Boilers & Pressure Vessels

Boilers, air receivers, autoclaves and pressure tanks store huge amounts of energy. A failure can explode with enough force to destroy a building, and steam leaks cause severe burns. Boilers and pressure vessels are safe when they are built, inspected, operated and maintained correctly, and they are dangerous the moment a safety device is disabled or an inspection is skipped.

• General Industry
• Facilities

Why they’re dangerous

• Stored energy: Water heated under pressure turns to steam with enormous expansion when the pressure is released. If a boiler fails, the water flashes to steam and the blast can throw the vessel and wreck the building. Compressed air and gas vessels store even more energy than liquid-filled ones, and are more dangerous to test with air.
• Steam and hot water burns: Live steam is invisible near the leak and can cause deep burns in an instant. A pinhole leak can cut like a blade.
• Furnace explosions: Fuel that builds up in the firebox and ignites late can explode.
• Low water: A boiler fired without enough water overheats and can rupture.
• Overpressure: A stuck, plugged or tampered safety valve lets pressure climb until the vessel fails.
• Corrosion and fatigue: Rust, scale and repeated heating and cooling weaken metal over time, often where you can’t see it.
• Carbon monoxide, noise, heat and confined spaces around boilers and in boiler rooms.

Types of equipment

Low-pressure heating boilers are generally those with steam at 15 psi or less, or hot water at 160 psi and 250°F or less. Higher pressure or temperature falls under stricter power boiler rules. Check your jurisdiction’s definitions.

Who regulates them

• State and local boiler laws do most of the regulating. Most states and some cities require registration, regular inspections by a state or insurance inspector, certificates posted at the equipment, and licensed operators or repair contractors. Rules differ, so check with your state boiler inspector.
• The ASME Boiler and Pressure Vessel Code sets how boilers and pressure vessels are designed, built and stamped. Look for the ASME stamp and the National Board number on the nameplate.
• The National Board Inspection Code covers inspection, repair and alteration. Repairs and alterations to a pressure-retaining item must be done by an authorized repair organization.
• OSHA has specific rules for air receivers (29 CFR 1910.169) and applies other standards to boilers and vessels, such as lockout, confined spaces, machine guarding and the General Duty Clause. OSHA expects employers to follow the ASME code and manufacturer instructions. See the OSHA Technical Manual.

Key safeguards

• Pressure relief (safety) valves sized correctly, tested on schedule, and never blocked, plugged, gagged or set higher than allowed
• Low-water cutoffs and other boiler controls working and tested
• Gauges, including the gauge glass, readable and accurate
• Flame safeguard and fuel shutoff controls working
• Inspections by the state or insurance inspector as required, and certificates posted. Many states require boiler operator licenses.
• Vessels built to the ASME code and marked with the maximum allowable working pressure
• Drains, blowdown lines and discharge pipes from relief valves routed safely, with no valves in the discharge

Daily and routine duties for operators

• Do a walk-around each shift. Look at gauges, water level, leaks, unusual noises, vibration, flame appearance and the area around the boiler.
• Follow the manufacturer’s schedule for blowdown, water column checks and low-water cutoff tests. Never skip them.
• Test safety valves as required, using the lift lever or a test, and have a qualified technician service them. Never add weight or pliers to hold a valve shut.
• Maintain proper water treatment to prevent scale and corrosion, and handle treatment chemicals safely. See Hazard Communication.
• Keep the boiler room clean, ventilated and clear. Keep combustibles away, and keep air openings unblocked so combustion has enough air.
• Keep a log of readings, tests, repairs and inspections.
• For air receivers, drain moisture from the bottom regularly, keep the relief valve free, and check the tank for rust, dents and leaks.

Starting up and shutting down

• Follow the written procedure for your boiler. Before lighting, a pre-purge clears unburned fuel from the furnace. Never relight a boiler that has gone out without a full purge.
• If you smell gas or fuel, stop, ventilate and call for help. See Gas Leaks.
• Bring up pressure slowly. Rapid heating and cooling stress the metal.
• For shutdown, follow the cooldown steps, and don’t drain a hot boiler quickly.

Emergencies

• Low water: shut off the fuel and let the boiler cool. Never add feedwater to an overheated, low-water boiler. Cold water on hot metal can cause an explosion.
• Relief valve popping or pressure rising: shut off the fuel, and call a qualified person. Don’t tamper with the valve.
• Steam leak: stay clear, and don’t reach into the steam. Find a leak by watching the insulation or using a broom handle, never your hand. Isolate and shut off the source if it’s safe.
• Explosion, fire or severe leak: evacuate, call 911, and follow the emergency plan. See Emergency Action Plans. Report deaths and serious injuries to OSHA. See Recordkeeping & Reporting.

Working on boilers and vessels

• Lock out fuel, electrical and feedwater. Isolate and bleed off steam and pressure, and let it cool. See Lockout/Tagout.
• Use blanks or double block-and-bleed isolation where required, and verify zero pressure before opening anything.
• Treat boiler and vessel entry as a confined space: test the air, ventilate, use a permit, an attendant and a rescue plan. See Confined Spaces.
• Open drains and vents slowly, and stand to the side. Loosen flanges and covers gradually.
• Wear eye protection, gloves, long sleeves and hearing protection, and protect yourself from hot surfaces.
• Use only qualified, authorized contractors for welding, repair or alteration of pressure parts.
• Be aware of insulation and gaskets in older boilers, which may contain asbestos. See Asbestos.

• Don’t weld, drill or modify a pressure vessel without an authorized repair and approval.
• Don’t test a vessel with compressed air unless a qualified person has approved a procedure. Use hydrostatic (water) testing, which stores far less energy.
• Don’t exceed the maximum allowable working pressure on the nameplate.
• Don’t use a vessel with dents, bulges, deep rust, leaks or cracks.

Training and records

Operators need training on their specific equipment, controls, and emergency steps, and many states require a license. Keep inspection certificates, test records, repair documents and operator logs where inspectors can find them, and post certificates as required. See Training Requirements.

Boiler room checklist

• Inspection certificate current and posted
• Relief valves free, tested and with unobstructed discharge piping
• Low-water cutoff and flame safeguard tested
• Gauges accurate; gauge glass clear
• Air receiver drained and relief valve working
• Room ventilated, with clear access, no combustibles and combustion air openings open
• Lockout procedure and emergency plan posted

Last updated: October 4, 2026