Generac Power Systems Stationary Emergency Generator, Installation Manual

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1.9 CALCULATING VOLUMETRIC
FLOW RATE AND PIPE SIZE
If the Owner's Manual identifies the fuel consumption requirements
by a volumetric flow rate (units of ft
3
/hr), skip the next section and
go on to the section, Calculating Pipe Size. If the Fuel Consumption
requirements are given in units of lb/hr, follow the next section,
Calculating Volumetric Flow Rate.
1.9.1 CALCULATING VOLUMETRIC FLOW RATE
Since the fuel consumption requirements (from the owner's
manual) are given as a mass flow (units of lb/hr), the volumetric
flow rate (ft
3
/hr) must be identified before the appropriate pipe
size can be selected. The relationship between the mass flow rate
(lb/hr), density (lb/ft
3
), and volumetric flow rate (ft
3
/hr) can be
described by Equation 1. According to Equation 1, the volumetric
flow rate can be calculated by dividing the mass flow rate by the
density. You may need to contact your fuel vendor to identify the
density or specific gravity for your fuel. If your vendor provides
the specific gravity (relative to air) Table 1 can be used to identify
the appropriate density. Since the density of natural gas and LP is
temperature dependant, the maximum temperature of the fuel (at
the fuel shutoff/regulator assembly) should be measured so the
fuel consumption requirements can be more accurately identified
for the generator. A sample calculation (Example 1) is included in
this section.
Equation 1:

=
ṁ
—
p
•
Where:
ṁ
= Mass Flow Rate (lb/hr)

= Volumetric Flow Rate (ft 3 /hr)
p = Density from Table 1 -
(T-60)
—
C
•
C = 11111 (for Natural Gas) or 3333 (for LP)
T = Temperature of the fuel into the fuel shut-off/regulator
Example 1: Determine the volumetric flow rate for a generator that
requires 91.34 lb/hr of natural gas at 100% load.
Solution:
Specific gravity according to the local vendor: 0.5
Density of Natural Gas from Table 1: 0.0383 lb/ft
3
Maximum Temperature of the fuel going into the generator: 90°F
C=11111 (for Natural Gas) from Equation 1

=
ṁ
—
p
and
ṁ
= 91.34 lb/hr
p = 0.0383 -
(90-60)
—
11111
p = 0.0356 lb/ft
3
•
Thus:

=
91.34
—
0.0356
•
And:

= 2566 ft
3
/hr
•
1.9.2 CALCULATING PIPE SIZE
Now that the volumetric flow rate has been identified, the mini-
mum pipe size can be determined by using Table 2. This table is
based on a specific gravity of 1.00 (specific gravity of air). For that
reason, a correction is required when the fuel used has a different
specific gravity. The fuel’s specific gravity can be obtained from
the fuel supplier. The table is also based on a pressure drop of
0.3, which allows for a nominal amount of restrictions from bends,
fittings, etc. Example 2 illustrates how to calculate the pipe size for
the generator.
Example 2: Determine the iron pipe size for a generator that
requires 2566 ft
3
/hr of Natural Gas. The unit is located 75 feet
from the fuel source.
Solution:
Specific gravity of natural gas according to the local vendor: 0.5
Multiplier for the given specific gravity from Table 1: 1.1
According to Table 2, a 2-1/2 inch pipe will deliver 1750 ft^3/hr
of air if it is located 75 feet from the fuel source. To determine the
volumetric flow rate of natural gas, multiply the given flow rate by
the multiplier (from Table 1).
Natural gas flow rate
= air flow rate (ft
3
/hr) * Multiplier
= 1750 ft
3
/hr * 1.10
Natural gas flow rate = 1925 ft
3
/hr
Since the flow rate through a 2-1/2 inch iron pipe is less than the
flow rate required by the generator (2430 ft
3
/hr), we must evaluate
the next larger pipe (3 inches) by the same method.
Natural gas flow rate
= air flow rate (ft 3 /hr) * Multiplier
= 3000 ft 3 /hr * 1.10
Natural gas flow rate = 3300 ft
3
/hr
A 3 inch pipe is required at the given distance of 75 feet. Pressure
drop does not have to be considered unless an unusual number
of fittings, bends or other restrictions are used. In such unusual
cases, the fuel supplier will usually specify which multiplier is
applicable.
NOTE:
The installed piping system shall be capable of providing the
Fuel Consumption requirements as identified in the specifica-
tions section of the Owners Manual.
10
General Information