© 2019 Department of Petroleum Resources. All rights reserved Department of Petroleum Resources (DPR) PROCEDURE GUIDE

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1.20.4 GAS CHROMATOGRAPH 
 
1.20.5 Functional requirements 
1.20.5.1 General 
 
The purpose of an on-line gas chromatograph (OGC) is to give continuous quantitative 
composition analysis of natural gas from a process stream. 
i. The ranges shall be restricted to the operational needs for each project. 
ii. The OGC shall quantify the concentrations of the main components in the gas 
composition, for gas accounting, calculation of calorific value and reference 
density in fiscal applications. The gas composition shall also be used for 
checking of gas quality conformity with gas quality specifications defined by 
commercial gas agreements, and as a base for calculation of the operating 
density. The calculation of the operating density will normally be handled by 
the flow computers using live input values of the gas composition from the 
gas chromatograph. 
 
iii. Calculation of flow weighted average composition will normally be handled 
by the fiscal measurement computer system. The concentration given in mole 
% shall be converted to mass % for such calculations. 
iv. The analytical results shall be traceable regarding sample point, time and 
date, calibration gas cylinder, calibration table, and time of last calibration. 
 
 
1.20.6 Computer unit 
 
i.  The computer unit shall control the chromatograph. Program software, 
interfaces and protocol should be robust with special consideration 
concerning automatic regeneration of all control and communicational 
functions after the event of a general power failure. 
ii.  Criteria for accepting the results from the calibration sequence shall be 
implemented. 
iii. The computer unit shall have the option of selecting an automatic calibration 
performed between a certain number of analysis, or executed at selected 
calendar dates or weekdays, or at a specified time during the day. 
iv. The computer system shall be capable of calculating the following figures 
based on the compositional data normalized values Xi: in accordance with ISO 
6976. 
 
• Compressibility factor at reference condition. 
 
• Gross calorific value. 
 
• Wobbe-index. 
 
• Relative density (real / ideal) 
 
• Density at reference condition. 
 
v. Algorithm and truncated/rounding errors for computations in the computer unit 
shall be less than ± 0.001 %. 
 
1.20.7 Calibration equipment. 
 
The calibrated gas shall be permanently connected to the analytical unit. The system 
shall have a valve arrangement that provides the possibility of automatically selecting 
gas samples from the WGMs. Selection of WGMs shall be possible from the central 
control unit.