roleum gases are also usually extracted be-
fore the natural gas is put into the pipeline to prevent
condensation during transmission.
The specific gravity of natural gas varies from 0.55 to 1.0
and the heating value varies from 900 to 1100 BTU/ft
3
(33.9 to 41.5 mJ/m
3
). Natural gas is nominally rated at
1000 BTU/ft
3
(37.7 J/m
3
), manufactured gas is nominally
rated at 520 BTU/ft
3
(20 mJ/m
3
), and mixed gas is nomi-
nally rated at 800 BTU/ft
3
(30.1 mJ/m
3
). Liquefied petrole-
um gases (LPG) are nominally rated at 2500 BTU/ft
3
(94.1
mJ/m
3
). Natural gas is transmitted from the fields to the lo-
cal marketing and distribution systems at very high pres-
sures, usually in the range of 500 to 1000 psi (3447.4 to
6894.8 kPa). Local distribution systems are at much lower
pressures. The plumbing engineer should determine the
specific gravity, pressure, and heating value of the gas
from the utility company or LPG provider serving the proj-
ect area.
This chapter covers fuel-gas systems on consumers
premisesthat is, upstream and downstream from the gas
suppliers meter set assemblyand includes system de-
sign and appliance gas usage, gas train venting, ventila-
tion, and combustion air requirements. Since natural gas
is a depletable energy resource, the engineer should de-
sign for its efficient use. The direct utilization of natural
gas is preferable to the use of electrical energy when elec-
tricity is obtained from the combustion of gas or oil.
However, in many areas, the gas supplier and/or govern-
mental agencies may impose regulations that restrict the
use of natural gas. Refer to the chapter Energy
Conservation in Plumbing Systems in Data Book Volume
1 for information on appliance efficiencies and energy
conservation recommendations.
Design Considerations
The energy available in 1 cubic foot (cubic meter) of nat-
ural gas, at atmospheric pressure, is called the heating (or
caloric) value. The flow of gas, expressed in cubic feet per
hour (cfh) or cubic meters per hour (m
3
/h), in the distribu-
tion piping depends on the amount of gas being consumed
by the appliances. This quantity of gas depends on the re-
quirements of the appliances. For example, 33,200 BTU/h
(35 mJ/h) are required to raise the temperature of 40 gal
(151.4 L) of water from 40 to 140°F (4.4 to 60°C) in 1 hour.
This value is obtained as follows:
Q
= m
×
C
p
×
T
Equation 1
where
Q
= Energy required, BTU/h (J/h)
m
= Mass flow, gal/h (L/h)
C
p
= Specific heat of water, 1 BTU/°F (J/°C) T = Temperature rise, °F (°C)
Q
= (40 gal/h)(8.33 lb/gal)(1 BTU/lb-°F)(100°F) =
33,320 BTU/h
[Q = (151 L/h)(1 kg/L)(6.1 kJ/kg-°C)(38°C) = 35 MJ/h]
If the water heater in this case is 80% efficient, then
41,650 BTU/h (43.8 mJ/h) of gas will be needed at the ap-
pliances burner (33,320 BTU/h/.80). If natural gas with a
heating value of 1000 BTU/ft
3
(37.7 mJ/m
3
) serves the ap-
pliance, the piping system must supply 41.7 cfh (1.2 m
3
/h)
of gas to the appliance with adequate pressure to allow
proper burner operation. The formula for the flow rate of
gas is shown below:
Q = Output
Equation 2
(Eff x HV)
where
Q = Gas flow rate, cfh (m
3
/h)
Output = Appliances output, BTU/h (J/h)
Eff = Appliances efficiency, %
HV = Heating value of the fuel gas, BTU/ft
3
(J/m
3
)
The difference between the input and the output is the
heat lost in the burner, the heat exchanger, and the flue
gases. Water heating and space heating equipment is usu-
ally 75 to 85% efficient, and ratings are given for both in-
put and output. Cooking and laundry equipment is usually
75 to 85% efficient, and ratings are given for both input and
output. However, cooking and laundry equipment is
Fuel-Gas Piping Systems
32
Plumbing Systems & Design May/June 2003
Continuing Education
Reprinted from American Society of Plumbing Engineers Data Book: Vol. 2. Plumbing Systems, 2000, Chicago: American Society of
Plumbing Engineers. Chapter 7, Fuel-Gas Piping Systems (pp. 173174, 176178, 183185), Joseph J. Barbera, PE CIPE, John P.
Callahan, CIPE, Paul D. Finnerty, CIPE, Ronald W. Howie, CIPE, Robert L. Love, PE CIPE, Steven T. Mayer, CIPE CET, Jon G. Moore,
& Rand J. Refrigeri, PE, Contributors. © 2000, American Society of Plumbing Engineers. usually rated only by its input requirements. When the in-
put required for the appliance is known, Equation 2 is ex-
pressed as follows:
Q = Input
Equation 3
HV
where
Q = Gas flow rate, cfh (m
3
/h)
Input = Appliances input, BTU/h (J/h)
HV = Heating value of the fuel gas, BTU/ft
3
(J/m
3
)
The gas pressure in the piping system downstream of the
meter is usually 5 to 14 in. (127 to 355.6 mm) of water col-
umn (wc). Design practice limits the pressure losses in the
piping to 0.5 in. (12.7 mm) wc, or less than 10%, when 5 to
14 in. (127 to 355.6 mm) wc is available at the meter outlet.
However, local codes may dictate a more stringent pressure
drop maximum; these should be consulted before the system
is sized. Most appliances require approximately 5 in.
(127mm) wc; however, the designer must be aware that large
appliances, such as boilers, may require higher gas pressures
to operate properly. Where appliances require higher pres-
sures or where long distribution lines are involved, it may be
necessary to use higher pressures at the meter outlet to satis-
fy the appliance requirements or provide for greater pressure
losses in the piping system. If greater pressure at the meter
outlet can be attained, a greater pressure drop can be allowed
in the piping system. If the greater pressure drop design can
be used, a more economical piping system is possible.
Systems are often designed with meter outlet pressures of 3
to 5 psi (20.7 to 34.5 kPa) and with pressure regulators to re-
duce the pressure for appliances as required. The designer
has to allow for the venting of such regulators, often to the
atmosphere, when they are installed within buildings.
When bottled gas is used, the tank can have as much as
150 psi (1044.6 kPa) pressure, to be reduced to the burner
design pressure of 11 in. (279.4 mm) wc. The regulator is
normally located at the tank for this pressure reduction.
To size the gas piping for a distribution system, the de-
signer must determine the following items:
1. The appliance requirements, including the gas con-
sumption, pressure, and pipe size required at the ap-
pliance connection (total connected load). Is the ap-
pliance provided with a pressure regulator?
2. The piping layout, showing the length of (horizontal
and vertical) piping, number of fittings and valves,
and number of appliances.
3. The fuel gas to be supplied, where and by whom; also
the specific gravity and heating value of the fuel gas
and the pressure to be provided at the meter outlet.
4. The allowable pressure loss from the meter to the
appliances.
5. The diversity factorthe number of appliances oper-
ating at one time compared to the total number of
connected appliances. This should be provided by the
owner and/or user.
Standard engineering methods may be used to determine
pipe sizes for a system, or the acceptable capacity/pipe size
tables may be used when such tables are available for the
specific operating conditions of the system under consider-
ation. The diversity factor is an important item when deter-
mining the most practical pipe sizes to be used in occu-
pancies such as multiple-family dwellings. It is dependent
on the type and number of gas appliances being installed.
Refer to the pipe sizing section later in this chapter.
The most common material used for gas piping is black
steel; however, many other materials are utilized, including
copper, wrought iron, plastic, brass, and aluminum alloy.
The proper material to be used depends on the specific
installation conditions and local code limitations. Any con-
dition that could be detrimental to the integrity of the pip-
ing system must be avoided. Corrosion and physical dam-
age are the most obvious causes of pipe failure. The pip-
ing material itself and/or the provisions taken for the pro-
tection of the piping material must prevent the possibility
of pipe failure. Corrosion can occur because of electrolysis
or because a corrosive material is in contact with either the
exterior or the interior surface of the piping.
Coatings are commonly applied to buried metallic pipe
to prevent corrosion of the exterior surface. The gas sup-
plier should be contacted to determine if the gas contains
any corrosive material, such as moisture, hydrogen sulfide
(H
2
S), or carbon dioxide (CO
2
). Due to the grave conse-
quences of leakage in the gas piping system, the designer
must carefully consider the piping material to be used and
the means to protect the piping and protect against leaks.
Gas piping should be installed only in safe locations.
Buried piping should be installed deep enough to protect
the pipe from physical damage. When piping is installed in
concealed spaces, care should be taken so that, in the
event of gas leakage, gas will not accumulate in the con-
cealed space. The installation of gas piping in an unventi-
lated space under a building should be avoided. Such con-
ditions have resulted in disastrous explosions. A gas leak
anywhere along the length of a buried pipe can flow in the
annular space around the pipe and accumulate in a cavity
under the building. Ignition of this accumulated gas can re-
sult in an explosion. For this reason, it is best to try to lo-
cate the gas main above grade at the point of entrance into
the building. If this is not feasible, the main can be installed
in a ventilated sleeve (containment pipe). The designer
should car