A blow-off vessel must withstand what percentage of boiler pressure, depending on boiler size?

• A. Thirty percent up to 300 psi
• B. One hundred psi for boilers over 300 psi
• C. Both thirty percent up to 300 psi and one hundred psi for boilers over 300 psi
• D. Fifty percent up to 250 psi

Answer: (C)

Blow-off vessels are designed to survive the pressure that can occur when you blow down a boiler, not the full boiler pressure itself. The idea is to limit the pressure the vessel and its discharge piping must withstand, based on how large or high-pressure the boiler is. For boilers rated up to 300 psi, the vessel is sized to withstand 30% of the boiler pressure. So if the boiler runs at 250 psi, the vessel should be able to handle about 75 psi during blowdown. For boilers above 300 psi, the requirement shifts to a fixed 100 psi. This provides a safe, lower-pressure limit for the vessel even when the boiler is much higher in pressure. Because the specification changes with boiler size, the correct option reflects both scenarios: 30% of boiler pressure for smaller boilers up to 300 psi, and a 100 psi rating for larger boilers over 300 psi.