NGL Sampling Method

NGL Sampling Method, samples should be taken from points on the line or vessel which have a constant flow. Select a sampling point at least several in

NGL Sampling Method

Samples should be taken from points on the line or vessel which have a constant flow. Select a sampling point at least several inches below the gas-liquid contact or mid-point of a liquid full line. Typically, the probe should be sampling from the middle third  of the pipeline. To assure representative samples, a static mixer may be installed upstream of the sample point. Liquid samples should never be taken from the top of a line. Before taking any sample, vent some liquid from the sampling valve to clean out the valve. The sampling cylinder temperature should also be lower or the same as the sample temperature to prevent flashing.

1. General
NGL sampling with displacement requires that outage be taken so that possible temperature increases during transport or storage will not rupture the container or safety valves. Outage is creating a space within the cylinder by bleeding off a portion of the displacement fluid. A minimum outage of 10% of the volume of the sample cylinder is needed. Outage must be displacement fluid, not part of the sample.

2. Equipment
Sample cylinders for NGL sampling are of two types. More common is the type which can be used for either gas or NGLs. However, in some locations the sample cylinders are designed only for taking NGL samples.

3. Sample Identification
As with gas sample identification, all liquid samples must be identified. Below is a list of typical
data needed for NGL samples :

• Date and time sampled.
• Sample source.
• Location - well, lease, plant.
• Method of sampling.
• Amount of outage.
• Volume of sample.
• Name of individual taking sample.
• Sampling container identification (number).
• Displacement fluid.
• Pressure and temperature of separator or line. If sample is taken at 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.
• NGL gravity.
• Analyses to be performed.

4. Sampling Methods
The two most commonly used methods by Acompany for NGL sampling are the fluid displacement method and the floating piston method utilizing fluid.

Fluid Displacement Method

As mentioned in Section 2, sampling by the fluid displacement method requires that a sample cylinder filled with a displacement fluid (usually glycol, water, or brine) be attached to a sampling valve by tubing. The sample is taken by bleeding the fluid and replacing it with the sample. Unlike gas, however, outage is necessary to prevent cylinder damage or loss of sample through a relief valve or rupture disk.

Floating Piston Method

The floating piston method of liquid sampling is similar in concept to the fluid displacement method. The floating piston within the cylinder helps to maintain the integrity of the sample by separating and preventing the absorption of sample components within the displacement fluid. Again, if the displacement fluid is a liquid outage, is necessary to prevent damage to the sampling cylinder or loss of sample. If inert gas is the displacement fluid, no outage is required.
Fluid Displacement Method

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

1. Be sure the sampling cylinder is filled with displacement fluid to 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 and cylinder
4. Connect a gauge and manifold to the outlet valve (Valve 4)
5. Open the sampling valve (Valve 1)
6. Gently crack the fitting at the inlet valve (Valve 3). Also partially open the vent valve (Valve 2) and slowly purge for two or three minutes
7. Tighten the fitting on the inlet valve and close the vent valve
8. Open the inlet valve (Valve 3)
9. Be sure the vent valve (Valve 5) is closed. Open the outlet valve (Valve 4). Pressure on Gauge A and Gauge B should be the same
10. Slowly crack the vent valve (Valve 5) and bleed out displacement fluid. Do not allow the manifold pressure (Gauge B) to drop. Vent the displacement fluid into a graduated container
11. Bleed out 80% of the cylinder volume, e.g., for a 1000 cc cylinder, bleed out 800 cc of displacement fluid

Floating Piston Cylinder Method (Liquid Displacement)

1. Open the intet valve (Valve 3) of the cylinder. Fill the displacement end of the cylinder with liquid through the outlet valve (Valve 4). Close the outlet valve
2. Evacuate the sample end of the cylinder and close the inlet valve
3. Connect tubing and cylinder to sample source. Attach manifold and gauge to the cylinder
4. Open the sampling valve (Valve 1)
5. Gently crack the fitting at the inlet valve (Valve 3) and crack the vent valve (Valve 2). Purge the line for two or three minutes
6. Tighten fitting at the inlet valve, close the vent valve (Valve 2) and open the inlet valve (Valve 3)
7. Make sure the vent valve (Valve 5) is closed. Open the outlet valve (Valve 4). There should be no pressure drop at Gauge B
8. Slowly crack the vent valve (Valve 5) and bleed out displacement fluid. Do not allow the manifold pressure (Gauge B) to drop. Vent the displacement fluid into a graduated container
9. Bleed out 80% of the cylinder volume, e.g., for a 1000 cc cylinder, bleed out 800 cc of displacement fluid
10. Close the inlet valve (Valve 3)
11. Bleed out 10% more of the displacement fluid (outage) through the vent valve, e.g., 1000 cc cylinder, bleed out 100 cc of displacement fluid
12. Close the outlet valve (Valve 4) and the sampling valve (Valve 1)
13. Open the vent valves (Valves 2 and 5) to bleed the sample line and manifold
14. Remove the cylinder
15. Check for cylinder leaks and plug or cap the valves
16. Complete the sample tag and attach to the cylinder

To take duplicate samples, you would repeat steps a-p with another cylinder.

Floating Piston Cylinder Method (Inert Gas Displacement)

1. Fill the displacement end of the cylinder (Valve 4) with inert gas to approximately 10 psig above the sampling pressure. Close the outlet valve (Valve 4)
2. Evacuate the sample end of the cylinder and close inlet valve (Valve 3)
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. Loosen fitting at the inlet valve (Valve 3) and slightly open vent valve (Valve 2) to slowly purge the line. Do not allow pressure to drop below line pressure. Tighten the fit-ting and close the vent valve
6. Be sure the vent valve is closed. Check pressure (Gauge B) of inert gas by fully opening the outlet valve (Valve 4). Adjust pressure (Gauge B) to equal sample line pressure (Gauge A). Bleed (Valve 5) if necessary
7. Open the inlet valve (Valve 3). No pressure drop should occur on the inert gas side (Gauge B)
8. Slowly crack the vent valve (Valve 5). Do not allow a pressure drop (Gauge A) on the tubing
9. Bleed out 80% of the cylinder volume
10. Close all valves

Incremental Sampling Method

Sometimes incremental sampling is necessary to determine product composition over an extended period. On some sales lines a continuous sampler operates for 30 days. Incremental sampling is usually done in 2, 4, 6 or 8 hour increments during the test and sampling period. Incremental samples are several spot samples taken into the same container.

1. Evacuate the 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 valves (Valve 3) are 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 (Valve 5), outlet valve (Valve 4), and inlet valve (Valve 3) on cylinder 1
8. For the duplicate sample, repeat steps f and g for cylinder 2
9. After the last increment, close the sampling valve (Valve 1). Bleed off tubing pressure
10. Remove the cylinders
11. Bleed out 10% of volume for outage. Do not bleed out sample liquid
12. Check the valves for leaks and plug or cap the valves
13. Complete the sample tags and attach to the cylinders

• [accordion]
• 1. Natural Gas and NGL Sampling
• 
• 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