Boiler Quality Plates
Large cylindrical pressure-holding components like drums, RH headers, blowdown tanks and so on are required to be made from BQ plates.
BQ plate is primarily a very high-quality, fully deoxidised, mild steel (MS) plate of medium to high thickness, suitably alloyed where necessary to withstand high p & t. The essential requirements are
- Very good weldability
- Assured and verified high-temperature properties
- High tensile strength even at high temperatures
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BQ plates are available in three classes of metallurgy
- CS (low, medium and high C)
- Low and medium AS
- High AS—not in ASME steels
CS Plates
Low CS has good weldability while high CS has good tensile strength.
The American practice prefers
- CS for both LP and HP applications and
- Thick CS to thinner AS drums
The European method is to avoid C > 0.25% and adopt AS plates for HP and have thinner drums.
High CS plates of SA 515, 516 and 299 are extensively used for generally thickness up to ~40, <100 and >100 mm for the three steels, respectively.
Low and Medium AS Plates
SA 302 is a very popular low-alloy American steel employed for high PS & TS. For SH and RH headers of large diameter, when they are required to be rolled out of plates, medium- and high-AS plates are regularly employed.
High AS Plates
High AS plates are not listed in ASME standards but are routinely used in Europe to reduce the drum and header thicknesses. A thin drum is helpful in cycling duty of boilers.
Alloy plates are preferred in European practice.
Boiler Quality Plates to ASME Specification are given below:
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