While this section covers many aspects of boiler operation, it does not contain all of the technical details of boiler design or function.
National Board of Boiler Pressure Vessel inspectors' statistics indicate that boiler and pressure vessel failures result in many injuries and deaths. In spite of sophisticated mechanical safety devices there is no substitute for constant vigilance by the Engineer or his/her immediate staff.
The most frequent causes of boiler accidents were noted as deferred repairs and maintenance, and improper feedwater treatment. It is important for engineers make certain that each school facility with boilers and pressure vessels is maintained and operated in accordance with established regulations and the preventive maintenance program.
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The normal life cycle of a high or low pressure boilers vessel/boiler can be extended for many years of beneficial service if the operators are property trained. As boiler design and technology improve there are less frequent reports of boiler failures. Fortunately, the Chicago Public Schools engineers have had a preventive maintenance program, well trained staff and a successful in-service program in operation for over 30 years. Our preventive maintenance (PM) program is effective in providing for safe use and occupancy of school facilities.
It must be noted that untrained and unconscientious boiler operators are a liability. Individual carelessness usually results in unnecessary costly boiler failures and nuisance down time. In any boiler maintenance program the operator must be apprised of the importance of the position and be thoroughly trained. The element of time does not provide for an informal training process. Individual programs must be guided to involve and motivate the employees interest in his/her assignment. The engineer must provide instructions for an emergency shutdown of the boilers, fire alarm procedures, and normal operations. A rehearsal and practice in shutdown procedures is cost effective. In addition, and most importantly, them is no substitute for constant attentiveness.
Automatic low-high water control equipment must be serviced on a daily basis when the boiler is in operation. A high frequency of boiler failures is the result of low water, and can be attributed to a careless boiler operator. A procedure must be established at your school to regularly clean the glass gauge column by "blowing down" the column at the start of the school day, during non-peak operating periods, and at the conclusion of the school day or shift. This ensures ability to determine the level of water in the boiler.
A major reason for damages incurred to low pressure steam boilers is the low water within the boiler. If the condition of low water exists it can seriously weaken the structural members of the boiler, and result in needless inconvenience and cost. Low pressure boilers can be protected by installing an automatic water level control device.
Steam boilers are usually equipped with automatic water level control devices. It must be noted, however, that most failures occur due to low water on boilers equipped with automatic control devices. The water control device will activate water supply or feed water pumps to introduce water at the proper level, interrupt the gas chain and ignition process when the water reaches the lowest permissible level, or perform both functions depending on design and interlocking systems. No matter how automatic a water control device may be, it is unable to operate properly if sediment scale and sludge are allowed to accumulate in the float chamber.
Accumulations of matter will obstruct and interfere with the proper operation of the float device, if not properly maintained. To ensure for the reliability of the device, procedures must be established in your daily preventive maintenance program to allow "blow-down" the float chamber at Ieast once a day. Simply open the drain for 3 to 5 seconds making certain that the water drain piping is properly connected to a discharge line in accordance with City Building Codes. This brief drainage process will remove loose sediment deposits, and at the same time, test the operation of the water level control device. If the water level control device does not function properly it must be inspected, repaired and retested to guarantee proper operation.
There are two very effective tests for low water controls on steam boilers. The first is the quick drain. or blowdown test, which should be performed at a time other than a peak steam generating period. As the water is drained from the column the firing sequence is interrupted, the low water alarm signal activates and the boiler operation shuts down.
The second, and more costly method is the slow-drain test. By opening the blowdown valves the water level can be checked to determine the water level in the column, the gauge glass, and the boiler. The boiler should shut down while you determine the level in the gauge glass.
As a safety precaution, the low water float chamber of hot water boilers should be tested daily, at the beginning of the shift, at the end of the shift, and once during non-peak firing periods. Time of tests and the boiler controls tested should be recorded on your Boiler Room Log.
Annually, or as required, a thorough inspection of all low water control parts shall be performed. The annual inspection should include opening and cleaning the water chamber.
Old, worn and obsolete feed water pumps are sometimes overlooked as potential problems. A centrifugal pump may have worn seal rings that allow the water to chum between the suction and discharge openings.
An indicator of the latter problem is low pressure discharge. Also, by comparing the time it takes to raise the boiler water level to a predetermined level or the time to empty the condensate tank to the time it formerly required, it is possible to determine if a pump is operating properly. Also, a pump that operates quietly does not mean it is functioning properly.
Safe operation of a boiler is dependent on a vital accessory, the safety valve. Failure to test the safety valve on a regular basis or to open it manually periodically can result in heavy accumulations of scale, deposits of sediment or sludge near the valve. These conditions can cause the safety valve spring to solidify or the disc to seal, ultimately rendering the safety valve inoperative. A constantly simmering safety valve is a danger sign and must not be neglected. Your preventive maintenance program includes the documentation and inspection of the safety valve. A daily test must be performed when the boiler is in operation Simply raise the hand operating lever quickly to its limit and allow it to snap closed. Any tendency of a sticking, binding or leaking of the safety valve must be corrected immediately.
Steam traps have play a very important role in steam distribution systems. The service performed by steam traps is primarily to discharge condensate. Normally a steam trap can be easily and quickly selected by considering only the average operating conditions. However, an exact analysis of these conditions will give the proper data necessary for selecting the type and size for greater savings and proper plant operation. After the careful selection of the steam trap, it must be properly installed, tested, periodically inspected, cleaned and maintained to keep it operating efficiently.
Traps need cleaning periodically. A simple way to prevent dirt from entering is to drop a short length of pipe vertically below the supply to the trap (called a dirt leg) which can be cleaned easily and frequently.
Traps can be seriously damaged by scale or pipe comings in lines. A good practice is to install strainers ahead of the traps which should be inspected and cleaned frequently.
Traps are subject to severe wear if steam blows through continuously. They should be inspected for worn valve parts or a change in operating conditions.
When a steam trap fails to discharge, inspect the heating system and be certain that all units are drained with separate traps, thus guarding against short circuiting, loss of energy, and reduction of operating efficiency.
Traps operating under high pressure or superheated steam are often insulated in a manner similar to adjacent pipe lines. In such instances, they shall be fitted with dirt pockets, test valves, and drains.
Steam traps installed in areas exposed to climatic conditions will lose heat if not insulated and may freeze unless adequately protected. Discharge lines should be short and self draining and traps should be fitted with a drain tapping and valves.
Steam traps handling large volumes of air require more frequent inspection and proper venting for efficient operation. Vents shall be used to avoid air binding and ensure positive drainage. Gauge glasses shall be kept in proper repair, for they indicate whether or not the trap is working. Periodic cleaning and gauge glass replacement shall be considered as a high priority in the maintenance of steam traps.
All steam traps require protection from corrosion to prevent unnecessary deterioration. All valves, joints, and gaskets should be kept tight to avoid steam leakage and ultimate energy losses. For continuous and efficient operation. steam traps require periodic inspection and maintenance for purposes of eliminating foreign matter and obstructions in supply and discharge lines. Each steam trap at an assigned work station should be inspected as specified by the preventive maintenance program.
Engineers are reminded that all equipment inspection, operation, and maintenance programs are designed to create a more efficiently operating heating plant. This type of operation is the only acceptable manner in which to heat a facility and will ultimately provide the means toward saving taxpayer money.
It is important to inspect the operation of steam traps frequently. There are many conditions under which traps may fail to operate property. The following are some of the most common reasons for trap failures:
1. Condensate does not flow into the trap:
a. Obstruction in line to trap inlet.
b.Valves leading to trap are closed.
c. Bypass open or leaking.
d. Trap may be air bound.
e. Insufficient pressure to blow condensate through orifice.
f. Improper installation of trap.
g. Accumulation of foreign matter within the trap.
h. Trap held closed by defective mechanism.
i. Strainer may be blocked.
2. Condensate fails to drain from trap.
a. Discharge valve may be closed.
b. Trap may not be large enough to handle condensate.
c. Pressure may be too low to blow the condensate through.
d. Improper installation for draining.
e. Check valve may not be holding.
e. Obstruction in return line or the line may simply be too small.
3. Trap does not shut off.
a. Trap is too small for the condensate load.
b. Trap held open by defective mechanism,
c. Overload due to excessive boiler foaming or priming.
d. Submerged steam coils leaking.
e. Differential pressure exceeds design of trap.
f. Scale or foreign matter lodged in orifice.
4. Steam blows through trap.
a. Valve mechanism does not close due to wear or defective valve.
b. Mechanism is held open by foreign matter.
c. Trap has not been properly primed or reprimed after clean-out or blow-off.
d. Bypass is open or leaking.
e. Excessive pressure for design of trap.
As soon as possible after the end of the heating season, take these steps, where applicable:
In recent years centrifugal pumps have been used to replace steam powered reciprocating pumps in Chicago schools and the respective heating plants. This reduces the dependency on steam generation for operation of pumping systems prior to attaining steam operating pressures. For a better understanding of the centrifugal pump we must fully understand the principle of operation. A centrifugal pump is one which depends on centrifugal force and the rotation of an impeller for its action. The type of pump employed depends on the service for which it is intended and varies with the capacity required, the variations in suction and discharge head, the type of water handled (whether hot or cold, clean or dirty), and the type of the drive to be employed.
Centrifugal pumps may be classified as follows:
The engineer must have the operating knowledge to properly instruct personnel and ensure continuity of maintenance schedules for centrifugal pumps. It should always be remembered that preventive maintenance of a pump begins the day the pump is selected. You must remember that full realization of life expectancy can only be attained through proper implementation of operations. The following guidelines will assist in attaining these goals:
The head of a pump is calculated to determine the height to which the liquid can be raised (vertical lift). Theoretical lift is 34 feet, however, this is not possible in general practice. At atmospheric pressure of 14.7 p.s.i. the theoretical lift is approximately: 14.7 / 0.433 = 33.9 feet. Temperature increases will decrease the suction lift.
1. Determine Total Static Lift - Measure the vertical height the liquid is raised with a tape measure or pressure gauge. Most pumps have 1/4 inch tap holes at pump nozzle flanges to permit use of gauges.
2. Determine Total Friction Loss - Losses depend on size, condition and length of piping system in relation to gallons pumped per minute, plus valves and fittings used. Charts and nomographs are readily obtained from pipe manufacturers and suppliers for determining friction loss.
3. Add Totals and Compare with Pump Ratings
If the total head (total static lift plus total friction loss) fails to correspond to the pump rating either a pump of different capacity should be substituted or friction losses should be reduced. Reduction of friction losses will be realized by use of larger pipe, new pipe, or elimination of high loss fittings.
4. Exception, Hot Water - At atmospheric pressure water turns into steam at 212F, which affects and limits the suction lift possible with hot water. The danger is that low pressure at the pump impeller eye will cause water to flash into steam at considerably lower temperatures. Thus, hot water cannot be lifted very high, and extremely hot water cannot be lifted at all. In many instances, the pump must be considerable lower than the source of hot water, so water will flow in under pressure. Pressure required depends on the temperature of the water, pump capacity, and type of impeller installed.
As the weather moderate, there is often a tendency to prop open the boiler room door leading to tunnels, plenums and interior portions of the school facility. In accordance with Building Codes, the interior boiler room door shall remain closed at all times. There are no exceptions.
Passage through boiler and engine room by employees, other than those of Department of Facilities, should be discouraged. The stairways, ramps and walks in these areas are not designed for general use or egress. Employees not specifically assigned to work in these areas, should be encouraged to use other entrances and exits which lead to and from the corridors.
Signs may be posted on boiler room doors indicating 'Authorized Board Personnel Only'. However, this does not mean that educational and operational staff do not have access to the boiler room for the purpose of communicating with the engineering and custodial staff.
Under no circumstances shall student class sessions be conducted in any boiler room or equipment area.
Engineers and Custodial Workers In-Charge are reminded that steam generating vessels and low and high pressure boilers must be inspected during the calendar year. If you have not scheduled an inspection with City of Chicago Boiler inspectors, contact their offices, at 312-744-3516, and establish an inspection.
High Pressure Boilers. Steingress and Frost American Technical Publishers, Inc.
Steam Plant Operation. 5th Edition. Lammers, E, Lammers, H., and Lammers, T. McGrawHill, 1984.
Monthly Boiler Mechanical Safety Checks
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