TECHNICAL JUSTIFICATION

Non‐road Diesel Fuel Engine Used for Periodic Testing and Maintenance on Natural Gas Pipelines

The Compliance section of the Air Quality Bureau requested an exemption relating to pipeline

maintenance.  Frequent variance requests were received to waive permitting requirements for

temporary use of a non‐road diesel engine used during periodic maintenance and testing.  The variances

were always granted as the emissions in the limited time frame were minor.  The equipment is not

subject to NSPS or NESHAPs.  Pipeline maintenance and testing is conducted at varying intervals of the

pipeline according to the protocol.  Details on pipeline inspection and testing are in the attachment,

Transportation of Natural Gas by the Natural Gas Supply Association.  No area is tested more than once

in a year.  Maintenance could not be classified as periodic if it exceeds the time limits in the exemption.   

The exemption is based on the small unit exemption of 5 tons of NOx.  Other pollutants do not reach

small unit thresholds.  The current language is based on a per unit interpretation, specifically that not

more than one engine  with a brake horsepower rating of up to 1,100 may be used per event.  It is

unlikely that more than two engines would be used at a time.  The language on the recordkeeping was

altered slightly (for rather than at) to indicate it would not be kept at the location where the event

occurred.  The exemption is intended for periodic testing and maintenance of the pipeline at any

location in the State.  There is no exclusion included for Muscatine or Scott Counties as little work is

anticipated in those areas.   

One key element of this exemption is the non‐road engine description. An engine that does not meet

the definition of a non‐road engine may be subject to a NSPS subpart. The non‐road definition has

several descriptions including an engine that is portable or transportable on wheels, skids, etc. that will

not remain at a single location for more than one year. An engine used to replace an existing stationary

engine even temporarily would not meet the definition of a non‐road engine. This exemption could not

be used for engines used to replace an existing engine at a compressor station.  

Through the dialogue it was indicated that the draft exemption would not be valid for replacement or

substitution of engines at compressor stations.  These stations are required to have continuous power in

the form of a backup generator.  When a component fails, it creates a situation where a replacement

must be at the station immediately.  A variance would still be needed for this situation and would be

needed urgently.  In the last instance, the engine was never used but was on site for almost a month

waiting for a needed part.  One possible option would be to draft a rule similar to 567 IAC 21.6

Temporary electricity generation for disaster situations.  It would have to be based on actual emissions

rather than a day time frame.   


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The Transportation of Natural Gas

The efficient and effective movement of natural gas from 

producing regions to consumption regions requires an 

extensive and elaborate transportation system. In many 

instances, natural gas produced from a particular well will 

have to travel a great distance to reach its point of use. The 

transportation system for natural gas consists of a complex 

network of pipelines, designed to quickly and efficiently 

transport natural gas from its origin, to areas of high natural 

gas demand. Transportation of natural gas is closely linked to 

its storage, as well; should the natural gas being transported 

not be required at that time, it can be put into storage

facilities for when it is needed. 

There are essentially three major types of pipelines along the 

transportation route: the gathering system, the interstate 

pipeline, and the distribution system. The gathering system 

consists of low pressure, low diameter pipelines that 

transport raw natural gas from the wellhead to the processing 

plant. Should natural gas from a particular well have high 

sulfur and carbon dioxide contents (sour gas), a specialized 

sour gas gathering pipe must be installed. Sour gas is 

extremely corrosive and dangerous, thus its transportation from the wellhead to the sweetening 

plant must be done carefully. Review the treatment and processing of natural gas. 

Pipelines can be characterized as interstate or intrastate. Interstate pipelines carry natural gas 

across state boundaries, in some cases clear across the country. Intrastate pipelines, on the other 

hand, transport natural gas within a particular state. This section will cover the fundamentals of 

interstate natural gas pipelines, but the technical and operational details discussed are essentially 

the same for intrastate pipelines. 

Natural gas pipelines are subject to regulatory oversight, which in many ways determines the 

manner in which pipeline companies must operate. 

Interstate Natural Gas Pipelines

The interstate natural gas pipeline network 

transports processed natural gas from 

processing plants in producing regions to 

those areas with high natural gas 

requirements, particularly large, populated 

urban areas. As can be seen, the pipeline 

network extends across the entire country. 

Interstate pipelines are the 'highways' of 

natural gas transmission. Natural gas that 

is transported through interstate pipelines 

travels at high pressure in the pipeline, at 

pressures anywhere from 200 to 1500 

pounds per square inch (psi). This 

reduces the volume of the natural gas 

being transported (by up to 600 times), as well as providing propellant force to move the natural 

gas through the pipeline. This section will cover the components of the interstate pipeline system, 

the construction of pipelines, and pipeline inspection and safety. For more information on 

interstate pipelines in general, visit the website of the Interstate Natural Gas Association of 

America. 

Source: Duke Energy Gas Transmission 

Canada

Interstate Natural Gas Pipelines

(Click Image to Enlarge)

Source: National Energy Technology Laboratory, DOE

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Pipeline Components

Interstate pipelines consist of a number of components which ensure the efficiency and reliability 

that is needed from a system that delivers such an important energy source year round, twenty 

four hours a day, and consist of a number of different components. 

Pipes

Pipelines can measure anywhere from 6 to 48 

inches in diameter, although certain component 

pipe sections can consist of small diameter pipe, as 

small as 0.5 inches in diameter. However, this 

small diameter pipe is usually used only in 

gathering and distribution systems. Mainline pipes, 

the principle pipeline in a given system, are usually 

between 16 and 48 inches in diameter. Lateral 

pipelines, which deliver natural gas to or from the 

mainline, are typically between 6 and 16 inches in 

diameter. Most major interstate pipelines are 

between 24 and 36 inches in diameter. The actual 

pipeline itself, commonly called 'line pipe', consists 

of a strong carbon steel material, engineered to 

meet standards set by the American Petroleum 

Institute (API). 

Pipelines are produced in steel mills, which are 

sometimes specialized to produce only pipeline. There are two different production techniques, 

one for small diameter pipes and one for large diameter pipes. For large diameter pipes, from 20 

to 42 inches in diameter, the pipes are produced from sheets of metal which are folded into a tube 

shape, with the ends welded together to form a pipe section. Small diameter pipe, on the other 

hand, can be produced seamlessly. This involves heating a metal bar to very high temperatures, 

then punching a hole through the middle of the bar to produce a hollow tube. In either case, the 

pipe is tested before being shipped from the steel mill, to ensure that it can meet the pressure and 

strength standards for transporting natural gas. 

Line pipe is also covered with a specialized coating to ensure that it does not corrode once placed 

in the ground. The purpose of the coating is to protect the pipe from moisture, which causes 

corrosion and rusting. There are a number of different coating techniques. In the past, pipelines 

were coated with a specialized coal tar enamel. Today, pipes are often protected with what is 

known as a fusion bond epoxy, which gives the pipe a noticeable light blue color. In addition, 

cathodic protection is often used; which is a technique of running an electric current through the 

pipe to ward off corrosion and rusting. 

Compressor Stations

As mentioned, natural gas is highly pressurized as it travels through an interstate pipeline. To 

ensure that the natural gas flowing through any one pipeline remains pressurized, compression of 

this natural gas is required periodically along the pipe. This is accomplished by compressor 

stations, usually placed at 40 to 100 mile intervals along the pipeline. The natural gas enters the 

compressor station, where it is compressed by either a turbine, motor, or engine. 

Turbine compressors gain their energy by 

using up a small proportion of the natural gas 

that they compress. The turbine itself serves 

to operate a centrifugal compressor, which 

contains a type of fan that compresses and 

pumps the natural gas through the pipeline. 

Some compressor stations are operated by 

using an electric motor to turn the same type 

of centrifugal compressor. This type of 

compression does not require the use of any 

of the natural gas from the pipe, however it 

does require a reliable source of electricity 

nearby. Reciprocating natural gas engines are also used to power some compressor stations. 

These engines resemble a very large automobile engine, and are powered by natural gas from the 

pipeline. The combustion of the gas powers pistons on the outside of the engine, which serves to 

compress the natural gas. 

Pipes in Transit

Source: Duke Energy Gas Transmission Canada

A Compressor Station

Source: Duke Energy Gas Transmission Canada

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In addition to compressing natural gas, compressor stations also usually contain some type of 

liquid separator, much like the ones used to dehydrate natural gas during its processing. Usually, 

these separators consist of scrubbers and filters that capture any liquids or other undesirable 

particles from the natural gas in the pipeline. Although natural gas in pipelines is considered 'dry' 

gas, it is not uncommon for a certain amount of water and hydrocarbons to condense out of the 

gas stream while in transit. The liquid separators at compressor stations ensure that the natural 

gas in the pipeline is as pure as possible, and usually filters the gas prior to compression. 

Metering Stations

In addition to compressing natural gas to reduce its volume and push it through the pipe, metering 

stations are placed periodically along interstate natural gas pipelines. These stations allow 

pipeline companies to monitor and manage the natural gas in their pipes. Essentially, these 

metering stations measure the flow of gas along the pipeline, and allow pipeline companies to 

'track' natural gas as it flows along the pipeline. These metering stations employ specialized 

meters to measure the natural gas as it flows through the pipeline, without impeding its 

movement. 

Valves

Interstate pipelines include a great number of valves 

along their entire length. These valves work like 

gateways; they are usually open and allow natural gas 

to flow freely, or they can be used to stop gas flow 

along a certain section of pipe. There are many 

reasons why a pipeline may need to restrict gas flow in 

certain areas. For example, if a section of pipe 

requires replacement or maintenance, valves on either 

end of that section of pipe can be closed to allow 

engineers and work crews safe access. These large 

valves can be placed every 5 to 20 miles along the 

pipeline, and are subject to regulation by safety codes. 

Control Stations and SCADA Systems

Natural gas pipeline companies have customers on 

both ends of the pipeline - the producers and 

processors that input gas into the pipeline, and the 

consumers and local distribution companies that take gas out of the pipeline. In order to manage 

the natural gas that enters the pipeline, and to ensure that all customers receive timely delivery of 

their portion of this gas, sophisticated control systems are required to monitor the gas as it travels 

through all sections of what could be a very lengthy pipeline network. To accomplish this task of 

monitoring and controlling the natural gas that is traveling through the pipeline, centralized gas 

control stations that collect, assimilate, and manage data received from monitoring and 

compressor stations all along the pipe. 

Most of the data that is received by a control 

station is provided by Supervisory Control and 

Data Acquisition (SCADA) systems. These 

systems are essentially sophisticated 

communications systems that take measurements 

and collect data along the pipeline (usually in a 

metering or compressor stations and valves) and 

transmit them to the centralized control station. 

Flow rate through the pipeline, operational status, 

pressure, and temperature readings may all be 

used to assess the status of the pipeline at any 

one time. These systems also work in real-time, 

meaning that there is little lag time between the 

measurements taken along the pipeline and their 

transmission to the control station. 

This information, relayed to a centralized control 

station, allows pipeline engineers to know exactly what is happening along the pipeline at all 

times. This allows quick reactions to equipment malfunctions, leaks, or any other unusual activity 

along the pipeline. Some SCADA systems also incorporate the ability to remotely operate certain 

equipment along the pipeline, including compressor stations, allowing engineers in a centralized 

A Ground Valve

Source: Duke Energy Gas Transmission Canada

Pipeline Control Station

Source: Duke Energy Gas Transmission Canada

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control center to immediately and easily adjust flow rates in the pipeline. 

Pipeline Construction

As natural gas use increases, so does the need to have transportation infrastructure in place to 

supply the increased demand. This means that pipeline companies are constantly assessing the 

flow of natural gas across the U.S., and building pipelines to allow transportation of natural gas to 

those areas that are underserved. 

Constructing natural gas pipelines requires a great deal of 

planning and preparation. In addition to actually building the 

pipeline, several permitting and regulatory processes must 

be completed. In many cases, prior to beginning the 

permitting and land access processes, natural gas pipeline 

companies prepare a feasibility analysis to ensure that an 

acceptable route for the pipeline exists that provides the 

least impact to the environment and public infrastructure 

already in place. 

Assuming a pipeline company obtains all the required 

permits and satisfies all of the regulatory requirements, 

construction of the pipe may begin. Extensive surveying of 

the intended route is completed, both aerial and land based, 

to ensure that no surprises pop up during actual assembly of 

the pipeline. 

Installing a pipeline is much like an assembly line process, with sections of the pipeline being 

completed in stages. First, the path of the pipeline is cleared of all removable impediments, 

including trees, boulders, brush, and anything else that may prohibit the construction. Once the 

pipeline's path has been cleared sufficiently to allow construction equipment to gain access, 

sections of pipes are laid out along the intended path, a process called 'stringing' the pipe. These 

pipe sections are commonly from 40 to 80 feet long, and are specific to their destination. That is, 

certain areas have different requirements for coating material and pipe thickness. 

Once the pipe is in place, trenches are dug 

alongside the laid out pipe. These trenches 

are typically 5 to 6 feet deep, as the 

regulations require the pipe to be at least 30 

inches below the surface. In certain areas, 

however, including road crossings and bodies 

of water, the pipe is buried even deeper. Once 

the trenches are dug, the pipe is assembled 

and contoured. This includes welding the 

sections of pipe together into one continuous 

pipeline, and bending it slightly, if needed, to 

fit the contour of the pipelines path. Coating is 

applied to the ends of the pipes (the coating 

applied at a coating mill typically leaves the 

ends of the pipe clean, so as not to interfere 

with welding), and the entire coating of the 

pipe is inspected to ensure that it is free from 

defects. 

Once the pipe is welded, bent, and coated, it can be lowered into the previously dug trenches. 

This is done with specialized tracked construction equipment acting in tandem to lift the pipe 

relatively uniformly and lower it into the trench. Once lowered into the ground, the trench is filled in 

carefully, to ensure that the pipe and its coating do not incur damage. The last step in pipeline 

construction is the hydrostatic test. This consists of running water, at pressures higher than will be 

needed for natural gas transportation, through the entire length of the pipe. This serves as a test 

to ensure that the pipeline is strong enough, and absent of any leaks of fissures, before natural 

gas is pumped through the pipeline. 

Laying pipe across streams or rivers can be 

accomplished in one of two ways. Open cut 

crossing involves the digging of trenches on 

the floor of the river to house the pipe. When 

this is done, the pipe itself is usually fitted with 

Surveying the Right-of-Way

Source: Duke Energy Gas Transmission Canada

'Stringing' the Pipe

Source: Duke Energy Gas Transmission Canada

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a concrete casing, which both ensures that the 

pipe stays on the bottom of the river, and add 

an extra protective coating to prevent any 

natural gas leaks into the water. Alternately, a 

form of directional drilling may be employed, in 

which a sort of 'tunnel' is drilled under the river 

through which the pipe may be passed. The 

same techniques are used for road crossings - 

either an open trench is dug up across the 

road and replaced once the pipe is installed, 

or a tunnel may be drilled underneath the 

road. 

Once the pipeline has been installed, and 

covered up, extensive efforts are taken to 

restore the pipeline's pathway to its original state, or to mitigate for any environmental or other 

impacts that may have occurred during the construction process. This often includes replacing 

topsoil, fences, irrigation canals, and anything else that may have been removed or upset during 

the construction process. 

For more information on natural gas pipeline construction, visit the Interstate Natural Gas 

Association of America's website. 

Pipeline Inspection and Safety

In order to ensure the efficient and 

safe operation of the extensive 

network of natural gas pipelines, 

pipeline companies routinely inspect 

their pipelines for corrosion and 

defects. This is done through the use 

of sophisticated pieces of equipment 

known as pigs. Pigs are intelligent 

robotic devices that are propelled 

down pipelines to evaluate the interior 

of the pipe. Pigs can test pipe 

thickness, and roundness, check for 

signs of corrosion, detect minute 

leaks, and any other defect along the 

interior of the pipeline that may either 

impede the flow of gas, or pose a 

potential safety risk for the operation 

of the pipeline. Sending a pig down a 

pipeline is fittingly known as 'pigging' the pipeline. 

In addition to inspection with pigs, there are a number of safety precautions and procedures in 

place to minimize the risk of accidents. In fact, the transportation of natural gas is one of the safest 

ways of transporting energy, mostly due to the fact that the infrastructure is fixed, and buried 

underground. According to the Department of Transportation (DOT), pipelines are the safest 

method of transporting petroleum and natural gas. While there are in excess of 100 deaths per 

year associated with electric transmission lines, according to the DOT's Office of Pipeline Safety in 

2001, there were 2 deaths associated with transmission pipelines, and 5 deaths associated with 

distribution systems. To learn more about pipeline safety, visit the DOT's Office of Pipeline Safety. 

A few of the safety precautions associated with natural gas pipelines include: 

z Aerial Patrols - Planes are used to ensure no construction activities are taking place too 

close to the route of the pipeline, particularly in residential areas. Unauthorized construction 

and digging is the primary threat to pipeline safety, according to INGAA 

z Leak Detection - Natural gas detecting equipment is periodically used by pipeline 

personnel on the surface to check for leaks. This is especially important in areas where the 

natural gas is not odorized. 

z Pipeline Markers - Signs on the surface above natural gas pipelines indicate the presence 

of underground pipelines to the public, to reduce the chance of any interference with the 

pipeline. 

z Gas Sampling - Routine sampling of the natural gas in pipelines ensures its quality, and 

may also indicate corrosion of the interior of the pipeline, or the influx of contaminants. 

Lowering Pipe

Source: Duke Energy Gas Transmission Canada

Pig - Pipeline Inspection Tool

Source: Duke Energy Gas Transmission Canada

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z Preventative Maintenance - This involves the testing of valves and the removal of surface 

impediments to pipeline inspection. 

z Emergency Response - Pipeline companies have extensive emergency response teams 

that train for the possibility of a wide range of potential accidents and emergencies. 

z The One Call Program - All of the states have instituted what is known as a 'one call' 

program, which provides excavators, construction crews, and anyone interested in digging 

into the ground around a pipeline with a single phone number that may be called when any 

excavation activity is planned. This call alerts the pipeline company, which may flag the 

area, or even send representatives to monitor the digging. 

While large interstate natural gas pipelines transport natural gas from the processing regions to 

the consuming regions and may serve large wholesale users like industrial or power generation 

customers directly, it is the distribution system that actually delivers natural gas to most retail 

customers, including residential natural gas users.

Overview of Natural Gas | Natural Gas - From the Wellhead... | Business Overview | Natural Gas Regulations | Environment & Technology | Focus on LNG

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