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CI Solutions...
Gas Sub-Metering / Utility Auditing |
| In any
industry, finding a better way to monitor and control your operating
expenses is a sure way to improve your competitive position. The ability
to monitor the amount of Natural Gas entering your facility and to accurately
track it’s use throughout the individual areas of consumption, can
give you the information you need to adjust for peak usage and to
correctly assign costs when reviewing general operating expenses and
specific product profitability. |
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| Natural Gas is typically delivered to the end user at a predetermined
pressure, through relatively small distribution lines. Consumption is
measured in cubic feet, through a volumetric flow meter supplied by the
gas company and then converted to Therms
for billing purposes.
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|
But,
beyond this general custody transfer, how much do you really know about
your true overall consumption? And... How do you determine where you can
find cost savings? |
| Two
ways... 1) Sub-metering at the
point of entry gives you an accurate indication of overall consumption
and 2) Sub-Metering at selected points within your facility provides a
clearer picture of who, when and
how much gas is being consumed at specific locations and/or work areas.
This is the kind of information you need to analyze your ongoing
expenses and to then refine your overall usage. |
| Why
should I audit the Gas Company’s readings for overall consumption? |
|
| Natural
gas entering your facility is usually measured with a Positive Displacement
(PD) flowmeter supplied by the Gas Company and metered in units of
hundreds of cubic feet (CCF). |
| Keep in mind
that this is a volumetric measurement of
the gas and merely serves as one part of the formula used to compute your
consumption charges.
What’s important for you to
know is that although Natural Gas is measured in Cubic Feet, it’s sold
in units of energy referred to as: Therms |
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|
Q)aaWhat's a Therm?
A)aaA Unit of Energy consisting of 100,000
BTU's |
| To
understand the concept of "Therms", you'll need to know
a little about British
Thermal Units or BTU's for short. One (1) BTU is defined as the amount of energy
required to raise the temperature of one pound of water, one degree (1°) at
atmospheric pressure.
One Cubic Foot of natural gas has an average
heating capacity of 1,025 BTU's. This value differs slightly in different
locations due to variations in the gas mix, but for our purposes, we'll
stick with 1,025. At that rate, a therm would consist of 97.56 Cubic
Feet of Natural Gas or roughly 100 Cubic Feet when rounded up. (CCF...
See the tie?) |
Gas Billing -
(Finally, we get to the math...)
Your monthly bill is a product of
net consumption
in cubic feet, multiplied by a factor representing the BTU content of the gas
and then divided by 100,000. Because the pressure at which your gas is
delivered has a huge effect on it's density, this figure is adjusted one
more time using a Density Factor.
The |
| The
process starts at your meter where the gas company records a net
consumption of Natural Gas in Cubic Feet. Next, Cubic Feet are
converted to Therms using the following formula: |
| (Cubic Feet x
1,025) / 100,000= Therms
Example: Acme Cookie
Company's gas meter shows a net consumption of 1,500.000 Cubic
Feet of gas used last month to fire the cookie ovens.
(1,500,000 x 1,025) / 100,000 =
15,375.00 Therms
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Remember...
A cubic foot of gas in this example contains about 1,025 BTU's and a Therm is equal to
100,000 BTU's.
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|
To correct for pressure,
the following formula is used to create a factor to compensate for gas
density: |
| (Line Pressure
+ 14.7) / 14.7 = Density Factor
Example:
(2.5 + 14.7) / 14.7 = 1.170
|
Usage/Density
Factor:
In this
example, we're assuming a line pressure of 2.5 psig at the meter.
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|
Your bill would now be
figured as follows: |
| (Therm
Value($) x Therms Consumed) x Usage / Density Factor = Gas Charge |
| ($1.00
x 15,375.00) x 1.170 = $17,988.75 |
To keep the
math simple, we'll assume a Per Therm Value of $1.00 |
| But,
what happens if the line pressure drops to 1/4 PSIG and your bill is
still being based on 2.5 PSIG?
|
| Gas
enters your
meter at 1/4 PSIG, but the density factor on your bill is based on a
system pressure of 2.5 PSIG. Does it make a difference? Let's
check back with the Bean Counters at Acme Cookie Company, who are
already steamed over the $17,988.75 gas bill they received last
month. That bill was based on an incoming pressure 2.5
PSIG, but in reality the pressure had dropped to 0.25 PSIG. Let's
do the math one more time... |
|
15,375.00 Therms @ 0.25
PSI |
15,375.00 Therms @ 2.5
PSI |
|
(0.25 + 14.7) / 14.7 =
1.01 (Density
Factor) |
(2.5 + 14.7) / 14.7 = 1.170 |
| ($1.00
x 15,375.00) x 1.01 = $15,528.75 |
($1.00
x 15,375.00) x 1.170 = $17,988.75 |
|
That's
a difference of almost $2,460.00-representing a billing error of nearly 15%. |
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Can this type of billing
error really happen? |
 |
|
You bet it can and it happens every day. Pressure
variations are inevitable in gas lines. Adjustments
are made for new construction, lines are shut down for repairs... The list
of reasons is endless! The Gas Companies do their best to correct for
these inconsistencies, but without a more accurate way to measure the flow
of gas into your facility, you're at their mercy at the end of each
billing period. |
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How can you be sure
you're only paying for your actual consumption? The answer is simpler than you
might think... |
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Gas measurement by mass rather than volume.
Thermal Mass
Flowmeters from EPI are temperature
compensated and insensitive to changes in system pressure, so no
additional calculations are required to obtain an accurate measurement of
your gas consumption. You
just can’t do that with a volumetric flowmeter! |
|
Thermal
Mass Flow
Technology... |
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Thermal mass
flowmeters are solid state instruments that use the principle of convective
heat transfer to directly measure mass flow. EPI's sensors consist of
two resistance temperature detectors (RTD's). A forced null,
Wheatstone bridge which preferentially heats one RTD; while the other acts as the
temperature reference. The process gas flow dissipates heat from the
first RTD, causing an increase in the power required to maintain a balance
between the RTD's. This increase is directly proportional to the molecular rate
of flow of the gas.
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EPI produces
a wide variety of thermal mass flowmeters for measurement and control of
gas.
EPI
Thermal Mass Flowmeters offer:
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Direct
measurement of mass flow or mass velocity without requiring temperature
or pressure corrections.
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Exceptional
Turndown: 100:1 or more.
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Integral
or Remote Electronics
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Both
4-20mA & 0-5Vdc output signals
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For
additional information on EPI Thermal Mass Flowmeters for Sub-Metering / Utility
Auditing, call the flow-specialists
at Cross Instrumentation!
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