Page 267 - Pipeline Rules of Thumb Handbook

254 Pipeline Rules of Thumb Handbook

content and are useful in process control applications such as With recent developments, these devices have been tied to
boiler feed forward controls. electronic flow computers to give flow-weighted composite
There are two classes of indirect Btu or energy measure- samples rather than time-weighted ones. An example of such
ment, both chromatographic devices, relying on the separa- an installation is in Fig. 2.
tion, identification and quantification of all the components Flow weighting generally gives even better results than
9
that make up the natural gas mixture. The two classes are: time-weighted samples if there are variations in the flowrate
laboratory chromatographs and on-line or process chro- at the sampled station. Great care should be exercised when
matographs. Through the years a number of different people analyzing a sample cylinder of natural gas obtained by any of
have compared calorimetry to chromatography with varied the above methods. For instance, the sample bottle should be
results. 10,11 heated to 120°F for at least two hours before the analysis is
The laboratory chromatograph (Fig. 1) is just that. It is made to ensure that all the heavier components are analyzed.
located in a laboratory. Somehow a gas sample must be A complete discussion of the problems associated with the
obtained and transported in a cylinder to the lab to be recommendations for bottle sample analysis have been
analyzed. published previously. 12,9
How the sample is obtained is the main difference between The process or on-line gas chromatograph (GC) (Fig. 3) is
lab and on-line gas chromatographs (GCs). The natural gas designed to continually purge a small amount of the gas
samples analyzed in a lab today usually are called spot or grab flowing through the station or pipeline to provide a repre-
samples. A cleaned vessel (cylinder), sometimes N 2 filled, is sentative sample at the time the GC is ready to make a new
purged and filled at least three times for reliable sampling analysis. Typically, this is once every six minutes.
site. The pressured-up bottle is carried to a lab to be analyzed. The main advantage of continuous sampling and analysis is
More and more samples today are being collected using a the documentation of the duration and the amount of devia-
continuous sampler. This device takes a small aliquot of gas tion of the energy content that has occurred during the mea-
at a predetermined time interval so the bottle is filled at the surement period. It is possible to assign a heating value to a
end of the sampling cycle required by the gas contract. specific volume of gas if the on-line GC is tied directly to an

Figure 1. Laboratory chromatograph used to measure gas samples.