BOILER OPERATIONS
Assignment Sheet Number 62B-207
INTRODUCTION
Proper boiler operation is essential to allow the ship to operate effectively and efficiently. This can be accomplished by operating the boilers within parameters. Boiler operations are basically the same for all steam propulsion ships. The parameters and sensing points vary by boiler design.
LESSON TOPIC LEARNING OBJECTIVES
Terminal Objective:
8.0 DESCRIBE the principles, construction, function, components, control and monitoring systems, and operation of a conventional steam propulsion plant and associated auxiliary support systems. (JTI:A)
Enabling Objectives:
8.31 DESCRIBE the requirements, procedures, and restrictions for:
a. Boiler surface blowdown
b. Boiler bottom blowdown
8.32 STATE the purpose, frequency, and restrictions for soot blowing tubes.
8.33 STATE the normal operating parameters and sensing points for the D-type boiler.
8.34 DESCRIBE the effects of deviating from normal boiler operating parameters.
8.35 DESCRIBE the effects of boiler load changes on boiler operating parameters.
8.36 STATE the effects of the following conditions on boiler operation:
a. Fouled firesides
b. Fouled watersides
8.37 DESCRIBE the boiler end points and the effects they may have on boiler operations.
8.38 STATE the safety precautions that apply to lighting off a naval boiler.
8.39 UNASSIGNED; reserved for future use.
8.40 UNASSIGNED; reserved for future use.
8.41 UNASSIGNED; reserved for future use.
STUDY ASSIGNMENT
1. Read Information Sheet 62B-207.
2. Outline Information Sheet 62B-207 using the enabling objectives for lesson 62B-207 as a guide.
3. Answer study questions and scenarios.
STUDY QUESTIONS
1. What are the normal operating parameters for the following:
a. Steam drum pressure: ______+/- ______psi.
b. Steam drum water level: ______ +/- ______".
c. Temperature rise across the economizer: ______°F-
______°F
d. Superheater outlet temperature: ______°F-
______°F
2. Explain the effects on the boiler caused by a low DFT outlet temperature.
3. Explain the effects on the boiler caused by a high DFT outlet temperature.
STUDY SCENARIOS
You are the E.O.O.W. underway, while making your rounds through the engineering spaces you notice that the DFT temperture is ten degrees higher than normal.
1. Is the ten degrees in temperature rise a concern? Why or why not?
After the DFT is corrected ,you find that the superheater temperture is low.
2. Is the low superheater temperature a concern? Why or why not?
INFORMATION SHEET
BOILER OPERATIONS
Information Sheet Number 62B-207
INTRODUCTION
Proper boiler operation is essential to allow the ship to operate effectively and efficiently. This can be accomplished by operating the boilers within parameters. The parameters and sensing points vary by boiler design.
REFERENCES
(a) Boilers NSTM Chapter 221
(b) Fireman NAVEDTRA 10520 series
(c) Boiler Technician 3&2 NAVEDTRA 10535 series
(d) Principles of Naval Engineering 10788 series
(e) Boiler Operation and Maintenance Manual NAVSEA 0951-LP-022-6010
(f) Boilerwater/Feedwater Test and Treatment NSTM Chapter
220 V2
INFORMATION
With the boiler design in mind, certain objectives should include protection of the boiler's pressure parts against corrosion, overheating, and thermal stress. Proper operation reduces the chance of damage and ensures the production of steam at the desired temperature, pressure, and purity.
Normal steady-state operating parameters of a D-type boiler and their sensing points:
Steam drum pressure Setpoint psig ± 5 psig
Steam drum water level Normal ± 1"
Superheater outlet temperature (600 psig) 800-850°F
Economizer inlet 240-250°F
Economizer outlet 340-450°F
At a steady rate of steaming the boiler steam drum pressure should remain steady at set point ±5 psig. If steam pressure becomes excessive, safety valves will lift to reduce the pressure to a safe limit. If steam pressure is allowed to drop lower than 85% of operating pressure, natural circulation will be disrupted and possibly cause overheating of the boiler tubes. When pressure has dropped this low, the forced-draft blowers (FDB) cannot supply enough combustion air to return the boiler to its set point. High or low pressures are usually caused by faulty automatic boiler controls (ABC) or improper acceleration or deceleration of the main engine throttle valve.
Steam drum water level is critical during boiler operation. Watchstanders monitor the boiler gage glass and remote water level indicators. The remote water level indicator also activates an audible alarm when boiler water level is at +7" or -6" to warn watchstanders of the condition. The boiler shall be secured when these alarms sound. The water level should remain at normal level in the gage glass at a steady steaming state, with a tolerance of ±1". High water in the boiler is when the water level is out of sight (high) in the boiler gage glass. This is the most damaging condition to both personnel and equipment, as the water may carry over with the steam into the superheater and main steam piping. This can damage the superheater and turbines. Water can cause corrosion or chemical build-up on turbine blades and could cause destruction of the turbine and injury to personnel. Low boiler water level is when the water level is out of sight (low) in the boiler gage glass. Without water in the boiler tubes, the tubes will quickly and ultimately rupture. Water level problems can be caused by faulty ABCs, a malfunctioning feedwater control valve, main feed booster pump, or main feed pump.
Due to the nature of a ship's operations, the steam demand on a boiler changes frequently. These frequent demand changes affect the boiler water level. When steam demand is increased, the water level temporarily increases. This increase is normally allowed to increase up to a +4" to +6", depending on the boiler design. The increase in demand will also decrease steam pressure. Steam pressure is allowed to deviate a maximum of 8% from the set-point pressure. The increase in steam flow through the superheater momentarily lowers the outlet temperature 10-20°F. As the boiler supplies more steam to the system, the main feed pump increases in speed to maintain feed pressure to the boiler at 150 psig above steam drum pressure. To accommodate the increase in demand, the fuel oil pressure and combustion air flow also increase proportionally. All these actions happen simultaneously through the use of the ABC system.
When steam demand is decreased, the water level in the boiler shrinks, the water level temporarily drops. The water level is normally allowed to decrease to a -3" to -4", depending on the boiler design. The decrease in demand also increase steam pressure. Steam pressure is allowed to deviate a maximum of 3% from set point pressure. The decrease in steam flow through the superheater momentarily increases the outlet temperature 10-20° F. As the boiler is supplying less steam to the system, the main feed pump slows to accommodate the decreased feed water requirements of the boiler. To accommodate the decrease in demand, the fuel oil pressure and combustion air flow will also decrease proportionally. All these actions happen simultaneously through the use of the ABC system.
During normal operation, the economizer inlet temperature should be maintained between 240-250°F. A low deaerating feed tank (DFT) outlet temperature will cause the economizer inlet temperature to be low. This causes the boiler to increase the firing rate to make up for the lower temperature. The steam flow from the boiler through the superheater remains the same. An increased firing rate and an unchanged steam flow cause the superheater temperature to increase proportionally. The opposite happens when the DFT outlet temperature is high. This causes the boiler to decrease the firing rate without a change in steam flow. This causes the superheater temperature to decrease proportionally.
Superheater outlet temperature normally should not exceed maximum except during transients. If the temperature were to remain excessive, the superheater will overheat and could suffer a ruptured boiler tube. Causes of a high superheater outlet temperature are:
Faulty temperature indicators
High levels of excess air
Improperly installed furnace gas baffles
Incorrect fuel oil burner settings and alignment
Improper automatic combustion control adjustments
Damaged fuel oil burner sprayer plates
Leaking fuel oil atomizers
Incorrect burner sequence and mixed sprayer plates
Leaky desuperheater
Low feedwater temperature
Dirty economizer firesides
Low boiler steam pressure
Gas side or fireside restrictions
Low steam flow
A low superheater temperature will cause the boiler to operate at an increased rate to compensate for the lower temperatures, thereby increasing fuel consumption and reducing efficiency. Causes of a low superheater outlet temperature are as follows:
Drum pressure too high
Feedwater temperature too high
Wrong burner combination
Levels of excess air too low or much too high
Excess moisture carry-over
Superheater tubes fouled either on the steam side or firesides
Improper gas baffles and improper bypass areas
The boiler is the "holding tank" for all impurities in the boiler water. These impurities enter the boiler from the feed system or result from the chemical treatment of the boiler water. Since the boiler may be thought of as an efficient evaporator, almost every impurity that enters, will remain in the boiler unless means are provided to rid the boiler of it. These impurities either form scum, which floats on the surface of the water in the steam drum, or sludge, which settles in the lower headers and water drum. To remove these impurities we use the blowdown system. The system is aligned in accordance with Engineering Operational Sequencing System (EOSS) from the hull or skin of the ship to the boiler. The securing is in the reverse order, from the boiler to the skin of the ship.
Surface blowdown provides the normal control of boiler water conductivity, chemical overtreatment, and nondetergent lubricating oil. Surface blowdown shall be conducted on a steaming boiler as necessary in accordance with NSTM 220 V2 as discussed in lesson 62B-203. The following procedures produce a 10% surface blow.
The EOOW must obtain permission to blowdown the boiler from the OOD (u/w) or CDO (inport). Align the blowdown system by first opening the overboard discharge valve and then the overboard guarding valve. By manual or remote manual control raise the boiler water level to three inches above the surface blow take-off pipe in the steam drum. This amount is standard; however, boiler design varies somewhat in the location of the surface blow take-off pipe. Maintain the water level for 4-5 minutes, then open the surface blow valve and monitor boiler water level. Close the surface blow valve quickly when water level has dropped three inches, then repeat these steps a second time, for a total of two blowdowns. After completing the surface blow, return water level to the normal operating level. Close the overboard guarding and overboard valves. Open the drain to the bilge carefully and allow the system to drain off residual water. Flow should stop to indicate the tight seating of the surface blow valve. Some systems have overboard guarding and overboard valves that require a second tightening of these valves after 15 minutes to ensure a tight seal.
A bottom blowdown removes sludge from the lower circuits of the boiler. The boiler must be secured and allowed to stand for a minimum of one hour prior to conducting a bottom blow. A successful bottom blow results in a reduction of the water level in the steam drum. Steam drum water level must be maintained in sight in the boiler gage glass. If deaerated feedwater is not available, discontinue the blowdown and record which valves were not blown in the boiler water chemistry log. Procedures for bottom blowdown are as follows:
The EOOW will obtain permission to conduct blowdown from the OOD or CDO. Raise the steam drum water level to +6 inches using the main feed system from an on-line boiler, or using residual steam from the boiler being blown to operate the main feed pump. Align the blowdown system in the same manner as for a surface blow, opening the overboard and overboard guarding valves. Starting with the water drum valve quickly and fully open the valve until steam drum water level has dropped two inches, then quickly close the valve. Repeat this procedure at each header valve, but allow the water level to drop only 1 to 1½ inches of water, then quickly close the header valve. After the blowdown is complete, secure the system in the same way as for a surface blowdown and check the drain for proper sealing of the bottom blow valves. Bottom blowing a steaming boiler can disrupt natural circulation and cause overheating and ruptured tubes and shall not be done. The 100 psig requirement on the secured boiler ensures there is sufficient pressure to overcome seawater pressure on the hull to perform a proper blowdown.
Soot blowing boiler tubes is necessary to remove the accumulation of fireside deposits from the tubes. Soot is an insulator and must be removed to ensure efficient operation and proper heat transfer. Procedures for soot blowing are as follows:
Permission must be obtained from the Officer Of the Deck (OOD) before starting to blow soot from the boiler. The OOD puts the ship on the best possible course so that soot from the stacks will clear the topside decks and equipment. While conducting soot blowing on a boiler that is supplying steam to the main engine, consideration must be taken that speed and maneuverability will be limited during this operation. Soot blowing shall begin when the boiler is at a firing rate of 50 percent or greater. This is so that the action of blowing steam into the firesides will not affect the stability of the flame on the burners. The burnerman should monitor flame stability carefully during soot blower operation. Align the soot blowing system in accordance to EOSS and ensure that the system is free of any condensate before commencing. As a precaution station a watchstander at the soot blow steam root valve for the entire evolution in case of system failure. Raise boiler air casing pressure two to three inches so that dislodged soot will be carried clear of the boiler and out the stack. To ensure maximum efficiency of the soot blowing operation, blow the economizer first. This will clear the economizer area of any soot that has collected since the last blowing operation. After this follow the sequence of blowing all the soot blowers, from the generating bank and upward in accordance with EOSS. The economizer soot blowers shall be blown again at the end of the operation. All soot blowers should be labeled in accordance with the manufacturer's technical manual for proper identification.
The capacity of any boiler is limited by three factors that have to do both with the design of the boiler and with its operation. These limitations are called "end points" and there descriptions are as follows:
End point for combustion: The process of burning fuel oil in a boiler furnace involves forcing the oil into the furnace through atomizers which breaks up the oil into fog-like spray, and forcing air into the furnace in such a way that it mixes thoroughly with the oil spray. The amount of fuel oil that can be burned is limited primarily by the actual capacity of the equipment that supplies the fuel (including the capacity of the sprayer plates) by the amount of air that can be forced into the furnace, and by the ability of the burner apparatus to mix this air with the fuel. The volume and shape of the furnace are also limiting factors. The end point for combustion for a boiler is reached when the capacity of the sprayer plates is reached, at the designed oil pressure for the system, or when the maximum amount of air that can be forced into the furnace is insufficient for complete combustion of the fuel. If the end point for combustion is actually reached because of insufficient air, the smoke in the uptakes will be black because it will contain particles of unburned fuel. This condition, however, is rare since the end point of combustion is artificially limited by sprayer plate capacity when fuel oil is supplied to the burner manifold at designed operating pressure. This artificial limitation upon combustion in the boiler furnace is the factor that would cause the end point of combustion to occur before either of the other two end points.
End point for moisture carry over: The rate of steam generation should never be increased to the point at which an excessive amount of moisture is carried over in the steam. In general, naval specifications limit the allowable moisture content of steam leaving the saturated steam outlet to ¼ to 1 percent. Excessive carryover can be extremely damaging to piping, valves, and turbines, as well as to the superheater of the boiler. It is not only the moisture itself that is damaging but also the insoluble matter that may be carried in the moisture. This insoluble matter can form scale on superheater tubes, turbine blades, piping and fittings; in some cases, it may be sufficient to cause damage to rotating parts. Because modern naval boilers are designed for high evaporation rates, steam separators and various baffle arrangements are used in the steam drum to separate moisture from the steam.
End point for water circulation: In natural circulation boilers, circulation
is dependent upon the difference between the density of the ascending mixture
of hot water and steam and the density of the descending body of relatively
cool water. As the firing rate is increased, the amount of heat transferred
to the tubes increases with a corresponding increase in the upward flow
of the steam/water mix. The number of tubes devoted to the downward
flow of water remains constant; therefore, a point would eventually be
reached when the downward flow would be insufficient to supply the upward
flow of steam/water mix, and the tubes could overheat and rupture.
This condition would determine the end point for water circulation.
The use of downcomers ensures that the end point for water circulation
will not be reached merely because the firing rate is increased.