Practical Considerations Of Gas Sampling Systems

July 2012, Vol. 239 No. 7

David J. Fish, Senior Vice President, Welker, Inc., Sugar Land, TX

A representative sample of a hydrocarbon product is necessary to ensure proper accounting for transactions and efficient product processing. The amount of hydrocarbon product that is transported between producer, processor, distributor and end-user is significant.

To be able to verify the exact composition of the product is important from an economic and product treatment standpoint. A small percentage savings made by correctly determining composition will quickly recoup the investment made in the purchase of a system designed to obtain an optimum sample. In addition, if the best sampling procedures are followed, the potential for disputes between supplier and customer will be greatly reduced.

The importance of properly determining hydrocarbon gas composition benefits all parties involved and will achieve greater significance as this commodity becomes a bigger player in the energy marketplace. Contracts, royalties, audits, allocation and similar issues are more and more concerned with quality as well as quantity.

From the Gas Processors Association publication GPA 2166-05, “The objective of the listed sampling procedures is to obtain a representative sample of the gas phase portion of the flowing stream under investigation. Any subsequent analysis of the sample regardless of the test is inaccurate unless a representative sample is obtained.” And from ISO-10715 a representative sample is, “A sample having the same composition as the material sampled, when the latter is considered as a homogeneous whole.” Finally, API 14.1 offers a similar statement in the latest revision, “a representative sample is compositionally identical or as near to identical as possible, to the sample source stream.” These standards are the most common and current ones referenced on gas sampling procedures.

Proper sampling is fundamental to the correct determination of the product composition. It is not altogether uncommon that a sample is used with an Equation of State to help with the determination of HCDP. In a majority of cases, the sample is also the source for the determination of the specific gravity of the gas. This figure is a critical component of the flow formula from which we derive the product quantity. An error in sampling affects both quality and quantity, and ultimately, profitability.

Most current gas chromatographs boast an accuracy level of 0.5 of a Btu, but that should not be the comfort zone for the measurement department. A faulty sampling method or improperly installed and maintained equipment may alter the Btu content of the flowing stream by 25+ Btu. While the accuracy of the GC may be considered as a given, the properly executed technique for taking the sample is certainly not a given.

In the last few years, the measurement of natural gas has encountered some challenges different from the traditional operational parameters of past decades. We have typically dealt with what the industry called “clean, dry natural gas” and our contracts addressed the occasional “water wet” aspect of gas flow and the seldom seen “hydrocarbon wet” carryover in a gas stream. Today, new production from shale formations, changes in operational procedures and a broader range of gathering systems have introduced the presence of “hydrocarbon wet” natural gas streams. This presents us with new challenges in the measurement of volume quantity, as well as the sampling and quality of the gas stream.

It is not only important that we sample the natural gas at flowing conditions, but that we also sample all components present in the pipeline, regardless of phase, since they are all part of the total stream. To filter or isolate portions of the stream from our sample for whatever reason is to leave us with a non-representative sample of the flowing stream.

API Committee on Gas Fluids Measurement in Chapter 14.1 and ISO TC 193 Committee on Natural Gas in SC 3, WG5 are looking closely at the sampling of wet natural gas. As these groups begin to look at the challenges surrounding sampling of wet gas, data and testing will be conducted and additions to current standards will be forthcoming.

Whether it is a lack of gas processing, or pulling hard on storage fields in peak periods, or production from richer formations creating hydrocarbon dew point issues, or a host of other reasons, we are nonetheless faced with wet gas systems and the challenge of extracting a meaningful and accurate sample for analysis. Even with a proper representative sample taken from the pipeline, proper handling and analysis are still required.

To date, there have been a few serious research efforts on this matter and some well-organized venues in which this matter has been discussed. All these sources are valuable and should continue to be pursued. One thing has been very clear in recent discussions on this subject – there appear to be at least two different concerns about wet gas, and perhaps more than two. The upstream community needs a sample that will give it the components for meter factors and basic operational information while the downstream community needs an accurate analysis for billing and custody transfer that dictates much finer resolution than that needed upstream. Therefore, before productive discussions can be pursued, it will be beneficial to have clarity on the scopes, clear definitions and clear focus on what is required for each group.

In the end, while some alterations will be required, the basics of sampling will still play an important part of quality determination of our natural gas system.

Gas Sampling
Natural gas sampling has been performed for years with techniques handed down from generation to generation. Most of the “old traditional” methods are not sufficient to meet today’s requirements of accuracy and repeatability; however, standards have been developed to reach toward these demands. The most widely known standards are GPA-2166-05, API 14.1 and ISO-10715. API has produced a revised API 14.1 which was revised and published in June 2001. It was updated slightly in 2006. This latest revision of the standard has already generated significant interest in proper sampling techniques due to a large volume of data produced during the revision work.

Proper maintenance of all sampling equipment is vital to the operations of all sampling methods. A review of relative sampling standards and the manufacturer’s operation, installation and maintenance manuals, is an important step of the total accurate sampling process. Dirty or poorly maintained sampling apparatus will adversely affect the final results and profitability of the gas company’s operation.

Sampling Components
Sampling can be accomplished primarily by three techniques: spot, continuous composite or continuous online sampling systems. The various components of a sampling system deserve individual consideration before the various sampling procedures are investigated.

Regulators – Online analysis should incorporate specifically designed regulators to reduce the pressure to the analyzer. They will reduce the gas volume to the sampler, thus minimizing the time delay between the sample point, via the regulator, to the analyzer. This will lessen any negative effect on the gas sample by ambient conditions.

Insertion probe-type regulators are preferable as they will be able to reduce the sample pressure in the flowing stream, enabling a minimization of the Joule-Thompson effect created by the pressure drop.

Valves – If shut-off/isolation valves present a restriction that causes a pressure drop, it is possible that condensation could occur. When used with a collection cylinder it is important that there are no leaks from the gland. Light ends will be the first to leak off, thereby causing the sample to be over-represented with heavy ends. It is wise to use valves with soft seals to give a positive shut-off. Large orifice valves should be used, as restrictive valve paths can cause fractionation of the sampled gas.

Filters – For online analyzers, it is sensible to install a fit for purpose designed filter. Proper selection of the filter flow capacity and the particle size capacity should be encouraged. A filter that is too small or does not have a sufficient drip pot capacity for gases that have entrained water is a recipe for high maintenance and off spec analysis. It is prudent to invest in a reasonable filter.

Relief valves – Regulators should have a relief valve installed downstream if the equipment downstream is not able to withstand full upstream pressure. Regulators will not always give a guaranteed shut-off and their lock-up pressure will climb to a dangerous level should there be failure to attain a good shut-off such as seal damage, diaphragm damage or impurity build-up on working parts and sensing lines.

Pipework – Should be as short and as small a diameter as possible. This will assist in minimizing the time delay from sample point to the analyzer or cylinder. It will also help maintain the sample integrity. When used with online analyzers, sample delivery lines should slope upward from the probe to the analyzer to prevent condensation and impurities entering the analyzer. Lead lines to continuous samplers should slope back toward the pipeline. Pipework should also be heat traced and insulated to ensure that the sample remains above the hydrocarbon dew point throughout the delivery system to the GC.

Heating Elements – There is sufficient evidence to show that heating all components of a sampling system is a prudent step in having a reliable and accurate sampling system. The hydrocarbon dew point of a natural gas stream is a critical issue in obtaining a representative gas sample.

Probes – The correct placement is at the top of the pipe, into the center one-third or at least 200 mm (8 inches) for larger diameter pipes; in an area of minimum turbulence, that is, away from headers, bends, valves, etc. Turbulence will stir up contaminates that usually reside at the bottom of the pipeline and are therefore not normally part of the gas stream. By having the probe at a point of turbulence these contaminates will be taken into the sample, giving a sample that is not representative. The key is to have the probe in the center of the line in the correct spot (positive velocity/no turbulence) with a proper valve on the outlet. Field applications have shown that mounting the probe on the top of the pipeline is the preferred location. Side or horizontal mounts can easily encourage free liquids (if present) to migrate into the sample system.

Sample Pump – These pumps are, of course, needed to extract the sample from the line and transfer the sample to the analyzer or collection cylinder. They should have the capability to be able to extract the sample under flowing conditions, maintain a consistent discrete size of sample, take a fresh purged sample every time and have the ability to be controlled by a timer or proportional-to-flow controller. This forms the heart of the continuous gas sampling system. If the pump or sampler is unable to perform all these functions, a representative sample will not be taken and the sampling exercise will be flawed.

Pumps can be either pneumatic or electric. The safety requirements of the electrical components such as motors and solenoid valves and the environmental protection rating, dictate careful selection and compliance with applicable codes. The selection options may well be limited if electrical components have requirements which are incompatible with the use of standard components elsewhere in the system.

Sample Cylinders – Used for the collection of gases and light liquid hydrocarbons, sometimes called “sample bombs”. The cylinders come in two forms; one is a plain single cavity cylinder with a valve at each end, and the other is known as a Constant Pressure Sample Cylinder, which takes the form of a closed end cylinder with an internal piston. Before using this cylinder, one side is pressurized forcing the piston to the sample end. When the sample is taken, the product is then collected and stored at whatever pressure is pre-charged at the back of the piston. Using the Constant Pressure Cylinder the sample can be collected at a pressure above the vapor pressure of the light ends. By having the piston at the end of the cylinder, the need for excessive purging is eliminated. Pulling a vacuum in the sample cylinder (which is often destroyed by technicians) or using the water outage method is not necessary. It can be guaranteed that the sample taken is composed entirely of the gas being sampled. The hook-up is simple and straightforward, making the operation easier for technicians and minimizing the possibility of an incorrect sample being taken.

Sample cylinders should be constructed with a material that is compatible with the gas. For instance, H2S can be absorbed into the structure of 316 stainless steel. This will necessitate coating the inside of the cylinder. The resultant sample will not be truly representative otherwise.

Sample cylinders are normally protected with bursting discs. They are less expensive and lighter weight than relief valves, though their proper selection and replacement should have more importance than is sometimes given them.

Author’s note: This concludes the first portion of the considerations we must address in providing an accurate and dependable sample of natural gas. We must acknowledge the changes in the industry and we must seriously consider the equipment that is available and designed for the purpose of extracting a representative sample.

In the next article, we will look at the various methods that we can utilize to provide a quality sample for the laboratory or directly to the online gas chromatograph and the transportation of the sample cylinder to the laboratory.

Ed. note: Part 2 will be published Oct. 30, 2012.

“Proper Sampling of Light Hydrocarbons”, O. Broussard, Oil and Gas Journal, Sept. 1977.

“Standard Cylinder vs. Constant Pressure Cylinders”, D.J. Fish, Gas Industries, Jan. 1994.

“Natural Gas Sampling”, T. F. Welker, AGA Annual Meeting, Anaheim, CA, 1981.

“Methods, Equipment & Installation of Composite Hydrocarbon Sampling Systems”, D.J. Fish, Belgian Institute for Regulation and Automation, Brussels, Belgium, 1993.

“Selection and Installation of Hydrocarbon Sampling Systems”, D.A. Dobbs & D.J. Fish, Australian International Oil & Gas Conference, Melbourne, Australia, 1991.

“The Importance of Discerning the Impact of New Measurement Technology,” D.J. Fish, Keynote Address, 25th Annual North Sea Flow Measurement Workshop, Oslo, Norway, 2007.

Various Standards of AGA, GPA, API, ASTM and ISO.