Heat Trace 030203f Flashcards
What are the 3 main reasons for using heat trace?
To prevent the freezing of liquids in a pipe or container.
To melt snow and ice.
To maintain a specific fluid temperature in a pipe or vessel.
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What are the 4 basic design approaches to electric heat trace systems?
Series fixed resistance.
Parallel fixed resistance.
Self-regulating variable resistance.
Skin effect.
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What principle does a series fixed resistance system use?
A resistor in series with a supply voltage.
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What are the two main types of cables used for a series fixed resistance system?
Mineral insulated.
Polymer insulated.
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What is used to make a mineral insulated (MI) cable?
The mineral insulation is either magnesium oxide or silicon dioxide.
The sheath is either copper or stainless steel.
Contains one or more bare conductors.
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When two-conductor single termination cables are manufactured, what are the two things that are changed?
Specific lengths.
Specific heat output.
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Can mineral insulated cables be repaired in the field?
Only by trained personnel with proper equipment and techniques.
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What does a one-conductor polymer insulated cable require?
Dual termination.
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What does a two-conductor polymer insulated cable require?
Single, one-end termination.
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How are three-conductor polymer insulated cables configured?
Three-phase wye heating cable with one-end termination.
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What length can polymer insulated cables get up to?
3500m long with a single end connection to a power source.
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What does the length of the cable in a parallel fixed resistance system affect?
The length of the cable will affect the current being drawn from the source.
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What does a self-regulating cable do?
Changes the resistance of the cable based on the temperature.
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What is the design of a self-regulating variable resistance cable?
Multiple parallel resistors between two bus conductors.
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What is the difference between parallel resistance cables and power limiting cables?
Both are constructed with parallel heating elements however with power limiting cables the heating elements are an alloy that changes resistance with temperature.
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What phenomenon does skin effect heat trace rely on?
That current flows only on the inside of a ferromagnetic tube.
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What distances can skin effect heat tracing be used for?
3 km up to approximately 15 km.
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What voltage is applied to the conductor in a skin effect heat trace cable?
2000 V to 5000 V
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What are the two main types of self-regulating cables?
Self-regulating polymer.
Power limiting.
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What are four common faults with heat trace?
Incorrect temperature settings.
Incorrect temperature sensing location.
Insulation breakdown.
Incorrect voltage applied.
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What are some methods of testing for troubles with heat trace?
Manually measure the temperature of a service.
Insulation testing the heat trace.
Check terminations and splices.
Ensuring the correct voltage is applied to the heat trace.
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How do you properly test the insulation of heat trace with a megohmmeter (megger)?
Disconnect both ends of the heat trace and perform an insulation test of the entire run.
If the test fails then move to a point in the middle of the cable.
Test in both directions to further pin point the fail location.
Continue to do this until the portion that needs to be replaced is found.
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What is a thermostat used to sense the temperature of for heat trace systems?
Outside ambient air temperature.
Temperature of the surface that it is heating.
Temperature of a fluid in a process pipe.
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How can a simple thermostat be connected in a heating trace system?
In series with the cable.
May also be connected in a control circuit of an electromechanical heating contactor.
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What type of switching does an electronic control use?
Solid state switching that has no moving mechanical parts.
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What are the advantages of electronic control switching over electromechanical switching?
No moving parts to wear out.
On/off switching time is in milliseconds.
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What are two applications of electronic control that a simple thermostat control cannot do properly?
Soft start a heat trace circuit.
Control average current for fine temperature control.
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What are the disadvantages of electromechanical switching?
Moving parts get worn out with repeated use.
Contactor may wear out from rapid switching.
Repair requires down-time and increases maintenance costs.
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What is soft starting?
Electronic controller gradually raises the voltage to a circuit. As a cable warms the resistance increases and voltage can then be increased without current increasing.
Avoids nuisance tripping from a high initial current.
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What must heat tracing circuits for surface heating have?
Ground fault protection.
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What are the two options for securing heat trace cable?
Fibre tape for non-metallic outer coverings.
Metal banding for metallic sheath MI cables.
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In what way is heat trace cable wrapped around valves and flanges?
Due to valves needing to be replaced and flanges opened, heat trace is wrapped in such a way that it can be removed without cutting the cables.
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Why do flanges, pipe supports and valves require more heat to keep the temperature at required levels?
Have greater heat loss than the pipe.
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