Page 266 - Pipeline Rules of Thumb Handbook

Gas—General 253

CONSIDERATIONS FOR SELECTING ENERGY MEASUREMENT EQUIPMENT

Keeping Track of Gas Heating Value is Important Phase of Today’s Market
Paul E. Kizer, Applied Automation, Inc., Houston, Texas

With natural gas energy determination becoming an inte- General methods
gral part of the transmission and distribution business in
today’s market, companies are placing increased emphasis on First, consider the direct methods of energy determination,
choosing the correct measuring device. Most current con- since this is the oldest method in use today. All direct methods
tracts have a Btu specification and many others use MMBtu involve the complete oxidation (burning) of gas to determine
instead of gas volume. its energy content. There are two basic classes of direct
Here is what happens at the burner tip. The following is measurement devices, calorimeter and thermtitrator.
the exothermic reaction of methane and oxygen (80 to 95% Early on, the term representing 100,000Btus was one
of natural gas): therm of energy. One dekatherm (10 therms) is equal to
21
+
CH 4 + 20 2 Æ CO 2 + 2 H O Heat ( 1010Btu cf ) 1 1,000,000Btu or 1MMBtu. Perhaps this is where the name
thermtitrator came from. However, as discussed earlier,
2
2
for the case of ethane (up to 3% of natural gas): MMBtu has supplanted the therm unit of measurement for
now.
1
+
2C H + 70 2 Æ 4CO + 6H O Heat (1 769Btu cf ) The calorimeter was adapted from a laboratory physical
2
6
2
2
chemistry method called “bomb calorimetry.” These were lab-
and for the case of propane (usually up to 0.5%, except where oratory instruments which yield precise results when used
propane/air peak shaving is used): under controlled conditions. However, each determination
(
+
C H 8 + 50 2 Æ 3 CO 2 + 4 H O Heat 2516Btu cf ) 1 required 4 to 8 hours to complete. This is fine for a research
3
2
project, but in a production setting it is easy to see how gas
Since natural gas is a mixture of these three hydrocarbons sample cylinders would start piling up. The American
3
plus small amounts of other compounds, these three chemi- calorimeter provided laboratory type results in less time, but
cal reactions account for 90 to 99% of the heat generated the lab conditions had to be tightly controlled with continu-
when the gas is burned. However, one mole (a mole is a ally manned equipment. For on-line energy content mea-
23
standard number of molecules (6.023 ¥ 10 , Avogadro’s surement, the Cutler-Hammer Recording Calorimeter stood
17
number)) of propane will generate over twice the heat of alone for decades. 3,4
8
3
one mole of methane (CH 4 = 1010Btu/cf vrs C H = 2516 While this device still requires rigid environmental condi-
Btu/cf at 60°F and 414.696psia). A small error in the heavier tions, it runs unattended for long periods of time (weeks).
or longer chain hydrocarbon components of natural gas leads Also it provides a recorder output, which allows the correla-
to a larger error in the Btu calculation. Consequently, a small tion of the heating value with the volume of gas. The record-
shift in the composition of any component can affect the ing calorimeter’s stated precision is ±5Btu out of 1,000Btus,
heating value of a cubic foot of carefully measured gas. although many operators can do better.
The term MMBtu first came into wide use after the 1978 With the introduction of solid-state electronics, micro
Natural Gas Policy Act changed the method of custody trans- calorimetry devices have been tried, but not with much com-
fer from Mcf to MMBtu. This method was upheld in a court mercial success. Hart Scientific, Provo, Utah, has used a cat-
case involving FERC Order 356 (Jan. 1984). 2 alyst bed to perform a controlled isothermal oxidation on a
How much is it worth to keep track of the Btu? Postulate very precise volume of gas. Initial results seem to show ±0.5
a ±5% error between doing a lab determination of the heating Btu at 1,000Btu precision. 5
value on a spot sample of gas and on-line (nearly continuous) The thermtitrator devices utilize a more elegant chemical
monitor of heating value, which results in a ±50Btu differ- method, stoichiometry—that is air to gas ratios. One instru-
ence at a 1,000Btu/cf level. A thousand Btu/cf is fairly average ment, the PMI thermtitrator, uses the temperature difference
for natural gas. between two flames to maintain the gas flow ratios at stoi-
On a station that has 50MMcf/d at $2.50 per Mcf or chiometric conditions. 6,7
MMBtu, this is $125,000 worth of gas per day. Five percent Accuracy is around ±1Btu at 1,000Btus and compares
8
of this is $6,250 per day. If a process chromatograph, $50,000 favorably with the recording calorimeter. The other stoi-
installed cost, is used to determine the energy content, a chiometric device is the Honeywell HVT. It uses a zirconium
payout of less than 10 days is obtained on a 50MMcf/d station. oxide oxygen sensor to determine stoichiometric conditions
This explains why most major interconnects have Btu mea- and automatically adjusts air/gas ratios to minimize excess O.
surement on-line today. Both devices yield rapid reaction to step changes in Btu