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Summary of Calculations
 During the MPE event, a total flow rate measurement of the process stream is
taken on the discharge side of the vacuum extraction pump and before off-gas
treatment. This measurement is performed using an averaging pitot tube (Dwyer
DS‑300) attached to a digital differential pressure sensor. This measurement is
used to calculate the removal rates and the off-gas emission rates and is
reported in actual cubic feet per minute. A separate flow rate is calculated for
the extraction well field, as well as for any additional ambient air introduced
into the influent stream. To determine the volume of hydrocarbon removed during
the event, samples of the extracted vapors are collected from a sample port
located before the vapor stream enters the ThOx unit. A second sample port
located on the exhaust stack of the ThOx unit provides access for determining
the destruction efficiency of the combustion process.
Concentration measurements are taken using a TVA‑1000A flame ionization detector
(FID) calibrated to methane. This FID instrument has a dynamic range of 0 to
50,000 parts per million (PPM) as methane, and 0 to 100,000 PPM as hydrocarbon.
The concentration measurements are made before any additional bleed or quench
air is added to the extracted vapors; however, these undiluted measurements
exceed the dynamic range of the FID instrument. To accurately record the high
concentrations commonly observed during an MPE event, a calibrated 10:1 dilution
assembly is used to accurately dilute the sample. This dilution assembly, along
with the FID instrument, is calibrated before the start of each event.
To account for naturally occurring
methane present during a typical MPE
event, two concentration
measurements are taken. One
unfiltered sample measures the total
volatile organic compounds (VOCs) in
the subsurface. The other sample is
collected using an in-line activated
carbon filter, which adsorbs the
hydrocarbon compounds, leaving only
methane. This methane-only result is
then subtracted from the total VOC
measurement for use in the mass
hydrocarbon removal calculation.
However, as with any FID instrument,
the non-methane organic compound
results are recorded as parts per
million by volume (PPMv) as if the
concentrations were equivalent to
methane gas. A conversion from
methane to hydrocarbon, and from
volume to weight, is necessary to
calculate the accurate hydrocarbon
removal. By using the TVA‑1000's
factory-certified response ratio for
various hydrocarbons, the
measurements are converted to
equivalent hydrocarbon in milligrams
per liter (mg/L). For example, a
TVA‑1000 FID has an average response
ratio of 600 PPMv per mg/L for
unleaded gasoline and 200 PPMv per
mg/L for diesel. Following is a
summary of calculations.
Flow
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Q = 128.8 x K x D x SQRT((P x dP) / (T + 460) x
Ss)
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Where:
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Q =
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Flow expressed in standard cubic feet per minute
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K =
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Flow coefficient (provided by Dwyer Instruments, Inc.)
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D =
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Inside diameter of process line in inches
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SQRT =
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Square root
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P =
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Static line pressure
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dP =
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Differential pressure expressed in inches of water column (WC)
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T =
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Temperature in degrees Fahrenheit (plus 460 equals degrees Rankine)
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Ss =
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Specific gravity at 60 degrees Fahrenheit
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Conversion of Field Data (PPMv
to mg/m3)
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C = (PPMv / R) x (1000 L / 1 m3)
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Where:
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R
=
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TVA response ratio supplied in The Foxboro Monitor, Volume 3, Issue 1A
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(600 PPMv / (mg/L) for gasoline and 200 PPMv / (mg/L) for diesel)
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Hydrocarbon Loading Rate
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M = Q x C x c
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Where:
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M =
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Contaminant loading rate (lbs/hr)
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Q =
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Air flow rate
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C =
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Contaminant concentration (mg/m3)
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c =
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(1m3 / 35.31 ft ) x (1 lb / 454 x 103)
x (60 min / 1 hr) = 3.743 x
10-6
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Conversion of Pounds of
Hydrocarbon to Equivalent
Gallons
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Equivalent Gallons = Ss x c
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Where:
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Ss =
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Specific gravity (0.74 = gasoline, 0.84 = diesel fuel)
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c =
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8.34 lbs/gallon
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