1、BRITISH STANDARD Gas analysis Preparation of calibration gas mixtures using dynamic volumetric methods Part 6: Critical orifices ICS 71.040.40 6145-6:2008 BS EN ISOThis British Standard was published under the authority of the Standards Policy and Strategy Committee on 16 May 2003 BS EN ISO 6145-6:2
2、008 ISBN 978 0 580 60360 0 National foreword This British Standard is the UK implementation of EN ISO 6145-6:2008. It is identical with ISO 6145-6:2003. It supersedes BS ISO 6145-6:2003 which is withdrawn. The UK participation in its preparation was entrusted to Technical Committee PTI/15, Natural g
3、as and gas analysis. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard canno
4、t confer immunity from legal obligations. Date Comments This corrigendum renumbers BS ISO 6145-6:2003 as BS EN ISO 6145-6:2008 28 February 2009 BSI 2009 Amendments/corrigenda issued since publicationEUROPEAN STANDARD NORME EUROPENNE EUROPISCHE NORM EN ISO 6145-6 August 2008 ICS 71.040.40 English Ver
5、sion Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 6: Critical orifices (ISO 6145- 6:2003) Analyse des gaz - Prparation des mlanges de gaz pour talonnage laide de mthodes volumtriques dynamiques - Partie 6: Orifices critiques (ISO 6145-6:2003) Gasanal
6、yse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-volumetrischen Verfahren - Teil 6: Kritische Dsen (ISO 6145-6:2003) This European Standard was approved by CEN on 30 July 2008. CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions
7、 for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN Management Centre or to any CEN member. This European Standard exists in three off
8、icial versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the official versions. CEN members are the national standards bodies of Austri
9、a, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom. EUROPEAN COMMIT
10、TEE FOR STANDARDIZATION COMIT EUROPEN DE NORMALISATION EUROPISCHES KOMITEE FR NORMUNG Management Centre: rue de Stassart, 36 B-1050 Brussels 2008 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN ISO 6145-6:2008: EForeword The text o
11、f ISO 6145-6:2003 has been prepared by Technical Committee ISO/TC 158 “Analysis of gases” of the International Organization for Standardization (ISO) and has been taken over as EN ISO 6145-6:2008 by Technical Committee CEN/SS N21 “Gaseous fuels and combustible gas” the secretariat of which is held b
12、y CMC. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by February 2009, and conflicting national standards shall be withdrawn at the latest by February 2009. Attention is drawn to the possibility th
13、at some of the elements of this document may be the subject of patent rights. CEN and/or CENELEC shall not be held responsible for identifying any or all such patent rights. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound t
14、o implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switz
15、erland and the United Kingdom. Endorsement notice The text of ISO 6145-6:2003 has been approved by CEN as a EN ISO 6145-6:2008 without any modification. BSENISO61456:2008OSI -54163002:6)E( ISO 3002 r llAithgs reservde iii Contents Page 1 Scope 1 2 Normative references 1 3 Principle 1 4 Application t
16、o preparation of gas mixtures 2 4.1 Description of the orifice system and the experimental procedure . 2 4.2 Area of validity . 4 4.3 Operating conditions 4 5 Calculation of operating parameters and results . 4 5.1 Selection of suitable orifice systems 4 5.2 Calculation of volume fraction . 5 5.3 So
17、urces of uncertainty . 5 5.4 Uncertainty of volume fraction 6 Annex A (informative) Premixed gases for preparation of mixtures of high dilution 7 Annex B (informative) Practical hints . 9 Bibliography . 11 iii BSENISO61456:2008blankOSI -54163002:6)E( ISO 3002 r llAithgs reservde v Introduction This
18、part of ISO 6145 is one of a series of standards that present various dynamic volumetric methods used for the preparation of calibration gas mixtures. v BSENISO61456:2008ANRETNIITOTS LANDNADRA OSI -54163002:6)E( ISO 3002 r llAithgs reservde 1 Gas analysis Preparation of calibration gas mixtures usin
19、g dynamic volumetric methods Part 6: Critical orifices 1S c o p e This part of ISO 6145 specifies a method for the continuous production of calibration gas mixtures, containing two or more components, from pure gases or other gas mixtures by use of critical orifice systems. By selection of appropria
20、te combinations of orifices and with the use of pure gases, the volume fraction of the calibration component in the calibration gas mixture can be varied by a factor of . Additionally, it can be changed by a factor of by changing the initial pressures in the orifice systems. The uncertainty of the m
21、ethod depends mainly upon the flow calibration method and the variations in temperature and outlet pressure. The relative expanded uncertainty in the volume fraction obtainable for a binary mixture (at a coverage factor of 2) is . If pre-mixed gases are used instead of pure gases, much lower volume
22、fractions can be obtained (see Annex A). The mass flow rates or volume flow rates, from which the mass or volume fractions are determined, can be calculated and can be independently measured by a suitable method given in ISO 6145-1. The merits of the method are that multi-component mixtures can be p
23、repared as readily as binary mixtures if the appropriate number of orifices is utilized, and that a large quantity of calibration gas mixture can be prepared on a continuous basis. The range of flow rates can be from several millilitres per minute to approximately . Although particularly applicable
24、to preparation of gas mixtures at barometric pressure, the method also provides a means of preparation of calibration gas mixtures at pressures above barometric pressure. Annex B gives practical hints on the use of the method. 2 Normative references The following referenced documents are indispensab
25、le for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 6143, Gas analysis Comparison methods for determining and checking the composition of calibrati
26、on gas mixtures ISO 6145-1, Gas analysis Preparation of calibration gas mixtures using dynamic volumetric methods Part 1: Methods of calibration 3 Principle When passed through a critical orifice at increasing upstream pressure , the volume flow rate of gas passing through the orifice will increase.
27、 When the ratio of the gas pressure upstream and the gas pressure downstream of the orifice has reached a critical value, on further increase of the volume flow rate of the gas becomes independent with respect to . 10 4 10 2 3% 10 l/min p 1 p 1 p 2 p 1 p 2 1 BSENISO61456:2008ISO -54162:6)E(300 2 ISO
28、 3002 All rithgs reresvde For a given gas, and at constant temperature the critical pressure ratio is: (1) where is the ratio of the molar heat capacities of the gas at constant pressure and at constant volume. For monatomic, diatomic and triatomic gases, this critical pressure ratio is approximatel
29、y 0,5. Orifice systems which are operated at pressure ratios less than are termed “critical orifices”. Use of such systems provides a means of maintaining constant flow rates of gases. In actual practice it is convenient to arrange the system so that is equal to the prevailing barometric pressure. T
30、o prepare calibration gas mixtures, the complementary gas is supplied at known flow rate, from a critical orifice, to meet the calibration component emerging from another critical orifice. The mixture is then allowed to pass along a mixing tube, at the end of which the flow rate is measured by a sui
31、table method as given in ISO 6145-1. Since the volume flow rate of the calibration component remains the same whether or not the complementary gas is flowing, it can be measured after the flow of complementary gas has been stopped. The concentration of the calibration gas mixture is calculated from
32、the two measured critical flow rates. 4 Application to preparation of gas mixtures 4.1 Description of the orifice system and the experimental procedure A schematic diagram of the arrangement for preparation of binary mixtures is shown in Figure 1. In Figure 1, the orifices 9 and 10, respectively, fo
33、r the complementary gas and the calibration component to be added are mounted in the orifice system (11). Cylinders 1 and 13, respectively, contain the complementary gas and the other gaseous component, and are connected to the mixing system via pressure-reducing valves (3 and 15) and metallic filte
34、rs (5 and 17), which provide protection against contamination. In each of the lines, and upstream of the filters, are a pressure-reducing valve and a pressure gauge. A shut-off valve (8) is provided in the complementary gas line and a venting valve is included in the other line. To operate the gas-m
35、ixing system, the valves of the gas cylinders are opened and the readings on the pressure gauges 4 and 16 are adjusted, each to a value approximately ( ) above the values anticipated at pressure gauges 7 and 20 respectively. The pressure regulators (6 and 18) are opened so that the complementary gas
36、 and the calibration component flow through the respective orifices. The flow of complementary gas is then stopped by closing the shut-off valve 8. The other line is then flushed with the calibration component by repeated opening and closure of the venting valve 19. Valve 19 is then closed and press
37、ure regulator 18 is adjusted so as to set the pressure at gauge 20 to the value necessary to produce the required flow rate (see below). The flow rate is then measured, by one of the methods given in ISO 6145-1, at outlet 12 of the orifice system (11). The shut-off valve 8 is then opened and pressur
38、e regulator 6 is adjusted to set the pressure indicated on pressure gauge 7 to the value necessary to produce the required flow rate. The combined flow rates of the complementary gas and the calibration component are then measured by a suitable method according to ISO 6145-1. p 2 p 1 crit = 2 + 1 1
39、p 2 /p 1 (p 2 /p 1 ) crit p 2 200 kPa 2 bar 2 BSENISO61456:2008OSI -54163002:6)E( ISO 3002 r llAithgs reservde 3 Key 1 pressure cylinder (complementary gas) 2 pressure gauge (inlet pressure) 3 pressure-reducing valve 4 pressure gauge (delivery pressure) 5f i l t e r 6 pressure regulator 7 pressure g
40、auge 8 shut-off valve 9 orifice (complementary gas) 10 orifice (calibration component) 11 orifice system 12 exit for calibration gas mixture 13 pressure cylinder (calibration component) 14 pressure gauge (inlet pressure) 15 pressure-reducing valve 16 pressure gauge (delivery pressure) 17 filter 18 p
41、ressure regulator 19 vent valve 20 pressure gauge Figure 1 Preparation of calibration gas mixtures with a critical orifice system 3 BSENISO61456:2008ISO -54162:6)E(300 4 ISO 3002 All rithgs reresvde 4.2 Area of validity The method is applicable to preparation of mixtures of non-reacting species, i.e
42、. those which do not react with any material of construction of the flow path within the orifice system or the ancillary equipment. Particular care shall be exercised if the method is considered as a means of preparation of gaseous mixtures that contain components which form potentially explosive mi
43、xtures in air. Steps shall be taken to ensure that the apparatus is safe, for example by means of in-line flame arrestors in addition to the items listed in 4.1 (unless the in-line metallic filters already present are in fact approved sintered metal flame arrestors). This is of particular importance
44、 in this method because the gases in the dilution system are at pressures appreciably above the prevailing barometric pressure. The method is not absolute and each orifice system shall be calibrated for the particular gas for which it is to be used. This is necessary because the formula for the volu
45、me flow rate of a gas includes the molar mass. 4.3 Operating conditions The general precautions common to all dynamic techniques of preparation shall be observed. It is essential that attention is paid to the materials used in construction of the flow system. Only materials of low porosity and which
46、 are non-adsorbing are suitable. The pipe work shall be clean and all joints secure. 5 Calculation of operating parameters and results 5.1 Selection of suitable orifice systems The volume flow rate of a gas through an orifice at is given by: (2) where (in the appropriate units) is the volume flow ra
47、te of the gas under normal conditions; is the standard atmospheric pressure ( ); is the normal temperature ( ); is the pressure of the gas upstream of the orifice; is the upstream temperature of the gas; is the universal gas constant ; is the molar mass of the gas, in grams per mole; is the ratio of
48、 the molar heat capacities of the gas at constant pressure and at constant volume; is the cross-sectional area of the orifice; is the pressure of the gas downstream of the orifice. At a constant temperature, , Equation (2) can be simplified to: (3) p 2 0,5p 1 q 0 = p 1 p n T n T 1 A RT n M 2 + 1 +1
49、1 q 0 p n 101,325 kPa T n 273,15 K p 1 T 1 R 8,314 510 J/(molK) M A p 2 T 1 q 0 = Dd 2 M 1 2 p 1 4 BSENISO61456:2008OSI -54163002:6)E( ISO 3002 r llAithgs reservde 5 where is the diameter of the orifice; is a constant. For polyatomic gaseous molecules, the ratio of the molar heat capacities, , is calculated from the known value of , the molar heat capacity at constant pressure, a
