Start-up, Raising Steam, and Shutdown

1. Why start-up is the most dangerous time

A boiler at steady load is a settled system. Everything is at temperature, the metal has expanded to its working dimensions, the flame is stable, and the water is circulating by itself.

A cold boiler is the opposite. It contains:

• Steel at ambient temperature, which will expand as it heats — by millimetres over the length of a drum.
• Water that is not yet circulating, because circulation depends on a temperature difference that does not exist yet.
• Refractory that will crack if heated too fast.
• A furnace full of air that may contain flammable gas from a previous imperfect shutdown.
• Protections that have not yet been proven.

Every start-up procedure exists to manage those five conditions in a controlled order. Skipping a step does not make the start faster; it removes the margin that protects the boiler when something else goes wrong.

Every start-up follows the same logical order: authorise → inspect → prove instruments → fill → vent → purge → ignite → warm → test protection → load.

2. Authorisation and handover

Before touching the boiler:

• Read the boiler log and the defect list. A boiler with an outstanding defect — a leaking tube, a faulty gauge glass, a bypassed trip — must not be started without the Chief Engineer's specific instruction and a clear understanding of the risk.
• Confirm the boiler is not under a permit to work, isolation, or repair.
• Confirm which boiler is being started where two are installed, and that the correct valves will be operated.
• Obtain authorisation from the duty engineer or Chief Engineer.
• Confirm the standby boiler is available or not, and note that in the log.

This is not paperwork for its own sake. Starting the wrong boiler, or starting a boiler that was isolated for a reason, is a plausible accident.

3. Pre-start inspection

3.1 External inspection

Walk round the boiler and look at it as an engineer, not as a checklist:

The mounting layout to confirm before lighting up: safety valves, water level indicators, water level controller, alarms and cut-out assembly, remote level transmitter, main steam outlet, feed inlet and blowdown connections. Every one of these must be in service and correctly lined up — a start-up with a mounting isolated or bypassed is a start-up without protection.

3.2 Instrument and protection check

• Both gauge glasses clean, correctly mounted, illumination working, and blown through.
• Gauge glass protection devices in place and free.
• Pressure gauge in service and reading zero.
• Feedwater control valve free through its travel; feed check valve accessible and correct.
• Feedwater supply available — tank level adequate, feed pumps ready, standby proven.
• Low-water alarm and low-low level trip available.
• Flame scanner clean, sight tube clear, and the scanner proved during the test.
• Burner management system healthy, with no unresolved lockout.
• Safety valves unobstructed, with their drains and waste steam pipes clear and easing gear free and correctly rigged.

Each gauge glass has steam, water and drain cocks in gunmetal bodies, with plate guards against shattering and plugs for renewing the glass. Prove both glasses through before filling: a glass with a choked connection reads a level that the drum does not have.

3.3 Internal inspection after a repair or survey

If the boiler has been opened, before closing it up:

• Internal surfaces clean and free from scale and sludge.
• Tubes clear, proved with a light or a rod, and not plugged beyond the permitted number.
• Internal fittings, separators, and baffles correctly secured.
• Refractory sound, with expansion gaps clear.
• No tools, rags, or scaffolding material left inside.
• Manhole and handhole doors correct, with new or sound joints, correctly seated and tightened.
• All plugs and fittings fitted and tight.

A foreign object left inside a boiler becomes a tube failure, a blocked gas passage, or a shutdown at sea.

4. Filling the boiler

4.1 Water quality

Fill with the best water available — distilled or treated water, not raw water. The water put in at start-up is the water the boiler will concentrate for the next weeks. Starting with bad water means starting with a treatment problem.

4.2 Filling procedure

4.3 Why overfilling is as bad as underfilling

High water level at start-up carries water into the superheater and steam line before the boiler has produced useful steam. That produces:

• Water hammer in the superheater and steam line.
• Thermal shock to hot superheater tubes.
• Wet steam conditions during the critical warming period.
• A false high level that hides the true level during the first load increase.

Set the level accurately, and confirm it before ignition.

4.4 Why filling must be slow

Filling a large cold boiler quickly is a thermal shock in reverse. Water entering at ambient temperature into a boiler whose lower parts are at a different temperature sets up differential expansion. On a boiler that has just been closed up, that can start a leak at a joint that would otherwise have sealed.

5. Purging the furnace

5.1 What purge is for

The furnace and gas path may contain flammable gas. Sources:

• Unburnt fuel from a previous shutdown or failed start.
• Leakage past a fuel valve that does not seat properly.
• Hydrocarbon vapour from fuel oil spills or from work carried out in the furnace.
• Gas from any incomplete combustion during the last firing period.

Lighting a burner into that mixture produces a furnace explosion. Pre-purge removes the mixture before ignition is attempted.

5.2 How purge works

• The forced-draught fan runs at the correct speed for purge.
• Dampers and air register are set to the purge position so air flows through the whole gas path, not just around the burner.
• The flow must be proven — the burner management system checks the airflow signal.
• The purge runs for the full time required by the system, typically several furnace volume changes.

Both the air quantity and the time are set by the burner management system for the specific boiler. Both must be satisfied.

5.3 If purge does not complete

6. Lighting the burner

6.1 The ignition sequence

6.2 Confirming the flame properly

The flame scanner proves the flame to the control system. The local observation proves it to you. Both are needed because the scanner can be fooled:

• A dirty lens may fail to see a good flame.
• A hot refractory surface may be seen as a flame.
• A poorly positioned scanner may lose the flame signal when the flame shape changes, even though the flame is good.

Look at the flame at every start. You are building the reference picture of a good flame for this burner, so that you can recognise a bad one later.

6.3 What a good flame looks like

• Stable, not flickering or pulsating.
• Correct shape and length for the furnace — not touching the tubes, the refractory, or the furnace floor.
• Bright, with the hottest zone in the position the design intends.
• Minimum smoke at the funnel.
• Furnace pressure steady.

6.4 If ignition fails

When the burner management system senses no flame, it must:

1. Shut off the fuel immediately.
2. Raise an alarm.
3. Post-purge the furnace.
4. Lock out until reset.

Do not attempt to relight without completing the post-purge. Each failed ignition attempt without purge adds more unburnt fuel to an already hazardous furnace.

Find the cause first:

7. Raising steam from cold

7.1 The physical problem

As the boiler heats, every pressure part expands. The drum grows in length, diameter, and wall thickness. If the whole boiler is at the same temperature, expansion is proportional and harmless. If one part is much hotter than another, the differential expansion sets up stresses that the design did not anticipate.

The worst case is a rapid start, where the furnace and lower tubes heat quickly while the upper drum is still cool. The result is differential expansion between drum and tubes, which produces:

• Leaking expanded tube joints.
• Stay and stay-tube cracking.
• Distortion of the drum or headers.
• Refractory cracking where it is restrained.

This is why raising steam is done slowly, and why a cold start takes hours on a large water-tube boiler while a hot restart takes much less.

Know which boiler you are starting. A Cochran vertical fire-tube boiler (shown) holds a large volume of water around a small furnace and smoke tubes and responds slowly; a water-tube boiler holds little water in its tubes and drums and responds fast but punishes fast firing with differential expansion, leaking tube joints and cracked refractory. The warm-up rate in the procedure reflects that difference.

7.2 The sequence

7.3 What happens if you raise steam too fast

• Leaking tube joints. Thermal stress opens expanded joints.
• Refractory damage. Rapid heating cracks and spalls refractory and loosens it from its anchors.
• Superheater overheating. At low steam flow with high firing, superheater tubes overheat.
• Drum level instability. Rapid changes drive the level control into oscillation, tripping the burner.
• Stress cracking in plates and stays. Repeated fast starts accumulate damage that eventually becomes a failure.
• Ogee ring and small-radius cracks. Sharp-section components with limited access are the most vulnerable to rapid heating and cooling, and to any deposits that collect in the narrow space around them.

8. Proving protection before load

The boiler must not be placed on load with unproven protection. This is the opportunity to test everything while a trip causes inconvenience rather than an incident.

8.1 Tests to perform

• Gauge glasses. Blow each glass through using the correct sequence: drain open, then water cock closed, then steam cock closed, then steam passage blown, then water passage blown, then drain closed and cocks reopened. Confirm both glasses refill correctly and agree.
• Low-water alarm and cut-out. Lower the water level by the approved method — with all burners shut off — and confirm the alarm sounds and the fuel cut-out operates.
• Flame failure shut-down. Mask the flame scanner or use the approved test method, and confirm the fuel shuts off, the alarm sounds, and the post-purge runs.
• High steam pressure cut-out where fitted.
• Fuel cut-off valve closure. Confirm the quick-closing valve actually closes and at the right time.
• Airflow and purge proving. Confirm the burner management system will not allow ignition without proven purge.
• Alarms and indications reach the engine control room, not just the local panel.

8.2 After testing

Reset everything to normal service condition. Confirm no test equipment, gag, or masking device has been left in place. Confirm the safety valves are unobstructed and their gagging gear, if used, is removed and accounted for.

9. Steam line warming and paralleling

9.1 Warming the line

9.2 Why the line must be drained

A cold steam line condenses the first steam that enters it. That condensate collects at low points and in horizontal runs. When steam reaches it, the condensate is accelerated to high velocity and strikes the first obstruction it meets — a bend, a valve, or a fitting. That is water hammer, and it can crack a pipe or a support.

The drain must be open before the valve, not after.

9.3 Paralleling two boilers

Where a second boiler is to be brought on line:

Instability often shows up here: two boilers with their own level control loops, connected by a common feed system, can interact through feedwater pressure fluctuations.

10. Normal shutdown

10.1 Emergency shutdown

• Stop firing immediately.
• Do not open furnace doors into a hot furnace; a hot furnace can admit air to unburnt fuel and cause an explosion, and it exposes personnel to flame and hot gas.
• Isolate fuel at the quick-closing valve.
• Keep the boiler vented and drained as the procedure requires.
• Maintain water level only if the condition and procedure permit. Feedwater into a boiler with overheated, uncovered tubes can cause a sudden contraction and rupture.
• Inform the duty engineer and log the event with times and indications.
• Do not restart until the cause is found and permission is given.

11. Lay-up

A boiler out of service corrodes faster than a boiler running, because the protective magnetite film needs the right chemistry and the absence of oxygen. Lay-up is a protection routine, not just a shutdown.

11.1 Choice of method

• Wet lay-up — for short to medium periods, or where the boiler must be available at short notice.
• Dry lay-up — for long periods, or where the boiler is opened for repair.

The decision depends on the duration out of service, the ambient conditions, whether the boiler will be entered, and the maker's instructions.

11.2 Wet lay-up

• Fill the boiler completely with treated water, with no air space.
• Maintain the correct chemistry — alkalinity and oxygen scavenger reserve.
• Top up periodically to replace losses and exclude air.
• Keep the water hot if the arrangement allows, because hot deoxygenated water is less corrosive than cold aerated water.
• A beneficial practice is to fit an alkaline dosing pot or a small circulation arrangement to keep the chemistry uniform.

11.3 Dry lay-up

11.4 The principle behind both methods

Corrosion needs water, oxygen, and an electrolyte. Lay-up works by removing one of the three:

• Wet lay-up removes the oxygen, by chemical scavenging and by excluding air.
• Dry lay-up removes the water, by drying and then keeping the space dry with desiccant.

Either method is effective if applied properly and monitored. Neither works if it is set up once and forgotten.

12. Start-up and shutdown reference sequence

START-UP
1.  Authorise the start; read the log; confirm no permit or outstanding defect
2.  External inspection: casing, fuel, atomising, air, gas path, steam line
3.  Prove instruments and protections: gauge glasses, alarms, trips, scanner
4.  Internal check if the boiler has been opened
5.  Fill with treated water, slowly, through the economiser; vent all air
6.  Set the level correctly; confirm both glasses agree
7.  Purge the furnace for the full time at proven airflow
8.  Light the approved burner: prove the igniter, confirm the flame locally and on the scanner
9.  Establish stable low fire; adjust fuel and air together
10. Raise pressure in steps on the low-capacity burner
11. Keep superheater vents and drains open until steam flow is established
12. Shut drum vents when steam issues freely
13. Drain and warm the steam line before opening the stop valve
14. Prove low water trip, flame failure trip, and fuel cut-off before loading
15. Open the main stop valve slowly; parallel if required
16. Watch level, pressure, temperature, flame, furnace pressure, and uptake temperature
17. Tighten manholes and handholes as the maker permits during first warm-up
18. Log all readings and test results

SHUTDOWN
1. Reduce load, then reduce firing in steps
2. Maintain water level through cool-down
3. Secure fuel; allow post-purge to complete
4. Close the main stop valve
5. Drain steam line, superheater, and economiser as directed
6. Vent to prevent vacuum as the boiler cools
7. Cool naturally; do not force-cool
8. Open up only when depressurised and cool
9. Protect the boiler in wet or dry lay-up
10. Record readings, defects, and events