GPA STD 2174-2014 Obtaining Liquid Hydrocarbon Samples for Analysis by Gas Chromatography.pdf

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1、 Obtaining Liquid Hydrocarbons Samples For Analysis by Gas Chromatography Adopted as a Tentative Standard 1974 Revised 2014 Gas Processors Association 6526 East 60th Street Tulsa, Oklahoma 74145 GPA Standard 2174-14 DISCLAIMER GPA publications necessarily address problems of a general nature and may

2、 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 intent of GPA to ass

3、ume 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 disclaims any liability or

4、 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 for any infringement of letters of patent regarding apparatus, equipment, or method so covered. Copyright2014 by Gas Processors A

5、ssociation. All rights reserved. No part of this Report may be reproduced without the written consent of the Gas Processors Association FOREWORD This 2014 revision of GPA Publication 2174 is based on the original 2174 publication, first published in 1974 and revised in 1983. It has been expanded and

6、 revised to include additional natural gas liquid sampling procedures. These changes are primarily the result of a cooperative analytical study directed by a Gas Processors Association work group through Technical Committee Section B. Testing was performed on eight (8) potential sampling methods. Te

7、n laboratories and eighteen volunteers were eventually involved in gathering 320 samples that covered over 7,000 data points. Criteria for an acceptable sampling method required that it demonstrated minimal variability, was representative of the source, and was user friendly. Details of the project

8、addressed experimental design, sample collection, logic of the evaluation process, and statistical analysis; final conclusions were presented in the Proceedings of the Gas Processors Associations 67th Annual Convention, March 1988, in Dallas, Texas. Sampling methods found to be acceptable from the c

9、hromatographic analysis of the 10 common components of the natural gas liquids and which are covered in the work group report are: 1. Floating Piston Cylinder (original GPA Standard 2174) 2. Water Displacement (total H20 removal- 80% replaced by hydrocarbons; 20% displaced for outage) 3. Water Displ

10、acement (partial H20 removal - 70% replaced by hydrocarbons; 20% displaced for outage; 10% remaining in cylinder) 4. Ethylene Glycol Displacement (total glycol removal- 80% replaced by hydrocarbons; 20% displaced for outage) Proper samples can be obtained when using any of the four methods listed, p

11、rovided a strict adherence to detail is maintained. It is recommended that the potential users of these methods study the work group report prior to selection of an applicable method. Obtaining Liquid Hydrocarbon Samples For Analysis by Gas Chromatography 1. SCOPE 1.1 The specific purpose of this me

12、thod is to describe the equipment and procedures for obtaining representative samples of natural gas liquids and the subsequent preparation of those samples for laboratory analysis by gas chromatography. The procedures described in this method may be used for obtaining samples for analysis by method

13、s other than gas chromatography. A SAMPLE PROBE MUST BE USED TO OBTAIN A REPRESEN-TATIVE SAMPLE. 2. OUTLINE OF METHODS 2.1 A hydrocarbon fluid sample is transferred under pressure from a source into a sample container by one of the following methods: a. Floating Piston Cylinder Method b. Water Displ

14、acement Method 1 (total H20 removal - 80% replaced by hydrocarbons; 20% displaced for outage) c. Water Displacement Method 2 (partial H20 removal - 70% replaced by hydrocarbons; 20% displaced for outage; 10% remaining in cylinder) d. Ethylene Glycol Displacement Method (total glycol removal - 80% re

15、placed by hydrocarbons; 20% displaced for outage) Note 1 Methods b, c, and d may not be applicable to the analysis of certain samples containing reactive non-hydrocarbons such as sulfur compounds, carbon dioxide, etc. 3. GENERAL INFORMATION 3.1 The objective of any sampling operation is to secure, i

16、n a suitable container, an adequate portion of a hydrocarbon fluid under pressure, having the same composition as the stream being sampled. 3.2 Particular emphasis should be given to the necessity of obtaining accurate, representative samples for analysis since the results, regardless of the care an

17、d accuracy of the laboratory tests, may be useless if the samples are not valid. 3.3 It is not possible, nor is it the intent of this method, to provide a procedure that will be applicable for all sampling situations. The sample source here is assumed to be a homogeneous, single-phase liquid. All sa

18、mples must be obtained using a probe designed to secure product from the center one-third of the flowing stream. The location of the 1 probe fitting should be on the top or side of the line rather than at the bottom. It is strongly recommended that the samples be obtained under the supervision of a

19、person knowledgeable in the phase behavior of hydrocarbon systems and experienced in all sampling operations. 3.4 The scope of this method does not include recommendations for the location of the sampling point in a line or vessel, although the importance of the proper sampling location cannot be ov

20、eremphasized. 3.5 A certain amount of information about a sample is necessary before it can be intelligently handled in the laboratory. Essential information includes the sample source, sample date, cylinder identification, sample source pressure and temperature, ambient temperature, type of analysi

21、s required, and the sampling method used. There may be additional related facts such as field-determined results and operating conditions which will assist in the evaluation of the analytical data. This information must accompany the filled sample cylinder. 3.6 If the hydrocarbon fluid samples are t

22、o be transported by common carrier within the United States, the sample containers must meet the specifications of and be labeled and packaged according to the Hazardous Materials Regulations of the Department of Transportation. 3.7 This method assumes all procedures begin with clean, leak-free samp

23、le cylinders. Cylinders must be thoroughly cleaned prior to sampling with an appropriate volatile solvent or by following manufacturers recommendations. The use of detergent/water solutions or steam is not recommended for the cleaning of floating piston cylinders. 3.8 For floating piston cylinders,

24、it is desirable in most cases to use an “inert” charge gas for the piston cylinder which is not present in the sample so that a leak in the cylinder itself can be easily detected during the analysis. The use of natural gas as a back pressure fluid is not recommended because a leak across the piston

25、may result in compromising the sample with hydrocarbons (or other components) which were not actually present at the source. A method to insure a leak-free cylinder is to pressure test both sides of the piston cylinder. This is accomplished by pressurizing the displacement chamber with inert gas to

26、a typical operating pressure with valve C opened (see Fig. 1). Next, close valves C and D and note the pressure reading on gauge N. After a period of five minutes, a decrease in pressure as indicated at gauge N and a corresponding increase in pressure at gauge M would indicate a leak across the pist

27、on. A pressure drop only at gauge N indicates a leaky fitting, valve, gauge, or rupture disc. The inert gas should be removed from the displacement end and the above procedure repeated for the sample side. It is also possible to analyze the inert gas side of the cylinder to determine hydrocarbon con

28、centration if leakage occurs in that direction. Cylinders should be tested at least on an annual basis. 2 Figure 1. Typical Visual Indicator Sampling System 3.9 Care must be exercised when sampling liquids having a vapor pressure higher than atmospheric pressure to prevent flashing of lighter compon

29、ents when transferring product from the source to a sample cylinder, or from a primary sample cylinder to a secondary cylinder. For floating piston cylinders, precautions should also be taken to ensure that the inert gas pressure in the piston cylinder never drops below the sampling pressure or the

30、products vapor pressure at the existing source temperature, thus preventing flashing of the sample. Theoretically, the product in the vapor state could be returned to liquid phase, but the physical restrictions of the piston cylinder (e.g., rupture disc, valve dead space, and stirring assembly desig

31、n) may prevent the flashed product from returning to the original homogeneous mixture. 3.10 Duplicate Samples 3 3.10.1 When re-sampling is difficult or impossible, it is advisable to take duplicate samples as a precaution against accidental loss. These duplicate samples must be taken using the same

32、sampling method and from the same sampling location. The sample sequence should be noted on the sample information tag. 4. APPARATUS 4.1 Sample Containers 4.1.1 Floating Piston Cylinders 4.1.1.1 The container required for this method is constructed of metal tubing, honed and polished on the inside s

33、urface. The cylinder is preferably closed with removable end caps to provide access to remove and service the moving piston. The end caps are drilled and tapped for valves, gauges, and relief valves. The cylinder is designed consistent with the maximum pressure anticipated during sampling and to be

34、nonreactive to materials being sampled, the pressurizing fluid, the cleaning solvents and the expected corrodants. The volume of the cylinder will depend on the amount of sample needed for the laboratory analysis. 4.1.1.2 The cylinder itself contains a moving piston equipped with a-rings, Teflon rin

35、gs, or other devices to effect a leak-free seal between the sample and the pressurizing fluid while allowing it to move freely within the cylinder. The use of guide rings is recommended to assure smooth piston travel. The piston and sealing device must be non-reactive to the sample, the pressurizing

36、 fluid, the cleaning solvents, and expected corrodants. CAUTION: Some types of lubricating fluids will absorb appreciable amounts of the C6+fraction from the gas being sampled, thus compromising the integrity of the sample. It is recommended that non-absorbing lubricating greases, such as, but not l

37、imited to DuPonts KrytoxAC or AD, be used to prevent this difficulty. Further details regarding non-absorbing lubricating greases can be obtained from the piston cylinder manufacturers. 4.1.1.3 All valves and safety devices must meet the appropriate material and pressure requirements for safe design

38、. The pressure reliefs may be of spring or rupture disc types. These allow a partial or complete loss of contents due to thermal expansion or over-pressurization. Should relieving occur, the sample should be considered compromised. 4.1.1.4 Some piston-type cylinders are fabricated from non-magnetic

39、materials such as the 300 series stainless steel. The piston likewise is fabricated of stainless steel but has magnets attached to the precharge side of the piston. As the piston moves the length of the cylinder, the magnetic field 4 generated by the magnets flips a series of bi-colored flags. This

40、system, or systems of similar configuration, indicates the piston position and the volume of product in the cylinder. 4.1.1.5 Some piston-type cylinders are fabricated with a rod attached to the piston which extends through the end cap on the inert gas back pressure chamber with appropriate sealing

41、devices to prevent the inert gas from leaking. The traveling rod provides an indication of the piston position and the volume of the product sample in the cylinder. Again, some variations of this style may exist. 4.1.1.6 Other types of floating piston cylinders are available which have no visual met

42、hod of determining the sample volume directly. For these cylinders, a displacement cylinder, cylinder Y (Fig. 2), is fabricated from metal tubing. This cylinder must be designed to meet the same pressure requirements as the piston cylinder and have a volume of no more than 80 percent of the pressuri

43、zing volume of the piston cylinder. 4.1.2 Double Valve Cylinders 4.1.2.1 Metal sample containers of a type which insure maximum safety and which are corrosion resistant to the product being sampled should be used. Stainless steel containers are recommended to minimize problems of surface adsorption

44、of heavy components (hexanes and heavier components) and to minimize the reaction of carbon dioxide or other contaminants with the container. Sample containers and valves must have a working pressure equal to or exceeding the maximum pressure anticipated in sampling, storage, or transportation of th

45、e sample container. Soft-seated valves are preferable to those having metal-to-metal seats. The size of the container depends upon the amount of sample required for the laboratory tests to be performed. Note 2 DOT regulations regarding the use of pressure relief devices on these cylinders must be fo

46、llowed. 4.2 Sample Transfer System 4.2.1 Transfer lines, valves, and gauges in the transfer system shall be designed consistent with maximum anticipated pressure and be resistant to all expected corrodants. (Stainless steel is preferred.) The transfer lines should have a minimum diameter of 1/4 inch

47、 (6.35 mm) and be as short as is practical. The use of filters and dryers is discouraged. 4.3 Composite Sampling Mechanism 5 4.3.1 A composite sampling mechanism is a device which is used to obtain a representative sample from a flowing product stream over a given period of time. The unit consists o

48、f a sample probe, either a flow-through sample injection valve, whether automatic or manual (see Figures 3 and 4) or a probe-mounted sample pump (Fig. 5) and utilizes a floating piston cylinder as its collection chamber. The system must include a method to mix the product sample in the collection ch

49、amber. This is necessary regardless of whether the original floating piston cylinder is removed and used for laboratory analysis or a transfer of the product sample is made into a secondary cylinder as described herein. (Figure 6 depicts a typical automatic sampler and the various parts required. Figure 5 depicts a typical proportional sampler using an injection pump and the various parts required.) Figure 2. Displacement Cylinder Sampling System 6 4.4 Sample Filters 4.4.1 The sample filter is an optional device used to protect the sampling valve from scoring due to the presence of fo