Section 2(a) of Indian Boiler Act, 1923 defines “Accident” as follows:
“Accident means an explosion of Boiler or steam pipe or any damage to boiler or steam pipe which is calculated to weaken the strength thereof so as to render it liable to explode.”
The root cause of majority of the accidents is non-following of the safety norms and safety procedures.
Therefore, the explosion of the boiler and the secondary combustion in the boiler are of prime concern as they resulted in severe accidents.
Coal combustion is a function of turbulence and air distribution. The normal residence time of coal particles inside the furnace is 3-5 seconds & the particle to exit from the chimney is 22 seconds. As a result, incombustible will remain inside the boiler for 22 seconds. Falling of heavy, high temperature deposited clinker to the bottom ash hopper leading to furnace pressurization and resulting in an explosion of the furnace and escaping out of the flue gases from the weakest portions of the boiler causing a severe accident.
If a steam boiler is properly operated and maintained, the likelihood of an explosion is virtually eliminated.
Improper purge can leave a combustible mixture in a boiler. Many explosions occur during attempts to relight a burner after it has tripped because of another problem and will result in large unburnt combustible gases in the furnace which leads to explosion.
This scenario can be avoided by investigating the cause of the trip and allowing the furnace to purge thoroughly before any attempt to relight.
Safety practices during the erection of boiler
Top girder jack-up, structure erection, installation of pressure part components, and hydrostatic tests are the crucial activities as far as erection is concerned.
- Prior to the commencement of the girder jack-up and structure erection work, the management team will ensure HSE (Health Safety and Environment) Plan and pep talk procedure which is to be applied
- All workers together with the frontline supervisor shall conduct safety task assignments every day before the commencement of the work
- Supervisors shall make sure that all workers under their supervision understand and shall fully comply with the assigned job and all requirements mentioned in the standard procedure
- No welding is to be carried out during jack-up operation and will be allowed without proper checking of earthing and welding equipment
- Lifting equipment must be with a valid certification
- The additional assisting cranes should be prepared in good condition with the certified crane operators
- All open holes should be well protected
- No lifting allowed under harsh weather condition
- Movement should be stopped if instructions are not clear
- There should be good communication with handy walkie-talkie of special frequency
- The working platform should be as per standard and with sound material
- The height work permit is required
- All structure erection must be finished as per requirement including bolt tightening
- Experienced riggers are required to be deputed to handle rigging
- The load should be lifted in equilibrium position and should be a constant load
- Approved critical lift plan must be available in hand prior to lifting work
- Only the competent and trained jack up operator will be allowed to operate the equipment
- Protection from sling’s sharp edge to be provided
- No unauthorized persons are to be involved
- A full body harness (IS 3521) and safety belts must be used while working at height
- Before commencement of the hydrostatic test, a specification sheet of the equipment and pipeline which mentions pressure and temperature must be available
- Hydrostatic test pressure and temperature should be clearly defined in the document
- Supporting drawings must be available during the hydrostatic test
- Hydrostatic pressure test planning must be distributed to all relevant sections on plant site at least two days before the execution date
- Make good coordination among the related sections that involve in the test
- All test equipment and tools shall be inspected for wear and damage
- The pressure gauges used in the test are recommended to have 150% maximum allowable working pressure
- Pressure gauges must be installed at a proper location so that they can be easily read and do not create additional hazards to the hydrostatic test or expose personnel to the vessels being tested
- All the temporary tools such as valves, fittings, hoses, flanges, and blind plates have to be rated more than maximum hydrostatic pressure
- Isolate other equipment and pipelines that do not include in the hydrostatic test
- Check the vent line before testing and make sure that it is not blocked
- Isolate the test area and surround it with a safety line. Put appropriate safety signal in that area
- Do not start the test if the problem is identified
- Issuing of work safety permit by authorized personnel is compulsory
- Proper illumination in and around the boiler and the temporary system has to be ensured
- Please ensure that before the commencement of tests, no work is going on inside the boiler. A clearance certificate shall be signed off by all parties before the test
- All the hangers should be in the locked position
- All turbine equipment like HP heaters and condensers are to be isolated, normalized for boiler hydro test and clearance to conduct the hydro test is to be obtained from turbine group prior to commencement of boiler hydro test
- Housekeeping is to be done in boiler area, particularly on top of the boiler, stairway, platform, ground level, furnace, corner water wall, superheater, reheaters, economizers, etc.
Modern trends of erection of boiler with utmost safety and minimum manpower
The assembly of ceiling girders with pressure part assembly weighing 3050 MT load is to be carried out at ground level, and the same is to be lifted with 16 strand jacks of 300MT capacity each up to 85.5 meters.
The same is to be connected to the main columns on LHS and RHS of the boiler.
This exclusive erection technique was adopted by M/s L&T for the erection of a supercritical boiler of 660MW capacity at Koradi thermal power stations, thereby reducing the chances of accidents during pressure part erections considerably.
Therefore, safety practices are required to be monitored, controlled and implemented at various pressure part components during the operation, maintenance and erection of the boiler also.
IBR codes for safety of Boilers
|Efficiency of ligament
|Shells and headers
|Permissible stresses for shells and headers
|Endplate with manhole
|Endplate subject to pressure on the concave side
|Boiler and superheater tubes, header, etc.- material and construction
|Cylindrical headers and attachments
|Minimum bend radius/ pipe fittings
|Aging of boilers and Residual Life Assessment
|Physical properties of ferritic and austenitic alloy steels
|Physical properties of carbon steel
Safety practices during maintenance of boiler
- Using sky climbers ensure electrical connections reliability – proper fitting – necessary care
- Ensuring proper safety rope
- Avoid water washing
- With cup-lock scaffolding, water jetting should be with thorough checking electric connections, proper scaffolding arrangement are to be ensured
- Railings and side supports must be used
- Perfect walkways and safety platforms are to be installed
- Use of safety belts to be made mandatory
2. Platen Super Heater
- While attending boiler tube leakage at P.S.H. zone scaffolding erection is critical
- From II Pass, erection is done & hanging platform is required to be erected
- The risky job is to be carried out with utmost safety and sturdiness
- Utmost care should be taken while erecting scaffolding from wrapper tube coil assembly to the required elevation
- Work is generally required to be carried out on slant portion and therefore safety precautions must be taken
- Utmost safety is to be followed due to restricted space
- High risk & proper holding supports are required for carrying out repairs
- Access platforms to be erected by maintaining perfect distance.
- Sometimes slant portion is full of ash. While attending BTL, care to be taken that ash is to be removed completely. Sudden severe burning of the legs can be observed.
4. Screen Tubes
- Utmost safety is to be followed due to restricted area
- Both side slant and narrow passage – safety norms to be followed stringently
- Tube Metal temperature must be checked before the commencement of the work
- Only 2-3 access modules for free movement should be permitted
- Access Platforms should be erected at proper distances
While carrying out Eco Coil / LTSH Coil Assembly replacement
- Rear steam cooled wall scaffolding must be made permanent
- Electrically operated switches & their supports must be proper
- PPE must be worn
- Safety belts must be fastened
- More precaution is in the vicinity of eco-hopper must be observed
- Fire in APH during II stage passivation is generally observed
- Best maintenance practice and preservation of CDS tubes to be maintained
- Ensuring healthiness of oil carryover detectors is must
7. Coal Mills
- Care to be taken if burning reject is observed
- Check on Alpha Quartz Particle
- Ensure sulphur quantity in coal, otherwise allied effects like vomitting, headache and unconsciousness can be experienced
- Check and ensure the breaker to be perfectly in “OFF” condition
- Observe necessary care during the start of the mill at local/remote level
Safety practices for safe removal of clinker:
Ministry of Commerce and Industry, Department of Industrial Policy and Promotion constituted a three-member committee under the chairmanship of the Technical Advisor (boiler) and Secretary. The committee has submitted its report which has been approved by the Hon’ble Commerce and Industry minister. As per this report, the following recommendations are made to all power plants for follow-up of the safe practices:
- Opening of the man-hole doors/scaffolding doors during operation is prohibited and should not be resorted to at any cost
- Water injection for dislodging the ash in the S-panel should be avoided
- Whenever there is a problem in the ash handling system, the problem should be addressed and rectified fully before resorting to normal operations
- Permission/protocol shall be obtained from relevant departments as per prescribed norms, before taking up maintenance of equipment
- If the system is not fully rectified due to constraints, the same is to be explicitly recorded and a timeline for further work to be indicated by the maintenance department
- If the problem is not fully resolved, the boiler should be prohibited from further maintenance
- Accumulation of ash should be controlled, and continuous ash removal should be ensured without blocking the furnace bottom opening and the ash hopper
- Operating personnel should monitor the various equipment conditions and report to the operation-in-charge and mention in the logbooks the condition of the equipment and corrective action if required, should be taken
- Proper communication should be made and insisted upon for shutting down the unit to avoid such incidents in the future to save lives and equipment
- The usage of coal of high ash quality should be studied for its effect on the boiler system and the ash handling system
- Only such quality of coal is to be used for which the boiler has been designed
- It is mandatory that qualified BOE should be engaged in boiler operation always to ensure the safe operation of the boiler
- Whenever people attend to any problem during boiler operation, there should be a clear practice of having a checklist for safe practices to be adopted. This should be approved by a committee of personnel/representatives from the operation and maintenance/safety departments with the final approval of the head of the plant. The persons working in the ash handling area are to be provided with thermal wear which can withstand high temperatures and flameproof clothing for people working near the furnace area
- There should be a permit system for people working in other areas around the boiler also depending upon the activities being performed in the pressure parts, pulverizer systems, boiler operating floor, safety valve floating, boiler start-up, etc., which should be endorsed by safety department apart from operation and maintenance. This should also be notified by the plant head
- All power plants are advised to follow the recommendations above for the prevention of accidents
Safety practices during operation of boiler
Furnace explosion is the main cause of severe boiler accidents that leads to escaping out of flue gas and ash from the furnace and allied pressure part components and resulting in fatal accidents. Therefore, utmost safety is required to be taken while working on boiler and boiler mountings. Other critical parameters like ensuring wind box to furnace Differential Pressure (DP), complete combustion, flame intensity, availability of clinker grinders, soot blowing as per schedule, mill fineness, furnace temperature mapping, and O2 grid measurement are also required to be monitored for prevention of explosion of the furnace.
Hazard identification and risk assessment study of every critical and crucial activity of carrying out the repairs of boiler is to be conducted for critical appraisal of all potential hazards involving personnel, plant, services, and operation methods and to prepare a risk register that will help in continuously monitoring these risks and to identify the existing safeguards available to control the risks due to the hazards.
Thus, safety practices are required to be implemented, monitored and controlled during the erection, commissioning, operation and maintenance of the boiler to minimize accidents.
- Photographs by 3×660 boiler erection team Mahagenco
- L&T MHPS boiler erection manual 3×660 MW Koradi-Nagpur
- Safety guidelines by HSE department L&T MHPS boilers
Shri Pramod P Kate, Boiler Technical Consultant