BOILERS DESIGN

Boilers are fuel-burning appliances that produce either hot water or steam that gets circulated through piping for heating or process uses. Boiler systems are major financial investments, yet the methods for protecting these investments vary widely. Proper maintenance and operation of boiler systems is important with regard to efficiency and reliability. Without this attention, boilers can be very dangerous.

Boiler designs can be classified in three main divisions –

1. fire-tube boilers
2. water-tube boilers
3. electric boilers.

Fire-tube boilers rely on hot gases circulating through the boiler inside tubes that are submerged in water. These gases usually make several passes through the tubes, thereby transferring their heat through the tube walls and causing the water to boil on the other side. Fire-tube boilers are generally available in the range of 20 through 800 boiler horsepower (bhp) and in pressures up to 150 psi.

Most high-pressure and large boilers are of this type. It is important to note that the small tubes in the water-tube boiler can withstand high pressure better than the large vessels of a fire-tube boiler. In the water-tube boiler, gases flow over water-filled tubes. These water-filled tubes are in turn connected to large containers called drums.

Water-tube boilers are available in sizes ranging from a smaller residential type to very large utility class boilers. Boiler pressures range from 15 psi through pressures exceeding 3,500 psi.

Electric boilers are very efficient sources of hot water or steam, which are available in ratings from 5 to over 50,000 kW. They can provide sufficient heat for any HVAC requirement in applications ranging from humidification to primary heat sources.

Component Description

Drums

The steam drum is the single most expensive component in the boiler. Consequently, any maintenance program must address the steam drum, as well as any other drums, in the convection passes of the boiler. In general, problems in the drums are associated with corrosion. In some instances, where drums have rolled tubes, rolling may produce excessive stresses that can lead to damage in the ligament areas. Problems in the drums normally lead to indications that are seen on the surfaces—either inside diameter (ID) or outside diameter (OD).

Assessment: Inspection and testing focuses on detecting surface indications. The preferred nondestructive examination (NDE) method is wet fluorescent magnetic particle testing (WFMT). Because WFMT uses fluorescent particles that are examined under ultraviolet light, it is more sensitive than dry powder type magnetic particle testing (MT) and it is faster than liquid dye penetrant testing (PT) methods. WFMT should include the major welds, selected attachment welds, and at least some of the ligaments. If locations of corrosion are found, then ultrasonic thickness testing (UTT) may be performed to assess thinning due to metal loss. In rare instances, metallographic replication may be performed.

Headers

Boilers designed for temperatures above 900°F (482°C) can have superheater outlet headers that are subject to creep – the plastic deformation (strain) of the header from long-term exposure to temperature and stress. For high-temperature headers, tests can include metallographic replication and ultrasonic angle beam shear wave inspections of higher stress weld locations. However, industrial boilers are more typically designed for temperatures less that 900°F (482°C) such that failure is not normally related to creep. Lower temperature headers are subject to corrosion or possible erosion. Additionally, cycles of thermal expansion and mechanical loading may lead to fatigue damage.

Assessment: The nondestructive examination (NDE) method should include testing of the welds by magnetic particle testing (MT) or by wet fluorescent magnetic particle testing (WFMT). In addition, it is advisable to perform internal inspection with a video probe to assess waterside cleanliness, to note any buildup of deposits or maintenance debris that could obstruct flow, and to determine if corrosion is a problem. Inspected headers should include some of the water circuit headers as well as superheater headers. If a location of corrosion is seen, then ultrasonic thickness testing (UTT) to quantify remaining wall thickness is advisable.

Tubing

By far, the greatest number of forced outages in all types of boilers are caused by tube failures. Failure mechanisms vary greatly from long term to short term. Superheater tubes operating at sufficient temperature can fail long term (over many years) due to normal life expenditure. For these tubes with predicted finite life, Babcock & Wilcox (B&W) offers the NOTIS® test and remaining life analysis. However, most tubes in the industrial boiler do not have a finite life due to their temperature of operation under normal conditions. Tubes are more likely to fail because of abnormal deterioration such as water/steam-side deposition retarding heat transfer, flow obstructions, tube corrosion [inside diameter (ID) and/or outside diameter (OD)], fatigue, and tube erosion.

Assessment: Tubing is one of the components where visual examination is of great importance because many tube damage mechanisms lead to visual signs such as distortion, discoloration, swelling, or surface damage. The primary nondestructive examination (NDE) method for obtaining data used in tube assessment is contact ultrasonic thickness testing (UTT) for tube thickness measurements. Contact UTT is done on accessible tube surfaces by placing the ultrasonic transducer onto the tube using a couplant, a gel or fluid that transmits from the ultrasonic transducer sound into the tube. Variations on standard contact UTT have been developed due to access limitations.

Examples are internal rotating inspection system (IRIS)-based techniques in which the signal from the ultrasonic transducer is reflected from a high rpm rotating mirror to scan tubes from the ID—especially in the area adjacent to drums. A second system is B&W’s immersion ultrasonic transducer, where a multiple transducer probe is inserted into boiler bank tubes from the steam drum to provide measurements at four orthogonal points. These systems can be advantageous in the assessment of pitting.

Piping

Main Steam
– For lower temperature systems, the piping is subject to the same damage as noted for the boiler headers. In addition, the piping supports may experience deterioration and become damaged from excessive or cyclical system loads.

Assessment: The nondestructive examination (NDE) method of choice for testing of external weld surfaces is wet flourescent magnetic particle testing (WFMT). Magnetic particle testing (MT) and penetrant testing (PT) methods are sometimes used if lighting or pipe geometry make WFMT impractical. Non-drainable sections, such as sagging horizontal runs, are subject to internal corrosion and pitting. These areas should be examined by internal video probe and/or ultrasonic thickness testing (UTT) measurements. Volumetric inspection (i.e., ultrasonic shear wave) of selected piping welds may be included in the NDE. However, concerns for weld integrity related to the growth of subsurface cracks is a problem associated with creep of high temperature piping and is not a concern on most industrial installations.

Feedwater – A piping system often overlooked is feedwater piping. Depending upon the operating parameters of the feedwater system, the flow rates, and the piping geometry, the pipe may be prone to corrosion or flow assisted corrosion (FAC). This is also referred to as erosion-corrosion. If susceptible, the pipe may experience material loss from internal surfaces near bends, pumps, injection points, and flow transitions. Ingress of air into the system can lead to corrosion and pitting. Out-of-service corrosion can occur if the boiler is idle for long periods.

Assessment: Internal visual inspection with a video probe is recommended if access allows. NDE can include MT, PT, or WFMT at selected welds. UTT should be done in any location where FAC is suspected to ensure there is not significant piping wall loss.

Deaerators

Overlooked for many years in condition assessment and maintenance inspection programs, deaerators have been known to fail catastrophically in both industrial and utility plants. The damage mechanism is corrosion of shell welds, which occurs on the inside diameter (ID) surfaces.

Assessment: Deaerators’ welds should have a thorough visual inspection. All internal welds and selected external attachment welds should be tested by wet fluorescent magnetic particle testing (WFMT).

Pilot and main burner flames

Assessment: Visually inspect pilot burner and main burner flames.

Pilot

- Proper pilot flame:Blue flame. Inner cone engulfing thermocouple. Thermocouple glowing cherry red.
- Improper pilot flame:Overfired – Large flame lifting or blowing past thermocouple. Underfired – Small flame. Inner cone not engulfing thermocouple.Lack of primary air – Yellow flame tip.Incorrectly heated thermocouple.

Main Burner

- Check burner flames—Main burner:
- Improper main burner flame:Overfired – Large flames.Underfired – Small flames. Lack of primary air – Yellow tipping on flames (sooting will occur).Yellow-orange streaks may appear (caused by dust).

Boiler heating surfaces

Assessment: Use a bright light to inspect the boiler flue collector and heating surfaces. If the vent pipe or boiler interior surfaces show evidence of soot, clean boiler heating surfaces. Remove the flue collector and clean the boiler, if necessary, after closer inspection of boiler heating surfaces. If there is evidence of rusty scale deposits on boiler surfaces, check the water piping and control system to make sure the boiler return water temperature is properly maintained. Reconnect vent and draft diverter. Check inside and around boiler for evidence of any leaks from the boiler. If found, locate source of leaks and repair.

Burner and base

Assessment: Inspect burners and all other components in the boiler base. If burners must be cleaned, raise rear of each burner to release from support slot, slide forward, and remove. Then brush and vacuum the burners thoroughly, making sure all ports are free of debris. Carefully replace all burners, making sure burner with pilot bracket is replaced in its original position and all burners are upright (ports up). Inspect the base insulation.

A boiler efficiency improvement program must include two aspects: (1) action to bring the boiler to peak efficiency, and (2) action to maintain the efficiency at the maximum level. Good maintenance and efficiency start with having a working knowledge of the components associated with the boiler and keeping comprehensive records, and end with details such as cleaning heat transfer surfaces and adjusting the air-to-fuel ratio.

General Requirements for a Safe and Efficient Boiler Room

Keep the boiler room clean and clear of all unnecessary items. The boiler room should not be used as a storage area. The burner requires proper air circulation in order to prevent incomplete fuel combustion. Use boiler operating log sheets, keep maintenance records, and monitor the production of carbon monoxide.

-Ensure that all personnel who operate or maintain the boiler room are properly trained on all equipment, controls, safety devices, and up-to-date operating procedures.

-Before start-up, ensure that the boiler room is free of all potentially dangerous situations, like flammable materials, or mechanical or physical damage to the boiler or related equipment. Clear intakes and exhaust vents; check for deterioration and possible leaks.

-Ensure that a thorough inspection is done by a properly qualified inspector.
After any extensive repair or new installation of equipment, make sure a qualified boiler inspector re-inspects the entire system.

- Monitor all new equipment closely until safety and efficiency are demonstrated.

-Use boiler operating log sheets, maintenance records, and manufacturer’s recommendations to establish a preventive maintenance schedule based on operating conditions, past maintenance, repair, and replacement that were performed on the equipment.

-Establish a checklist for proper startup and shutdown of boilers and all related equipment according to manufacturer’s recommendations.

-Observe equipment extensively before allowing an automating operation system to be used with minimal supervision.

-Establish a periodic preventive maintenance and safety program that follows manufacturer’s recommendation

Reference :
Website

http://www.eere.energy.gov/femp/techassist/operations_maintenance/technologies/boilers/types.cfm

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