High Risk Work Licence - Standard Boiler (BS) cheat sheet

Fire-tube boilers pass hot combustion gases through tubes surrounded by water; water-tube boilers pass water through tubes surrounded by hot gases, allowing higher pressure and larger capacity.

Standard boiler terminology: shell, drum, furnace, tubes, tubesheet, crown sheet, firebox, uptake, superheater, economiser, air heater.

Heating surface is the gas-side area exposed to products of combustion, expressed in square metres; it determines evaporative capacity.

Working pressure is the maximum permitted internal pressure; design pressure exceeds working pressure to provide margin for the safety valve setting.

Evaporation rate (kg/h) = steam flow; equivalent evaporation relates actual output to evaporation of feedwater at 100 °C from and at 100 °C.

Water capacity and steam space volume govern response to load swings: small water capacity gives faster pressure rise but less tolerance to fluctuating demand.

Boiler horsepower (from and at 100 °C) equals approximately 15.65 kg/h of steam; 1 kW is about 1.0 kg/h of steam at typical conditions.

Shell thickness for cylindrical shells is governed by pressure, diameter, joint efficiency and allowable stress; riveted seams require higher thickness than welded seams.

Access openings (manholes, handholes) are sized and located to permit internal inspection of shell, tubesheets, crown sheets and mud drums.

Two independent low-water fuel cutouts, or one cutout plus a check of water level, are required on automatically fired boilers; each must be tested by evaporation or drain test.

Safety valves: at least one per boiler, set at or below design pressure, with total relieving capacity at least equal to maximum evaporation; the lowest-set valve is the controlling valve.

A try-cock or gauge glass with blowdown valve gives an independent means of verifying water level; gauge glass connections must be kept clear and blown through regularly.

Steam pressure gauge must be connected to the steam space, have a siphon or pigtail to protect the Bourdon tube, and be marked with a red line at the maximum allowable working pressure.

Main steam stop valve is a non-return or screw-down valve at the boiler outlet; a non-return valve prevents backflow from the header into a depressurised boiler.

Blowdown valves: a slow-opening valve nearest the boiler plus a quick-opening valve downstream; the boiler-side valve is opened first and closed last.

Fusible plugs, where fitted, melt at about 230–250 °C to warn of low water or overheating of the crown sheet.

Feedwater check valve and stop valve are fitted between feed pump and boiler; the check valve prevents backflow of boiler water into the feed line.

Feedwater must enter the boiler through a non-return valve; two independent feed arrangements (or one pump plus an injector) are required for unattended or automatically fired boilers.

Injector operation relies on a steam jet condensing in feedwater to create a pressure head; it needs clean, cool feedwater and will not lift hot water well.

Centrifugal feed pumps must be primed, have suction valves fully open, and a recirculation line to prevent overheating at low flow.

Positive-displacement (reciprocating) pumps require an unloader or bypass relief to protect against overpressure on a closed discharge.

Water level control: modulating control maintains level within a band; on/off (two-position) control is used on smaller boilers; low-water cutout is a separate safety function, not part of level control.

Normal water level is at the centre of the gauge glass; the lowest visible level in the glass must be at least 50 mm above the highest part of the heating surface.

Feedwater temperature affects thermal shock: cold feedwater entering a hot boiler causes differential contraction and can damage tubesheets and seams.

Feedwater regulators should be checked by raising and lowering level; the pump should start and stop at the correct levels with the burner interlocked.

Stoichiometric (theoretical) air is the minimum air for complete combustion; excess air is supplied to ensure complete burning and is measured as a percentage above theoretical.

Typical excess air: about 10–15% for gas, 15–25% for light oil, 25–40% for coal or heavy oil; too much excess air wastes heat up the stack, too little produces CO and smoke.

Combustion products: CO2, H2O, SO2/SO3, N2, plus excess O2; CO indicates incomplete combustion and is a safety hazard.

Flue gas CO2 reading falls as excess air rises; a falling CO2 with steady load suggests excess air or air ingress, not improved combustion.

Natural draft relies on the density difference between hot flue gas and cool ambient air; mechanical draft uses forced-draft, induced-draft or balanced-draft fans.

Forced draft pressurises the furnace; induced draft maintains negative pressure in the furnace and draws gases through the boiler; balanced draft combines both.

Furnace draught should be slightly negative (about −0.05 to −0.2 kPa) on balanced-draft units to prevent flame escape and air leakage.

Purge (pre-purge) of the furnace with air is required before ignition to clear explosive gases; typical purge volume is at least 4–5 furnace volume changes.

Flame failure protection must shut off fuel within a few seconds of flame loss; the flame scanner must be proven before fuel is admitted.

Scale is precipitated hardness (calcium and magnesium) that insulates heating surfaces and can cause overheating and tube failure.

Total alkalinity and pH: boiler water is normally kept alkaline (pH about 10–11) to inhibit corrosion; excessive alkalinity causes caustic embrittlement and foaming.

Dissolved oxygen causes pitting; it is removed mechanically by a deaerator and chemically by an oxygen scavenger such as sodium sulphite or hydrazine.

Total dissolved solids (TDS) control by blowdown prevents carryover, foaming and priming; TDS limits are set by the boiler manufacturer and are typically lower for high-pressure boilers.

Bottom blowdown removes sludge and sediment from the mud drum; surface (top) blowdown removes dissolved solids and scum from the water surface.

Blowdown rate percentage = (feedwater TDS / boiler water TDS limit) × 100; increasing blowdown lowers TDS but wastes heat and treated water.

Hardness should be essentially zero in softened feedwater; a hardness test result above the limit indicates softener failure and requires immediate action.

Phosphate treatment controls residual hardness in the boiler by forming a soft sludge that is removed by bottom blowdown.

Caustic embrittlement occurs in riveted seams and stressed areas at high alkalinity; nitrate treatment or coordinated phosphate control is used to prevent it.

Pre-start checks: water level at normal in the gauge glass, all valves in correct position, safety valves free, burner and controls proved, purge completed, and no personnel in the furnace.

Cold start: warm the boiler slowly, keep air vents open until steam appears, and raise pressure gradually to avoid thermal stress on the shell and tubes.

On raising steam, the gauge glass should be blown through and the water level re-checked; the low-water cutout should be tested before putting the boiler on line.

When paralleling with a header, open the stop valve slowly and drain the connecting line; the boiler pressure should be slightly above header pressure before opening.

Normal operation: maintain water level at the gauge glass centre, steam pressure within the set band, and combustion with correct excess air; record readings at fixed intervals.

Before shutdown, reduce load gradually, blow down the boiler at the correct water level, then shut off the burner and close the main steam valve.

After shutdown, keep the boiler full and warm for a short time, then allow it to cool slowly; do not dump water from a hot boiler or admit cold feedwater to a hot shell.

Emergency shutdown: trip the burner, close the fuel valve, close the main steam stop, and do not admit water to a boiler that has been overheated through low water until it has cooled.

Low water: if the level is not visible in the gauge glass, shut off fuel immediately, do not feed water, and allow the boiler to cool; feeding water to an overheated boiler can cause a violent shell failure.

High water or priming: carryover of water into the steam line causes water hammer, erratic level and possible turbine damage; reduce firing, blow down, and check the level control.

Foaming is indicated by a rapidly fluctuating water level and dirty steam; causes include high TDS, high alkalinity, oil contamination and overloading.

Water hammer in steam lines is caused by accumulated condensate; warm and drain lines before admitting steam, and keep steam traps and drains working.

Furnace explosion prevention relies on pre-purge, proven flame scanner, correct fuel/air ratio and interlocked fuel valves; never attempt to relight a hot furnace without purging.

Safety valve lifting continuously indicates overpressure, a faulty valve seat or a valve set too low; investigate before operating at reduced pressure.

Tube or shell leaks show as falling water level, inability to maintain pressure, or water in the flue gas; shut down and isolate the boiler for repair.

Accident prevention includes keeping the boiler room clear, guarding rotating parts, using correct PPE, and following permit and lock-out procedures for maintenance.

Statutory inspection intervals: external inspection typically every 12 months and internal inspection typically every 2 years, or as required by the jurisdiction; the certificate must be current for operation.

Internal inspection covers shell, tubes, tubesheets, crown sheet, stays, firebox, mud drum, and the condition of refractory and insulation.

Hydraulic test pressure is normally 1.25 to 1.5 times the maximum allowable working pressure; the test is done with the boiler isolated and all safety valves removed or gagged.

Safety valve setting and accumulation test: the valve must relieve at the set pressure, and with the valve fully open the pressure must not rise more than 10% above set pressure.

Non-destructive testing includes ultrasonic thickness measurement, magnetic particle and dye penetrant inspection of welds and stays.

Maintenance records should include date, work done, parts replaced, test results, and the name of the person performing the work; records are kept for the statutory period.

Daily and weekly checks include gauge glass blowdown, low-water cutout test, safety valve try-lever, and burner flame scanner test.

A boiler log records water level, steam pressure, feedwater temperature, blowdown, chemical test results and any abnormal condition, signed by the operator.

A boiler operator must hold the appropriate high-risk work licence for the boiler class and must operate only within the scope of that licence.

The person conducting a business or undertaking (PCBU) has the primary duty to ensure health and safety, so far as reasonably practicable, including safe plant, safe systems of work and adequate training.

Workers have a duty to take reasonable care of their own health and safety and that of others, follow instructions, and report hazards and incidents.

Boiler registration and current inspection certificate must be maintained; operating an unregistered or out-of-certificate boiler is an offence.

The operator must follow documented procedures and not defeat or bypass safety devices such as low-water cutouts, safety valves or flame scanners.

Incidents, near misses and dangerous occurrences must be reported to the regulator within the required time and recorded in the incident register.

Hazard identification and risk control follow the hierarchy: eliminate, substitute, isolate, engineer, administrate, then PPE as the last line of defence.

Emergency procedures, including evacuation and shutdown, must be displayed, practised and understood by all boiler room personnel.

Pre-start checks: water level at normal in the gauge glass, all valves in correct position, safety valves free, burner and controls proved, purge completed, and no personnel in the furnace.

Cold start: warm the boiler slowly, keep air vents open until steam appears, and raise pressure gradually to avoid thermal stress on the shell and tubes.

On raising steam, the gauge glass should be blown through and the water level re-checked; the low-water cutout should be tested before putting the boiler on line.

When paralleling with a header, open the stop valve slowly and drain the connecting line; the boiler pressure should be slightly above header pressure before opening.

Normal operation: maintain water level at the gauge glass centre, steam pressure within the set band, and combustion with correct excess air; record readings at fixed intervals.

Before shutdown, reduce load gradually, blow down the boiler at the correct water level, then shut off the burner and close the main steam valve.

After shutdown, keep the boiler full and warm for a short time, then allow it to cool slowly; do not dump water from a hot boiler or admit cold feedwater to a hot shell.

Emergency shutdown: trip the burner, close the fuel valve, close the main steam stop, and do not admit water to a boiler that has been overheated through low water until it has cooled.