Natural Gas Sampling Method

Natural Gas Sampling Method, Before any sampling lines are attached, all accumulated liquid and solids in the sampling point valve should be blown out

1. General
Samples should be collected at stabilized flow conditions. The temperature of the sampling cylinder should be equal to or greater than the gas temperature. Before any sampling lines are attached, all accumulated liquid and solids in the sampling point valve should be blown out by venting some gas through the valve.

2. Equipment
Stainless steel sample cylinders are used to minimize the problem of surface absorption of heavy components and to minimize the reaction of acid gases within the cylinder. These cylinders may be designed only for gas samples or may be constructed to be used for gas or NGL samples. The size of the container depends upon the amount of sample required for the laboratory tests.

The sample cylinders are connected to the flow lines in one of two ways. Some locations have a fitting permanently installed to which the sample cylinder is directly connected. At other locations the cylinder is connected to the flow line using a flexible tubing. Stainless steel tub-ing is preferred; a non-reactive metal tubing like copper or steel may be used.


Two-valve cylinders are used for gas samples. Cylinders, valves and tubing must have working pressures equal to or greater than the maximum possible pressure which can develop during sampling, storing, or transporting the sample.

When entrained liquids or suspended liquids are possible at a sample point, a liquid separator may be required between the sample point and the sample cylinder. The purpose of this device is to remove liquids which may otherwise cause a non-representative sample to be taken. To avoid sample contamination, the separator should be made of stainless steel.

3. Sample Identification
All samples must be identified and tagged. This information is needed before a sample can be handled in the laboratory. Sample tags should include certain basic information along with any special instructions or remarks which may help laboratory personnel in properly handling the sample and in evaluating the analysis results. Below is a list of typical data needed for gas samples :

1. Date and time sampled.
2. Sample source.
3. Location - well, lease, plant.
4. Method of sampling.
5. Name of individual taking sample.
6. Pressure and temperature of separator or line. If the sample is taken at a point remote from actual separation vessel and the temperature and/or pressure are different, both separator pressure and temperature, sampling point pressure and temperature should be given.
7. Sampling pressure - if taken at reduced pressure.
8. Gas gravity and H2S and C02 content (if tests were run in the field).
9. Sampling container identification (number).
10. Analyses to be performed.

Sampling into an evacuated cylinder is the most commonly used sampling method for gas and is applicable for dry and wet gases. There are also several other methods of sampling which are applicable under certain conditions. Company commonly uses three methods for natural gas sampling, as following:

I. Evacuated Cylinder Method

In the evacuated cylinder method, a sample cylinder is cleaned, dried, and evacuated. An evacuated Cylinder is basically a sample cylinder which has had the air removed from it through the use of a vacuum pump. The cylinder is attached directly to the sample point with a manifold. The sampling line valve and manifold valve are opened and the sample is taken.

1. Check the cylinder with a vacuum gauge to be sure that the valves have not leaked.
2. Open valve at sampling point and thoroughly vent the line.
3. Install sample line (manifold) and sample cylinder, leaving the connection to the inlet valve (Valve 3) loose.
4. Slowly purge the sample line with gas to displace air by partially opening the vent valve (Valve 2) and the sampling valve (Valve 1). Check for gas flowing out of the loose connection (Valve 3) and out of the vent valve (Valve 2).

5. Tighten the loose connection at the sample cylinder and then close the sampling valve (Valve 1).
6. Allow the sample line to vent to atmospheric pressure and then close vent valve (Valve 2).
7. Fully open sampling valve (Valve 1).
8. Slowly open the cylinder inlet valve (Valve 3). The cylinder can be filled to line pressure or any pressure below line pressure.
9. Close the cylinder inlet valve (Valve 3) and the sampling valve (Valve 1).
10. Open the vent valve (Valve 2) to bleed the sample line.
11. Remove sample cylinder.
12. Check for leaks, m. Cap or plug valves.
13. Complete sample tag and attach to sample cylinder.

A duplicate sample can be taken by repeating steps a-n or, by installing a manifold by which two or more sample cylinders can be filled simultaneously.

II. Purge Method

This method is acceptable only when gas is lean and there is little chance of condensation. Lean gases are gases which are made up of light hydrocarbon vapors. When using the purging method for wet gases, the sample container temperature must be equal to or greater than the source temperature. The source pressure must be greater than atmospheric.

1. Open valve at sampling point and thoroughly vent the line.

2. Install the sample’line (manifold) and sample cylinder. The sample cylinder should be in a vertical position.
3. Slowly purge the line and sample cylinder with gas. (Valves 2, 3, and 4 should be fully open with Valve 1 partially open.)
4. Close the extension line valve (Valve 4).
5. Allow pressure to rapidly build up to the line pressure.
6. Close the inlet valve (Valve 2) and slowly vent the sample cylinder through the extension valve (Valve 4).
7. Open inlet valve (Valve 2).
8. Repeat steps c-g according to the table below

III. Fluid Displacement Method

When using the fluid displacement method, the fluid being displaced should be warmer than the gas stream being sampled. Glycol, water, or brine can be used as displacement fluid.

1. Be sure the sampling cylinder is filled with displacement fluid with pressure less than or equal to the line pressure.
2. Open the valve at the sampling point and thoroughly vent the line.
3. Install the sample tubing (manifold) and sample cylinder.
4. Crack the fitting at the inlet valve (Valve 3) and partially open vent valve (Valve 2).
5. Open the sampling valve (Valve 1) and vent the tubing for one minute.
6. Close the sampling valve (Valve 1), the vent valve (Valve 2), and tighten the fitting at the inlet valve (Valve 3).
7. Attach gauge and tubing to outlet valve (Valve 4). Be sure the vent valve (Valve 5) is closed.
8. Open the sampling valve (Valve 1), the inlet valve (Valve 3) and outlet valve (Valve 4).
9. Slowly crack the vent valve (Valve 5) and bleed out displacement fluid. Do not drop the tubing pressure. The fluid may be vented into a graduated container. While all of the displacement fluid may be vented, none of the sample should be vented. Close the vent valve (Valve 5).
10. Close the inlet and outlet valves (Valves 3 and 4).
11. Open the vent valve (Valve 5) and bleed to zero psig.
12. Close the sampling valve (Valve 1).
13. Open the vent valve (Valve 2) and bleed the sampling tubing.
14. Remove the cylinder.
15. Check cylinder valves for leaks and plug or cap the valves.
16. Complete sample tag and attach to sample cylinder.

IV. Inert Gas Displacement Method

This procedure assumes that the materials of construction allow the monitoring of the piston movement during sample entry.

1. Evacuate sample end of the cylinder and close inlet valve (Valve 3).
2. Fill the displacement end of the cylinder (Valve 4) with inert gas to or above the sampling pressure. Close the outlet valve (Valve 4).
3. Connect tubing and sample cylinder to sample source. Attach tubing and gauge (Gauge B) to the outlet valve (Valve 4).
4. Be sure the vent valve (Valve 2) and the inlet valve (Valve 3) are closed. Open the sampling valve (Valve 1) and observe gauge (Gauge A) pressure.
5. Crack fitting at the inlet valve (Valve 3) and slightly open vent valve (Valve 2) to purge the line. Tighten the fitting and close the vent valve.
6. Check pressure (Gauge B) of inert gas side by opening the outlet valve (Valve 4). Adjust pressure on Gauge B to be equal to pressure in Gauge A. Bleed the inert gas through the vent valve (Valve 5) if necessary.
7. Open the outlet valve (Valve 4) with the vent valve (Valve 5) closed. Then slowly open the inlet valve (Valve 3). There should be no pressure drop at Gauge B.

V. Liquid Displacement Method

Liquid displacement fluids can be used to take gas samples. When using liquid displacement fluids, the exact volume of the sample is known regardless of construction of the floating piston cylinder. The floating piston cylinder may be completely or partially filled with sample. No outage is necessary since the sample is a gas. The sampling procedure would be the same as used in NGL Sampling, Floating Piston.

VI. Incremental Sampling Method

On some sales lines a continuous sampler is used and operates for 30 days. Sometimes it is necessary to know gas composition over a 24- 48 hour period. Incremental sampling is usually done in 2, 4, 6, or 8 hour increments during the 24-48 hour test and sampling period. As previously defined, incremental samples are several spot samples taken into the same container.

Incremental Sampling Method - Fluid Displacement
With this sampling method, duplicate samples should always be taken. This can be done by using a second sampling cylinder and an expanded manifold.

1. Evacuate sample end of both cylinders. Close the inlet valves of both.
2. Fill the displacement end of both cylinders with displacement fluid, using the outlet valve.
3. Connect the manifold and the cylinders to the sample source. Attach tubing and a gauge (Gauge B) to each cylinder.
4. Be sure the inlet valve (Valve 3) is closed on both cylinders. Open the vent valve (Valve 2) and crack the fitting at the inlet valve (Valve 3) of each cylinder. Purge the line by opening sampling valve (Valve 1).
5. Tighten the fittings and close the vent valve (Valve 2).
6. Open the inlet (Valve 3) and outlet (Valve 4) valves of cylinder 1. Crack vent valve (Valve 5) on cylinder 1. Bleed the desired volume of displacement fluid.
7. Close the vent valve, outlet valve and inlet valve on cylinder 1.

Incremental Sampling - Pressure Increments
With this sampling method, duplicate samples should always be taken. This can be done by using a second sampling cylinder and an expanded manifold.

1. Check the sample end of each cylinder to be sure it is evacuated.
2. Connect the manifold and cylinders to the sampling source. Only one manifold gauge is required.
3. Check line pressure to determine pressure increments. For example, if you wish a 24- hour test in 4-hour increments and the line pressure is 880 psia, then the pressure in-crement is 880 -r 6 or 147 Ibs./increment. There are six four-hour increments in 24 hours.
4. Crack the fittings at the inlet valves (Valve 2) of both cylinders. Open the sampling valve (Valve 1) to purge the manifold.
5. Bleed the manifold pressure to 0 psig (Gauge A) for the first increment and to the preceding pressure for all subsequent pressure increments.
6. Slowly open the inlet valve (Valve 2) of cylinder 1. Let gas into the cylinder to the desired pressure (Gauge A). Using the above example, the desired pressure for the first increment would be 132 psig, for the second, 279 psig, for the third, 426 psig, etc.
7. Close the inlet valve on cylinder 1.
8. Repeat steps f and g for cylinder 2.
9. Bleed pressure from all sample tubing. Remove the cylinders.
10. Check for leaks and plug or cap the valves.
11. Complete the sample tags and attach to the cylinders.

• [accordion]
• 1. Gas Sampling Procedure
• 
• 2. ISO 19230-2020 Gas analysis Sampling guidelines
• 3. GPA Standard 2166-05 Obtaining Natural Gas Samples
• 4. API MPMS 14.1Collecting and Handling of Natural Gas Samples
• 5. BS EN ISO 10715-2001 Natural gas Sampling guidelines
• 6. D5287_082015_Standard_Practice_Sampling of Gaseous Fuels
• 7. ASTM D4057 - 22 Standard Practice Manual Sampling of Petroleum