Why non-pressure parts are equally important?
Boiler pressure parts supplied in India are designed as per Indian Boiler Regulations, 1950 and there is check and approval of IBR authority at every stage of Engineering, Procurement, Manufacturing and Erection, which ensures operational safety.
The non-pressure part thus becomes the responsibility of the manufacturer & user to supply and maintain and ensure no adverse effect on boiler operation. Non-pressure parts equally contribute to successful boiler installation and are to be designed keeping operational safety as a motto.
Non-pressure parts in any boiler:
- Structure: columns, support girders, beams, bracings, railings, staircases, ladders, access platforms, grills, canopy
For Indian Installations, follow IS 800 for steel structure design, IS 875 for wind speed and IS 1983 for earthquake design.
Columns and heavy girders shall have to lift arrangements for safe erection. Column sections shall be selected such that proper access for painting is available. Bolted connections are preferred as it saves erection time.
Chances of welding missouts are avoided. Skilled manpower like welders is not required. And overall quality is better than welded connections. The cost factor shall be accommodated by the user considering the advantages of bolted connection.
The angle of staircases shall be limited to 37°. Railings shall have a height of 1100mm up to 30m and 1500mm above them for safety reasons. Proper access to all manholes and Instruments shall be provided.
Every level platform shall have a minimum of two accesses from the safety point of view. If due to layout constraints two staircases are not possible, provide an escape ladder at least at one end.
Electro-forged gratings with self-tapping screws have better quality than traditional welded types. The canopy shall be designed in such a way that it avoids rain ingress in the working area.
2. Pressure part supports: rigid or spring supports
Most high-pressure boilers are top-supported boilers. Small capacity and biomass boilers having special furnaces are bottom-supported boilers. In top-supported boilers, calculation of pressure part weight and expansions are important to design supports. High temperature and pressure piping like main steam lines are Stress analysed in software like CAESAR and necessary supports are provided accordingly.
Critical lines like steam lines connecting to turbine flanges require experience in line behaviour at high temperatures and routing with spring supports is done accordingly.
3. Steam drum internals
In any high-pressure boiler, a steam drum is a part where feed water and saturated steam from furnace walls are collected.
Feed water is fed through a pipe across the steam drum length. Holes on these pipes are oriented such that proper distribution, mixing with HP dosing and no obstruction to separate water from saturated steam is achieved.
Cyclone separators and scrubbers are provided to separate water from saturated steam and this steam is passed to superheaters. The life of the superheater depends upon the efficiency of the separators.
Blow-down connections and HP dosing are also part of the drum internals.
Drum internal play’s vital role in maintaining steam quality and enhancing superheater life
4. Buckstay, explosion doors, emergency vents
Buckstay: boiler furnace water wall panels are slender and try to buckle outside due to furnace pressure. Buckstay form a flexible support around the 4 furnace walls which allows thermal expansion and at the same time restricts any buckling of a water wall panel. These are specially designed structural sections attached to the furnace without any welding and a simple pin-holding arrangement is provided. As per NFPA guidelines, design pressure is selected to size the buckstay member cross-section.
Explosion Doors: In case of sudden pressure rise in the furnace due to explosion or implosion, buck stays may not save furnace damage. Explosion doors designed for specific pressure get open due to actual pressure rise and save pressure parts from any Damage.
CO vent: In CFBC boilers during sudden blackout conditions, these vents help to evacuate CO accumulated in the furnace wind box.
5. Fuel silos, bunkers, fuel feed chutes
The sizing of a bunker is very important for any plant operation. Many times, water volumes of the bunker are not fully utilized if feeding from the conveyor is not designed in a proper way.
Circular Silos are better for coal flow compared to Rectangular Bunkers and choking of Coal in rainy seasons is minimized. Air Blasters are commonly used for this purpose
For the coal feed chute, wear-resistant material, the refractory lining is provided for better availability.
6. Ducting and its auxiliaries
Ducting sizing is maintained according to the required air or flue gas velocity. Round ducting works better than conventional rectangular ducting where flue gas contains more ash content. A round cross-section with a slope avoids ash accumulation.
Fixed and sliding support shall be ensured during erection at the site. Welding of supports shall be checked on a regular basis.
Expansion bellows, dampers and special supports need regular maintenance. Ensure there are drains provided where chances of condensation are there. Adequate manholes and vents are required for safe working during erection and shutdowns
7. Furnace or duct refractory
- Refractoriness: It is a property at which a refractory will deform under its own load and it is generally determined by the composition of material used to make a refractory.
- Porosity: It is the property to resist chemical attack (usually by fuel and water). A low value of porosity means high strength and good thermal conductivity.
- Refractory strength: It is the resistance of the refractory to compressive loads, tension and shear stresses.
- Specific gravity: The specific gravity is associated with the weight of the refractory after application. A higher specific gravity will have more strength.
- Spalling: spalling is a type of defect, also known as fracture of refractory, which is caused by excessive thermal and/or mechanical load on the refractory.
- Permanent Linear change (PLC) on reheating: Permanent change in the property of the refractory mostly caused by high temperature.
- Thermal conductivity: It indicates the general heat flow characteristics of the refractory
- Thermal expansion: an important factor determining the ability of refractory products to expand during high temperatures and contract during cooling
- Bulk density: it is measured as the weight of a given volume of the refractory and relates to the apparent porosity of the material used. A refractory with higher bulk density is better in quality.
- Abrasion resistance: resistance of refractory to the surface wear caused by the mechanical action of moving solids.
Apart from the above properties, the design and spacing of refractory holding anchors are important. Any refractory sample shall be checked and approved before using it at any installation. Application method and dry-out cycles are to be followed as per recommendation.
8. Insulation and Cladding
- Thermal insulation is required for the boiler and its auxiliaries for safety purposes. insulation thickness is calculated as per IS 14164.
- The selection of Insulation holding anchors, Anchor Pitch, Washers, MOC suitable to operating temperature, and other special attachments plays a vital role in the Insulation application and its performance.
- New insulation materials giving lesser thickness and better surface temperature are available in the market and are to be used more than conventional materials.
- For better aesthetics, coloured cladding is used nowadays than the conventional plain aluminium cladding.
Author
Abhay Ramrao Pati
(Thyssenkrupp Industries India Pvt. Ltd.)
Co-Author
Santosh Asangi
(Thyssenkrupp Industries India Pvt. Ltd.)
