1、GPA Standard 2198-03Adopted 1998Revised 2003Gas Processors Association6526 East 60th StreetTulsa, Oklahoma 74145Selection, Preparation, Validation, Care and Storage of Natural Gas and Natural Gas Liquids Reference Standard Blends1GPA Disclaimer GPA publications necessarily address problems of a gene
2、ral nature and may be used by anyone desiring to do so. Every effort has been made by GPA to assure accuracy and reliability of the information contained in its publications. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. It is not the in
3、tent of GPA to assume the duties of employers, manufacturers, or suppliers to warn and properly train employees, or others exposed, concerning health and safety risks or precautions. GPA makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaim
4、s any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal regulation with which this publication may conflict, or any infringement of letters of patent regarding apparatus, equipment, or method so covered. “Copyrignt 2003 by
5、Gas Processors Association. All rights reserved. No part of this Report may be reproduced without written consent of the Gas Processors Association.” 2GPA Standard 2198-03 for Selection, Preparation, Validation, Care and Storage of Natural Gas and Natural Gas Liquids Reference Standard Blends 1. SCO
6、PE 1.1 This method covers the recommended procedures for selecting the proper Natural Gas and Natural Gas Liquids Reference Standards, preparing the standards for use, verifying the accuracy of composition as reported by the manufacturer, and the proper care and storage of those standards to insure
7、their integrity as long as they are in use. 2. SUMMARY OF METHOD 2.1 It is highly recommended that the proper reference blend be obtained for a particular application. The accuracy of its reported composition must be authenticated when received and periodically during its use. 2.2 Special care needs
8、 to be taken to ensure the integrity of any reference standard from the time it is received until it is taken out of service. 2.2.1 It is a good Laboratory practice to order a replace-ment standard and confirm its validity well in advance of the depletion or deterioration of the reference standard b
9、lend that is presently in service. 2.3 A constant pressure cylinder used to obtain a Natural Gas Liquids Reference Standard must be properly prepared to avoid contaminating, changing or losing its contents. 2.4 All necessary Department of Transportation (DOT) requirements must be followed when trans
10、porting a Natural Gas or Natural Gas Liquids Reference Standard blend. 3. SELECTION OF A PROPER REFERENCE STANDARD BLEND 3.1 The selection of a proper reference standard blend depends upon the composition of the samples that are to be analyzed. A reference standard composition should resemble the sa
11、mples that are to be analyzed. 3.1.1 A known blend prepared by weights of high purity components (99+ per cent with known impurities) to an accuracy, as shown in Table 1, for each component is required for calibration runs. The concentration of a component in the calibration blend should be not less
12、 than one-half of the concentration of the corresponding component in the un-known. Corresponding components in the calibration blend and in the unknown should not differ by more than twenty (20) mol percent (of the total mixture) when the peak area calculation method is used. In applying these rule
13、s the values for the hexanes and heavier components may be totaled and considered as a single component. 3.1.2 In a Natural Gas Reference Standard blend containing hexane-plus components, a full spectrum of the heavies should be used when blending the reference standard. 3.1.3 A properly blended Ref
14、erence Standard should be prepared gravimetrically by the manufacturer and its composition verified by chromatographic analysis before shipment to the user. 3.2 It may be necessary for the blend user to check the linearity of an instrument before a range of instrument accuracy can be determined. (Re
15、fer to Appendix A of this standard.) 3.2.1 The linearity of the instrument will determine the range of samples that can be analyzed with a single reference standard. It may be necessary to obtain more than one reference standard blend. Table 1 Required Blending Accuracy % of Concentration % of Accur
16、acy 0.00 - 0.099% 5% Relative 0.10 - 9.999% 2% Relative 10.0 - 100% 1% Relative NOTE - Hydrogen and/or Helium are more difficult to blend. Contact the blend manufacturer for the accuracy determination of these components. 34. PREPARATION FOR ANALYZING REFERENCE STANDARDS 4.1 Before a reference stand
17、ard can be used for calibrating instruments, it needs to be properly prepared and the contents must be verified. Improper preparation can lead to inaccurate calibrations and verifications. 4.2 A Natural Gas Reference Standard must be heated to a minimum of 20F (11C) above its hydrocarbon dew point f
18、or at least 12 hours before attempting to analyze it. 4.3 The pre-charge end of a Natural Gas Liquids (NGL) Reference Standard cylinder should be pressured to not less than 200 psi (1379 kPa) above the vapor pressure of the sample at the temperature of the chromatograph injection valve (A pressure o
19、f 1000-1200 psig should be sufficient for most liquid blends). 4.3.1 The contents of a Natural Gas Liquids Reference Standard blend should be thoroughly mixed immediately before attempting to analyze it. (Refer to manufacturers information for mixer type and proper mixing instructions.) 5. NATURAL G
20、AS REFERENCE STANDARD ACCURACY DETERMINATION 5.1 Before a new reference standard blend can be used for calibrating instruments, it is highly recommended that the composition be authenticated. If the composition of the new blend does not match the gravimetric composition provided by the vendor, it sh
21、ould not be used for calibration purposes. To determine the accuracy of a new reference standard blend, these steps should be followed. 5.1.1 All regulators, gauges, and fittings that are attached to the blend cylinder should be tight and leak tested. The sample line between the blend cylinder and t
22、he gas sample valve should be as short as practical and constructed of one size tubing only (no larger than 1/8“ O.D.). CAUTION: It may be necessary to heat the sample line from the cylinder to the sample valve to prevent the loss of heavy ends. 5.1.2 Analyze the composition of the blend cylinder at
23、 least two (2) times, as an unknown, to determine the repeatability of the chromatograph. Repeatability can be confirmed by comparing the component mol percents of the analyses. Component mol percents should agree within the repeatability requirements as stated in section 8 of GPA Standard 2261-95.
24、5.1.3 Once the repeatability of the instrument has been confirmed, compare the composition of the latest analysis with the gravimetric composition supplied by the blend manufacturer. The two compositions should agree within the reproducibility values as stated in GPA 2261-95. If the two compositions
25、 do not agree, refer to section 5.1.6. 5.1.4 Calibrate the instrument and obtain updated response factors. 5.1.5 Using log/log paper or an appropriate computer software application, plot the new response factors on the vertical scale versus the component molecular weights on the horizontal scale. Th
26、is resultant plot will be essentially a straight line with a negative slope. (Refer to Appendix B of this standard.) 5.1.6 If the fidelity plot does not approximate a straight line or the analysis does not agree with the blending manufacturers gravimetric analysis, another laboratory should analyze
27、the blend cylinder contents and confirm that the blend composition is in error. Do not use the bad blend. Consult with the blend manufacturer. 5.1.7 Natural Gas Reference Standard Blend cylinders should be returned to the blending manufacturer under positive pressure to maintain the cylinders integr
28、ity. 6. NATURAL GAS REFERENCE STANDARDLONG-TERM CARE AND STORAGE 6.1 Special care needs to be taken to insure the integrity of a natural gas reference blend after it has been verified and placed into service. If improperly handled, the composition of the blend will begin to show noticeable deteriora
29、tion as the pressure of the blend cylinder drops, due to usage, and should be removed from service at that time. A few outlined steps will help ensure the prolonged integrity of the original composition of the natural gas reference blend. 6.2 After the gas reference blend has been verified as to the
30、 proper composition for calibration purposes, it should be stored with the cylinder valve tightly closed, a plug inserted in the valve and tightly secured to eliminate possible leakage. Blends that contain hexane and heavier mole components should be stored at a temperature that is above the hydroca
31、rbon dew point temperature of the gas composition (preferably heat blanketed, or in some type of heating cabinet). The cylinder temperature should be monitored on a routine basis. 6.3 Fidelity plots (as described in section 5.1.5) should be made on all calibration runs. For a specific instrument, th
32、e 4Figure 1 - Control Chart for Methane on 11-Day Chart plot angle should remain essentially constant. A change in the angle usually indicates a change in the gas blend composition. Should a Fidelity Plot indicate a change in composition, the blend should be removed from service (Refer to section 5.
33、1.6). 6.4 Comparison of recent component peak area ratios with corresponding peak area ratios of the original calibration will indicate if the gas blend is beginning to deteriorate. 6.5 Another good practice is the use of quality control charts, as in Figure 1, with a chart for each component in the
34、 blend. Control charts should be plotted for components from verification runs. (Refer to Appendix C). 7. CONSTANT PRESSURE CYLINDER PREPARATION 7.1 The constant pressure cylinder that is to be used to obtain a Natural Gas Liquids (NGL) Reference Standard blend should be properly prepared before the
35、 cylinder is sent to the blend manufacturer to be filled with the appropriate blend. Lack of proper preparation can result in the contamination of the new blend or the product leaking from the cylinder. To insure the integrity of the NGL Reference Standard blend that was ordered, these outlined step
36、s should be followed. 7.2 The constant pressure cylinder should be completely dismantled for proper cleaning and visual inspection. (It is highly recommended that the cylinder to be used to obtain an NGL blend be equipped with a mixing apparatus. If the cylinder does not contain such a device, anoth
37、er cylinder should be used.) CAUTION: Disassembly of the constant pressure clyinder for maintenance requires special precautions. If either end cap is removed while pressure is on the cylinder, the end caps and the piston can be ejected with such force as to cause serious injury to personnel and dam
38、age to adjacent equipment. 7.2.1 All internal parts of the cylinder should be cleaned with a non-petroleum based product. (Acetone works well and does not leave a film on the cylinder.) 7.2.2 Seals and O-rings should be inspected for tears and scratches, and replaced as necessary. (Replacement kits
39、are available from the cylinder manufacturer.) 7.2.3 A small amount of lubricant (fluoride/petroleum free) should be used to lubricate the o-rings and seals. A silicone vacuum grease is good for this purpose. (Do not use products derived from petroleum distillates.) 7.2.4 When installing the floatin
40、g piston into the cylinder, care must be taken to insure the piston is inserted in the proper direction. (Check the manufacturers literature for proper assembly instructions.) 7.2.5 Check the valve threads for smoothness. (When installing or removing fittings from the valves, care must be taken to m
41、inimize any damage to the valve threads.) Remove any foreign material such as teflon tape, dirt, rust, etc. 7.3 After the cylinder has been reassembled, it needs to be pressure tested for leakage. 7.3.1 Slowly pressurize (using helium or nitrogen) the pre-charge end of the cylinder to 1000-1200 psig
42、 with the product end valve open slightly, making sure the piston travels smoothly through the bore of the cylinder. Once the cylinder is pressurized, close the product end valve. 7.3.2 Check the valve stems, packing nuts, and rupture discs for leaks. Check purge valves and purge fittings if applica
43、ble. 7.3.3 Allow the cylinder to set overnight if possible. Then, check the product end gauge for an increase in pressure. If no increase in pressure is observed, seals are holding properly in that direction. Depressurize the pre-charge end of the cylinder. Repeat this procedure pressurizing the pro
44、duct end of the cylinder and letting it set overnight if possible. If no increase in pressure is observed on the pre-charge end gauge, the seals are holding properly. (Refer to GPA Standard 2174-95.) 7.4 Install caps or plugs to protect threads of the valves and to prevent any foreign material from
45、entering the valve opening. 69.9370.0770.2170.3570.491234567891011DateLowerLimit-Target-UpperLimit12345678910157.5 The cylinder is now ready for shipment to the blend manufacturer to be filled with an appropriate NGL reference standard blend. 7.6 See section 10 for proper DOT shipping procedures. 8.
46、 NATURAL GAS LIQUIDS REFERENCE STANDARD ACCURACY DETERMINATION 8.1 Before a new standard blend can be used for calibrating instruments, the accuracy of the blend must be determined. Liquid reference standards are difficult to prepare and are subject to change in composition during use. It is highly
47、recommended that the composition of the reference standard be authenticated. If the composition of the new blend does not match the gravimetric composition provided by the blend manufacturer, it should not be used for calibration purposes. To determine the accuracy of a new standard blend, these ste
48、ps should be followed. 8.1.1 Connect a high pressure source (helium or nitrogen) to the pre-charge end of the constant pressure cylinder and slowly apply a pressure not less than 200 psi (1379 kPa) above the vapor pressure of the sample at the temperature of the sample injection valve (A pressure of
49、 1000-1200 psig should be sufficient for most liquid blends). This pressure should be maintained on the cylinder at all times. NOTE - It is a good Laboratory practice to use a high pressure source that is not present in the sample. 8.1.2 Mix the sample thoroughly immediately before each use. There are several different types of mixers in the various constant pressure cylinders such as rods, balls, wafers, etc. Be sure to know the type of mixer in your cylinder and how it operat
