LNG CARGO TRANSFER CALCULATION METHODS AND ROUNDING-OFFS

Transcription

1 CARGO TRANSFER CALCULATION METHODS AND ROUNDING-OFFS CONTENTS 1. Method for determnng transferred energy durng cargo transfer. Calculatng the transferred energy.1 Calculatng the gross transferred energy.1.1 Calculatng the transferred volume V.1. Calculatng the transferred densty ρ.1.3 Calculatng the transferred gross heatng value H. Calculatng the return gas energy..1 Calculatng the return gas volume V NG.. Calculatng the return gas gross heatng value H NG.3 Calculatng the energy consumed by the carrer s engne.3.1 Determnng the gross heatng value of the gas consumed by the carrer s engne H EG.3. Determnng the volume of the bol-off gas or return gas consumed by the carrer s engne, at reference combuston condtons, V EG.4 Calculatng the net transferred energy 3. Unloadng (or reloadng) certfcate and unloadng (or reloadng) assessment Appendx 1 Data from NBS - Techncal note 1030, December 1980 Appendx Data from ISO Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 1/14 Verson of Aprl 015

2 1. Method for determnng transferred energy durng cargo transfer A schematc representaton (Fgure 1) of an cargo transfer from an carrer to the termnal (unloadng) or from the termnal to an carrer (reloadng) llustrates the amount of energy transferred. Before UNLOADING Before RELOADING After UNLOADING After RELOADING Fgure 1: Transfer of an cargo (before and after) Durng these transfer operatons: cargo unloadng: the volume of unloaded s replaced by evaporaton gas sent back by the termnal; cargo reloadng: the volume of reloaded replaces the gas phase present n the carrer s tanks when arrvng ; gas s sent to the termnal durng transfer. In ths document, ths gas (sent by or to the termnal) wll be referred to as «return gas» (NG). From a general pont of vew, t s consdered that an heel: remans n the carrer s tanks at the end of a cargo unloadng; s present at the bottom of the carrer s tanks before startng a cargo reloadng. The net energy transferred, E, s equal to: cargo unloadng: the energy of the unloaded potentally reduced by the return gas (gas returned from the termnal to the carrer, NG) and by the engne gas (gas consumed by the carrer s engne, EG): E = E - E NG - E EG cargo reloadng: the energy of the reloaded potentally reduced by the return gas (gas returned from the carrer to the termnal, NG) and ncreased by the engne gas (gas consumed by the carrer s engne, EG): E = E - E NG + E EG Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs /14 Verson of Aprl 015

3 These energes are evaluated by determnng the transferred volumes and/or masses, the densty, and the mean gross heatng value on a volumetrc and/or mass bass for the duraton of the cargo transfer: : E = V.. H V : volume of measured n the carrer s tanks; : mean densty of the calculated from the chromatographc analyss of the ; H : mean gross heatng value (on a mass bass) of the calculated usng the mean value of the chromatographc analyses of the. Return gas (NG) : E NG = V NG. H NG V NG : volume of natural gas replacng the unloaded n the carrer s tanks (f cargo unloadng) or sent back from the carrer to the termnal (f cargo reloadng). Ths volume, converted at normal condtons (0 C and bar), s calculated from the volume of transferred and the pressure and temperature condtons of the gas phase n the carrer s tanks at the end (f unloadng) or at the begnnng (f reloadng) of cargo transfer; H NG : mean gross heatng value of the gas (on a volumetrc bass) calculated from the chromatographc analyss of the return gas. Fgure llustrates the prncple for determnng the transferred energy based on the transferred and the return gas. Nota: the natural evaporatons of the cargo durng transfer are not taken nto account as the mssng quantty of s balanced by a lesser quantty of return gas. Bol-off gas (BOG) or return gas consumed by carrer s engne (EG) : Several cases can be consdered: E EG = 0 (no gas s consumed), E EG = 0.04 % of the transferred (f no meterng system for engne gas consumpton onboard), E EG : calculated from the volume (or mass) of BOG or return gas consumed and measured onboard and from heatng value on a volumetrc (or mass) bass. Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 3/14 Verson of Aprl 015

4 Fgure : Determnng the transferred energy (usng cargo measurement data) Source: Custody Transfer Handbook, 3 rd edton, GIIGNL, 011. Calculatng the transferred energy.1 Calculatng the gross transferred energy The calculaton of the gross transferred energy E s a functon of: V : volume of transferred : densty of the transferred H : gross heatng value (on a mass bass) of the transferred E = V.. H.1.1 Calculatng the transferred volume V Calculaton method The volume of transferred s calculated as the dfference between the volumes of contaned n the tanks before and after the cargo transfer. The calculaton of the volume of contaned n a tank, at a gven moment, s determned by readng from a measurement table accordng to the corrected level of. Ths corrected level of s obtaned from the level measured n a tank (by means of level gauges), to whch are appled, f necessary, the correctons referred to above. Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 4/14 Verson of Aprl 015

5 The volume of onboard at a gven moment s the sum of the volumes contaned n each of the carrer s tanks. Unts and roundng-offs The volume s expressed n m³. The volume of, before and after cargo measurement, s determned wth three decmal places; the net volume of transferred s the dfference between these measurements and s taken wth three decmal places for calculatng the energy..1. Calculatng the transferred densty ρ The densty s calculated from varous models based on state equatons, correspondng state equatons, etc. wth as startng data: the composton of the taken from the chromatographc analyss after samplng and vaporzaton of the sample; the molar composton values are taken wth 5 decmal places; the mean value of the temperature of the measured n the carrer s tanks; the temperature of the s taken n C wth 1 decmal place. Elengy uses the revsed Klosek-McKnley (KMK) method 1 to determne the densty of the. Klosek-McKnley method: range of applcaton The lmts of the Klosek-McKnley method regardng the composton and the temperature of are the followng: Methane (CH 4 ) Iso and normal butanes (C 4 + nc 4 ) Iso and normal pentanes (C 5 + nc 5 ) Ntrogen (N ) Temperature (T) > 60 % mol. < 4 % mol. < % mol. < 4 % mol. < 115 K (equvalent to < C) Klosek-McKnley method: formula The method for calculatng the densty of s based on an emprcal evaluaton of the molar volumes of the mxture n the consdered thermodynamc state. The densty s calculated as follows: ρ M V : densty of n kg.m -3 x M M : molar mass of n g.mol -1 1 Klosek-McKnley method: Four mathematcal models for the predcton of denstes - NBS Techncal Note December Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 5/14 Verson of Aprl 015

6 wth: M : molar mass of component accordng to table 1 of ISO (cf. appendx of ths note) x : molar fracton of component V : molar volume of n L.mol -1, defned as: wth: V x. V K 1 x 0,045 N K K1 x CH4 and V : molar volume of component at temperature K 1, K : correcton factors V mol x. V The values of K 1 and K, expressed n L.mol -1, are determned by tables accordng to the molar mass and the temperature (between 105 K and 135 K). Tables ndcatng the molar volumes n L.mol -1 for the hydrocarbons C 1 to C 5, as functons of temperatures n the range 106 K K, are used by the calculaton method (see values n appendx 1 of ths note). No roundng-off s performed durng these calculatons of K 1, K and V mol. Unts and roundng-offs The densty s expressed n kg.m -3. The densty calculatons wll be performed wthout any roundng-off usng the KMK calculaton codes (note NBS 1030, December 1980)..1.3 Calculatng the transferred gross heatng value H Calculaton method The gross heatng value on a mass bass of the s calculated from the molar composton, the molar mass and the gross heatng value on a molar bass of the varous components. These values are taken from standard ISO «Natural gas - Calculaton of calorfc values, densty, relatve densty and Wobbe ndex from composton». The relaton used s produced thus: H : gross heatng value on a mass bass x : molar fracton of component M : molar mass of component H N x H (t1) 1 M M M N x M 1 : molar mass of Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 6/14 Verson of Aprl 015

7 o H (t 1 ) : gross heatng value (on a mass bass) of the component calculated wthout roundng-off from «table 3 - calorfc value on a molar bass» (kj.mol -1 ) and «table 1 - molar mass of consttuents» (kj.mol -1 ) of standard ISO accordng to the relaton: o H (t ) 1 o (t1 H M ) (see data n appendx of ths note) Unts and roundng-offs The gross heatng value (on a mass bass) s expressed n MJ.kg -1 or n other unts such as kwh.kg -1 n the reference combuston condtons at 0 C at atmospherc pressure of bar. The physcal constants of gross heatng value on a mass bass and the molar masses of the components are taken from standard ISO No roundng-off of H s made to calculate the gross transferred energy.. Calculatng the return gas energy The calculaton of the returned energy (gas return NG) E NG s based on the followng terms: the volume of return gas, V NG, the gross heatng value (on a volumetrc bass) of the return gas H NG. E NG = V NG. H NG..1 Calculatng the return gas volume V NG The volume of NG s calculated by dfference from the volume of transferred, corrected accordng to: the temperature of the gas phase, the pressure n the gas phase. Between two cargo measurement operatons, the natural evaporatons are taken nto account when determnng the volume of transferred, snce the reducton n the correspondng level s measured. Before and after cargo measurements (.e. before startng and after endng cargo transfer), these evaporatons are not taken nto account, although they may be rencorporated by the termnal. Calculaton method The volume of return gas between two cargo measurement operatons, correspondng to the geometrc volume of the transferred, must be converted at the temperature and pressure condtons of 0 C and bar respectvely; t must be corrected accordng to the temperature and pressure condtons of the return gas phase to the carrer (unloadng) or accordng to the temperature and pressure condtons of the ntal gas phase n the carrer (reloadng). The correcton of the compressblty factor of the return gas s not taken nto account n ths calculaton because of ts neglgble mpact on the measurement of the volume of the return gas. V NG V t P V NG : volume of gas converted at normal condtons of pressure and temperature, expressed n m 3 ; no roundng-off s performed for the return gas calculaton; P : absolute pressure, expressed n bar, n the tanks of the carrer; the measurement s taken to the nearest mbar (10-3 bar) for the calculaton; Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 7/14 Verson of Aprl 015

8 t : temperature of the gas phase expressed n C. The value of ths temperature s equal to the mean of the temperature means from each tank calculated as the mean of the values ndcated by the temperature sensors not mmersed n the left n the carrer s tanks; the temperature s taken to the nearest 0.1 C for the calculaton. Unts and roundng-offs The volume of the return gas V NG s expressed n m³ n normal condtons of temperature and pressure (0 C ; bar), wthout roundng-off when calculatng the energy of the return gas... Calculatng the return gas gross heatng value H NG The gross heatng value (on a volumetrc bass) of the NG s calculated from the molar composton and the gross heatng value (on a molar bass) of the components. These values are taken from standard ISO «Natural gas - Calculaton of calorfc values, densty, relatve densty and Wobbe ndex from composton». In all cases, the termnal operator takes nto account the gross heatng value of the return gas and use ths when calculatng the transferred energy. Calculaton method The gross heatng value (on a volumetrc bass) of the deal gas for a combuston temperature t 1 of a component measured at a temperature t and under pressure P s calculated usng the followng equaton: 1 H ~ t,v t, P H t 1 H ~ t,vt,p H t 1 1 P R T : deal gross heatng value (on a volumetrc bass) of component : deal gross heatng value (on a molar bass) of component R : molar constant of the gases beng equal to 8, J.mol -1.K -1 T = (t ) : absolute temperature n K, wth t n C For a mxture of known composton, the deal gross heatng value (on a volumetrc bass) s expressed thus: H ~ mel N x H ~ t 1,Vt, P where x s the molar fracton of the component n the mxture. 1 The deal gross heatng value (on a volumetrc bass) of the return gas s then: H NG H ~ Z mel mel N 1 x H ~ 1 N 1 t 1,V t x b,p Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 8/14 Verson of Aprl 015

9 H NG : actual gross heatng value (on a volumetrc bass) of the return gas N Z mel : compressblty factor of the return gas equal to 1 x b 1 wth b : so-called summaton factor of component Unts and roundng-offs The actual gross heatng value (on a volumetrc bass) s expressed n MJ.m -3 or n other unts such as kwh.m -3 n reference combuston condtons at 0 C at atmospherc pressure of bar and reference volume condtons at 0 C at atmospherc pressure of bar. The physcal constants of gross heatng value are on a molar bass and the molar masses of the varous components are taken from standard ISO No roundng-off s performed when calculatng the energy of the return gas..3 Calculatng the energy consumed by the carrer s engne Cas n 1 : no gas consumpton by carrer s engne durng cargo transfer: E EG = 0 Cas n : gas s consumed by carrer s engne durng cargo tranfer: Gas consumed by the carrer s engne s evaporaton gas or return gas. Its heatng value s thus the same as the heatng value of the evaporaton gas or of the return gas. As a consequence: H EG = H NG where H EG s the gross heatng value of the gas consumed by the carrer s engne and H NG s the gross heatng value of the evaporaton gas or of the return gas. A) no gas meterng at carrer s engne or malfunctonng flowmeter; a fxed rate of 0.04 % of the gross energy of the transferred s appled: E EG = E B) mass flowmeter at carrer s engne; the energy of the BOG or of the return gas consumed at carrer s engne (engne gas) s calculated from the measured mass n kg usng the followng equaton: E EG = H EG. m EG H EG = H NG :gross heatng value (on a mass bass) of the NG (BOG or return gas) n MJ.kg -1 m EG : mass of engne gas n kg. The gross heatng value (on a mass bass) of NG s calculated from standard ISO , accordng to the followng formula: H NG (H x M ) (x M ) Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 9/14 Verson of Aprl 015

10 H NG : gross heatng value (on a mass bass) of NG n MJ.kg -1 at T reference combuston = 0 C, H : gross heatng value (on a mass bass) of component n MJ.kg -1 at T reference combuton = 0 C, equal to the gross heatng value (on a molar bass) of the component dvded by ts molar mass M (see appendx of ths note), x : molar fracton of the component n NG, M : molar mass of component (see appendx of ths note). C) volume flowmeter at carrer s engne; the energy of the BOG or of the return gas consumed at carrer s engne (engne gas) s calculated from the measured volume n m 3 usng the followng equaton: E EG = H EG. V EG H EG = H NG : gross heatng value (on a volumetrc bass) of the NG n MJ/m 3 (n), V EG : volume of engne gas at combuston reference condtons n m 3 (n)..3.1 Determnng the gross heatng value of the gas consumed by the carrer s engne H EG As mentoned above, H EG s equvalent to H NG. See.. of ths note for gross heatng value calculaton..3. Determnng the volume of the bol-off gas or return gas consumed by the carrer s engne, at reference combuston condtons, V EG The actual volume of engne gas (BOG or return gas) V EGactual s converted to combuston reference condtons by means of P, T, Z correcton, usng the followng equaton: V EG Z Z ref NG T T ref NG P P NG ref V EGactual Z ref, Z NG : compressblty factor of BOG or return gas, at reference condtons and actual condtons respectvely ; T ref, T NG : respectvely reference temperature and BOG or return gas actual temperature (measured at the volume gas flowmeter); P ref, P NG : respectvely reference pressure and BOG or return gas actual pressure (measured at the volume gas flowmeter). Generally, n case of a volumetrc-type flowmeter, the nformaton transferred to the carrer s cargo control room s already expressed at the reference combuston condtons. Therefore, no correcton s necessary, except for checkng the homogenety of the reference condtons of the measured volume of gas wth gross heatng value on a volumetrc bass of the gas. Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 10/14 Verson of Aprl 015

11 .4 Calculatng the net transferred energy Calculaton method In summary, the net transferred energy, E, s expressed accordng to the followng formula: cargo unloadng: cargo reloadng: P ρ H H NG E V - E EG t E V Unts and roundng-offs P ρ H H NG t All calculatons that lead to the net transferred energy are made wthout roundng-off va the calculator and use as startng data those mentoned below: V : volume of transferred, expressed n m 3 to 3 decmal places, + E EG : densty of the, expressed n kg.m -3 wthout roundng-off for the calculaton; no roundng-off for the calculatons of K 1, K and V mol ; the molar composton of the s gven to 5 decmal places or f %molar three decmal places; the temperature of the n C s gven to one decmal place, H : gross heatng value (on a mass bass) of the expressed n MJ.kg -1 or kwh.kg -1 wthout roundng-off for the calculaton; the molar composton of the s gven to 5 decmal places or f %molar three decmal places, t : temperature of the evaporaton gas or of return gas expressed n C, gven to one decmal place, P : pressure of the evaporaton gas or of the return gas expressed n bar to three decmal places, or n mbar to the nearest mbar, H NG : gross heatng value of the evaporaton gas or of the return gas expressed n MJ/m³(n) or kwh/m³(n) wthout roundng off for the calculaton; the molar composton of the s gven to 5 decmal places or f %molar three decmal places, E EG : energy calculated wth no roundng-off, usng gross heatng value (on a mass or a volumetrc bass), calculated wth no roundng-off from a gross heatng value on a molar bass; mass and volume n kg or m 3 to be rounded to nearest kg or m 3. If no gas meterng, a fxed rate of 0.04 % of E s appled. If no gas s consumed at carrer s engne, E EG = 0. E : net transferred energy expressed n MJ or kwh wth no roundng-off. N.B. In case of CO traces n the, the CO molar fracton s added to the N molar fracton for all energy calculatons. Conversons from MJ to kwh : 1 Wh (T reference combuston ) = J (T reference combuston ) from MJ to MMBtu (ASTM E380-7) : 1 MMBtu (T reference combuston ) = MJ (T reference combuston ) wth T reference combuston = 0 C or 15 C or 0 C or 5 C or 60 F ( C) For other T reference combuston, converson factors are not the same. For nstance: 1 MMBtu (T reference combuston = 15 C) = MJ (T reference combuston = 60 F) Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 11/14 Verson of Aprl 015

12 3. Unloadng (or reloadng) certfcate and unloadng (or reloadng) assessment For unloadng (or reloadng) certfcates and unloadng (or reloadng) assessments, the characterstcs of the cargo are gven as follows Volume of before transfer Volume of after transfer Volume of transferred : n m 3 to (3) three decmal places : n m 3 to (3) three decmal places : n m 3 to (1) one decmal place Mass of (before/after transfer, transferred) : n kg to (1) one decmal place temperature before or after transfer Carrer s tank pressure before or after transfer NG temperature before or after transfer composton Return gas composton Wobbe ndex Gross heatng value (on a volumetrc bass) Gross heatng value (on a mass bass) densty Gas phase densty Gas phase relatve densty Gross transferred energy Return gas energy Energy consumed by carrer s engne Net transferred energy : n C to (1) one decmal place : n mbar to the nearest (1) one mbar : n C to (1) one decmal place : n % molar to (3) three decmal places : n % molar to (3) three decmal places : n kwh.m -3 to () two decmal places : n kwh.m -3 to () two decmal places : n kwh.kg -1 to () two decmal places : n kg.m -3 to (1) one decmal place : n kg.m -3 to (3) three decmal places : wthout unt, to (3) three decmal places : n kwh to nearest kwh (no decmal places) : n kwh to nearest kwh (no decmal places) : n kwh to nearest kwh (no decmal places) : n kwh to nearest kwh (no decmal places) Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 1/14 Verson of Aprl 015

13 APPENDIX 1 Data from NBS - Techncal note 1030, December 1980 COMPONENT MOLAR VOLUMES (V ) Component Molar volume*, L.mol K 116 K 114 K 11 K 110 K 108 K 106 K Molar mass**, kg.kmol -1 CH C H C 3 H C 4 H nc 4 H so + neo-c 5 H C6 + + n-c 5 H N (+ CO ) *Source: NBS - Techncal note 1030, December 1980 **Source: ISO Table 1 VOLUME CORRECTION FACTOR - K 1 x 10-3 Molecular weght of mxture g.mol -1 Volume reducton, L.mol K 110 K 115 K 10 K 15 K 130 K 135 K Source: NBS - Techncal note 1030, December 1980 Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 13/14 Verson of Aprl 015

14 VOLUME CORRECTION FACTOR - K x 10-3 Molecular weght of mxture Volume reducton, L.mol K 110 K 115 K 10 K 15 K 130 K 135 K Source: NBS - Techncal note 1030, December 1980 APPENDIX Data from standard ISO Component Molar mass (kg.kmol -1 ) M Gross heatng value on a molar bass (kj.mol -1 ) H t 1 Methane Ethane Propane n-butane Methyl propane n-pentane Methyl butane ,-Methylpropane C Ntrogen Elengy Fosmax Cargo Transfer Calculaton methods and roundng-offs 14/14 Verson of Aprl 015