Marine boilers and steam systems

In short

A marine boiler converts the chemical energy of fuel oil (or the waste heat of main-engine exhaust) into steam, which on a motor ship heats fuel oil and cargo, runs cargo and feed pumps, supplies domestic hot water and the inert-gas/tank-cleaning services, and on a steam ship drives the propulsion turbine. Every merchant vessel carries at least one oil-fired auxiliary boiler plus an exhaust-gas economiser, so boiler operation, water treatment and survey are core competence for a marine engineer. The topic is examined in B.E. Marine Engineering marine-machinery papers, in DG Shipping MEO Class IV (Motor) and Class II oral and written examinations, and in interviews at Indian shipping and ship-management employers (SCI, Great Eastern, Anglo-Eastern, Fleet Management, Synergy, V.Ships, Wilhelmsen) as well as at IRS and DG Shipping survey roles.

Explained

Classification and types

• By water/gas path: water-tube — water inside the tubes, hot gas outside; fire-tube (smoke-tube) — gas inside the tubes, water outside the tubes in a shell.
• Water-tube: high pressure and steam temperature possible (40–60 bar, 450–515 °C), small water content, fast steam-raising, light for its output, but needs good feed-water quality and skilled operation. Typical designs: D-type (two drums, bent tubes), Foster Wheeler ESD II/III, Babcock radiant types; on motor ships a small vertical water-tube auxiliary boiler (Aalborg Mission/OS type) with forced circulation.
• Fire-tube: low pressure (up to about 7–10 bar), large water volume so it tolerates fluctuating load and poorer water, cheap and simple, but heavy and slow to raise steam. Types: Scotch marine (cylindrical multi-tubular), Cochran vertical, donkey boiler.
• Composite boiler: one shell with two gas passes — an oil-fired furnace for use in port and an exhaust-gas tube nest for use at sea; one unit does both duties.
• Exhaust-gas economiser (EGE)/exhaust-gas boiler: finned tube bank in the main-engine uptake, with forced circulation from a separate steam drum; produces most sea-going steam free of fuel cost. If steam demand is low, surplus steam goes to a dump condenser.
• By circulation: natural circulation (density difference between downcomer and riser water), forced circulation (circulating pump — all EGEs and some small boilers), once-through (large steam plants).
• By use: main propulsion boiler (steam tankers, LNG carriers with steam turbines), auxiliary boiler, waste-heat boiler, incinerator-boiler combination.

Construction, mountings and accessories

• Shell/drums and tubes: steam drum (steam and water), water drum, risers and downcomers, furnace with refractory and casing, uptake, access/manhole doors, stays in fire-tube shells.
• Mountings (fitted on the boiler, required by class and regulation): two safety valves, two independent water-level gauge glasses (with shut-off and drain cocks), pressure gauge with siphon, main steam stop valve, auxiliary stop valve, feed check valve(s) (two in series on main feed line), bottom blow-down valve, scum/surface blow valve, air release (vent) cock, salinometer/sampling cock, low-water level alarm and fuel-cut-out, fusible plug on cylindrical boilers, whistle/steam connection.
• Accessories (improve efficiency, not on the shell): superheater, desuperheater, air pre-heater, economiser, feed pump, soot blower, automatic combustion and feed control, flame-failure (photocell/flame eye) safeguard.
• Superheater: convection type (temperature rises with load) or radiant type (temperature falls with load); dry pipe or steam separator prevents moisture carry-over into it. A desuperheater returns part of the steam to near-saturated condition for heating services.
• Safety valves: improved high-lift or full-lift spring-loaded type, set at or just above working pressure, fitted with easing gear led outside the boiler space; the pair must be able to pass all the steam the boiler can produce.

The shipboard steam system

• Feed/condensate circuit: boiler → steam services → condensate → steam traps → observation/cascade (hotwell) tank (deaerates and lets oil contamination be seen) → feed filter → feed pump → feed heater → feed check valve → boiler. Low-pressure auxiliary systems are usually an open feed system; turbine plants use a closed feed system with main condenser, air ejector, deaerator and extraction/booster/main feed pumps.
• Steam consumers on a motor ship: HFO settling and service tank heating, HFO heaters before purifier (about 98 °C) and before injection (to give 10–15 cSt at the engine inlet), lub-oil and fuel-tank heating coils, cargo-oil pump turbines and tank cleaning heaters on tankers, inert-gas scrubber service, domestic calorifier and air-conditioning pre-heater, galley and laundry, steam smothering lines, soot blowing, fuel-tank/deck sealing steam.
• Pressure levels: boiler 7–10 bar saturated on a typical motor ship, reduced through pressure-reducing valves to 3–4 bar for heating services; drains returned through traps (thermostatic, thermodynamic or float type).
• Automatic controls: single-element (level only) feed control on small boilers, three-element control (level + steam flow + feed flow) on large boilers to beat shrink-and-swell; combustion control modulates burner firing on steam pressure; burner sequence has purge, ignition, flame-proving and lock-out interlocks.
• Burners: pressure-jet (simple, poor turn-down), rotary cup (wide turn-down, common on auxiliary boilers), steam/air-assisted atomising (best for HFO at low load). Good atomisation plus correct excess air (about 10–20 % for oil firing) gives complete combustion.

Operation: raising steam, running and shutting down

1. Pre-start: check water level (just above the lowest visible level), open air vent, check gauge glasses, fuel oil temperature and viscosity, verify safety valve easing gear free, drains open on steam lines.
2. Light up on low firing and raise pressure slowly — typically over several hours for a water-tube boiler — so that drum and tubes expand evenly; uneven heating causes thermal stress and leaking tube ends.
3. Close the air vent when steam issues steadily (about 1–2 bar); as pressure rises, blow through and test the gauge glasses, check for expansion and leaks.
4. Warm through steam lines with drains open before opening the main stop valve slowly, to avoid water hammer.
5. In service: maintain level and pressure, carry out daily water tests and dosing, blow down as the tests require, soot blow at high load (good gas flow carries the soot clear), check combustion (clear funnel haze, O₂/CO₂ in uptake).
6. Shutdown: reduce firing, secure fuel, keep feeding until steam pressure falls, keep the boiler closed to cool slowly; for lay-up either keep it full with treated water and a nitrogen/hydrazine blanket (wet preservation) or dry it out with desiccant (dry preservation).

Boiler water treatment and testing

• Why: feed water must carry no hardness (scale), no dissolved oxygen (pitting), and controlled alkalinity (protective magnetite film). Scale 1 mm thick can raise tube metal temperature by over 100 °C and cost several per cent of efficiency.
• Tests carried out on board: pH/alkalinity, phosphate reserve, chloride, hydrazine (or sulphite) reserve, hardness, dissolved oxygen, condensate pH and oil/salinity check. Indicative limits for a low-pressure auxiliary boiler: pH 9.5–11, P-alkalinity 100–300 ppm CaCO₃, phosphate 20–50 ppm PO₄, chloride below about 300 ppm, hydrazine reserve 0.1–0.3 ppm, hardness nil. Exact limits depend on boiler pressure and on the chemical supplier's and class society's schedule — always follow the ship's own water-treatment manual.
• Chemicals: disodium/trisodium phosphate (precipitates hardness as a soft sludge and holds alkalinity), caustic soda for pH, hydrazine or sodium sulphite as oxygen scavenger (hydrazine is preferred at higher pressures as it leaves no solids; it is toxic and handled with care), amines for condensate-line corrosion, sludge conditioner/antifoam.
• Blow-down: bottom blow-down removes settled sludge; scum/surface blow removes oil and foam. Blow down only at low load with a steady, high water level, one boiler valve at a time, and never leave the blow-down valve unattended.
• Corrosion and contamination faults: oxygen pitting, caustic cracking (embrittlement) at stressed seams from over-high free alkalinity, acid corrosion from CO₂ in condensate, oil contamination from a leaking heating coil or fuel heater — oil blankets the tubes and causes overheating; the boiler must be de-oiled/boiled out.
• Priming, foaming and carry-over: high water level, high dissolved solids, oil or sudden load increase throw water with the steam — wet steam reduces heating value, damages the superheater and causes water hammer. Cure: dump/blow down, correct level, check for oil.

Faults, safety and survey

• Low water: the single most dangerous fault — tubes/crown overheat, bulge or collapse; never feed cold water into a boiler whose level is lost — secure the fuel, let it cool, then investigate.
• Uptake (soot) fire in the EGE: soot plus moisture ignites at around 300 °C and raises exhaust temperature and smoke. Action: reduce engine load/stop, keep the circulating pump running, soot blow or water wash, cover the air supply. A severe fire can become an iron/hydrogen fire at over 1,000 °C in which the steel tubes themselves burn and water dissociates — water must not be applied once the fire has reached that stage; smothering and boundary cooling only. Prevention: avoid prolonged slow steaming without soot blowing, good combustion in the main engine, regular water washing.
• Tests: hydraulic test at 1.5 × working pressure (new boiler, or after major repair, with safety valves gagged and the boiler cold and full of water); accumulation of pressure test — boiler on maximum firing with the main stop shut and the safety valves blowing, pressure rise must not exceed 10 % of working pressure for the period in the class rules (of the order of 7–15 minutes depending on type); steam test / safety valve setting after the hydraulic test.
• Indian and international framework: ships' boilers come under the Merchant Shipping Act, 2025 (in force 15 March 2026; it repealed the 1958 Act) and its Survey, Audit and Certification Rules, 2026, with DG Shipping (now DGMA)/recognised-organisation survey, with construction and survey to the rules of a class society such as the Indian Register of Shipping (IRS) and to SOLAS Chapter II-1 (machinery, boiler feed, safety valves, level indication, means of escape) and Chapter II-2 for fire safety of oil-fired units. Shore and shipyard boilers are governed by the Boilers Act, 2025 (in force from 1 May 2025; it repealed the Boilers Act, 1923), with the Indian Boiler Regulations, 2026 (in force from October 2026, superseding the Indian Boiler Regulations, 1950) — with registration, annual inspection by the state Boiler Inspectorate and IBR-approved welders and materials; marine boilers on ships under the Merchant Shipping Act are outside the Boilers Act's scope.
• Seafarer competence is governed by STCW/the M.S. (STCW) Rules; MEO Class IV and II candidates are examined orally on mountings, start-up, water tests, accumulation test and EGE fires.

Key points

• Water-tube boiler - water in tubes, gas outside; high pressure/temperature, quick steaming, needs pure feed water.
• Fire-tube boiler - gas in tubes, large water volume; low pressure, robust, slow to raise steam, heavy.
• Composite boiler - one shell fired by oil in port and by main-engine exhaust at sea.
• Exhaust-gas economiser - forced-circulation waste-heat boiler in the uptake; gives nearly free steam at sea, surplus to a dump condenser.
• Mountings vs accessories - mountings (safety valves, two gauge glasses, stops, feed checks, blow-down) are required fittings on the boiler; accessories (superheater, economiser, air heater, soot blower) improve efficiency.
• Two safety valves must between them pass the whole evaporation; proved by the accumulation of pressure test, pressure rise ≤ 10 %.
• Hydraulic test at 1.5 × working pressure, cold, water-filled, safety valves gagged.
• Low water is the worst fault - secure the fuel and let the boiler cool; do not feed a boiler with no visible level.
• Three-element feed control (level, steam flow, feed flow) overcomes shrink and swell on large boilers.
• Feed water limits - hardness nil, pH 9.5–11, phosphate reserve, chloride low, oxygen scavenged by hydrazine/sulphite (exact figures from the ship's manual).
• Scale insulates, oxygen pits, caustic cracks - the three mechanisms boiler chemistry exists to prevent.
• Priming/foaming - water carried over with steam from high level, high solids or oil; cures wet steam and water hammer.
• Soot fire at ~300 °C in the EGE; an iron/hydrogen fire above ~1,000 °C must not be fought with water.
• Factor of evaporation = heat added per kg ÷ 2,257 kJ/kg; equivalent evaporation restates output on a common basis.
• Boiler efficiency = heat to steam ÷ fuel energy; 80–88 % typical for a well-run oil-fired marine boiler.
• Steam on a motor ship mainly heats fuel and cargo and drives tanker cargo pumps; the EGE carries the sea-going load.
• Raise and drop pressure slowly - thermal stress, not pressure, cracks tube ends and drum seams.
• Steam traps and a cascade/observation tank return condensate and show up oil contamination early.

Formulas and examples

Example 1 — boiler efficiency, factor and equivalent evaporation. An oil-fired auxiliary boiler makes 4,500 kg/h of steam at 16 bar, 300 °C (h = 3,035 kJ/kg) from feed at 85 °C (h = 356 kJ/kg), burning 350 kg/h of HFO of CV 42,000 kJ/kg.

• Heat per kg of steam = 3,035 − 356 = 2,679 kJ/kg.
• Heat to steam = 4,500 × 2,679 = 12,055,500 kJ/h = 12,055,500/3,600 ≈ 3,349 kW.
• Fuel energy = 350 × 42,000 = 14,700,000 kJ/h ≈ 4,083 kW.
• η = 12,055,500/14,700,000 = 82.0 %.
• Factor of evaporation F = 2,679/2,257 = 1.187; equivalent evaporation = 4,500 × 1.187 ≈ 5,340 kg/h, i.e. 5,340/350 ≈ 15.3 kg of steam per kg of fuel from and at 100 °C.
• Cost sense: at ₹45,000 per tonne of fuel (bunker prices vary daily), 350 kg/h costs about ₹15,750 per hour; recovering just 1 percentage point of efficiency saves roughly 4.3 kg/h ≈ ₹190 per hour, about ₹1.4 lakh over a 30-day voyage of continuous firing.

Example 2 — exhaust-gas economiser output. Main-engine exhaust of 8,000 kg/h at 320 °C is cooled to 190 °C in the EGE; c_pg = 1.05 kJ/kg·K. Steam is raised dry saturated at 7 bar (h = 2,763 kJ/kg) from feed at 90 °C (h = 377 kJ/kg).

• Heat available = 8,000 × 1.05 × (320 − 190) = 1,092,000 kJ/h ≈ 303 kW.
• Heat per kg of steam = 2,763 − 377 = 2,386 kJ/kg.
• Steam generated = 1,092,000/2,386 ≈ 458 kg/h — enough for fuel and tank heating at sea, so the oil-fired boiler can be shut down.
• The outlet must stay above the acid dew point (about 160–170 °C with high-sulphur fuel) to avoid cold-end corrosion.

Example 3 — blow-down to hold the chloride limit. A boiler evaporates 4,000 kg/h; 95 % of the condensate returns, so makeup is 200 kg/h of distilled water containing 25 ppm chloride. The boiler-water limit is 300 ppm chloride.

• Chloride entering = 200 × 25 = 5,000 mg/h.
• Continuous blow-down needed = 5,000/300 ≈ 16.7 kg/h, i.e. about 0.4 % of the steam rate.
• If the condensate return fails and makeup rises to 2,000 kg/h, the requirement becomes 2,000 × 25/300 ≈ 167 kg/h — ten times the blow-down and ten times the chemical and heat loss, which is why condensate return is guarded.

Example 4 — safety valve and hydraulic test pressures. Boiler working pressure 18 bar.

• Hydraulic test = 1.5 × 18 = 27 bar (cold, water-filled, valves gagged).
• Safety valves set at 18 bar; in the accumulation test the pressure must not exceed 18 × 1.10 = 19.8 bar with the main stop shut and full firing.

Example 5 — ideal Rankine efficiency of an auxiliary turbine. Steam at 16 bar, 300 °C (h1 = 3,035 kJ/kg, s1 = 6.884 kJ/kg·K) expands to a condenser at 0.1 bar (h_f = 191.8, h_fg = 2,392.8, s_f = 0.649, s_fg = 7.501).

• x = (6.884 − 0.649)/7.501 = 6.235/7.501 = 0.831 (83.1 % dry).
• h2 = 191.8 + 0.831 × 2,392.8 = 191.8 + 1,988.4 = 2,180.2 kJ/kg; turbine work = 3,035 − 2,180 = 855 kJ/kg.
• Pump work ≈ 0.00101 × (1,600 − 10) = 1.6 kJ/kg; heat added = 3,035 − (191.8 + 1.6) = 2,841.6 kJ/kg.
• η = (855 − 1.6)/2,841.6 = 30.0 % — real plant is lower; feed heating and reheat are used to improve it.

Where it is used

• Motor ships (tankers, bulkers, container ships): oil-fired auxiliary boiler plus EGE for HFO heating, cargo heating, domestic services — maintained by the 3rd/4th engineer under the 2nd engineer.
• Steam-driven tonnage: LNG carriers with steam turbines and some product tankers use high-pressure water-tube boilers; cargo-oil pump turbines and tank-cleaning heaters on crude tankers are steam-driven.
• Indian shipyards and ship repair (Cochin Shipyard, Mazagon Dock, GRSE, Hindustan Shipyard): boiler manufacture, mounting overhaul, hydraulic testing and class survey with IRS or DG Shipping surveyors.
• Shore boiler plants in refineries, fertiliser, power and process industries: same water-treatment, blow-down and safety-valve practice, governed by the Indian Boiler Regulations and state Boiler Inspectorates — a common second career for ex-marine engineers.
• Examinations and teaching: IMU, DG Shipping-approved marine engineering colleges, and the MEO Class IV/II oral syllabus (mountings, water tests, accumulation test, uptake fires).
• Technical superintendent and fleet roles: budgeting boiler surveys, tube renewals, chemical supply contracts and EGE cleaning across a fleet.

Interview questions

• Q: Why does a motor ship need a boiler at all? A: To heat heavy fuel oil to its pumping and injection viscosity, to heat cargo and lub oil, and to supply domestic hot water, tank cleaning and inert-gas services; the exhaust-gas economiser covers most of this load at sea.
• Q: Difference between a boiler mounting and a boiler accessory? A: Mountings are the fittings on the boiler needed for safe operation (safety valves, gauge glasses, stop and feed check valves, blow-down); accessories are external items that improve efficiency (superheater, economiser, air heater, soot blower, feed pump).
• Q: What would you do on finding the water level lost? A: Secure the fuel and the fans, shut the main steam stop, do not feed water in, let the boiler cool naturally, then inspect for bulged or leaking tubes before relighting.
• Q: What is the accumulation of pressure test and why is it done? A: With the main stop shut and the boiler on maximum firing, the safety valves must hold the pressure rise within 10 % of working pressure — it proves the valves can pass the full evaporation.
• Q: Why is a boiler hydraulically tested at 1.5 times working pressure with water and not with steam? A: Water is practically incompressible, so a failure releases very little stored energy; the same test with compressed steam could burst explosively.
• Q: Why must pressure be raised slowly when lighting up a water-tube boiler? A: To let drum, tubes and refractory expand evenly; rapid firing sets up thermal stress that leaks tube ends and cracks the drum at welds and ligaments.
• Q: What causes priming and what harm does it do? A: Too high a water level, high dissolved solids, oil contamination or a sudden load increase; it carries water into the steam line, damaging superheaters and turbines and causing water hammer.
• Q: How do you deal with an uptake fire in the exhaust-gas economiser? A: Reduce or stop the engine, keep the circulating water pump running, close the air supply and smother; if it has developed into an iron/hydrogen fire above about 1,000 °C, do not apply water — water dissociates and feeds it; cool the boundaries and let it burn out.
• Q: Which tests do you carry out on boiler water and why? A: pH/alkalinity, phosphate, chloride and hydrazine (or sulphite) reserve, plus hardness and condensate checks — to keep hardness out (scale), oxygen scavenged (pitting) and alkalinity protective without going so high as to cause caustic cracking.
• Q: Difference between bottom blow-down and scum blow? A: Bottom blow-down discharges settled sludge and dissolved solids from the water drum; scum (surface) blow removes oil and foam from the water surface.
• Q: What is a composite boiler? A: A single-shell boiler with both an oil-fired furnace and an exhaust-gas tube nest, so it can be fired in port and run on main-engine exhaust at sea.
• Q: Why is a fusible plug fitted on cylindrical boilers and not on modern water-tube boilers? A: It is a last-resort low-water device that melts and quenches the fire in a large-water-volume shell boiler; water-tube boilers rely on low-level alarms and automatic fuel cut-outs instead.
• Q: Which legal framework covers a ship's boiler in India? A: The Merchant Shipping Act, 2025 (which replaced the 1958 Act from 15 March 2026) with DG Shipping (now DGMA)/class (for example IRS) survey and SOLAS Chapter II-1 requirements; the Boilers Act, 2025 and the Indian Boiler Regulations, 2026 (which superseded the 1950 Regulations) govern shore and shipyard boilers, not boilers on ships under the Merchant Shipping Act.

Common mistakes

• Using total steam enthalpy instead of (h_steam − h_feed) in the efficiency calculation, which inflates the answer.
• Dividing by the latent heat at boiler pressure instead of 2,257 kJ/kg when finding the factor of evaporation.
• Mixing kJ/h with kW, or kg/h with kg/s — always divide kJ/h by 3,600.
• Calling the superheater or economiser a "mounting", or forgetting that two safety valves and two gauge glasses are required.
• Saying water should be fed to a boiler that has lost its level, or that water is the remedy for every uptake fire.
• Treating water-test limits as universal numbers; they vary with boiler pressure, class rules and chemical supplier.
• Taking the dryness fraction as 1 after turbine expansion in a Rankine problem instead of computing x from entropy.
• Confusing the hydraulic test (1.5 ×, cold, water) with the accumulation test (full firing, steam, ≤10 % rise).

Related topics

• Power cycles: Rankine, Otto, Diesel and Brayton · Heat transfer: conduction, convection and radiation · Marine diesel engines · Fuel and lubricating oil systems · Marine pumps and piping systems · Marine auxiliary machinery.