Constant Pressure Sample Containers

Thomas F. Welker
Welker Engineering Company
P.O. Box 138
Sugarland, TX 77487-0138

18th Annual Measurement Short Course
Acadiana Flow Measurement Society
April 29-30, 1997
Lafayette, LA

With the development of newer and more accurate measurement techniques, the demand for an improved product analysis is required. In order for this product analysis to be more representative, the collection and handling of the samples should be done with the utmost care.

Products are sold and pipelines are balanced on a component basis. For the component analysis, the chromatograph must have a representative sample to analyze. This representative sample will be collected, transported, mixed, and analyzed with the product in the liquid phase only.

Light liquid products are measured in the liquid phase and should be sampled the same way. Allowing the light liquid product to change phase from a liquid to a gas encourages the components to further fractionate and stratify. The sampling technique will have a direct bearing on the quality of sample itself, and contaminants in the product should be considered when handling the sample.

A sample container will have the following characteristics if it is to maintain the sample without allowing a phase change:

1. Pressure: The sample must be maintained at a pressure above the vapor pressure of the components in the product. If the pressure is not controlled, the sample will flash (liquid to a gas) and the composition will be changed.
2. Expansion: The sample container must allow for expansion of the product without phase change and without overpressuring the sample collection container.
3. Stratification: Product or daily stratification must be broken (mixed) at the end of a sample period. There is considerable controversy about stratification of hydrocarbons in streams and containers. The secret is to determine whether the light end contaminants, CO, natural gasoline or other components might rise to the top or sink to the bottom. If this is a possibility, an effective mixer must be employed to mix the products. The Vortex Mixer has been proven to be the most effective mixer.

Transferring: The technique used to transfer a sample from a stationary sample collection container in to a transportation container is important. Subsequently, the transfer of the sample from the transportation cylinder in to the analyzer would utilize the same. In order for this transfer to be effectively and easily done, the following steps are recommended:

1. The cylinder must be clean and checked for leaks.
2. The transportation container must be filled with precharge gas on the precharge side of the piston at a pressure higher than the pressure expected in the pipeline or storage vessel.
3. Connect the transport container as closejy as possible to the draw-off valve. Install a tee with a valve on the branch if a purge valve Is not available on your sample container.
4. Using the Vortex Mixer built into the cylinder, mix the sample three times to break any stratification that might occur.
5. Immediately purge the connecting piping by fill and empty method to clear air or other contaminants from the connection. If your cylinder has a purge valve, open it slowly and purge the piping using the purge valve.
6. Open the connecting valves fully.
7. Open the precharge valve slowly and bleed the precharge gas off until the cylinder begins to fill. Let the product push itself into the cylinder.
8. Fill the cylinder 80% full.
9. Disconnect. Check for leaks. Plug all valves.
10. Transport according to DOT specifications.

The same rules apply in the lab for transfer or injection of the sample from the cylinder in to the analyzer. The constant pressure sample container eliminates the vapor space in the sample container. This allows complete control of the phase of the products being handled. The constant pressure cylinder also eliminates the phase change or fractionation of products as the vapor/liquid volumes change as in standard sample containers. With vapor space in a container. regardless of the pressure applied to the products in that cylinder, fractionation will occur. This is supported by Henry's law of solubilities and the studies of phase equilibrium of multi-component systems. The phase distribution will become more pronounced as the liquid level is reduced and the vapor volume gets greater.

The constant pressure cylinder eliminates the problem of phase change completely. The Vortex Mixer, in the cylinder, further ensures the analysis will be more accurate because stratification is completely eliminated. A note of caution -- when water, natural gasolines or heavier components are part of the sample, special precautions must be observed. Mixing effectively may only be done with the Vortex type mixer for effective results. Other types of mixers should be tested and verified.

The use of the constant pressure sample container with the Vortex Mixer ensures the most accurate sample available to inject into a chromatograph or other type of analyzer.

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