SAFETY VALVES FOR HIGH-PRESSURE BOILERS

• At high pressure, the steam is denser and the valve must handle a large mass flow with a small lift; the valve must be designed to pass the required capacity with a limited lift.
• The valve must lift cleanly and reseat tightly (no chattering/ fluttering) at high pressure.
• The valve must be stable and not "hunt" (oscillate) between open and closed.
• The valve must be resistant to erosion/ corrosion from the high-velocity steam.
• The valve must be designed to avoid water/ condensate accumulation and to discharge safely.
• The valve must have adequate capacity to relieve the full boiler output without excessive pressure rise.
• The valve must be set accurately and remain set (no drift) under service conditions.
• The valve must be designed for the high temperature and pressure, with suitable materials and sealing.

(i) The relationship between the working pressure of the boiler and the setting pressure of the safety valve:

• The safety valve is set to lift at a pressure above the working pressure (typically the working pressure, or a set margin above it, e.g. the valve lifts at the working pressure or at a small percentage above it, as per Class requirements). The relationship is determined by the need to protect the boiler from overpressure while allowing normal operation at the working pressure. The setting is typically at or slightly above the working pressure (e.g. the valve lifts at the working pressure, or at a set value such as 3-5% above, per Class).

(ii) The blowdown setting of a safety valve:

• The blowdown is the difference between the pressure at which the valve lifts and the pressure at which it reseats (closes). It is determined by the need to prevent the valve from chattering (opening/ closing rapidly) and to allow the boiler pressure to drop sufficiently before the valve closes. The blowdown is set (typically 2-4% of the set pressure, or as per the maker/ Class) to give a stable, clean reseat.

(i) The superheater safety valve:

• The superheater safety valve is set to lift at a pressure lower than the drum safety valve (so the superheater is protected first and the steam flows through the superheater). Its setting is determined by the need to protect the superheater from overheating (if the drum valve lifts first, steam flow through the superheater stops and the superheater overheats). It is set to lift at a pressure below the drum valve setting.

(ii) The drum safety valve:

• The drum safety valve is set to lift at the boiler's working/ design pressure (the maximum allowable working pressure) to protect the boiler drum from overpressure. Its setting is determined by the boiler's design pressure and the Class requirements.