Page 268 - Pipeline Rules of Thumb Handbook

Gas—General 255

Figure 2. Example of on-line sampler connected to electronic flow computers.

repeatability values of ±0.5Btu at 1,000Btu/cf. While the pre-
cision of a GC makes it well-suited for custody transfer appli-
cations, the 6 to 10 minute analysis time makes it difficult to
use for burner feed forward control. Generally, the on-line
analyzers do not require much environmental protection at
all. Most require protection from rain.

How chromatographs work
The way a modern microprocessor controlled on-line chro-
matograph works has been thoroughly discussed in previous
papers. 13,14,15 However, it would be helpful to review briefly
just how a GC works. Again, except for the sample condi-
tioning system, this discussion generally applies to lab GCs
as well as on-line GCs. For purposes of this discussion, the
on-line GC has been divided into four sections. These are:

Figure 3. Process sampler continually purges gas moving • Sample conditioning system
through a station or pipeline. • Gas chromatograph oven
• Chromatograph controller
• Input/output chassis.
electronic flow computer. This, of course, allows actual billing
on an MMBtu custody transfer basis. The sequence also is the route the sample takes from the
Gas chromatographs—lab or on-line—using the comput- pipeline to the printer. This makes it a logical flow for dis-
ing power of today’s modern microprocessor are capable of cussion as well.