BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
INTRODUCTION
There are a number of very important
reasons why everyone connected to the natural
gas industry should understand the Basic Gas Laws. (1) Designing, building, and maintaining
safe gas distribution
and transmission systems can only be done by understanding and applying
the laws of physics governing the behavior of natural gas. (2) Effective job performance
in the
natural gas industry requires an understanding of the technical language we encounter every
day, a substantial portion of which is directly and indirectly related to the behavior of natural
gas. (3) Safety is perhaps the most important driving force in our industry and many of the
safety issues employees face relate to understanding how gases behave. (4) Many of the
service issues arising from interaction with gas customers involve questions of measurement
accuracy, pressure, and terminology
that are not at all understood by end users but are related
to the application of Basic Gas Laws.
This class is not intended to cover the subject matter in depth but instead will focus on
(3) Basic Gas Laws that everyone should conceptually understand.
BOYLE'S LA W - (PRESSUREJ
Of the (3) Basic Gas Laws discussed in this presentation,
Boyle's Law describing the
effect of pressure on gas is the most commonly applied and far-reaching in its impact in our
industry. Therefore it will get the most attention
engineers or mathematically
in this presentation.
For those who are not
inclined, a very simple example of Boyle's Law at work in the real
world is the use of scuba tanks by divers that need to work (and breathe) underwater.
A scuba
tank's internal volume capacity is relatively small (~ypically less than % cubic feet) so how is it
that a diver can stay underwater for so long breathing from this tank? The answer is that the
tank is pressurized to between 3000 and 4000 psig. While the tank volume remains constant,
the compressibility
of air means that we can "cram" a lot of air into the tank by applying lots of
pressure. Boyle's Law is a way to measure the change in volume that occurs with a change in
pressure.
"At a constant temperature.
pressure is applied."
Mathematically,
of an ideal gas decreases when an increase in
the volume
(Wikipedia)
this theorem can be expressed as follows:
Pl x Vl
=
1
P2 x V2
BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
In plain language, Boyle's Law states that if we take a given volume of gas at a constant
temperature
and increase the pressure applied against that volume, the molecules of gas will
move closer together and the gas will take up less space. In gas distribution
and transmission
systems, the physical space within the piping systems is fixed and constant. Therefore, when
we increase pressure on the system, we are increasing the density of gas traveling through the
piping network.
One of the terms regularly used in the gas industry is "pressure factor".
This term is
expressed numerically and represents the relative increase in standardized volume that a
system can deliver at a given higher pressure. Before citing an example of how pressure factors
are calculated and what they mean, it is important to note that there are several kinds of
pressure to understand.
The first is atmospheric pressure (atm.), which exists all around us.
This is the pressure the weatherman talks about on the news and it is affected by both local
weather conditions and altitude.
For simplicity purposes, most companies use an average
annual atmospheric pressure value calculated for their service area; here in New England
14.73psia is often used for rough calculation purposes. However, by industry convention most
manufacturers
use an atmospheric pressure value of 14.4psia when calculating flow and
volume capacities of their products used under varying line pressure scenarios.
A second type of pressure is gauge pressure, which is what most people in the industry
think ofwhen
we discuss the pressure within a gas piping system. This "line" pressure is usually
expressed in pounds per square inch gauge (psig). However, when talking about old, lowpressure gas systems (usually cast iron), you will also hear the term "inches of water column".
One pound of pressure is approximately
equal to 28 inches (27.72 to be precise) of water
column (w.c.) pressure. Therefore, when talking about low pressure delivery systems you may
hear people use such terms as .25PSIG or 7"w.c. to describe their operating conditions.
A third type of pressure is absolute pressure, expressed as pounds per square inch
absolute (psia). This pressure is the sum of both the atmospheric pressure and the gauge
pressure in the pipeline. When discussing Boyle's Law and using it to calculate pressure factors,
it is important to remember that this law uses absolute units of pressure, not gauge units, as its
reference point.
A fourth type of pressure is called base pressure, and it is an arbitrary pressure value
used to establish tariff rates for custody transfer measurement purposes (as a general rule of
thumb, the most frequently
used base pressure value in the natural gas industry is 14.73). The
best way to think about base pressure is that it is a standardized set of conditions used to
measure gas volumes that are being exchanged.
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
Getting back to our original discussion point, what exactly is a pressure factor, how is it
calculated, and how is it used? A pressure factor is a number that reflects the increase in
standardized volume capacity that occurs when we are able to raise system pressures above
"standard operating conditions {usually 7"w.c.}. Boyle's Law tells us that the amount of gas we
can push through a pipeline, adjusted to standardized conditions, is directly proportional
to the
increase in pressure applied to the pipeline. The pressure factors calculated from a change in
operating conditions are regularly used to quantify potential system capacity, the size of piping
needed for mains and services and to assess the proper sizing and selection of pressure
regulators and meters to ensure the equipment can safely operate under the new conditions.
Following is the formula used to calculate pressure factors, along with a brief table of
some of the most common pressure factors:
Atmospheric
pressure (14.4) + line pressure (2)
----------------------------------------------------------------
=
pressu re factor (1.113)
Base pressure (14.73)
PSIG
PF
1
1.045
2
1.113
5
1.317
10
1.656
20
2.335
30
3.014
60
5.051
Pressure is absolutely essential to the operation of gas distribution
and transmission
systems. Without it, gas does not flow. But higher pressures create problems as well, including
higher risks of gas leaks and/or explosions. You may have heard the term IIMAOP" {Maximum
Allowable Operating Pressure} used in conversations about gas systems. This is a pressure
rating designation that stipulates the maximum pressure at which a system can be operated,
based on the current design of the system. There are many factors that influence a system
MAOP, among them the pressure rating and capability of the existing piping and other
equipment {regulators, gas meters} that will be operating under those pressures. Damage
and/or repairs to system piping as well as equipment
replacements can also impact MAOP. The
MAOP is really a function of the lowest pressure-rated components in the system, which means
that a lot oftime,
man-hours, and money go into designing and maintaining accurate MAOP
ratings.
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
If higher pressures pose more risks, why don't LDCs (local gas distribution
companies)
and pipeline operators just reduce their MAOPs? There are (2) primary reasons. First, at lower
pressures, the volume capacity of pipelines is also much lower, which means that larger
diameter piping must be used to transport the gas. For example, in many old distribution
systems, you will find very large diameter cast iron pipe, typically as large as 42", in use and
capacities are still constrained.
Second, the cost to install large diameter pipelines is much
greater than the cost of smaller diameter piping. Increasing system capacities by increasing
system pressures overall is the most cost effective solution to transporting
larger volumes of
gas and to maintaining desired delivery pressures.
CHARLE'S LA W - TEMPERATURE
The impact on the natural gas industry of changes in gas temperature
is not as
significant as that of pressure changes. However, it is important to understand that
temperature
is a very significant factor in overall measurement accuracy and in applications
where large reductions in gas pressure (pressure cuts) occur. Unlike the effects of Boyle's Law,
where increases in pressure correspond to decreases in gas volume, the inverse occurs due to
increases in gas temperature.
"At constant pressure, the volume of a given mass of an ideal gas increases or decreases bv
the same factor as its temperature on the absolute temperature scale" (Wikipedia)
Mathematically,
this theorem is expressed as:
VlxT2 = V2xTl
In plain language, Charles' Law states that if we take a given volume of gas at a constant
pressure and increase the temperature
applied against that volume, the molecules of gas will
move further apart and the gas will take up more space. In gas distribution
and transmission
systems, the physical space within the piping systems is fixed and constant. Therefore, when
we increase temperature
in the system, we are decreasing the density of gas traveling through
the piping network.
The scientific work behind Charles' Law is based on the Kelvin temperature
temperatures
are expressed in degrees Celsius (0C) rather than degrees Fahrenheit (OF)with
which we are all familiar.
For our purposes in the U.S. gas industry, we typically use degrees
Rankin (OR)instead of Celsius because conversion is much easier to remember.
a temperature
scale where
in OF,we convert it to OR,for example, 32°F
4
If we add 460 to
= 492°R or 60°F = 5200R.
BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
Rather than getting hung up on calculations to determine the impact oftemperature
changes
on gas volumes, there is a simple rule of thumb to remember:
For every 5°F change in gas temperature.
The other situation where temperature
the volume
of gas changes by 1%.
impacts gas system design involves locations where
large pressure cuts occur, e.g. city gate stations and farm taps. Without going into too much
detail, the general rule of thumb to remember is:
For every 100 psig reduction in gas pressure. there occurs a temperature
The reason this is important
is that these temperature
drops, in combination
drop
of rF.
with winter
operating conditions, can often result in ice buildups on pressure regulating and metering
equipment, increasing the chance that equipment operation may be impaired.
Where these
conditions occur, you will often find the equipment fitted with some type of heating system or
placed inside heated buildings.
SUPERCOMPRESSIBIUTY
(the "fudge" factor}
You may recall that both Boyle's and Charles' Laws define the behavior of "ideal" gases.
In the real world, these "ideal" gases do not exist. As a result, we have learned that the
behavior of gases does vary from what those "Laws" mathematically
predict.
In fact the
deviation from those models is rather complex and NOT linear at all. The amount of deviation
that occurs is a function both of high pressures and high temperatures
and can result in very
large changes in volume. The measurement of the effects of super compressibility
has
substantially improved due to the use of more sophisticated pressure and temperature
and the complex algorithms written in software programs.
need to be aware that the effect of super compressibility
sensors
For our purposes today, you just
is approximately
a 1% increase in
volume at 60 psig and can increase to over 10% at transmission operating conditions.
BASIC GAS LAWS vs. VOLUME
One of the most widely used and frequently
misunderstood
concepts in the gas industry
is the term "volume", typically expressed in terms of a rate of flow in cubic feet per hour (cfh).
By now, you should have recognized that natural gas volumes are different from volumes of
liquids or solids because they are more significantly impacted by changes in pressure and
temperature.
In order to design and build gas piping systems and exchange gas among
different parties, it is essential that we define volume in terms that are consistent and
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
understood by all parties. For this purpose, the gas industry uses what is called a
of volume (scO. It is defined as the amount of gas found within a
cubic foot (cO volume container when the temperature of the gas is fixed at 60°F and the base
"standardized
cubic foot"
pressure conditions are adjusted to 14.73 psia. It is important to remember that the natural
gas network functions in cubic feet units of volume but system engineering design and custody
transfer require mathematical
conversion ofthose
units to standardized cubic feet.
COMMON GAS INDUSTRY DEFINITIONS
Cubicfoot
the volume of a container that is l' high, l' wide, and l' deep (cf)
Standard cubic foot
the volume of a container that is l' high, l' wide, and l' deep when held
of GO°Fand base pressure conditions of 14.73
at a constant temperature
psia (scf).
BTU
British thermal unit, represents the amount of heat required to raise the
temperature
of 1 pound of water from 39°F to 40°F.
Heating Value
1 scf of natural gas contains
ccf
the unit of volume recorded on gas meter indexes, representing
N
1,028 Btus
hundreds of cubic feet (not standardized cubic feet)
therm
a function of both the energy value and the volume of natural gas, it is
equal to 100,000 Btus. (NOTE: based on average heating values, a ccf is
approximately
equal to 1 therm.)
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