Practical Considerations Of Gas Sampling Systems: Part 2

Another major factor in correct sampling procedures is an awareness of the hydrocarbon dew point of the gas stream being sampled. The importance of knowing the HCDP is related to 1) The ambient temperature; 2) The temperature of the equipment being used to collect the sample; and 3) The temperature of the flowing stream. The creation of liquids due to equipment design and equipment temperature must be avoided. Determination of the HCDP of the gas stream can be done by the chilled mirror method or by the use of a number of equation-of- state models for hydrocarbon dew point determination. There are several programs available such as Peng-Robinson or SRK. The variations of the calculated results between different equations of state are so wide that it is strongly recommended to add at least 30 degrees F (16 degrees C) to the answers. This is to ensure the operator that he is designing his sampling system temperature requirements above the actual hydrocarbon dew point.

Spot Sampling
While there are several methods for spot sampling natural gas, two common methods in use today are the fill and purge method detailed in GPA-2166-05 section 7.1 and the piston cylinder method detailed in section 7.7.

Spot sampling was the primary method of acquiring a sample for analysis until the early 1970s. This method is still widely used. In today’s world of growing trends toward therm measurement and therm billing, this method is increasingly expensive in analytical cost and man-hours, as well as a very questionable method of assessing an accurate heating value to volume sales. It is at best a “spot” sample of what was present at the moment the sample was taken. Minutes before and minutes after become unknown guesses. While this may be a reasonable risk if the gas source is known by a long historical data base, most gas being consumed today is a combined gas from several origins, or is switched from source to source by contractual updates; in some cases by daily or even hourly arrangements.

This author has been on location and witnessed a 62-Btu increase at a single sample point within a one-hour time frame. It was mainly attributed to both a substantial increase and decrease in flow rate along with well selection changes within the gathering grid. Also, we find typically that the older the well and the longer it stays in production, the higher the Btu value will become. Natural gas is an extremely fragile product and almost every step in its production, transportation and distribution will have an adverse effect on its quality. Switching wells, pressure and temperature changes and storage vessels are only a few of the items that can add or subtract Btu values on the gas moving through measurement stations. Thus, a spot sample may not even represent the correct source in question.

In early years, the spot sampling method was used whereby the gas was introduced into the cylinder until it reached line pressure, and then was transported to the laboratory for calorimeter or chromatograph analysis. As the known quality of the gas (Btu value) became more important, tests were conducted to determine if the gas was being altered by the procedure used to fill cylinders.

It was determined that contaminates such as air were being introduced to the collected sample and a new filling method was needed. The fill-and-purge method was adopted. Afterward it was determined that this process was causing retrograde condensation and thus a newer method was created. This is known as a GPA method using a sampling manifold for filling the standard cylinder. The GPA method reduced the negative effects of the “filling only” procedure. The manifold allows for gas to be “trapped” in the cylinder at full pressure, rather than simply “dead ended” into the cylinder, i.e. zero pressure up to line pressure.

As the quality of gas became a critical part of billing, along with volume (std. cu. m. or std cu. ft.), the industry again reviewed the various sampling methods and the collection cylinders used for the collection and transportation of the samples.

The need for maintaining the gas at full line pressure from beginning to end became evident. Any reduction in pressure and change in temperature from the line condition at the time of sample was deemed to alter the gas analysis in almost every case. Only low Btu gas (975 Btu and below) seemed to possibly escape alteration.

It became evident that when the standard cylinder was being filled, the heavy ends dropped out as condensate in the cylinder until higher pressures were reached in the filling process. The GPA method helped eliminate this problem. However, when it was being bled into the chromatograph, there was no way to keep the pressure elevated in that cylinder. As the cylinder was opened, the light ends escaped first, thus giving a certain Btu value. As the analysis continued, that value increased because of the heavy ends remaining in the cylinder, thus altering the Btu value in a higher direction. As it is normal that more than one test is performed, due to concerns of accuracy or custody transfer, repeatability was usually impossible. It became clear the decrease in pressure was altering the gas composition.

It was in this environment that the constant pressure cylinder was designed and created. With an internal piston with seals, it was possible to pressurize (pre-charge) the cylinder with an inert gas supply (or the pipeline gas itself), and then turn the cylinder around and fill it slowly from the opposite end. By letting the gas push against the piston while slowly venting the pre-charge gas, the sample was taken at full line pressure from start to finish. Then, in the laboratory, a gas supply could be connected to the pre-charge side equal to the pipeline pressure. As the sampled gas is injected into the chromatograph, the piston is being pushed by the pre-charge gas. While the cylinder is being emptied, full pressure is being maintained and the gas composition is not being altered as a result of pressure reduction. The cylinder can be stored, or sent to another laboratory for confirmation, and when the remaining gas is analyzed, it will give repeatable results because the condition of the gas is maintained by the constant pressure cylinder.

The cylinder is equipped with valves, safety reliefs and gauges on both ends, thus the pressure can be controlled and monitored at all times on both ends. The temperature is maintained just as with standard cylinders, i.e. heating blankets, ovens, or water baths.

This procedure has proven to give extreme accuracy in both spot sampling procedures as well as in automatic sampling systems. The constant pressure cylinder has been tested against the laboratory chromatograph and online chromatographs, and has shown to maintain the integrity of the sample to within one-half Btu of the pipeline gas. This method consistently performs with excellent repeatability and reliability. Also, the richer the gas, the more alteration occurs with older methods.

The constant pressure cylinder also provides additional safety in handling the sample. No longer do you have to purge the cylinder and vent large amounts of gas to the atmosphere. A brief purge of the sample line up to the cylinder is all that is required. The piston is at the sample end of the cylinder when you commence to fill so there is no “dead volume” to purge.

And because of the design of the cylinder with seals on the end of caps, it cannot be over-pressured to the point of danger to the integrity of the cylinder. If the cylinder is over-pressured, the safety reliefs will allow the pressure to escape. In the rare event they fail to work, the cylinder will swell and the seals will stop sealing, allowing the product to escape safely.

Constant pressure cylinders have served the industry for 35 years to provide accurate sampling procedures, better sampling systems, repeatability, safer handling, accurate analysis and storage of samples as well as storage of gas and liquid calibration standards for the laboratory.

Because of the increasing cost of one Btu, more companies are improving their methods and departing from older spot sampling practices.

All updated ISO, GPA, ASTM and API standards and committee reports address the proper usage of standard and constant pressure cylinders for the gas and liquids industry.

Composite Sampling
Composite sampling is the proven middle ground between spot sampling and the continuous online analytical gas chromatographs.

Composite or grab sampling is the collection of the gas by direct introduction into a sample cylinder from a probe/valve combination or by means of a timed or proportional-to-flow sampler.

A composite gas sampler or gas sampling system consists of a probe, sample collection pump, instrumentation supply system, timing system and collection cylinder for sample transportation. Its sole objective is to collect and store a representative composite sample at line conditions, allowing it to be transported to the laboratory for complete analysis.

This package will typically mount on a pipeline and collect samples over a desired sample period unattended. For the sake of illustration, a description of a common system is provided here.

A probe should be installed which extends into the middle third of the flowing stream. This location should be chosen to provide a representative sample of the gas stream, thus devoid of stagnant gas, i.e. blow-down stack, and devoid of free liquids and aerosols, i.e. downstream of piping elbows or orifice fittings which cause turbulent flow. The probe should have a large ported outlet valve to prevent fractionation, resulting in compositional changes in the gas.

A self-purging sample collection pump designed to operate under line conditions should be located above and as close to the probe as is practical and possible. Filters, drip pots, screens, regulators and such conditioning equipment shall not be placed between the probe and the sampler, as this will affect the representative nature of the sample which is taken. Inlet check valves can also cause the gas to fractionate due to the restriction it causes in the line.

The sampler instrumentation source can be from the pipeline itself (the most common installation) or from an auxiliary instrument supply.

The timing system can be a simple function timer and solenoid, a proportional-to-flow signal conditioner and solenoid, or simply, a solenoid ready to be connected to field RTUs or other electronic devices capable of providing the desired signal.

The sample collection cylinder can be either a conventional single cavity sample cylinder or the more contemporary piston style, constant pressure sample cylinder. As these cylinders will be transported, they should meet design criteria such as ASME Section 8 or carry approvals from recognized agencies such as DOT, DNV, Lloyds, etc. A typical system would include a 500-ml cylinder which would be used on a monthly basis to contain 2,200+ bites of .2-cc size during the sample period.

Using the grab sampler, it is possible to obtain a representative sample over a pre-determined period. It is the only practical method for collecting a continuous sample. The grab sampler will introduce a set volume, taken in equal amounts to the collection cylinder over a set period, and is the preferred method when a representative sample has to be taken over time. It has the advantage of being able to measure precisely a predictable amount over a given period when using a timer, and can also take samples proportional-to-flow when taking a modified signal from a flow meter.

In addition, the sample is taken from the flowing stream at the system pressure and can be fed into the sampler or sample cylinder at the flowing pressure; thus any change in composition is avoided. Another required feature is that it should not have areas or pockets where residue of previous samples can accumulate and must take a fresh grab or bite of gas each time it samples.

This describes a typical continuous composite sampling system which has been proven to provide a representative sample for analysis. Such systems have been tested against continuous online gas calorimeters and gas chromatographs with + 1 Btu accuracy for the total sample period at considerably less cost and maintenance than online GCs.

Online Analyzers
In the realm of gas sampling there are the continuous online analytical units – the calorimeter and the chromatograph. These units have their place in the past, present and will continue to have an important place in the future of gas sampling. It is their cost, power requirements and typical upkeep that precludes their use in thousands of locations. Online analysis is convenient though is dependent on the accuracy of the analyzer, its correct calibration and the quality of the sample reaching it. It tends to be expensive to install and maintain. Economics, remote location, and downtime for service dictate the use of spot or composite sampling techniques at a majority of sample points and installations. It is also important to point out that with online units there is no second or third chance at analysis, and no second opinion option, as is the case with a sample in a sample cylinder.

On the immediate horizon, a new technology is emerging. Energy meters are soon to be introduced as an online, instant Btu meter. They will not provide analysis in the manner of the existing GCs, but will provide immediate Btu values. This new technology will fill a current void in real-time billing and plant operations. Their value is in reduced costs compared to online GCs, reduced maintenance and calibration costs, and in providing real-time information for operations.

Transportation
The transportation of natural gas samples is a very important issue for both the companies that are involved and the individual personnel who transport the samples. The U.S. Department of Transportation covers the carrying of samples in document CFR-49. Everyone involved in transporting sample cylinders and other sampling apparatus, both to and from collection locations, should be familiar with the rules and regulations set forth in CFR-49.

As well as the safety issues, markings and forms that are to be filled out for DOT purposes, other considerations should be addressed. Among these are:

• Proper tagging of the cylinder for time, date, location of the sample;
• Pressure and temperature of the pipeline source;
• Technician who took the sample;
• Method used to obtain the sample;
• Plugging of the valves and checking for leaks prior to transport;
• Protection of the cylinder and sample apparatus during transport, both to and from the sample location;
• Temperature concerns during transport, both to and from the sample location – if necessary or required; and
• Other company procedures that will assist in the success of a quality sample being delivered to the laboratory for an accurate analysis.

Conclusion
The methods, techniques, and designs of today’s sampling systems should be considered by every producer, shipper, buyer and end-user. Regardless of the application or installation, there is a system which meets your needs, and will affect your company in the profit-and-loss column. Sampling and metering create the cash register of your company. Sampling is an art! Examine your methods, procedures and needs closely.

The Author
David J. Fish is Senior Vice President of Welker, Inc., Sugar Land, TX. He can be reached at 281-491-2331, www.welkereng.com.

References
“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, Presented at AGA Annual Meeting, Anaheim, CA, 1981.

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

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

“The Importance of Discerning the Impact of New Measurement Technology”, David J. Fish, Key Note Address, Presented at The 25th Annual North Sea Flow Measurement Workshop, Oslo, Norway, 2007.

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