys a solid state sensor to sense therotation of the impellers and produce a high-


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resolution input (three pulses per impellerrotation) of the non-compensated (NC)metered gas volume. These volumetricinput pulses are converted to the basetemperature condition (60F or 15C), usingthe programmed routines in themicroprocessor and the temperature valuefrom the sensor, to perform the necessarycalculations.

Instrument Drive (STD ID or DCID)

Instrument drive meters permit the mountingof a variety of mechanical or electronic(electronic volume correctors) conversiondevices. The instrument drive rotationprovides a specific non-compensatedvolume per rotation to the instrument.

Temperature Compensated InstrumentDrive (TCID)

Temperature compensated instrument drivemeters convert the non-compensatedvolume to a volume converted to the basetemperature (60F or 15C) condition andprovide this compensation volume throughthe instrument drive. The instrument driverotation provides a specific temperaturecompensated volume per rotation to theinstrument.

ACCURACY:

A rotary meter provides accurate gasmeasurement over a wide range of flowrates within its rated capacity. The followinggraph illustrates the typical accuracy curvefor a rotary meter.

CAPACITY:

The maximum rated capacity of a rotary gasmeter (Q max) is marked on the meter. Thisrating is indicated in actual units of volume(acfh or am/h) since rotary meters measuregas by positive displacement. Natural gas ismeasured in standard units of volume (scfhor sm/h). The standard unit of volume issimply the volume of gas after being

converted to the base conditions. Thefollowing formula is employed to correctlysize a rotary meter to the flow requirement.

Q max = Flow (scfh or sm/h) / PF x TF

Where the pressure factor PF is:

PF = Pressure Factor

= (PATM+ PACT) / PBASE

Where:PATM = Atmospheric pressure (psia or abar)

PACT

= Actual gas pressure (psia or abar)

PBASE = Base pressure (psia or abar)

TF = Temperature Factor

= TABS + TBASE / TABS + TACT

Where:TABS = 459.67F or 273.15C

TBASE = 60F or 15C

TACT = Actual gas temperature F/C

Example:The maximum flow requirement for a factoryis 40,000 scfh at 40 psig. The peak loadoccurs in the wintertime when the averageflowing temperature of the gas is 38F.

PF = (14.65 40) / 14.73

= 3.71

TF = 459.67 + 60 / 459.67 + 38

= 1.04Q max = 40,000 / 3.71 x 1.04 = 10367 acfh


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Therefore, the recommended meter wouldbe and RM11000 with a maximum flowcapacity of 11,000 acfh.

MEASUREMENT CANADA:

Each type or model of rotary gas meter thatis employed to measure the custody transferof natural gas in Canada is required toundergo an approval of type evaluation byMeasurement Canada. If the meter modelor type complies with the requirements asspecified in LMB-EG-08, MeasurementCanada will issue a Notice of Approval(NOA) to the meter manufacturer. ThisNotice of Approval acknowledges that theapplicable meter type and/or model may beverified and sealed for custody transfer by aMeasurement Canada inspector or otheraccredited body.

The verification of a rotary gas meterinvolves the following steps:

Compliance with markingrequirements

Meter body accuracyDepending on the meter module type, thefollowing additional steps may be required:

Temperature conversion accuracy

Volumetric pulse output(s)

After a rotary gas meter passes all theapplicable verification requirements, the

meter is sealed by an approved method.Rotary meters may be sealed so that thepressure body and module are eitherseparately or integrally sealed.

TESTING:

The verification of a rotary meter body isperformed at two flow rates with anallowable error of 1%.

95% 5% of the meter capacity

20% 5% of the meter capacityThe verification of the meter body accuracy

is performed on a Measurement Canadacertified prover system. The two main typesof systems currently employed in Canadaare:

Bell

TransferA bell prover determines the meter accuracyby comparing the known volume of the bellto the registered volume of the rotary meter.By measuring the travel of the bell, the

volume being passed through the meter isdetermined and compared to the registeredvolume from the meter.

A transfer prover verifies the test metersaccuracy by comparing the known volumepassed through the master meter of the

prover to the volume registered by the testmeter. The flow for the prover is developedby a fan with a motor control to adjust theflow rate.

If the rotary meter has a temperatureconversion module, the accuracy of themodule must also be verified. In the case ofmechanical (TC) modules, the temperatureconversion accuracy is verified at threetemperatures (32, 60 & 86F or 0, 15 &30C) for a specific test volume. Electronictemperature conversion modules (ECM2)

require the verification of the temperaturesensor accuracy at the above three testpoints and a temperature conversion test atonly one temperature. In all of the tests, theallowable error is 1%.

GENERAL INSTALLATION PRACTICESFOR MECHANICAL AND ELECTRONICROTARY METERS:

Remember that a rotary meter represents asubstantial cash register for a gas utility.Good installation practices are the key toobtaining the accurate gas measurement.

Rotary gas meters can be installed in eithera horizontal or vertical pipe. Therecommended installation is in a verticalpipe to permit any contaminants in the gasto pass through the meter with the minimumof damage.

FIELD INSPECTION AND PRE-INSTALLATION PROCEDURE:

Prior to installing a rotary meter, inspect forany damage that may have occurred during

transport to site. Remove the protectiveplugs from the inlet and outlet of the meter.Blowing into the meter will confirm that theimpellers are rotating freely and withoutresistance. Ensure that the piping manifoldof the meter station is free of foreignmaterial (dirt, tap shavings, pipe dope, weldslag, etc.) and scale. Strainers and filtersare recommended upstream of the meter to


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ensure gas quality. Check that the mountingflanges for the meter are level (within 1/16per foot or 5 mm per meter), parallel to eachother and the correct FL distance apart.

INSTALLATION:

The manufacturers recommended hardwarespecifications relating to the mountingflanges and flange bolts, washers andgaskets should be checked for compliance.

*The dimension between the pipe flangesshould equal the meter dimension plus twotimes gasket thickness.

Install the rotary meter with full-face flangegaskets. Tighten the flange bolts to themanufacturers recommended torque value.Install the meter oil supplied by themanufacturer as required: way on sightglass.

*Note: Gasket thickness vary.

MAINTENANCE:

Under normal operating conditions, the oilshould be inspected every 5-10 years andchanged or topped up as required. Thechange period will vary with the cleanlinessof the gas being measured.

LEVEL

FL

METER

INLET

LEVEL

METER

INLET

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