Page 270 - Pipeline Rules of Thumb Handbook

Gas—General 257

Chromatograph controller How Btu is calculated

All the timing functions such as sample inject, column The methods for calculation of the natural gas Btu from
reverse, auto-calibration and log results must be performed analysis data are well documented. Calculations have been
by the microprocessor controller board. In addition, this published previously. 2,3,9,16,18 Nevertheless, it is good to review
device must detect each peak, open and then close integra- these methods. Although they look complicated, they really
tion gates, identify the peak, and assign the correct response are straightforward. Below is the general formula for the Btu
factor. The controller must then calculate the mole fraction calculation:
of each component. Since these peaks are quite arbitrary in
size due to equipment variability setup and environment,
high-quality standards should be used to calibrate these Btu cf = X * C + X * C + X * C 3 L X * C 11
2
3
1
1
11
2
instruments. This calibration assures accurate determination
of mole fraction possible. All Btu determination methods
require high quality standard samples. Where:
On a Btu GC, the peaks also must be normalized to com-
pensate for barometric pressure changes (which cause sample X 1 = Mole fraction of C 1 or methane
inject size variations). This allows the calculation of Btu to
repeatability of ±0.5Btu at 1,000Btu/cf. This processor must X 2 = Mole fraction of C 2 or ethane
also calculate the specific gravity, compressibility, condens-
able liquids, wobbie index, as well as the Btu. Calculation X 3 = Mole fraction of C 3 or propane
results must be made available to the outside world. This is " "
the job of the input/output chassis.
" "
" "
Input/output chassis
X 11 = Mole fraction of C 6 + hexanes and heavier
The device houses communications option cards such as
printer interface cards, analog output cards and computer and:
interface cards. These usually can be configured based on
the customer’s needs. However, caution is advised as specific C 1 = Molar heating value of C 1 , or methane (1010Btu/cf)
computer protocols may or may not be supported by
one vendor or another. One function for this input/output C 2 = Molar heating value of C 2 , or ethane (1769Btu/cf)
chassis, for example, is to hold an RS232 output card. This
card would log results to an analyzer interface unit (AIU), " "
which would then be connected to up to 30 flow computers " "
(Fig. 7). " "

C 11 = Molar heating value of C 6 + or hexanes and heavier
(4943Btu/cf)

The calculation of specific gravity and condensable liquids
is handled in the same manner. For example:

Specific Gravity = X 1* G 1 + X 2* G 2 + ...X 11* G 11

Where: X 1 , X 2 , is the mole fraction of each natural gas com-
ponent and: G 1 , G 2 , is the molar gravity contribution of each
respective component.
This calculation yields gross or total heating value and is
not compensated for compressibility Z. A short method for
calculation of Z is:

Figure 7. Output card logs results in an analyzer interface unit
.
connected to at least 30 computers. Z = 1 00369. - (0 010 * S G) + (0 007. . . * X inert )