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47 Cards in this Set

  • Front
  • Back
Define Combustion:
A chemical reaction in which oxygen combines with some type of fuel to produce heat
What are the required components for combustion?
Fuel, Oxygen, Ignition source, & Uninhibited chemical reaction
What are the main products of a normal combustion reaction?
The uninhibited reaction produces one carbon dioxide molecule, two water molecules, and heat
What are the products of an incomplete combustion reaction?
carbon monoxide, hydrogen, carbon, % nitrogen oxides
What is excess oxygen and why is it required?
Excess oxygen is the amount of oxygen in excess of the amount oxygen needed for sustained combustion.
Excess oxygen is needed to ensure complete combustion of all fuel
What are the adverse effects of incomplete combustion?
Increased fuel consumption
Afterburning
Flame impingement
Explosion hazard
Environmental exceedance
What is the normal target for excess oxygen?
Excess oxygen is typically targeted between 2.5 to 4.0% depending on the fired equipment and the type of fuel.
How is excess oxygen primarily controlled?
Excess oxygen is primarily controlled with air control mechanisms such as:
Air registers
Inlet air dampers
What is the function of a burner?
Mix fuel and combustion air, allowing combustion within a desired, fixed volume when an ignition source is provided.
How does a raw gas burner work?
Characterized by interaction of fuel gas in the combustion area only after the fuel leaves the burner tips.
Combustion air is admitted into the burner through air registers.
How does a premix burner work?
Premix burners have two sources of combustion air – primary and secondary.
Primary air - Drawn into the fuel line through an inspirating device and mixes with the fuel before it reaches the burner tip. May supply up to 60 percent of the total combustion air
Secondary air - Remaining combustion air enters the burner through secondary air registers
What is the function of a pilot?
Pilot burners are premix burners used to provide ignition and re-ignition of heater burners.
How does a pilot re-ignite itself?
Uses a metal that stays red hot for an extended period of time to allow for relighting
Why is NOx production within a heater?
NOx is formed by the combination of nitrogen and oxygen in the high temperature environment of the burner.
How does a low-NOx burner work?
Primary and Secondary Fuel Gas Risers – fuel gas is injected in two stages to reduce NOx emissions.
How does a low-NOx burner reduce NOx production?
NOx production decreases with decreasing peak flame temperature
How does an ultra-low-NOx burner work?
The addition of internal flue gas recirculation to a staged fuel burner reduces NOx production by a greater extent than staging the fuel alone.
What is the purpose of refractory?
Prevents escape of heat
Protects metal casing of the fired heater
Reflects radiant heat back to the radiant section tubes
What is a double-fired tube?
Burners are on both sides of the tubes.
Why do convection section tubes have extended surfaces?
Extended surface provide more surface for heat transfer. They increase the amount of heat absorbed from the flue gas.
What is the “shock bank”?
The shock bank protects the convection area tubes from overheating, and degradation or coking of the process fluid in the convection area tubes.
What is a heater pass?
Tube Pass – A continuous flow circuit consisting of one or more tubes from inlet to outlet, in series with or without connecting fittings
Where does process fluid normally enter a heater? Why?
process fluid normally enters the convection section. The fluid is coldest at this point, and is able to absorb more heat from the flue gas.
What is draft?
Draft is the difference between the atmospheric pressure and the pressure inside fired heater at an equivalent height.
What is the normal target for draft?
-2 inches of water
Why is draft important?
A positive pressure inside the heater will cause flue gas leakage and damage to the
fired heater casing and structure. And, a positive pressure can be hazardous to the
operating personnel.
Where is draft normally measured?
Draft is normally measured at the top of the bridgewall section.
How is draft primarily controlled?
Draft is primarily controlled by the stack damper.
Explain natural draft.
A natural draft heater allows the buoyancy of the hot gases to provide the movement of air and flue gas through the heater.
Explain forced draft.
A forced draft system relies upon mechanical means, typically forced draft (FD) fans, to supply the needed combustion air pressure at the burners
Explain balanced draft.
A balanced draft system uses a forced-draft fan to supply combustion air to the burners and an induced-draft fan to create the necessary draft.
What is the purpose of an air preheater?
Air preheater systems reduce fuel consumption by recovering heat from the flue gas leaving the convection section. Heats incoming combustion air (from the forced draft fan) with heat from the flue gas being drawn out of the heater (by the induced draft fan).
What will cause an emergency shutdown of a heater?
Low fuel gas pressure
Low process flow
Low combustion air
High bridgewall pressure
What happens when a heater ESD is activated?
Fuel gas supply is shut off.
What is afterburning?
The burning of fuel outside the combustion zone.
What should you look for when making rounds on a heater?
Draft%, excess O₂%, flame shape size and color, fuel gas pressure, refractory, impingment.
What is the typical appearance of a proper gas flame?
Stable flame exhibits a solid shape off the burner tips
Straight, not tilting
Proper height
Proper color
Short non-smoking
Not pulsating
Why are pass flows typically balanced in a heater?
To keep heat distribution even across all tubes. To prevent damage to the tubes and prevent coking.
What is the problem with having too much excess O₂ in a heater?
Excess oxygen above target range contributes to heater inefficiency.
The excess oxygen is passed through the burners and not totally burned and must be heated along with the desired process fluid to achieve desired temperatures.
How is coil outlet temperature normally controlled
Fuel gas flow controler
Explain how taking out a burner affects fuel gas pressure
Coil outlet temperature goes down and fuel gas pressure goes up.
Explain flooding.
Flooding is incomplete combustion
What are the causes of flooding?
Causes of flooding are too much fuel and too little oxygen.
What are the symptoms of flooding?
The symptoms of flooding lower coil outlet temperature, lower excess oxygen reading, higher fuel gas pressure, heater will be huffing and puffing, & heater will have smoke from the stack.
What actions must be taken to correct flooding?
Cut back fuel gas supply to the burners by 25% of the steady state.
ESD
When is it considered corrected?
When coil outlet temperature makes a turn and starts to recover
What are the steps to starting a heater?
Inspect
Process flow
Purge
Pilot
Burner