Basic Gas Laws

BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL

INTRODUCTION

There are a number of very important reasons why everyone connected to the naturalgas 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 everyday, 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 theservice 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 relatedto 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 LAW - (PRESSUREJ

Of the (3) Basic Gas Laws discussed in this presentation, Boyle's Law describing theeffect 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 in this presentation. For those who are notengineers or mathematically inclined, a very simple example of Boyle's Law at work in the realworld 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 ofpressure. Boyle's Law is a way to measure the change in volume that occurs with a change inpressure.

"At a constant temperature. the volume of an ideal gas decreases when an increase inpressure is applied." (Wikipedia)

Mathematically, this theorem can be expressed as follows:

Pl x Vl = P2 x V2

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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 willmove 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 thepiping 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 factorsare 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 localweather conditions and altitude. For simplicity purposes, most companies use an averageannual atmospheric pressure value calculated for their service area; here in New England14.73psia is often used for rough calculation purposes. However, by industry convention most

manufacturers use an atmospheric pressure value of 14.4psia when calculating flow andvolume 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 usuallyexpressed in pounds per square inch gauge (psig). However, when talking about old, low-

pressure 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 mayhear 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 inchabsolute (psia). This pressure is the sum of both the atmospheric pressure and the gaugepressure 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 tomeasure gas volumes that are being exchanged.

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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 PF1 1.045

2 1.113

5 1.31710 1.656

20 2.335

30 3.014

60 5.051

Pressure is absolutely essential to the operation of gas distribution and transmissionsystems. Without it, gas does not flow. But higher pressures create problems as well, includinghigher 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 pressurerating 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 otherequipment {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. TheMAOP 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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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 assignificant as that of pressure changes. However, it is important to understand that

temperature is a very significant factor in overall measurement accuracy and in applicationswhere 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 bvthe 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 willmove further apart and the gas will take up more space. In gas distribution and transmissionsystems, 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 scale wheretemperatures 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 degreesRankin (OR)instead of Celsius because conversion is much easier to remember. If we add 460 toa temperature in OF,we convert it to OR,for example, 32°F = 492°R or 60°F = 5200R.

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Rather than getting hung up on calculations to determine the impact oftemperature changeson gas volumes, there is a simple rule of thumb to remember:

For every 5°F change in gas temperature. the volume of gas changes by 1%.

The other situation where temperature impacts gas system design involves locations where

large pressure cuts occur, e.g. city gate stations and farm taps. Without going into too muchdetail, the general rule of thumb to remember is:

For every 100 psig reduction in gas pressure. there occurs a temperature drop of rF.

The reason this is important is that these temperature drops, in combination with winteroperating 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 orplaced 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 thedeviation 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 hassubstantially improved due to the use of more sophisticated pressure and temperature sensorsand the complex algorithms written in software programs. For our purposes today, you just

need to be aware that the effect of super compressibility 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 andtemperature. In order to design and build gas piping systems and exchange gas amongdifferent parties, it is essential that we define volume in terms that are consistent and

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understood by all parties. For this purpose, the gas industry uses what is called a

"standardized cubic foot" 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

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 heldat a constant temperature of GO°Fand base pressure conditions of 14.73psia (scf).

BTU British thermal unit, represents the amount of heat required to raise thetemperature of 1 pound of water from 39°F to 40°F.

Heating Value 1 scf of natural gas contains N 1,028 Btus

ccf the unit of volume recorded on gas meter indexes, representing

hundreds of cubic feet (not standardized cubic feet)

therm a function of both the energy value and the volume of natural gas, it isequal to 100,000 Btus. (NOTE: based on average heating values, a ccf is

approximately equal to 1 therm.)

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Basic Gas Laws