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
Back