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Obtaining Liquid Samples by Water Displacement Procedure (GPA-2174)

          Alternative Method for Obtaining Liquid Hydrocarbons by Piston Cylinder



SCOPE: To obtain samples of Natural Gas Liquids by using the alternative sampling method to GPA-2174, Obtaining Liquid Hydrocarbons by Piston Cylinder Method.  This guide should serve to assist those who need to obtain Liquid Hydrocarbon samples, using Water as the Piston to maintain compression of Volatile Liquid Hydrocarbons.  This procedure can be used on Light NGL (Propane, Butanes), as well as heavier Hydrocarbons such as Gasoline, Condensates and BG Mixes.




1. Always review the sampling point prior to obtaining the sample. Check to make sure you have adequate flow of product at the sampling point. Also check to assure valves are positioned at the proper point on the sampling line (ie, at bottom or lower side of pipeline, or sufficient sampling point on vessels). Make sure valves are in good operating order before attaching sample manifold. Make notations on sample tag if anything is determined that could affect the results of the testing (ie, leaking valves, bad sample point location, etc.).

2. Remove end caps from cylinder and put Teflon tape on end valves of cylinder. Also put Teflon tape on end of manifold which will be connected to sample source.

3. Attach sampling manifold to sample source and place cylinder on end of manifold.

4. Open valve to sample source and purge product through manifold. This is done by rotating product from sample source through hose relief valve. Purge sufficiently for 30 seconds, or until all air is removed from hose.

5. Close relief valve on manifold. With cylinder held in vertical position (up and down), open top valve of cylinder to full line pressure from sample source.

6. With sample entering through top of cylinder, slowly open bottom cylinder valve to allow water in cylinder to be discharged. The water from the cylinder should be removed at a rate of about 100 cc per minute (slightly faster than a drip). Water should be displaced into a volumetric cylinder so that 80% outage can be determined (if you are using a 300 cc cylinder, remove 240 cc of Water from cylinder. If you are using a 500 cc cylinder, remove 400 cc of Water).

7. When 80% of Water is displaced from cylinder, close bottom valve. Then, close top valve of cylinder (It is important to keep full line pressure on cylinder until both cylinder valves are closed. Do not close valve to sample source until you have closed both cylinder valves.)

8. Close main sample source valve and blow down manifold through relief valve. Make sure you do not have any pressure on manifold before you disconnect cylinder.

9. Disconnect cylinder from manifold. With cylinder held in upright (vertical) position, slowly open bottom valve and remove approximately 30 milliliters (10%) of the remaining Water in cylinder. This will allow for vapor expansion during transit of sample cylinder. remove all of the Water from the cylinder. It is critical to maintain full line compression and pressure. When the cylinder is received at the lab, it will be recompressed to 1000 psig with Water, so that any expansion of vapors will be recompressed back into liquids.

10. After removing your final outage of Water from the cylinder, place end caps back on to cylinder and secure. Do not over tighten end caps.

11. Fill out all sample tag information and attach sample tag to cylinder. It is critical to note sample point pressure, temperature, location of sample point, plant information, state and county info. Also, make notations at bottom of sample tag regarding any Hydrogen Sulfide measured during sampling, or any other notes relative to the integrity of the sample.

12. Place cylinders into pelican case and ship or transport back to lab.

     This procedure is to be used in place of Piston cylinders. If the sample is taken properly, there shouldn’t be any difference in the analysis, when comparing the Piston cylinder method to the Water Displacement Method.


     Contact our laboratory at (307) 235-4590 if you have any questions regarding this sampling procedure, or if you encounter any problems during sampling.



                                                                                       James A. Kane, President

                                                                                       American Mobile Research, Inc.

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