Standard Operating Procedure (SOP) for Hydrocarbon gas sampling

 Standard Operating Procedure (SOP) for Hydrocarbon gas sampling

GPA-2166-05 and ISO-10715.

Purpose

The need to be able to take a representative sample of a hydrocarbon product is necessary to ensure proper accounting for transactions and efficient product processing. The various sampling methods that are available and the options and limitations of these methods are investigated; the most appropriate equipment to use; the reasons for its use and correct installation of the equipment are also addressed.

Gas Sampling

Natural Gas sampling has been performed for years with techniques handed down from generation to generation. Most of the 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 and ISO-10715. API has produced a revised API 14.1.

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 (NGL samples), 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 are lighter weight than relief valves, though their proper selection and replacement should have more importance than is sometimes given them.

With all of the notes on the various components should go the comment which is one of the basic rules of sampling. The materials of construction of the sampling equipment that come into contact with the sample are to be compatible with the product being sampled. It is normally reasonably safe to use 316 stainless steel and Viton elastomeric components. One should look for these materials in selecting equipment, and ask questions of suppliers about material selections.

Transportation

The transportation of natural gas samples is a very important issue for both the companies that are involved and the individual personnel who are transporting the samples. Everyone involved in transporting sample cylinders and other sampling apparatus, both to and from sample collection locations, should be familiar with the rules and safety regulations.

As well as the safety issues, markings and forms that are to be filled out, other considerations should be addressed as well. 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

• Other company procedures that will assist in the success of a quality sample being delivered to the laboratory for an accurate analysis.

Procedure

1. Connect the cylinder to sample point.

2. Open the Isolation valve on pipeline. (operator Preferable)

3. Open sample point valve.

4. Open cylinder valves.

5. Purge to flash pipeline from main pipe to sample cylinder. Observed the temperature and wind direction and stand against wind direction.

6. Close the cylinder outlet valve

7. Close the inlet sample cylinder.

8. Disconnect the cylinder.