I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
2.7 Troubleshooting the Oil Burner
3. Lightly lubricate the labyrinth seal of the atom- izer taking care to not load up labyrinths or fill exit port or steam ports.
4. Lightly lubricate the threads on the 1 in. (25 mm) SCH 40 steam tube and replace firmly onto the oil body. This procedure should result in the atom- izer end being 1/4 ± 1/8 in. (6 ± 3 mm) inside of the sleeve (see Figure 2.26).
5. Lightly lubricate the threads on the oil tip and install onto the oil gun making sure that the beveled portion of the tip firmly seats on the bev- eled portion of the sleeve.
The oil gun is now ready to be reinstalled into the burner.
Steam is the best method to atomize heavy oil. Light oils such as No. 2 and naphtha can be atomized with air in some cases. For the majority of oils, steam is the best alternative for clean oil fires and proper atomization. The condition of the steam is very important to good atomi- zation. This means that the steam must be dry and deliv- ered to the burner at a higher temperature than the oil.
Normally the standard 150 psig (11.4 barg) steam, avail- able in most plants, will work for oil-firing applications.
In some cases, such as a heavy pitch, the use of 400 psig (30 barg) steam may be required to ensure the steam is hotter than the oil temperature. We generally label oil as
“pitch” if the API gravity is less than 10.
The steam has to be delivered to the burner at the right pressure for the oil gun to work as designed. Most John Zink guns are designed to operate with a constant differential pressure of 20–30 psig (2.4–3.1 barg). This means the steam pressure will be 20–30 psig (2.4–3.1 barg) higher than the oil pressure. In some cases, a higher dif- ferential may be required for proper atomization. Steam consumption varies with the type of oil gun. Normal rates are between 0.15 and 0.3 lb/lb.
Two of the most common problems, when using steam atomization, are the lack of insulation on the steam pip- ing and faulty steam traps that allow condensate into the burner. The steam piping should be fully insulated all the way to the oil gun on the burner. Leaving just three to four feet of pipe uninsulated can cool the steam enough to produce condensate.
When a steam trap is working properly, there will be substantial difference between the temperature of the inlet and outlet piping to the trap. A difference of 50°F–70°F (10°C–21°C) usually indicates normal operation. A downstream temperature within a few degrees of the upstream temperature indicates that the trap may not be working as it should. The down- stream temperature should always be lower than the inlet temperature.
Another “best operation” recommendation is to pro- vide steam with a slight amount of superheat such as 40°F–50°F (22°C–33°C). This will keep the steam dry and free of condensate.
John Zink provides a very detailed “trouble- shooting” guide in the “Installation and Operating Manual” provided with all of its oil-fired burners.
The operators and maintenance people can use this to resolve many of the common problems associated with oil firing.
In general, poor oil firing will exhibit six problems to the field operator, either singly or in combination (see Figure 2.29):
1. Smoky flame
2. Fireflies or sparklers in the firebox 3. Lazy or uncontrollable flame pattern 4. Instability
5. Coke formation 6. Oil spills 2.7.3 insufficient air
Insufficient air in a process heater is often one of the most difficult problems to solve because it can be either the main problem, whose solution is simply to increase the air/fuel ratio, or it may be the result of any one or a combination of the other listed causes.
In the case of multiple burner operation, to address air deficiency as a cause, review the single burner opera- tion with respect to the total heater operation. A single burner operating with insufficient air will still smoke even though the total firebox may exhibit sufficient excess air for completed combustion. This is due to the additive excess air from all other burners.
Specifically, an overall review of all burners, noting:
• Gas pressure on gas-fired burners
• Oil and atomization pressure on oil fired burners
• Position of hand block valves on
• Gas
• Oil
• Atomization medium
• Register settings
Figure 2.28 Fouled oil guns.
should give a good indication of whether any particular burner is being operated at a capacity greater than would seem suitable for its air supply. Neglecting airflow distri- bution in any forced-draft system and minor tolerance differences in burner throat tile installations, all burners with equivalent fuel supplies should have approximately equivalent register settings. It should be noted that this superficial preliminary review is not valid unless all manual block valves are fully open.
If the aforementioned review yields no major variations in fuel/air supply systems and air shortage is still sus- pected, a physical inspection of primary and secondary (and tertiary where applicable) air throats should be made to ensure no foreign materials are obstructing proper air- flow. This inspection can usually be made through sight ports and/or register assemblies; however, in some cases, a probe may be required. If no obstruction is evident in the tile throat(s), inspection and manipulation of the air reg- ister assembly(s) is indicated to rule out register blockage.
Given the aforementioned conditions are shown not to be a problem, the final step is to make a simple static pressure reading within the register assembly. This reading should entail no less than three locations, one of which should be made within the primary air sup- ply zone, downstream from the primary air supply on a regen-style burner and within 6 in. (15 cm) of the back- side of the diffuser in a diffuser-style burner. These static pressure readings are a valid indicator of plenum air dis- tribution and/or furnace draft variations and should be compensated by register settings.
In some cases, spot checks of oxygen levels within the radiant box can also be used for register adjustment to achieve balanced multiple burner operation.
Once the air supply side of the burner(s) in question has been determined to be balanced, the fuel supply becomes questionable.
2.7.4 lack of atomizing Pressure
Check the manufacturer’s capacity curve supplied with the burner. Each oil gun capacity curve should indicate both the type of oil used for design sizing and the sug- gested atomization pressure for that oil.
Generally, the low viscosity oils require lower atom- izing pressures than the higher viscosity oils. If a change in viscosity has been experienced either through a change in oil from design or a change in operating temperature, it is possible that a revision in atomization pressure may be required.
Since the “EA” series gun uses an internal mixing cham- ber, this type of gun is subject to variations in oil flow at any single oil pressure. A small variation in atomizing medium supply pressure can significantly change the internal pres- sure of the mixing chamber. Therefore, low atomizing sup- ply pressure will increase the net oil flow, increasing the firing rate, thereby causing that burner to be “overfired” at a pressure which it normally would have sufficient air.
2.7.5 lack of atomizing Flow
Plugging of atomizer steam ports with pipe scale, dirt, and/or particulates will cause a reduction in atomizing medium flow, effectively reducing the mixing cham- ber back pressure, resulting in the same effect as low atomizing pressure. A full discussion of disassembly and cleaning of the “EA” series oil gun is covered in the maintenance section.
2.7.6 Wet Steam
Specifically, on those burners designed to use steam as the atomizing medium, it should always be clean, dry steam. A suggestion of 20°F–40°F (11°C–22°C) superheat
Problem Possible causes
1. Smoky flame 2. Fire flies 3. Lazy flame 4. Instability 5. Coke 6. Oil spill
X X X X X X X X X
XX XX X X X X X XX X
XX X X X X XX XX XX
XX XX X
X XXX
X X X X X X X X
A. Insufficient air B. Lack of atomizing pressure C. Lack of atomizing flow D. Wet steam E. Cold oil (heavy oil) F. Tip/atomizer failure G. Diffuser/regen failure H. Low oil flow I. Low oil pressure J. High atomizing pressure K. Leaking cross over valve L. High steam temperature M. Oil tip mispositioned
Figure 2.29
Oil-firing problems and possible causes.
should ensure this situation. However, all atomizing steam systems should be trapped.
Since water is lower in energy than steam, atomiza- tion with wet steam is not as effective as with dry steam.
This lower-energy atomization will result in a larger droplet size and some oil-coated water droplets being dispersed into the combustion zone. These larger drop- lets and oil-coated water droplets are slower burning and can often be seen floating on the internal firebox currents. These “fireflies” are a major source for ash and soot buildup on radiant and convective tubes.
A second effect is that since this lower-grade atomi- zation is slower burning and accumulation (or sludg- ing) of water is not combustible, burner stability may be severely reduced.
2.7.7 Cold Oil (Heavy Oil)
On those burners operating on heavy or viscous oils, the oil should be heated to a temperature sufficient to achieve 200–250 SSU. This suggested viscosity range is sufficient for proper atomization and any reduction in oil temperature will increase the oil’s viscosity.
Viscosity is a measure of the oil’s resistance to break up, and as the viscosity increases, the quality of atomi- zation and combustion decreases.
2.7.8 Tip/atomizer Failure
The “EA” series oil gun operation is dependent on a number of machined orifices, channels, and seals. These pieces are subject to high-velocity abrasive flows and corrosive action dependent on the type of oil fired.
Clearly these orifices, channels and seals are subject to some “normal” wear, making them a “maintenance item.” Additionally, this condition is aggravated by the common, and some not so common, contaminants found in many oils. Coke or carbon particles, catalyst fines, and silica particles have a highly erosive action on metal parts when subjected to high-pressure, high-velocity metering, while sulfur, chloride compounds, and, in some cases, anhydrous acids will severely attack, through corrosion, the materials of the atomizer and dispersion nozzle.
The use of hardened tip and atomizer materials for erosive oils and 300 series stainless steel or higher for corrosive oils is suggested.
Some typical effects of tip and/or atomizer deteriora- tion are as follows:
1. Enlargement of oil orifice—high oil flow, low atomizing medium ratio, poor atomization, and burner overfiring
2. Enlargement of atomizing orifices—high atom- izing medium flow, low oil flow, reduction in oil gun capacity, and reduction in low fire stability
3. Enlargement of atomizer exit—lowered mixing chamber pressure, reduced atomization quality, and burner overfiring
4. Deterioration of atomizer labyrinth seal—steam bypassing of atomization chamber, poor atomi- zation, instability, and unsymmetrical flame patterns
5. Deterioration of dispersion chamber—reduc- tion of exit port L/D, deterioration of dispersion pattern, coking, and oil spills
6. Enlargement of exit ports—reduction of exit port L/D
See the maintenance section for details of disassembly, inspection, and cleaning of the oil gun.
2.7.9 regen/Diffuser Failure
Commonly, a failure in either the refractory of the regen tile or the metallurgy of a diffuser cone is the result of some other oil-firing problem. However, it should be noted that these parts are integral and necessary to the proper function of their burner. Failure of these parts should be acted upon with replacement immediately.
As a secondary consideration, nonconcentricity of these parts with respect to secondary tile throat and oil tip will cause poor air distribution, nonuniform flame patterns, coking, and oil spills.
2.7.10 low Oil Flow/low Oil Pressure
Extreme reduction in oil-firing rate and/or plugging of the oil orifice from pipe scale or oil-borne contami- nants can cause severe burner stability problems, while the lowered exit port velocities can cause dripping or internal oil spills. If, for any reason, the burner capac- ity requirements are reduced by any appreciable per- centage, new reduced capacity oil guns are suggested.
2.7.11 High atomizing Pressure
As discussed in Section 2.7.4, the converse is true. High atomizing medium pressure will increase the mixing chamber back pressure, thereby reducing the oil flow.
In many cases, this raised atomizing medium/oil ratio can cause severe stability problems.
2.7.12 leaking Crossover Valve
Since the “EA” series oil gun is commonly operated with the atomizing medium at a higher pressure than the oil, a leaking purge crossover valve can cause severe disruption in oil flow to the oil gun and be detrimental to the atomization of the oil supplied.
The bypassing of atomizing medium into the oil sup- ply is typically characterized by what is commonly called “motor boating.” This continuous disruption of oil flow is clearly audible, thus deriving its name from the similar sounds.
2.7.13 High Steam Temperature
High temperatures on the atomizing medium cause problems in two separate ways, but these problems can be directly tied to the medium temperature.
Light oils often can be adjusted to very clear, yellow fires; however, this same fuel will often exhibit insta- bility, pulsation in flow, clear blue/bright yellow flame envelope, and haze at the flame boundary. These are all indicators of fuel oil vaporization within the oil gun.
This two-phase or vapor flow, through orifices origi- nally designed for liquid flow, will severely reduce the oil gun capacity and stability.
Heavy oils that contain residual or added light oil will exhibit these same problems, as the light oils flash, with the added problems of heavy oil slug flow and resulting in smoke and poor atomization of the heavy ends.
2.7.14 incorrect Positioning of the Oil Tip
By far, the most common oil-firing problem and the most detrimental condition to oil firing is the incorrect position- ing of the oil tip with respect to its air supply/stabilization source. Due to variations in oil, atomizing medium, oil temperature, atomizing medium quality, burner airside pressure, operating oil/atomizing medium pressures, and furnace requirements for flame pattern, all John Zink Company oil guns are supplied with oil tip adjustability.
While the tip position is located in the John Zink Company burner assembly drawing supplied with every job, the final position is a field operator adjust- ment for optimum operation.
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