{"id":1461,"date":"2022-02-16T02:30:00","date_gmt":"2022-02-16T01:30:00","guid":{"rendered":"https:\/\/as-schneider.blog\/?p=1461"},"modified":"2022-01-27T14:52:42","modified_gmt":"2022-01-27T13:52:42","slug":"main-issues-obtaining-representative-sample-ng-applications","status":"publish","type":"post","link":"https:\/\/as-schneider.blog\/2022\/02\/16\/main-issues-obtaining-representative-sample-ng-applications\/","title":{"rendered":"What are the main issues in obtaining a representative sample in NG applications?"},"content":{"rendered":"\t\t
In this article, I present the major issues associated with obtaining a representative sample in NG applications. The article explains what is representative sampling, and why sampling is important in the natural gas industry. I discuss factors that determine what should be a suitable sampling frequency in a process, and which sampling standards can be followed to ensure representative sampling.<\/strong><\/p> Several major issues in obtaining a representative sample, and common design flaws that lead to poor sampling are presented and discussed. In addition, I present how a suitable probe length can be determined and how the probe should be installed. The different types of commonly used sampling probes are also compared and discussed.<\/strong><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t A sampling process allows one to draw conclusions about the overall system using a small subset.\u00a0To do this, however, the sample must be representative of the overall system. This is the case if the sample exactly reflects the characteristics of the overall system.<\/p> In the chemical industry, sampling helps plant operators manage product quality & improve efficiency.\u00a0The sampling can help reduce waste, and ensure compliance with the process parameters and regulations.<\/p><\/div><\/div>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t The frequency with which you should take samples depends on the expected fluctuations in the composition of the process stream. Depending on the process, the fluctuations may be hourly, daily, monthly, or seasonal.<\/p> Facilities and gathering systems that\u00a0are composed\u00a0of combined gas streams must\u00a0be sampled\u00a0more\u00a0frequently.\u00a0This is because NG comes from several wells with different quality, quantity and composition.<\/p> Based on this, I recommend that you take and analyze samples as often as necessary.\u00a0This way, you can ensure that all essential fluctuations\u00a0are recorded\u00a0within a specific period of time.<\/p><\/div><\/div>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t Raw natural gas is a mixture of methane, various hydrocarbons, nitrogen, along with some other inert compounds.<\/p> The heating value of the gas can vary depending on the composition of the mixture. Components like, nitrogen, carbon dioxide and inert compounds have no heating value. But the heating value of hydrocarbon components depends on their molecular structure.\u00a0The\u00a0overall\u00a0heating value can,\u00a0therefore, only\u00a0be determined\u00a0through an accurate analysis of the gas composition.<\/p> You can use compositional analysis to determine the extent of recoverable liquids in the mixture. Application to identify contaminants is also possible.<\/p> In the oil and gas sector, allocation refers to an ongoing process.\u00a0It\u00a0is based\u00a0on flow or volume measurements, that determines the distribution of contributing sources.<\/p> Sampling is used for the analysis of various gas streams flowing into a gathering system. Different streams can be classified as lean\/rich or sweet\/sour after a representative sample analysis.<\/p> Each type of gas has a different economic value depending on its composition. In order to fairly compensate the stakeholders, it is important to correctly measure the volume and quality of gas from each source.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t The development of sampling standards for the NG industry\u00a0is based\u00a0on the results of rigorous field testing. This\u00a0is supplemented by\u00a0recommendations from stakeholders based on decades of practical experience.<\/p> These standards provide details of potential causes of sample distortion. They describe recommended sampling equipment, and specify sampling techniques to obtain representative samples. The two most-cited industry standards are:<\/p><\/div><\/div> API 14.1<\/strong>:<\/span> This standard is developed by the American Petroleum Institute (API). The standard\u00a0is published\u00a0in the Manual of Petroleum Measurement Standards.\u00a0Chapter 14 of this manual, titled\u00a0Natural Gas Fluids Measurement<\/em>, specifies the standards for\u00a0Collecting and Handling of Natural, Gas Samples for Custody Transfer<\/em>.<\/p> GPA 2166<\/strong>:<\/span> This standard has been developed by the Gas Processors Association (GPA) and specifies techniques and procedures for\u00a0Obtaining Natural Gas Samples for Analysis by Gas Chromatography.<\/em><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t In the natural gas industry, representative sampling can be described as the sampling of a limited volume of gas. The volume sampled is representative of the process medium actively flowing through the system.<\/p> The sampling system must be designed so that a representative sample can be taken each time. And this is where the challenge lies. Using an inappropriate technique for sampling or poor design of the sampling system can lead to errors in the measurement of key parameters.<\/p> I have summarized for you the most common issues that lead to incorrect sampling in NG applications:<\/strong><\/p><\/div><\/div>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t The design of a representative sampling system must take into consideration the nature of the application and the process conditions under which the sample must be collected.<\/p> The piping and instrumentation diagram (P&ID) of a process generally identifies the sampling points within the process. As well as the required process conditions.\u00a0These can include operating temperature and pressure, product flow rate, product viscosity, and the size of the lines.\u00a0In order to\u00a0ensure process safety during sample collection, these operating conditions must all\u00a0be factored\u00a0in.<\/p> Significant temperature and pressure variations may lead to a material phase change causing measurement errors.\u00a0To avoid such issues, sampling must\u00a0be conducted\u00a0as close to the production process point as possible.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t A sampling system is usually connected to the main process via a return line. In this way, any uncollected sample can be returned.<\/p> With such installations, the gas sampling lines to and from the main process must be purged. This ensures that residual substances in the lines do not influence the test results.<\/p> A basic design flaw is to tie in the sampling system return line too close to the sampling supply inlet.<\/p> If there is no significant pressure differential between the inlet and the outlet, there will be effectively no flow through the sampling system.<\/p> A good design should incorporate what is known as a \u201cspeed loop\u201d or a \u201cfast loop\u201d. The pressure at the inlet should be higher than at the outlet, creating a continuous flow through the sampling system. A fast loop should typically have a pressure differential of 5-15 psig between the inlet and the outlet.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t When choosing a location for the sampler, you should consider the turbulence factors in the nearby area.<\/p> Flow restricting devices can create swirls or re-circulation eddies. These may have a composition that is significantly different from the flowing stream.<\/p> The recommended mounting location is at least five pipe diameters downstream from elbows, fittings, valves, or any other restricting devices.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t The API standard recommends that the sample probe tip extend into the center third of the flowing stream. By keeping the tip away from the pipe walls, you avoid drawing in migrating hydrocarbon fluids.<\/p> On the other hand, probes that are too long can mechanically fail due to vibration. The GPA standard specifies that probes must not be inserted more than 25 cm into the pipe to avoid vibrations. API 14.1 section 7.4.1 also contains recommendations regarding probe length.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t Installation of isolation valves at the inlet and outlet of the sampling loop can also lead to errors during sampling. This happens when they are\u00a0normally\u00a0kept closed and opened only for sampling.<\/p> If there is not enough time to flush the stagnant material in the loop, this can also result in an unrepresentative sample.<\/p> Each process presents its own sampling challenges.\u00a0It is advisable that design teams work with an expert in sampling systems who knows how to avoid potential pitfalls.<\/p><\/div><\/div>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t Gas sampling probes provide an important interface between the process and the analysis system.\u00a0The probe extracts the sample gas from the process stream and supplies it to the collection system.\u00a0As an operator, you must ensure that the collection system has a\u00a0properly\u00a0installed sample probe. A well-designed sample probe has a shorter response time and eliminates dead volumes.<\/p> Sample probes are available with direct mounted single flow, dual flow, or insertable\/retractable options. The two most\u00a0commonly\u00a0used gas sampling probes are single flow and dual flow.<\/p><\/div><\/div>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t A single flow sample probe is generally constructed from a piece of 316SS straight tubing connected to a \u00bd\u201d to 1\u201d bushing or valve. And it is the simplest sample probe design with a straight cut or beveled tip.<\/p> Most single flow probe designs provide for a hot loop connection to a downstream point. The differential pressure between the two points drives the flow through the sampler.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t The dual flow probe eliminates the need for a downstream pressure drop.<\/p> It uses the differential pressure between the probe tip and the return port at the bottom of the probe connection to create a slipstream loop. The loop provides a flowing stream of gas from the pipeline to the sample system.<\/p> Dual flow probes are commonly used when the sampler is remote mounted, but they also work with direct mount systems. When mounted remotely, the distance between the sample pump and the probe connection should be minimized.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t Proper sampling techniques are necessary in the NG industry to check the quality and heating value of natural gas. Representative gas sampling techniques must be employed for reliable, accurate, and repeatable gas analysis results. The API and GPA standards provide recommendations and guidelines for safe and representative sampling.<\/p> When setting up a sampling system, it is important to ensure that the sampling conditions correspond to process conditions. A continuous flow through the sampling system must be ensured and turbulent zones with recirculation eddies should be avoided during selection of sampling points. It should also be ensured that the isolation valves are flushed before a sample is drawn. The probe should extend well into the flowing stream but should not be too long to become a source of vibration.<\/p> Installing suitable sampling equipment at the correct location is critical to obtaining representative samples which can help ensure proper allocation, reduce maintenance costs, and ensure process safety.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t Image Source: \u00a9<\/span>Sergey<\/a> <\/span>(Adobe Stock), <\/span>dreampicture<\/a>, (Adobe Stock), matrivgraphics<\/a> (Fiverr) & AS-Schneider<\/span><\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":" In this article, I present the major issues associated with obtaining a representative sample in NG applications. The article explains what is representative sampling, and why sampling is important in the natural gas industry. I discuss factors that determine what should be a suitable sampling frequency in a process, and which sampling standards can be followed to ensure representative sampling. <\/p>\n Several major issues in obtaining a representative sample, and common design flaws that lead to poor sampling are presented and discussed. In addition, I present how a suitable probe length can be determined and how the probe should be installed. The different types of commonly used sampling probes are also compared and discussed.<\/p>\n","protected":false},"author":20,"featured_media":1486,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[401,411],"tags":[1239,1250,1251,566,1249,1248,1243,1240,778,1237,1264,1263,1242,1245,1234,1265,1244,1241,1236,1246,1238,1247],"class_list":["post-1461","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-cost-savings","category-safety","tag-api-14-1","tag-conventional-dbb-valve-configuration","tag-custom-designed-dbb-gas-sampling-valves","tag-dbb-valve","tag-double-isolation","tag-dual-flow-sample-probes","tag-flushing","tag-gpa-2166","tag-isolation-valve","tag-natural-gas-industry","tag-ng","tag-ng-application","tag-pressure-differential","tag-probe-length","tag-representative-sample","tag-representative-sample-in-ng-applications","tag-sample-probe","tag-sampling-conditions","tag-sampling-frequency","tag-sampling-location","tag-sampling-standards","tag-single-flow-sample-probes"],"acf":[],"yoast_head":"\nWhat is a representative sampling?<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
Sampling frequency<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
Why natural gas sampling should be undertaken?<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
What are the sampling standards for the NG industry?<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
What are the main issues in obtaining a representative sample in NG applications?<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#1<\/font><\/font><\/br><\/br>Sampling conditions don\u00b4t correspond to the production condition<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#2<\/font><\/font><\/br><\/br>No significant pressure differential between the inlet and outlet<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#3<\/font><\/font><\/br><\/br>The sampling location don`t consider the nearby turbulence factors <\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#4<\/font><\/font><\/br><\/br>Probe length doesn`t comply with standards<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#5<\/font><\/font><\/br><\/br>Not enough time for flushing of isolation valves<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#6<\/font><\/font><\/br><\/br>The sample probe isn`t installed correctly<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#7<\/font><\/font><\/br><\/br>Single flow sample probes<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
#8<\/font><\/font><\/br><\/br>Dual flow sample probes<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
Conclusion<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
\n\t\t\t\t\t\tInfographic - 7 major issues associated with obtaining a representative sample in NG applications.\t\t\t\t\t<\/h2>\n\t\t\t\t\n\t\t\t\t\n\t\t\t\t\t\t\t\t\t