इंटरनेट मानक Disclosure to Promote the Right To Information Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public. “जान1 का अ+धकार, जी1 का अ+धकार” “प0रा1 को छोड न’ 5 तरफ” Mazdoor Kisan Shakti Sangathan Jawaharlal Nehru “The Right to Information, The Right to Live” “Step Out From the Old to the New” IS 15125 (2002): Natural Gas - Sampling Guidelines [PCD 3: Petroleum, Lubricants and their Related Products] “!ान $ एक न’ भारत का +नम-ण” Satyanarayan Gangaram Pitroda ““IInnvveenntt aa NNeeww IInnddiiaa UUssiinngg KKnnoowwlleeddggee”” “!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता हहहहै””ै” Bhartṛhari—Nītiśatakam “Knowledge is such a treasure which cannot be stolen” ,, J&_ & -. ,$ IS 15125:2002 ‘~ 1s0 10715:1997 [ $llKfh Wm7 n Iqml * *–@-j*& WT&?ff r%iii’rl $ ,:: Indian Standard ,’ ;. ‘1 NATURAL GAS — SAMPLING GUIDELINES ICS 75.060 @ 61S 2002 BUREAU OF INDIAN STANDARDS MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG NEW DELHI 110002 April 2002 Price Group 12 Natural Gas Sectional Committee, PCD 24 NATIONAL FOREWORD This Indian Standard which is identical with ISO 10715:1997 ‘Natural gas — Sampling guidelines’ issued bythe International Organization forStandardization (ISO) was adopted bythe Bureau of Indian Standards on the recommendation of the Natural Gas Sectional Committee and approval of the Petroleum, Coal and Related Products Division Council. The text of ISO Standard has been approved as suitable for publication as Indian Standard without deviations. Certain conventions are, however, notidentical tothose used inIndian Standards. Attention is particularly drawn to the following: a) Wherever the words ‘International Standard’ appear referring tothis standard, they should be read as ‘Indian Standard’. b) Comma (,) has been used as a decimal marker while in Indian Standards, the current practice isto use a point (.) as the decimal marker. 4% IS 15125:2002 1s0 10715: 1997 Contents 1Scope ................................................................................................................................................ ............ ............ 1 2 Definitions ............................................................................................................................................................ ....1 3 Principles ofsampling ............................................................................................................................................3 4 Safety precautions ...................................................................................................................................................5 5Technical considerations .............................................................!..........................................................................7 6 Materials used in sampling ................................................................................................................................... IO 7 General preparation of equipment .......................................................................................................................11 8 Sampling equipment ............................................................................................................................................. 12 9 Spot sampling ................................................................................................................................................ ........ 18 10 Direct sampling .................................................................................................................................................... lg 11 Incremental sampling ...m..o...................................................................................................................................2l Annexes A Use of a block valve in direct sampling ..............................................................................................................23 B Cleaning of steel sampling cylinders ................................................................................................................25 C Procedure for low-pressure sampling into glass cylinders ..............................................................................26 D Procedure for sampling by the fill- and-empty method .....................................................................................28 E Procedure for sampling by the controlled-rate method ....................................................................................3O F Procedure for sampling by the evacuated-cylinder method .............................................................................32 G Guidelines for the calculation of the residence time ........................................................................................34 H Student’s t.table ....................................................................................................................................................38 J Bibliography ...........................................................................................................................................................39 i IS 15125:2002 1s0 10715: 1997 Indian Standard NATURAL GAS — SAMPLING GUIDELINES - WARNING — The use of this International Standard may involve hazardous materials, operations and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability or regulatory limitations prior to use. All sampling activities shall comply with local safety regulations. 1 Scope The purpose of this document is to provide concise guidelines for the collection, conditioning and handling of representative samples of processed natural gas streams. It also contains guidelines for sampling strategy, probe location and the handling and design of sampling equipment. Itconsiders spot, composite (incremental) and continuous sampling systems This document gives consideration to constituents such as oxygen, hydrogen sulfide, air, nitrogen and carbon dioxide in the gas stream. This document does not include sampling of liquid streams or streams with multiphase flow. Traces of liquid, such as glycol and compressor oil, if present, are assumed to be intrusive and not a part of the gas to be sampled. Their removal is desirable to protect the sampling and analytical equipment from contamination. This document can be used for custody transfer measurement systems and allocation measurement systems 2 Definitions For the purposes of this International Standard, the following definitions apply: 2.1 direct sampling: Sampling In situations where there is a direct connection between the natural gas to be sampled and the analytical unit. 2.2 floating-piston cylinder: A container which has a moving piston separating the sample from a buffer gas. The pressures are in balance on both sides of the piston. 2.3 flow-proportional incremental sampler: A sampler which collects gas over a period of time and at a rate that is proportional to the flow rate in the sampled pipeline. 2.4 high-pressure natural gas: Natural gas with a pressure exceeding 0,2 MPa. NOTE— The maximum forthis International Standard is15MPa IS 15125:2002 1s0 10715:1997 2.5 hydrocarbon dew point: The temperature, at a given pressure, at which hydrocarbon vapour condensation begins. 2.6 incremental sampler: A sampler which accumulates a series of spot samples into one composite sample. 2.7 indirect sampling: Sampling in situations where there is no direct connection between the natural gas to be sampled and the analytical unit. 2.8 liquid separator: A unit, in the sample line, used to collect liquid fall-out. 2.9 low-pressure natural gas: Natural gas having a pressure between Oand 0,2 MPa, 2.10 purging time: The period of time during which a sample purges a piece of equipment. 2,11 representative sample: A sample having the same composition as the natural gas sampled when the latter is considered as a homogeneous whole. 2.12 residence time: The time ittakes for a sample to flow through a piece of equipment. 2.13 retrograde condensation: Retrograde behaviour describes the non-ideal phase properties of hydrocarbon gas mixtures, such as natural gas. Retrograde condensation is the production of a liquid phase of heavy hydrocarbons at a particular pressure and temperature where, at that same temperature, the gas stays in a single phase at a higher pressure as well as at a lower pressure. NOTE—Seealso 5.2 2.14 sample container: A container for collecting the gas sampl e when indirect sampling is necessary. 2.15 sample line: A line provided to transfer a sample of the gas to the sampling point. It may include devices which are necessary to prepare the sample for transportation and analysis. . 2.16 sample probe: Adevice inserted into the gas line to be sampled and to which a sample line isconnected. 2,17 sampling point: A point in the gas stream where a representative sample can be collected. 2.18 spot sample: Asample of specified volume taken at a specified place at a specified time from a stream of gas. 2.19 transfer line: A line provided to carry the sample to be analysed from th=esample point to the analytical unit. 2.20 water dew point: The temperature, at a given pressure, at which water vapour condensation begins. 2 4* IS 15125:2002 1s0 10715: 1997 3 Principles of sampling 3.1 Sampling methods The main function of sampling isto take an adequate sample that is representative of the gas. The main distinction in sampling is between direct and indirect sampling methods Inthe direct sampling method, the sample is drawn from a stream and directly transferred to the analytical unit Inthe indirect sampling method, the sample is stored before it istransferred to the analytical unit The main classifications of the indirect sampling method are spot sampling and incremental sampling. Sampling - -l Q Indirect ml Time Flow ml Figure 1— Survey of direct and indirect sampling methods ---... The information needed from the analysis of natural gas falls into two basic categories: averaged and limit values. 3.1.1 Averaged values A typical example is the calorific value. Custody transfer requires the time- or flow-averaged calorific value. Commercial agreements determine the period and method of averaging. 3.1.2 Limit values Most gas custody transfer contracts contain specification limits on composition or on gas properties. Direct sampling can be applied, but often the requirements are such that also indirect sampling has to be applied. 3.2 Sampling frequency This subclause gives guidelines for the establishment of the sampling frequency. Mostly the sampling frequency is a matter of common sense. Information on the properties of the gas stream in the past and about expected (systematic) future changes determines the sampling frequency. Generally, pipeline gas composition will have daily, weekly, monthly, semi-annual and seasonal variations. Compositional variations will also occur because of gas treatment equipment and reservoir changes. AHof these environmental and operational considerations shall be taken into account when selecting a sampling interval. The statistical approach in this paragraph is only intended to support the common-sense approach. In this context, the required sampling frequency is the number of samples to be taken in a certain period of time in order to obtain meaningful results. 3 IS 15125:2002 1s0 10715: 1997 The formula for calculating the number of samples is: where d istheerror margin required; n isthe number of samples; s isthe standard deviation; t is Student’s t-factor (see table H.1 in annex H). This equation shall be solved by iteration: an initial value of fis estimated, and used to calculate a revised value of n, which is used, in turn, to give a new value of t. The error margin, the number of samples and the standard deviation shall be taken over the same period of time. 3.2.1 Error margin There are two different cases of error margins. One case is related to the determination of averaged values. In most custody transfer contracts, these values are given as an indication of the accuracy. The other is related to the determination of limit values, Custody transfer contracts specify the limits but rarely give an indication of the accuracy. In these cases, the difference between the last measured value, or the last year’s average, and the limit value isthe error margin. 3.2.2 Number of samples The number of samples is the number of samples to be taken in a defined period. It is equivalent to the number of partial samples in incremental sampling. 3.2.3 Student’s t-factor Student’s t-factor allows for the finite sample size, and is to be found in standard statistical tables.. The value depends on the claimed certainty (t ypically 95 and the “degrees of freedom”, here to be taken as the number of Y.) measurements minus one (n-1). EXAMPLE 1 Determination of the monthly average caloric value d =0,4 O/. (error margin required from custody transfer contract for monthly averaged value) s = 0,6 0/~ (estimated variation over a one-month period) First estimate, taking n= 7: t= 2,45 for 6 degrees of freedom and a certainty of 0,975 single-sided (equals 0,95 double-sided) 1 0,6 n: = 2,45 X— 0,4 n=14 First iteration, taking n= 14: 4