Boiler Fittings, Valves, Gauges and Safety Devices
Core concepts
Every standard boiler carries a statutory set of fittings. Learn the function, location and testing of: safety valves (usually two, one set lower), main steam stop valve, feedwater check and stop valves, blowdown valves, water level gauges (two independent), pressure gauge with siphon and cock, fusible plug (where required), low-water cutoff, high-steam-pressure limit, flame failure device and alarm systems.
Understand safety valve accumulation of pressure (typically 10 % above set pressure), simmer, pop action, lift and blowdown (reseating). Know why the siphon protects the gauge from steam temperature and why gauge glasses need try cocks or test cocks and guard glass.
Traps
- Believing the lowest-set safety valve should be the one nearest the boiler; the lower-set valve is usually the first to lift, and the higher-set valve is set at or below MAWP.
- Confusing accumulation test with pop test.
- Assuming a low-water cutoff also protects against high water level.
- Forgetting that gauge glasses must be blown through and that both water and steam connections can block.
Worked rule of thumb
If MAWP is 1 000 kPa, the highest safety valve setting must not exceed 1 000 kPa, and with all valves discharging the pressure must not rise more than 10 % - 1 100 kPa maximum. Any valve set above MAWP is a defect.