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Best practices.

For All Measurement Techniques--The general methods required to produce good accuracy begin with proper sampling. Proper sampling begins right at the sample tap. The sample should be drawn upwards from a region sufficiently away from the inner walls and five diameters downstream of any components, elbows and valves, which might modify the flow profile within the pipeline. This sample must be drawn off through heat traced tubing from the point of extraction through to the analyzer. This is a critical issue since all surfaces contacting the sample gas must be maintained at a temperature higher than any dew point or the accuracy will be compromised. Fast or speed loops should be used for maximum speed of response. Sample filtration must remove all particulates and liquid aerosols. This can sometimes be done as part of the sample extraction probe. However, any required pressure reduction should be taken immediately before delivery to the measurement section of the analyzer.

Best Practices for the Manual Visual Analysis Method--In addition to the general best practices, this method requires a well-trained operator and patience. The optical device must be clean before starting any measurements. The sample pressure should be at the approximate cricondentherm of the specific gas or the contract pressure. The sample should be allowed to bleed through the device per the ASTM standard D1142. Chill the mirror down gradually at a rate no greater than 1[degrees]F (0.6[degrees]C) per minute, per the recommendations of ASTM Standard D1142 (ASTM, 1995)5 until a visible condensate forms on the optical surface. Once this image is identified as the HCDP, the thermometer should read the HCDP temperature. The mirror temperature should then be allowed to elevate slightly and then be cooled again to "home in" on the actual reading. These readings should be repeated a minimum of three times with reasonable agreement to qualify as being accurate.


Best Practices for the Gas Chromatograph (GC) Analysis with EOS Method--GC best practices include using a [C.sub.9]+ GC and then adding data to [C.sub.12] from periodic laboratory analysis to improve accuracy of the EOS calculations. These results should periodically be compared to actual manual visual measurements to further enhance predictability. Using multiple EOS may also provide data comparison review over time that will determine the historical significance of one formula over another for a specific field or supplier. Keep in mind that field GC installations may not comply with all of the above general best practices and may produce less accurate results. GC samples are analyzed at very low pressures compared to pipeline pressures and are predicting values by measurements at conditions far different from those of the actual pipeline.

Best Practices for the method using Automatic Dew Point Analyzers

* Reliable Detection Method--A reliable detector is a given for all instruments. Rough or etched surfaces will be able to discriminate the HCDP because the condensate will make the optical surface more reflective and the image easier to detect.

* Close Proximity to Pipeline Sample Point--Automatic units should be able to be mounted near the sample tap with internal heaters and insulated housings. Using a sample already piped to an instrument house may be convenient, but the resulting delay in the update may cause serious lag in reaction time for control purposes. Since each manufacturer has different operating temperature specifications, environmental conditions often dictate this choice.

* Trap the Sample during the Measurement--A sample that is allowed to flow continuously creates an abnormal build up of the heavier hydrocarbons on the optical surface. Blocking in the sample during the measurement cycle will produce more accurate readings.

* Controlling Measurement Pressure--The derivation of the word cricondentherm is critical condensation thermal curve--also called the "phase envelope." The cricondentherm is the point on this curve where pressure and temperature indicate that the maximum HCDP is to be found (diagram).

Many tariffs are written with this point as the measuring point for the maximum allowable HCDP in the gas. Tariffs written with the reference to the maximum HCDP at any pressure, are describing the same point. The cricondentherm pressure is not as critical as may be anticipated. Since the profile of this region of the curve is nearly vertical, a change of fifty to a hundred psi either way can be shown to produce very little change in the accuracy of the measurement. In the expanded graph example, a change of 100 psi results in influencing the HCDP a maximum of only 2[degrees]F. In contrast, missing just 1 ppmv of a [C.sub.10] component in the sample can change the HCDP by as much as 10[degrees]F? It is however, always good practice for the measurement to be performed at the contract pressure which is often the cricondentherm pressure.


* Heat the Optical Surface Between Measurements--Without sensor heating the total cycle time can be three times that of the heated one and result in less reliability of the measurement.

* Keep Internal Volumes Small--When the volume of sample in the measuring chamber is reduced, it will speed the measurement and allow faster purging of the measurement chamber.

* Frequent Sampling--Many of the above practices will allow automatic dew point analyzers to make more frequent measurements. Frequent measurement cycles provide for better response to changes in the gas conditions and allow control functions to be implemented in a more timely fashion.

* Capability for Harmonizing With Contract Data--Historically the working definitions of HCDP have been slightly modified and standards have been rewritten to incorporate them. If this trend continues and changes come into effect, it is essential to have the ability to adjust the analyzer to align with newly refined standards.
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Title Annotation:measurement techniques
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Jul 1, 2010
Previous Article:Hydrocarbon dew point is a critical consideration for pipeline operations.
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