Boiler Fittings, Valves, and Appurtenances
The required set
A high-pressure boiler must carry a defined set of appurtenances: a safety valve (or valves) of certified capacity, a steam pressure gauge with siphon and three-way cock, a water column and gauge glass with try cocks, a low-water fuel cutoff, main steam stop valve, feedwater stop and check valves, a bottom blowoff arrangement, a nameplate, and - on hot-water boilers - an ASME relief valve, thermometer, and expansion provisions.
Valve logic questions love
A check valve prevents backflow; a gate/OS&Y valve is a stop valve; a globe valve throttles; a slow-opening blowoff valve protects piping from water hammer. Where pressure exceeds 100 psig or heating surface exceeds 500 sq ft, two blowoff valves in series are required, one of them slow-opening, with the upstream valve attached directly to the boiler. No valve may be placed between the boiler and its safety valve, and safety-valve discharge must be piped to a safe location without restriction, with drainage at the low point.
Traps
Operators do not adjust or plug safety valves - they are set, sealed, and tested by qualified persons. A low-water cutoff is a safety device, not a level control. Gauge-glass valves must be in the correct position; closed upper and lower valves give a false reading. Try cocks verify the glass rather than replace it. Blowoff piping is short, straight, and supported - never reduced in size.
Rule of thumb
Set pressure is never above the boiler MAWP, and total safety-valve capacity must equal or exceed the boiler's maximum steaming capacity. A quick field check: if the nameplate says 150 psig MAWP, no safety valve on that boiler may be set above 150 psig, and the stamp on the valve should match the capacity stated on the certificate.