Page 275 - Pipeline Rules of Thumb Handbook

262 Pipeline Rules of Thumb Handbook

From the Matthews and Hurd superheat table: 13,14

at 15psia
T 1 = 110 H 1 = 426.6
T 2 = 90 H 2 = 415.8

426 6 - 415 8
.
.
.
Then C p = = 054 Btu lb ∞F
110 - 90
The procedure given above is adequate for liquid heat
capacities when enthalpy values are taken from the table for
saturated methane. This works because the effect of pressure
on liquid heat capacity is essentially negligible.
Alternatively, instantaneous values of heat capacity may be
determined. 15

o -3
+
.
.
.
¥
C p = 0 213 1 11 10 T - 2 59
2
¥ 10 -7 T Btu lb F (6)
∞
Note carefully that Equation 6 yields the heat capacity at low
pressure, 1atm; at elevated pressures a correction may be
required. If Equation 5 above is used, no pressure correction
is necessary.
Heat capacity ratio

The ratio C p /C v = g, useful in process calculations (adapted
from reference 14), is for atmospheric pressure:

t°F g = C p /C v
-150 1.39
-100 1.36
- 50 1.33
0 1.31
50 1.28
100 1.27
150 1.25

Flammability characteristics
Since methane is a combustible gas, consideration of any
process involving its use or storage must include studies of the
associated fire hazard. Basic data are flammable limits and the
autoignition temperature. For methane in air these are 16,17 at
atmospheric pressure:

Figure 1. Cold box where liquefaction of natural gas is accom- a. Lower Flammable, Limit @ 25°C about 5% by vol
plished. b. Upper Flammable, Limit @ 25°C about 15% by vol
c. Autoignition Temperature 1,004°F