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ACCOL II Reference Manual Page AGA3Iter-1 AGA3Iter American Gas Association Report No. 3 - Iterative Module The AGA3Iter Module computes natural gas volume flow rate in MSCF/h for orifice plates, with flange taps ONLY, according to the factors method explained in the American Gas Association (AGA) Report #3 of August, 1992, 3rd Edition (Part 3, Equations 3-B-2 and 3- 7). This method retains factors such as Fpb , Fpv , Ext, and others described in the American Gas Association (AGA) Report #3 of 1985, but provides greater accuracy by using a new equation to calculate the coefficient of discharge. PIPE_DIAM TAP_LOC POINT ORIF_DIAM OUTPUT TRACK 3ITER ADJ _PRESS STAT_ PRESS FLOW_TEMP SPEC_GRAV THERM_COEF1 BASE _TEMP BASE _PRESS INPUT_1 LIST THERM_COEF2 VISCOSITY ISEN_COEF INPUT_2 INPUT_3 Z_BASE Z_FLOWING DIFF_ PRESS AGA3Iter ACCOL II Reference Manual Page AGA3Iter-2 AGA3Iter American Gas Association Report No. 3 - Iterative Module The paragraphs below outline the rules under which the AGA3Iter performs its computations: The output volume flow rate is computed in thousands of standard cubic feet per hour (MSCF/h), at base conditions, according to tech- niques described in the 1992 AGA Report #3, Part 3, Equations 3-B-2 and 3-7. The orifice coefficient and slope coefficient are calculated by iteration using the Reader-Harris equation shown in the 1992 AGA Report #3, Part 3. These two factors are combined into a coefficient of discharge, CD. Internal factors used in the calculations can be stored in an ACCOL signal list for examination. Input terminals to the AGA3Iter Module allow the user to substitute factors for CP and Ext, and to specify a standard compressibility, Z s . A standard cubic foot is one which exists at the standard pres- sure of 14.73 psia, and a temperature of 60 degrees Fahrenheit. When contract base conditions are not standard conditions, the module BASE_PRESS (base pressure) and BASE_TEMP (base temperature) terminals should be changed so that flow rate will be computed at base conditions. The base compressibility, Zb , is input to the module via the Z_BASE terminal. Zb must come from one of the AGA8 Modules (AGA8Gross, or AGA8Detail) and must have been computed at the same base conditions specified by the BASE_PRESS and BASE_TEMP terminals of the AGA3Iter Module. ❏ Module Equation The equation given in the 1992 American Gas Association (AGA) Report #3, Part 3, Equation 3-B-2 is as follows: ACCOL II Reference Manual Page AGA3Iter-3 AGA3Iter American Gas Association Report No. 3 - Iterative Module Qv = Fn (Fc + Fsl) Y Fpb Ftb Ftf Fgr Fpv . hw Pf1 Equation 3-B-2 calculates flow rate at standard conditions (14.73 psia and 600 F). The AGA3Iter Module calculates flow rate at base conditions, as follows: Q b = CP Ext BCF F m where Q b = Flow rate at base conditions CP = F n CD Y F pb F tb F tf F gr F pv Ext = h w P f1 F m = additional correction factor on the POINT terminal. BCF = Base correction (Zb/Zs) for Zb other than AGA report Zs value. where Fn = A numeric conversion factor which includes Ev the velocity of ap- proach factor. CD = Orifice coefficient of discharge, which is the sum of the orifice calculation factor, Fc and the orifice slope factor Fsl. Y = Expansion factor. ACCOL II Reference Manual Page AGA3Iter-4 AGA3Iter American Gas Association Report No. 3 - Iterative Module Fpb = Pressure base factor. Ftb = Temperature base factor. Ftf = Flowing temperature factor. Fgr = Specific gravity factor. Fpv = Supercompressibility factor, computed as Z b / Z f . The Z b and Zf values MUST come from an AGA8Detail or AGA8Gross calcu- lation. hw = Differential pressure, in inches of H 2 O. P f1 = Absolute static pressure, in psia. NOTE This equation is for flange tapped orifice plates ONLY. If pipe taps are being used, the AGA3TERM Module must be used. ❏ Module Terminals DIFF_PRESS Default: None, entry required Format: Analog signal or constant Input/Output: Input is the differential pressure, hw , in inches of water across the orifice ACCOL II Reference Manual Page AGA3Iter-5 AGA3Iter American Gas Association Report No. 3 - Iterative Module plate. Negative input values are clamped at zero, and zero produces a zero flow rate. STAT_PRESS Default: None, entry required Format: Analog signal or constant Input/Output: Input is the Upstream or Downstream static pressure, specified in psig. Negative input values are clamped at zero, and zero produces a zero flow rate. When it is a Downstream pressure, the TAP_LOC terminal should be non-zero. TAP_LOC Default: 0 Format: Analog signal Input/Output: Input indicates the pressure tap location for flange taps. A value of 0 on this terminal indicates that the tap location is Upstream, and a non-zero value indicates that the tap location is Downstream. When Downstream is specified, the differential pressure is converted to psi and added to the Downstream static pressure to obtain an Upstream value for use in calculations. See 'Module Usage Notes.' ADJ_PRESS Default: 14.73 psia Format: Analog signal or constant Input/Output: Input is the site barometric pressure in psia. This value is added to the value on the STAT_PRESS terminal to obtain absolute pressure. ACCOL II Reference Manual Page AGA3Iter-6 AGA3Iter American Gas Association Report No. 3 - Iterative Module ORIF_DIAM Default: None, entry required Format: Analog signal or constant Input/Output: Input is the orifice bore diameter in inches at 68 degrees Fahrenheit. Output of the module will be zero if this terminal is unwired, or if the value entered is zero, negative, or larger than 80% of the value on the PIPE_DIAM terminal. PIPE_DIAM Default: None, entry required Format: Analog signal or constant Input/Output: Input is the inside diameter of the pipe in inches at 68 degrees Fahrenheit. Output of the module will be zero if this terminal is unwired, or if the value on the terminal is zero or negative. THERM_COEF1 Default: 0.00000925 (stainless steel) Format: Analog signal or constant Input/Output: Input is the orifice coefficient of thermal expansion in inches per inch-degree Fahrenheit. THERM_COEF2 Default: 0.0000062 (carbon steel) Format: Analog signal or constant Input/Output: Input is the pipe coefficient of thermal expansion in inches per inch-degree Fahrenheit. ACCOL II Reference Manual Page AGA3Iter-7 AGA3Iter American Gas Association Report No. 3 - Iterative Module BASE_PRESS Default: 14.73 psia Format: Analog signal or constant Input/Output: Input is the base pressure Pb in psia. This value is used to calculate the factor Fpb. BASE_TEMP Default: 60o Fahrenheit Format: Analog signal or constant Input/Output: Input is the base temperature, Tb , of the gas in degrees Fahrenheit. This value is used to calculate the factor Ftb . FLOW_TEMP Default: 60o Fahrenheit Format: Analog signal or constant Input/Output: Input is the temperature, Tf , in degrees Fahrenheit. This value is used to calculate the factor Ftf . VISCOSITY Default: 0.0000069 lbm/foot-second Format: Analog signal or constant Input/Output: Input is the dynamic viscosity of the gas at flowing conditions in pound mass per foot-second. Viscosity is used to calculate the Reynolds number. This terminal should normally be left unwired unless the gas viscosity is known to be outside the range of .0000059 to .0000079 given in the 1992 American Gas Association (AGA-3) Report, Part 3, page 11. ACCOL II Reference Manual Page AGA3Iter-8 AGA3Iter American Gas Association Report No. 3 - Iterative Module SPEC_GRAV Default: 0.6 Format: Analog signal or constant Input/Output: Input is the specific gravity, Gr, of the gas at standard conditions. This value is used to calculate the factor Fgr. ISEN_COEF Default: 1.3 Format: Analog signal or constant Input/Output: Input is the gas Isentropic exponent. This value is used in the calculation of the expansion factor, Y. This terminalshould be left unwired unless the gas isentropic exponent is known to be other than the 1.3 value given in the 1992 American Gas Association (AGA-3) Report. Z_FLOWING Default: 1.0 Format: Analog signal or constant Input/Output: Input is the flowing compressibility factor, Zf , generated from an AGA8 calculation referenced to the Upstream conditions. NOTE When TAP_LOC is non-zero, this module internal- ly adds the DIFF_PRESS (in psia) to the STAT_PRESS value to calculate Upstream pres- sure for ALL calculations. Any AGA8Detail module calculating Zf MUST use an equivalent Upstream pressure. ACCOL II Reference Manual Page AGA3Iter-9 AGA3Iter American Gas Association Report No. 3 - Iterative Module Z_BASE Default: 1.0 Format: Analog signal or constant Input/Output: Input is the base compressibility factor, Zb , from an AGA8 calculation. POINT Default: 1.0 Format: Analog signal or constant Input/Output: Input is used internally as the meter correction factor, Fm , to compensate for external equipment calibration error or local variations in condi- tions such as gravity, or Downstream tap compressibility. TRACK Default: 1.0 or ON Format: Analog or logical signal Input/Output: Input is used to force the module output to zero. When a logical signal is wired to this terminal, ON enables calculation, and OFF forces the output of the module to zero. If an analog signal is wired to this terminal, it is used as a differential pressure threshold value or 'cutoff' level; calculations cease and the output of the module is forced to zero when the DIFF_PRESS terminal value is less than the TRACK value. OUTPUT Default: None Format: Analog signal Input/Output: Output is the gas flow rate, Qb , at base conditions in MSCF per hour. ACCOL II Reference Manual Page AGA3Iter-10 AGA3Iter American Gas Association Report No. 3 - Iterative Module LIST Default: None Format: Analog Signal or constant Input/Output: Input is the number of a signal list into which the factors used in the equa- tion are to be moved. Factors are moved to the list in the following order: CP, Fn , CD, E, Y, Fpb , Ftb , Ftf , Fgr , Fpv , Fm , Ext, Reyn, and BCF, where Reyn is the pipe Reynolds number computed by iteration as part of the CD calculation. The output list is not written to when the CP value factor comes from an input. INPUT_1 Default: None Format: Analog signal or constant Input/Output: Input allows the substitution of an alternate value for CP in situations where the factor is required to be different and internal calculation of that factor is not desired. If this terminal is wired, and its signal value is not less than zero, the value will be substituted for CP in the equa- tion, and the internal calculation of that factor will not be done. If the terminal is unwired, or has a negative value on it, the factor will be calculated internally. INPUT_2 Default: None Format: Analog signal or constant Input/Output: Input allows the substitution of an alternate value for Ext in situations where the factor is required to be different and internal calculation of that factor is not desired. If this terminal is wired, and its signal value is not less than zero, the value will be substituted for Ext in the equation, and the internal calculation of that factor will not be done. If the terminal is unwired, or has a negative value on it, the factor will be calculated internally. ACCOL II Reference Manual Page AGA3Iter-11 AGA3Iter American Gas Association Report No. 3 - Iterative Module INPUT_3 Default: 0.997971 Format: Analog signal or constant Input/Output: Input is the standard compressibility Zs for the gas composition in use. Zs is used in the calculation of the flow rate correction factor BCF (the ratio of Zb to Zs). A default value of 0.997971 is supplied which corresponds to the set of conditions defined in Appendix 3-C, Section 3-C.2 of American Gas Association (AGA) Report #3 of August, 1992, 3rd Edition. If this terminal is wired, and its signal value is not less than zero, the value will be substituted for Zs in the equation, and the internal default value of that factor will not be used. If the terminal is unwired, it is set to its default value of 0.997971. If the terminal has a negative value on it, the factor is set to 1.0. For most uses, this termi- nal will be wired with a Zs signal. ❏ Module Usage Notes Conditions Which Cause Zero Flow Rate to be output by the Module: The following conditions can force the flow rate output of the AGA3Iter Module to zero. These are: a. Negative or zero differential pressure. b. Negative or zero static pressure. c. Negative or zero orifice diameter. d. Negative or zero pipe diameter. e. Orifice diameter larger than 80% of pipe diameter. f. Track terminal conditions are satisfied. Bounded Inputs: The isentropic exponent value used internally will be forced to 1.3 if the input value is less than 1.0 or larger than 2.0. The viscosity value used internally will be forced to .0000069 if the input value is zero or negative, or larger than 1.0. ACCOL II Reference Manual Page AGA3Iter-12 AGA3Iter American Gas Association Report No. 3 - Iterative Module Compressibility: In cases where both the AGA8Detail and AGA3Iter Modules use standard conditions (14.73 psia, 600 F) on the BASE_PRESS and BASE_TEMP terminals, only one AGA8Detail module is needed because Zb is the same as Zs, and the Zb signal will be wired to the Z_BASE and INPUT_3 terminals. Where base conditions are other than standard, then an AGA8Detail module is used to obtain Z_FLOWING (Zf) and Z_BASE (Zb) signal values at base conditions. Another AGA8Detail module will be needed to compute a Zs at standard conditions. This Zs is to be wired to INPUT_3. This Zs comes from the AGA8Detail module Zb output, but it is the same as Zs because the BASE_PRESS and BASE_TEMP termi- nals are set at 14.73 and 600 F. Static Pressure Tap Location: If the static pressure is from a downstream tap, the TAP_LOC should be non-zero, in which case the DIFF_PRESS is converted to psia and added to the STAT_PRESS to obtain an upstream pressure. Users must ensure that the static pressure used for the AGA8Detail and AGA8Gross module is the sum of the downstream pressure and the differential pressure (in psia) or the Z factors will be incorrect. Return to the Table of Contents Return to the List of Manuals
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