Boiler safety devices — high steam and low water safety valve: For the combined “high steam and low water” safety valve, under which condition(s) will it blow off steam?

Introduction:
Safety valves are the final protective devices preventing overpressure and dangerous overheating in steam boilers. The combined high-steam and low-water safety valve integrates two protective functions to safeguard both pressure and water level conditions.

Given Data / Assumptions:

• Combined valve with mechanisms or features for pressure relief and low-water actuation.
• Normal firing and feed arrangements.

Concept / Approach:
When boiler pressure rises above the setpoint, the valve opens to discharge steam and limit pressure. Additionally, if water level drops too low, the heat transfer surfaces risk overheating; the low-water feature causes the valve to open (or trips firing) to prevent damage. Hence, it responds to both high steam pressure and dangerously low water level.

Step-by-Step Solution:

Verification / Alternative check:
Standards and practice endorse independent low-water protection (e.g., fusible plugs, low-water cutouts), but the named combined valve explicitly provides both protections in one assembly.

Why Other Options Are Wrong:

• Only pressure or only level: ignores the combined design intent.
• None of the above / pump trip only: not aligned with the device’s function.

Common Pitfalls:
Relying solely on pressure relief while neglecting low-water hazards; assuming feed pump trips are the only trigger for safety actions.

Final Answer: