Routine maintenance is what keeps a marine boiler safe and efficient. This lesson covers boiler cleaning, why and how a boiler is blown down, the blowdown valve sequence, gauge glass blowdown, burner maintenance and the defects found in a boiler furnace. Watch the video lesson below, then work through the two simulators and the self-test at the end of the page.
Module 1 — Boiler cleaning: washing, tubes and soot blowing
If the boiler water is treated properly and the chemical concentration stays within range, deposits on the water and steam side walls stay minimal. A boiler running on fresh water rather than hard water also builds few deposits in the tubes, and those can be removed by boiling out. Hard deposits inside the tubes are removed with a scrubber and brush from the upper drum.
With a healthy, well-maintained burner the soot inside an oil-fired boiler stays low, but it cannot be avoided completely: more soot forms during start-up and it collects over time. It is washed off with a 10% soda solution, sprayed with a water hose through the inlet door in the uptake.
Water washing
Water washing keeps the heat transfer surfaces of a water-tube boiler and economizer clean. Deposits from improper combustion or poor fuel are mostly non-soluble particles held together by a water-soluble bonding material. Water dissolves the binder and the particles simply wash away.
- Make sure the boiler is shut down and completely tagged out.
- Cool the boiler so its temperature stays below 100 °C during washing.
- Clean the tubes with high-pressure fresh water.
- Keep the drain that carries away the washing water clear and open.
- After washing, clean the refractory at the furnace bottom with alkaline water, since washing may cause corrosion.
- Before closing the doors, remove all washing water and loose deposits.
- Try out the boiler immediately after washing to avoid corrosion damage to the refractory and other heat transfer surfaces.
- On the waterside, power-wash through all uncapped ports to dislodge scale and debris.
Keep the refractory dry. Cover it with a plastic sheet and drain water continuously through the bottom drain. If the refractory does get wet, light the boiler very slowly — otherwise it cracks, drops down, and the result is overheating and deformation.
After chemical cleaning, drain the solvent from the unit completely and straight away. Leftover iron oxides can give black water samples and high iron readings. Dose dispersants for a while so the residue leaves through the normal drum blowdown.
Fire-tube cleaning
By hand it is slow work: a long rod with a brush is pulled through each tube to loosen the soot, then the tube is flushed with high-pressure water. Tube cleaning machines with mechanical brushes or scrapers make it easier and usually connect to a powerful vacuum that draws the soot out as it loosens, keeping the tubes, the boiler room and the operator clean. Newer systems use air motors to drive flexible shafts fitted with cleaning tools; water runs through the shaft casing to the tool, so a tube is cleaned and flushed in one pass.
Soot blowing
Poor-quality fuel or inefficient combustion leaves soot on the heat exchange surfaces, mainly the tubes, and the heat transfer rate drops. A soot blower clears it with steam, air or both. For the economizer, soot-blowing granules are used instead of steam or air.
Procedure, recommended every 24 hours:
- Check the steam pressure is at normal working pressure.
- Check the main steam valve on the soot blow line is closed and the top and bottom manual ball valves are shut.
- Open the drain to remove condensed water from the line.
- Crack open the main steam line valve; steam will come out of the drain.
- Shut the drain valves and fully open the main steam line valve.
- Open the bottom ball valve and blow for 5–10 s, then close it. Open the top ball valve and blow for 5–10 s, then close it.
- Repeat these four steps 3–4 times, then shut the main steam line valve.
Excess steam in soot blowing causes erosion damage to the tubes. Draining condensate first means dry steam, not water, reaches the tube metal.
Module 2 — Why and how a boiler is blown down
Boiler water carries Total Dissolved Solids (TDS) plus other dissolved and undissolved solids. They cause scaling, corrosion and erosion, and they carry over with the steam and deposit inside heat exchangers. Blowdown discharges part of the dirty water overboard.
Blowdown is used to:
- remove precipitates formed by chemical treatment of the boiler water;
- remove solid particles, dirt, foam or oil from the water (scumming);
- reduce the density of the water by lowering the water level;
- remove excess water in an emergency.
| Type | What it does |
|---|---|
| Intermittent (manual) | The operator blows down at regular intervals under the operating programme. It removes sludge and suspended solids and helps when oil enters the boiler water through a leaking heat exchanger. |
| Continuous (automatic) | Automation monitors feed and boiler water quality and opens the blowdown valves when TDS exceeds the limit. Precise control removes the most impurities with the least heat and water loss. |
| Bottom blowdown | Through the valve at the bottom of the boiler; removes sludge. Often simply called "boiler blowdown". |
| Scum blowdown | Through the valve at the water surface; removes foam, oil and floating dirt. Known as "scumming". |
Order matters: scum first, bottom second. A bottom blowdown stirs the water, and the scum and oil on the surface would mix back into the boiler water.
Module 3 — The blowdown procedure
Never blow down a modern boiler while it is steaming at high rates. Open the ship side valve first and the blowdown valve second, so the operator keeps control if a pipe bursts. Valve 2, next to the boiler, is opened fully — a partly open non-return seat is cut by the high-speed flow — and the rate is set with valve 3. Step through the sequence in the first simulator below.
Warning sign: a hot drain pipe with all valves closed means a leaking blowdown valve. If the boiler is blown down for inspection, stop firing first and let it cool, opening the vent to allow natural cooling at atmospheric pressure. The overboard non-return valve must work properly: if seawater enters the line, the steam condenses suddenly, a vacuum forms and the pipe can burst. After blowdown the belly plug is opened to drain the remaining water into the engine room bilges.
Module 4 — Gauge glass blowdown
Blow the gauge glass before lighting up, after stopping the boiler, and whenever the level reading is in doubt. The labels are S for the steam cock, W for the water cock and D for the drain valve. The water side is blown 2–3 times to flush out mud and deposits; the steam side is opened for 1–2 seconds only, because steam damages the sealing and shortens its service life. Work through the second simulator below for the full cock sequence.
Module 5 — Burner and furnace
The burner has three systems: fuel delivery, ignition and combustion control. Regular maintenance cuts downtime, reduces fuel consumption, lowers accident risk and keeps the burner compliant.
- Shut down the burner to prevent accidental ignition.
- Fuel delivery: inspect for wear or damage; check filters and replace if needed; check pumps and lines for leaks.
- Ignition: check it works and clean carbon deposits off the spark electrode.
- Pilot burner: check it works, is clean and is not dripping.
- Combustion control: check the control panel for faults; check sensors and replace if needed.
- Clean the burner of soot and debris — a dirty burner means inefficient combustion and higher fuel consumption.
- Reassemble, restart, and follow a set maintenance schedule rigorously.
Furnace defects and their causes
| Furnace defect | Cause |
|---|---|
| Overheating distortion of the furnace crown | Oil, scale or sludge on the heating surface, or shortage of water. Sludge distorts the lower section of the crown. |
| Welding cracks where the furnace joins the shell | Rapid fluctuation of thermal and mechanical stress from improper start-up or shutdown. |
| Crown deformation, local bulge in wall tubes opposite the burner opening | Direct flame impingement. |
| Dry, deep cracks in the furnace mouth, crown or furnace tubes | Flame impingement combined with waterside scale and forcing the boiler. Areas with poor circulation or weak cooling are most vulnerable. |
| Pitting of the furnace crown | Poor feed water. Found by careful examination through the bottom manhole door. |
| Distortion of bottom plating under the furnace | Damaged brickwork protecting the foundation. |
Repairs
| Defect | Repair |
|---|---|
| Deep crack in plating | Drill a hole at each end so it stops growing, open it up, then weld. |
| Corrosion on plating | Chip clean and brush clear; build up weakened areas with electric welding. |
| Pitting | Hard to protect from further corrosion — the protective magnetite layer is difficult to maintain inside the pits. |
| Damaged brickwork | Remove it first and inspect the bottom plating for distortion. |
Prevention comes down to regular cleaning of the heating surface, proper start-up and shutdown, watching for flame impingement, repairing brickwork, inspecting the furnace for corrosion, pitting and cracks, and keeping the feed water treatment right.
The boiler asks for clean surfaces, clean water and an unhurried start. Give it all three and it will hold its pressure for years.
For training only. On board, follow the boiler maker's manual and your company's SMS procedures.