Boilers safety controls

Local water conditions and the introduction of treatment chemicals to a boiler will vary the amount of sediment accumulation in a control float bowl or a water column. For heating boilers and power boilers it is recommended that the boiler safety control be blown down regularly at least once a week when the boiler is in operation however, power boilers may require a more frequent blowdown depending on operating and water conditions. When blowing down a control, it is advisable to check the operation of the low-water cutoff at a low-fire burner setting. 

In almost all countries today, safety codes and regulations exist for the construction, operation, and inspection of all boilers and associated pressure vessels and boiler systems. Both HW and steam-raising plants are provided with several vital boiler appurtenances (appliances or fittings) and various subsystems containing auxiliaries (accessories) that must be maintained, monitored, and controlled. However, for small HW and LP steam boiler plants the inspection process may be rather cursory with regard to the pressure vessel internals and tends to concentrate primarily on ensuring the proper operation of the various appurtenances that provide for boiler safety. 

Chemical sensors are widely used to monitor hazardous and combustible gases [65]. Applications include safety control in industrial applications, surveillance of boilers and other devices which are operated with natural gas as well as more sophisticated areas like cooking control and odor determination [66]. 

Burner management controls, complete with flame scanners, purging, and cooling should be provided to ensure boiler safety. Supplementary firing should be limited to a fraction of the gas turbine load. The applicable safety codes are a function of the presence of fuel gas in the proximity. Precautions include the use of double stuffing glands with bleeds between the glands on... 

A nonprofit association of insurance companies that provides services for participating companies. It provides statistical, actuarial, and underwriting information for numerous affiliated insurance companies and more than a dozen lines of insurance. It maintains one of the largest private databases in the world for insurance premiums and losses paid. Included in these lines is liability, automobile, boiler and machinery, homeowners, farm, and commercial fire insurance. ISO is a voluntary, nonprofit, unincorporated association of insurers. Previous to 1971, the functions performed by ISO were undertaken by various insurance organizations in different states. ISO gathers data that are used to establish rates for fire protection policies for residential and commercial properties. ISO developed the municipal grading schedule, which is commonly used to establish a basis of insurance rates for municipalities. It is an evaluation of the fire protection features of cities and towns based on seven factors climatic conditions, water supply, fire department, fire service communications, fire safety control, building codes, and a survey report. 

For example, the Power Engineer s Act [6] for the Province of Manitoba in Canada, although it does not apply to nuclear plants licensed by the AECB, allows for the operation of certain steam plants without constant supervision for periods not exceeding 72 hours with written authorization from the provincial Minister of Labour. However, such operation is permitted only for small systems up to a power level of 500 kW, and a pressure of up to 1030 kPa, provided the boiler is installed in an unoccupied building, is equipped with a full set of safety controls and an approved visual readout system, and the plant and each safety device are tested by a power engineer of the class required. 

Boilers are subject to extensive code and regulatory requirements. All boilers should have the following minimum safety controls ... 

On the boiler that exploded the manual individual burner valves were not closed when the boiler shut down. After the purge period, fuel gas was admitted to the header from remote manual controls in the control room and into the firebox. Low fuel gas pressure tripped the master safety valve after each attempt to pressure the fuel header. Three attempts were made to purge the boiler and on each of these occasions fuel gas was dumped into the furnace through the open manual burner gas valves. On the third attempt a severe explosion occurred. 

All reactors should meet or exceed ASME Boiler and Pressure Vessel Codes with respect to appropriate materials of construction. The unit should also be sized properly and equipped with safety relief valves and/or rupture discs and contain appropriate piping and controls. 

Shell boilers are supplied with controls making them suitable for unattended operation, although the insurance companies to comply with safety recommendations call for certain operations such as blowdown of controls. 

Sometimes it is not practical to blowdown the level controls and shut down the incinerator. In this case, the situation should be discussed with the insurance company and the boiler supplier. It is possible to include for an extra high working water level giving a safety margin above the heating surfaces. The controls may then be blown down and checked for satisfactory operation with a predetermined time delay before it shuts down the incinerator or operates a bypass in the event of a fault. 

Continuous conductivity measurement controlled with the electrode placed in the boiler. This method is not recommended because of potential safety and liability issues. In addition, there are difficulties with cleaning and maintaining the electrode, and the intense heat to which the electrode is constantly subjected may cause failure. FT boiler installations generally provide for the electrode to be placed above the first set of tubes but 4 to 6 inches below the waterline. 

More formal inspections also verily the operation of safety and other controls such as low water and fuel cutoffs, level controls, fusible plugs, pressure gauges, water glasses, gage cocks, stop valves, safety/relief valves, and BD valves and lines. Also, FW pumps, flue and damper arrangements, combustion safeguards, name plate specifications, set pressures, boiler connections, floats, mercury switches, bellows, and other components may be inspected. 

Rusche, S. Kostrzewa, G. Intelligent Combustion A Gas boiler with a new control and safety device using the signals of a semiconductor-sensor. Proceedings Eurogas 99, Bochum 1999. 

NFPA 85, Boiler and Combustion Systems Hazards Code, provides guidance for steam boilers and similar high reliability automatic combustion systems. In general, NFPA 85 combustion system control and safety instrumentation systems requirements exceed those defined in NFPA 86 and in API RP 556. 

Nuclear Boiler Assembly. This assembly consists of the equipment and instrumentation necessary to produce, contain, and control the steam required by the turbine-generator. The principal components of the nuclear boiler are (1) reactor vessel and internals—reactor pressure vessel, jet pumps for reactor water circulation, steam separators and dryers, and core support structure (2) reactor water recirculation system—pumps, valves, and piping used in providing and controlling core flow (3) main steam lines—main steam safety and relief valves, piping, and pipe supports from reactor pressure vessel up to and including the isolation valves outside of the primary containment barrier (4) control rod drive system—control rods, control rod drive mechanisms and hydraulic system for insertion and withdrawal of the control rods and (5) nuclear fuel and in-core instrumentation,... 

Table 1.4 lists the priority R D needs that were identified for burners, boilers, and furnaces. Essentially all of these needs require some amount of testing. These needs were generated from the following end-use requirements increased system efficiency reduced NOx, CO, CO2, and particulate emissions increased fuel flexibility more robust and flexible process control and operations better safety, reliability, and maintenance lower capital and operational costs faster, low-cost technology development and enhanced system integration. Coupled with these needs are some barriers to improvement financial risk, inability to accurately predict the performance of new systems, lack of industry standards, and the wide gap that often exists between the research done at a small scale that needs to be applied to industrial-scale systems. Testing is often required to address some of these barriers. 

High boiler efficiency is always the result of optimal combination of design and operating criteria, including enough heating surface and passes, good burner-boiler compatibility, reliability of combustion controls and safety, as well as cost-effective implementation of advanced waste heat recovery techniques. 

Flame safeguarding is a very important part of combustion control. Statistics show that the bulk of boiler explosions occur during start-up, either cold or hot. On-off operation such as setting safety... 

An in-service test program will be developed that includes preservice (baseline) testing and a periodic inservice test program to insure that all "safety-related" valves will be in a state of operational readiness to perform their principal radionuclide control function throughout the life of the plant. The test program will be based on the ASME Boiler and Pressure Vessel Code, Section XI, Division 2, Subsection IGV (primarily IGV-1000). 

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