Describe and Operate Indirect-Fired Line Heaters

Training Module
Describe and Operate
Line Heaters
Human Development
Consultants Ltd.
Cover Photo
Courtesy of Presson ENERFLEX and Petro-Canada
Describe and Operate
Indirect-Fired Line Heaters
© 2006 and 2014 HDC Human Development Consultants Ltd.
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ISBN 1-55338-040-1
Canadian Cataloguing in Publication Data
1. Pipelines—Equipment and supplies. I. HDC Human Development Consultants.
TP757.F48 2005 665.7’44 C2005-906879-5
This training kit consists of the following parts:
 Training Module and Self-Check
 Performance Check
 Knowledge Check and Answer Key
 Exercise
 Blank Answer Sheet
 Job Aid
Published by HDC Human Development Consultants Ltd.
Published in Canada
HDC Human Development Consultants Ltd.
(780) 463-3909
August, 2014
Training Objectives
Heat Medium Components
2.1 Heater Shell
2.2 Produced Gas Flow Coil
Combustion Components
3.1 Fuel Gas Supply
3.2 Fuel Gas Conditioning System
3.3 Burner Fuel Gas
3.4 Burner Assembly
3.5 Fire Tube Assembly
Combustion Safety Controls
4.1 Hazardous/Upset Conditions that Shut Down the
Line Heater
4.2 Pneumatic Combustion Safety Controls
4.3 Microprocessor-Based Combustion Safety Controls
Site Layouts
5.1 Gas Wellsites
5.2 Gathering System Sites
Choke Operation
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Contents (continued)
Site Systems and Equipment
7.1 Cold-Climate Enclosure Building
7.2 Chemical Injection Systems
7.3 Downhole Heat String
7.4 Flow Measurement
7.5 Combustible Gas, Toxic Gas, and Fire Detection
7.6 SCADA (Supervisory Control and Data
Acquisition) System
7.7 Electrical Power System
7.8 Emergency Shutdown (ESD) Pushbuttons
Line Heater Safety
8.1 Safety While Traveling to the Line Heater Site
8.2 Safety at the Line Heater Site
8.3 Safety at the Line Heater Building
8.4 Safety When Leaving the Line Heater Site
Operating Variables
Prepare Line Heater for Startup
10.1 Investigate Line Heater Shutdown
10.2 Restart Line Heater
Light Line Heater Burner Manually
Self Check
Answer Key
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Describe and Operate
Indirect-Fired Line Heaters
Upon completion of this training kit, you will be able to:
 Describe the purpose and importance of indirect-fired line
 Describe line heater:
 components and their function
 combustion safety controls
 Describe line heater site layouts
 Describe line heater site systems and equipment
 Describe line heater safety
 Describe manual lighting of line heater burner
 Describe line heater operating variables and operations
 Troubleshoot line heaters
1 Introduction
Indirect-fired line heaters are used to heat natural gas before
the gas is shipped by pipeline for processing or for distribution
to residential and industrial customers.
During shipping, processing, and distribution, the natural gas
cools as:
 heat is lost to cooler surroundings (e.g., a subsurface gas
pipeline loses heat to the surrounding soil)
 the pressure of the gas is reduced
Gas cooling can cause problems downstream on the pipeline:
 the formation of hydrates which can block the pipeline (see
the textbox on page 4 about hydrates and their hazards)
 hydrates formed in meters can cause inaccurate readings
 the chilling of downstream equipment:
― in mechanical equipment (e.g., meters, regulators, and
valves), cold temperatures may prevent lubricated
internal components from operating effectively
― in gas analysis instrumentation, cold temperatures may
contribute to inaccurate sampling results
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Figure 1—Typical Line Heater Locations in Natural Gas Processing/Distribution
Nearby Gas Field
Gas Processing
Remote Gas Field
Gas Gathering System Pipeline
Typical Line Heater
1 Gas Wellsites
2 Gas Gathering Pipelines
3 City Gate Stations
Drawoff to
Local Distribution
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Residential and
Industrial Customers
City Gate Station
(metering, regulating,
heating, and odorizing)
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Describe and Operate
Indirect-Fired Line Heaters
Indirect-fired line heaters heat the natural gas to prevent
hazardous hydrate blockages and frozen downstream
Line heaters are typically installed:
 at gas wellsites where the high wellhead pressure must be
reduced so that gas can flow via lower pressure piping to a
downstream gas plant. A major drop in gas pressure causes
significant cooling of the gas to the point where hydrates
could form. Line heaters are installed at the site to heat the
gas to prevent hydrate formation.
 along gas gathering system pipelines that link remote gas
fields to gas processing plants. Gas cools as it passes
through the pipeline and loses heat to the surrounding
subsoil. Line heaters along gas gathering system pipelines
reheat the gas to prevent hydrates from forming.
 at city gate stations where local distribution companies
purchase, meter, regulate, and odorize the processed
natural gas received from transmission pipelines. The city
gate station’s pressure regulators reduce the gas pressure
from its high transmission line pressure to the lower local
distribution line pressure. Line heaters at city gate stations
provide enough heat to prevent chilling of downstream
This module focuses on the operation of indirect-fired line
heaters at gas wellsites and along gas gathering system
Figure 2— IndirectFired Line Heater at
(Courtesy of Presson
ENERFLEX and PetroCanada)
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Hydrates and Their Hazards
Hydrates are a mixture of water and gas that forms crystals under certain temperature
and pressure conditions. In a hydrate, gas molecules (such as methane, ethane,
propane, butane, hydrogen sulphide, and carbon dioxide) are embedded inside a
lattice of water molecules. Hydrate crystals deposit on solid surfaces and restrict gas
flow. Hydrates can plug valves, meters, instruments, and flow lines.
- can form at high pressure, even when the temperature
of the flowing gas is above the freezing point of water
- can form when free water condenses in the gas stream
- tend to form at sharp bends in flow lines and at locations
where the pressure drops. However, hydrates can occur
anywhere the temperature drops below the hydrate
formation point.
Hydrate formation can be prevented by:
- operating at high gas temperatures
- operating at low gas pressures
- operating at a lower water content
- injecting a hydrate inhibitor, such as methanol
Preventing hydrates from forming is important:
- Clearing hydrate blockages is a high-risk activity. Incorrect hydrate removal can
cause pipeline/equipment rupture and put people in harm’s way.
- Hydrate blockages interrupt business: a hydrate blockage means that gas is unable
to flow.
Indirect-Fired Line Heater––Operating Principle
In an indirect-fired line heater, a fire tube immersed in a heat
medium (HM), such as glycol, heats the HM (Figure 3). The
natural gas passes through a flow coil that is also immersed in
the HM bath and is heated indirectly.
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In a direct-fired line heater, the fire tube is immersed in the
product being heated.
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Describe and Operate
Indirect-Fired Line Heaters
In this module, the terms line heater and heater refer only to
indirect-fired line heaters.
Figure 3––Line Heater Operating Principle
The fire tube heats the HM bath as follows:
 A fire tube is submerged in the lower part of the HM bath. A
flame travels down the fire tube and transfers heat to the
HM. Cooled combustion gases flow from the fire tube and
rise up a stack.
 As the HM’s temperature increases, its density decreases to
create thermal circulation in the bath:
― Hot, less dense HM rises to the upper part of the shell.
― The gas flowing in the gas flow coil absorbs heat from
the HM.
― The HM’s temperature decreases and its density
increases. The cooled HM sinks to the bottom of the
shell to be reheated.
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Line Heater Application and Site Requirements
A line heater must be appropriate for site conditions: it must be
able to heat the gas enough to prevent hydrate formation.
Line heaters are usually skid-mounted and can be transferred
between sites. Before a line heater is transferred to a new site,
your company will ensure that:
 the heater’s design specifications match conditions at the
new site
 the heater and its components comply with regulatory
requirements for the new site
This module describes line heaters that are correctly designed
for site conditions and meet regulatory requirements.
Indirect-Fired Line Heater––Operator’s Role
Careful monitoring and control of indirect-fired line heaters
contribute to the safe and efficient operation of the heater and
prevents hydrate formation in downstream piping. Operations
 monitoring operating variables (temperature, pressure, bath
level, flow) to assess/optimize heater operation
 starting up and shutting down the heater
 troubleshooting heater problems and rectifying minor
 promptly reporting major problems
Additional Operator responsibilities are described in Section 10.
Module Contents
For indirect-fired line heaters, this module describes:
 heat medium components
 combustion components
 combustion safety controls
 site layouts
 site systems and equipment
 safety
 line heater burner lighting
 operating variables and operations monitoring
 troubleshooting
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Describe and Operate
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2 Heat Medium Components
This section describes the indirect-fired line heater’s shell and
produced gas flow coil.
In this module:
 the term produced gas refers to the natural gas flowing
through the coil and being heated
 the term fuel gas refers to the gas that fuels the burner
 the term instrumentation gas refers to the natural gas or
propane used to operate pneumatic controls
2.1 Heater Shell
The heater shell is a horizontal, cylindrical vessel that operates
at atmospheric pressure. The shell holds the large volume of
HM that transfers heat from the fire tube to the produced gas
flow coil. The heater shell has two end flanges:
 the produced gas flow coil is installed through one end
 the fire tube is installed through the other end flange
For colder climates, the shell may be insulated. On older line
heaters, asbestos may have been used for insulation. Follow
your company procedures for handling asbestos.
Expansion Tank
The heater shell has a top expansion tank (also called a surge
tank or reservoir). The tank has sufficient volume:
 to ensure that the heater shell is completely filled with HM
(i.e., the liquid fully covers the produced gas flow coil)
 to accommodate HM expansion when the HM temperature
increases during cold startup
The expansion tank has a top hatch (thief hatch) that serves as
both an HM fill port and a vent/breather. Because the shell can
only withstand atmospheric pressure, the hatch’s vent/breather
provides both overpressure and vacuum protection:
 Venting prevents shell rupture:
― During line heater startup, the HM is heated to operating
temperature and expands. Air is pushed out the hatch.
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― In case the flow coil ruptures, venting protects the shell
from overpressure rupture.
Breathing prevents shell vacuum collapse. After the line
heater is shut down, the HM cools and contracts. Air is
drawn in through the hatch to compensate for the decrease
in HM volume.
For line heaters that have water baths, the expansion tank
minimizes water vapor loss. This type of expansion tank is
known as a water saver. The lower temperature at the top of
the expansion tank condenses the steam rising off the liquid
Heating Medium
The HM is either water or a glycol-water solution. A glycolwater mixture has a lower freezing point and a higher boiling
point than water. The type of HM used depends on the climate
and required heating:
 Water is used in hot climates where the HM does not have
to be protected from freezing. When using a water bath, a
line heater is operated below water’s boiling point to protect
against boiling off the bath water.
 Distilled water may be used in colder climates. Compared
to water with impurities, distilled water freezes at a lower
 Glycol-water bath solutions are used:
― in cold climates where the bath must not freeze (in case
the line heater shuts down during cold weather
― in any climate where a high bath temperature is required
The water and glycol-water solutions used in line heaters are
treated with corrosion inhibitors to prevent internal fouling and
corrosion. Your company may stipulate that:
 a specific glycol-water mixture be used instead of vehicle
 HM be sampled regularly to verify glycol/water
concentration, freezing/boiling points, and corrosion inhibitor
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The level of the heat medium is determined by two factors:
 The fire tube and gas flow coils must be covered
 Some empty space is required for thermal expansion
Follow the manufacturer’s recommendations for the required
level of the heat medium.
2.2 Produced Gas Flow Coil
The produced gas flow coil (also called the tube bundle) is a
removable length of piping installed above the fire tube in the
HM bath. Pressurized produced gas from the wellhead or
pipeline is directed through the flow coil for heating.
The gas flow coil must always be completely submerged in the
HM during line heater operation. Keeping the coil submerged
ensures that the produced gas in the coil is effectively heated.
The coil must remain submerged even after the line heater is
shut off and has cooled. Remaining submerged ensures that
the coil is not exposed to the corrosive effects of the air (drawn
in through the vent/breather during cool down).
Engineering and/or regulatory codes dictate how the produced
gas flow coil is manufactured, pressure-tested, and re-tested/
inspected during the coil’s service life to ensure the coil can
withstand the high gas pressures.
The configuration of the coil depends on both:
 the number of heating stages needed by the produced gas
heating application (either one or two stages)
 the number of coils needed to handle the produced gas flow
rate (either one coil for low flow rates or multiple parallel
coils for higher gas flow rates)
Heating Stages
One heating stage is used on line heaters which must provide
only a moderate temperature increase (such as those on
gathering system pipelines).
Figure 4 on the following page show a one heating stage line
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Figure 4—One-Stage
Produced Gas Flow
One Stage
gas flow coil
Two heating stages are used on line heaters which must
provide a great amount of heating to counter the intense
cooling caused by a large pressure reduction. (Large pressure
reductions commonly occur at sites where the wellhead gas
pressure must be reduced to match lower gathering system
pressure.) On two-stage line heaters (see Figures 5 and 6), the
coil provides a preheating stage before the pressure reduction
and a reheating stage after the pressure reduction. (Preheating
the gas before a large pressure reduction ensures that the
cooling caused by the pressure drop does not chill the gas to
below hydrate-formation temperature.)
The produced gas flows through the two-stage heater as
 the produced gas flow enters the preheat coil and is heated
 the heated produced gas flow leaves the preheat coil and
passes through the pressure reduction valve (choke)
 after pressure reduction (and resulting cooling), the
produced gas flows into the line heater’s reheat coil, is
heated to the final temperature, and then exits the line
heater. (The reheat coil is also known as the post-heat coil.)
For line heaters equipped with preheat and reheat coils:
 The preheat coil is built to withstand higher pressure than
the reheat coil.
 The configuration of the pressure reduction valve (choke)
between the preheat and reheat coils can vary:
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― The valve may be a manual valve whose body is
immersed in the HM bath.
― The valve may be an external control valve that is
automatically positioned as part of a wellhead pressure
control or flow control strategy to maintain flow and/or
pressure setpoint (as described in Section 6).
For a detailed description of process control and control loop
strategies, refer to these HDC training kits:
 Describe Basic Instrumentation and Control Strategies
 Describe Process Control Modes and Process Control
Figure 5—Two-Stage
Produced Gas Flow
Two Stage
gas preheat
coil (gas in)
valve (choke)
gas reheat
coil (gas out)
Refer to the next page for a picture of a two stage gas coil and
Number of Produced Gas Coils
Depending on the gas flow to be heated, a line heater’s
produced gas coil may consist of a single coil or multiple,
parallel coils:
 single coil. A single coil may be sufficient to heat a small
gas flow. A single coil is configured as a spiral-wound
bundle or as a tube-type bundle (where the coil makes
numerous passes back and forth before leaving the line
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Figure 6—Line Heater with Two-Stage Produced Gas Coil and Choke
(Courtesy of Larsen & D’Amico Mfg. Ltd.)
multiple, parallel coils. To heat a large gas flow, the flow
to the line heater branches outside of the heater to feed
multiple, parallel, tube-type coils. After passing through the
heater, the flows recombine.
3 Combustion Components
The burner directs a flame into the submerged fire tube to heat
the HM. The following components support the heater’s
combustion operation (see Figure 7):
 fuel gas supply
 fuel gas conditioning system
 burner fuel gas train
 burner assembly
 fire tube assembly
 flame arrestor/wind box
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3.1 Fuel Gas Supply
A line heater needs a reliable source of fuel gas. The fuel gas
source may be:
 a drawoff from the line heater produced gas outlet
 a natural gas utility distribution network or nearby gas
processing plant
 a propane storage system
Figure 7—Line Heater’s Combustion Components
Drawoff from Line Heater Outlet
The produced gas heated in the gas flow coil may be of
sufficient quality to support efficient combustion. In this case, a
slipstream of heated produced gas is withdrawn downstream of
the line heater. The produced gas may contain toxic
contaminants at trace levels that do not impair reliable
combustion. However, some contaminants are hazardous/toxic,
even at trace levels. Refer to Section 8.3 for safety precautions
at line heater sites.
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In applications where produced gas is used as the fuel gas
source, the fuel gas drawoff may be equipped with:
 a manual isolation block valve
 a pressure regulator to reduce the gas pressure
When produced gas is used as fuel gas, the fuel gas typically
does not contain a leak detection odorant. Although the fuel
gas may have a petroleum odor, don’t assume that you will be
able to smell a fuel gas leak.
Natural Gas Utility or Nearby Gas Plant
If the produced gas is not suitable, fuel gas may have to be
supplied from offsite. Two sources are:
 commercial-quality natural gas piped from a gas utility
distribution network
 sales-quality natural gas from a nearby gas plant
At some remote gas plants, a slipstream of sales gas is
withdrawn to supply equipment in the surrounding gas fields.
In applications where the fuel gas arrives from offsite, the fuel
gas piping may be equipped with:
 a manual isolation block valve
 an ESD (emergency shutdown) valve to close automatically
in case of a site emergency
 a pressure regulator to reduce the gas pressure to an
intermediate pressure
 a flowmeter to record fuel gas consumption
Find out if the fuel gas at your site is odorized:
 Fuel gas arriving from an offsite location may contain a leak
detection odorant.
 Fuel gas from a gas utility is typically odorized, whereas
fuel gas from a gas processing plant may not be. Don’t
assume that you will be able to smell a fuel gas leak.
Propane Storage System
If the produced gas is not suitable for use as fuel gas, another
option is to use propane supplied from a storage system. The
propane storage system consists of an outdoor propane
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Indirect-Fired Line Heaters
storage tank that holds a large volume of liquid propane under
pressure. The propane is withdrawn from the tank using either:
 propane liquid drawoff (for larger capacity systems)
 propane vapor drawoff (for smaller capacity systems)
Propane Liquid Drawoff
Propane liquid flows or is pumped from the bottom of the tank
through the tank’s manual liquid isolation valve. The propane
liquid is then heated in a specialty heater known as a propane
Propane Vapor Drawoff
Propane vapor leaves the top of the tank through the tank’s
manual isolation valve and is directed through a regulator to
reduce the propane pressure.
A propane tank configured to supply propane vapor may have
difficulty supplying sufficient vapor during cold weather. A
propane tank’s ability to discharge vapor declines as the
outdoor temperature drops and the tank’s propane level drops.
To prevent an interruption in propane vapor flow, the propane
tank may be winterized with an external flameless heater (a
catalytic heater, described in Section 7.1). The heater provides
radiant heat to the tank’s wall to ensure sufficient propane
Propane System Monitoring
The Operator checks the propane system to ensure continued
fuel gas supply:
 monitors propane storage tank pressure and requests a
propane refill, if necessary
 confirms that the propane system’s valves are correctly
 during the winter, confirms the operation of the propane
tank’s heater and flow of propane vapor from the tank
August, 2014
At sites where propane is used as fuel gas, your personal
combustible gas detector must be able to detect:
 propane fuel gas leaks
 methane produced gas leaks
Consult your company’s health and safety practices.
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3.2 Fuel Gas Conditioning System
All fuel gas undergoes conditioning (see Figure 7):
 fuel gas preheating
 fuel gas scrubbing
 additional fuel gas filtration
Fuel Gas Preheating
The fuel gas passes through a preheat coil immersed in the HM
bath. After heating, the fuel gas flow branches and flows
through pressure regulators in the main burner and pilot burner
supply lines. Preheating the fuel gas before it flows through the
pressure regulators prevents hydrates from forming.
Fuel Gas Scrubbing
Liquids entrained in the fuel gas must be removed to prevent:
 poor burner operation; the liquids may not burn consistently:
― the burning liquids may produce a smoky flue gas
― the liquids may not burn and may pool at the bottom of
the firebox housing. This pooled liquid may drain out of
the firebox flame arrestor, creating a fire hazard. The
pooled liquid may suddenly ignite or explode.
 discharge of flammable droplets out the top of the flue stack
which may ignite surrounding vegetation
 liquid entry into instrumentation lines. Liquids interfere with
the operation of pneumatic controls and instrumentation.
The fuel gas scrubber is a small separator where liquids in the
incoming fuel gas settle to the bottom and the gas rises to the
top outlet.
In case of a high liquid level in the scrubber, the scrubber’s
protective controls stop the flow of fuel gas, shutting down the
line heater burner. The scrubber’s high level protective controls
may consist of:
 an internal float ball that rises with the liquid until the ball
seals off the scrubber’s fuel gas outlet
 a float level switch that is triggered at a preset liquid level to
stop the flow of fuel gas by closing a shutoff valve.
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The scrubber’s drain valve depends on the liquid content of the
fuel gas:
 At sites where the fuel gas normally contains liquid, the
scrubber has a drain valve that automatically opens at a
preset level to empty the scrubber to a liquid drain system.
 At sites where the fuel gas does not normally contain
liquids, the scrubber may have only a manual drain valve.
Because liquid accumulation in the scrubber is abnormal,
any liquid would be drained manually to a portable container
and disposed by the Operator. The Operator must perform
liquid drainage/disposal in accordance with company safety
procedures and company/regulatory waste/effluent disposal
The fuel gas scrubber has a pressure safety valve (PSV) that
opens to relieve excessive pressure to prevent scrubber
rupture. Overpressurizing of the scrubber can be caused by
upstream regulator failure.
Additional Fuel Gas Filtration
Some line heaters may be equipped with fuel gas filtration,
depending on the contaminant(s) to be removed:
 To remove solids, a metal mesh strainer or a particulate
filter is used. The strainer’s removable mesh element can
be cleaned and reinstalled; a particulate filter’s internal
elements are replaceable.
 To remove liquid mist, a coalescing filter is used. The
coalescing filter’s elements are replaceable.
3.3 Burner Fuel Gas
The burner fuel gas piping/valving assembly:
 receives conditioned fuel gas
 branches to direct the fuel gas to the heater’s main burner
and pilot burner
 controls the flow of fuel gas to ensure safe operation of the
heater’s main burner and pilot burner
The specific fuel gas valving/control configuration is determined
by regulatory and insurance requirements and company
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The more stringent requirements incorporate a higher level of:
 safety redundancy
 burner safety controls to automate burner operation and the
burner’s safe startup/shutdown
 instrumentation to enable local and remote monitoring
This section describes fuel gas valving; Section 4 describes
how the valving is controlled as part of safe line heater
operation, startup, and shutdown. Figure 8 shows the main
burner and pilot burner fuel gas flows.
Figure 8—Main Burner and Pilot Burner Fuel Gas Flows
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Fuel Gas Flow to the Main Burner
Conditioned fuel gas flows to the burner fuel gas train and
passes through the following components to feed the main
 a manual block valve used to isolate the line heater from
the fuel gas supply when the line heater is shut down
 a pilot gas drawoff. As described later, this drawoff may be
located either upstream or downstream of the burner safety
shutoff valve.
 a pressure regulator that reduces the fuel gas pressure to
the optimum main burner pressure. The regulator is typically
equipped with a pressure gauge.
 one or two main burner safety shutoff valves that
automatically closes to shut off the fuel gas flow during
abnormal operation. Shutting down the flow prevents an
accumulation of fuel gas, which could lead to a line heater
explosion. Abnormal operation includes:
End of Sample
A full licensed copy of this kit includes:
• Training Module and Self-Check
• Knowledge Check and Answer Key
• Blank Answer Sheet
• Performance Check
• Job Aid
• Exercise
The full version of this kit can be purchased at:
August, 2014
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