11.3 Marine Boilers, Exhaust Gas Economizers & Steam Systems

11.3 Marine Boilers, Exhaust Gas Economizers & Steam Systems

Steam systems are indispensable on motor merchant ships. Although primary propulsion is provided by large diesel engines, thermal energy in the form of low- and medium-pressure steam (typically $6\text{--}16\text{ bar}$) is constantly required to heat Heavy Fuel Oil (HFO) bunker tanks, settling tanks, and service tanks so the fuel remains fluid enough to be pumped. Steam is also required to preheat fuel prior to purifiers and main engine fuel injectors, heat liquid cargoes on product/chemical tankers, warm accommodation blocks in cold climates, and supply galley and domestic hot water.

1. Marine Auxiliary Boilers and Exhaust Gas Economizers

+-----------------------------------------------------------------------------------------+
|                        MARINE AUXILIARY BOILER TYPES                                    |
+-------------------+--------------------------------+------------------------------------+
| Boiler Type       | Water / Gas Configuration      | Key Operational Characteristics   |
+-------------------+--------------------------------+------------------------------------+
| **Auxiliary       | Water and steam circulate      | - Rapid steam raising (15-30 min)  |
| Water-Tube        | inside high-pressure tubes;    | - Handles high pressures (>16 bar) |
| Boiler**          | hot combustion gas surrounds   | - Low water volume; highly sensitive|
|                   | tube exteriors.                |   to feed water quality & scale.   |
+-------------------+--------------------------------+------------------------------------+
| **Smoke-Tube /    | Hot combustion gas passes      | - Large water volume; excellent    |
| Fire-Tube         | inside tubes; water completely |   thermal buffer capacity.         |
| Boiler**          | surrounds tubes in outer shell.| - Moderate pressure (6-10 bar).    |
| (e.g. Cochran)    |                                | - Robust, tolerant of minor scale. |
+-------------------+--------------------------------+------------------------------------+
| **Composite       | Combines oil-fired combustion  | - Highly versatile for general cargo|
| Boiler**          | chamber and separate exhaust   | - Uses main engine exhaust at sea; |
|                   | gas tube bank in one shell.    |   burns oil in port/maneuvering.   |
+-------------------+--------------------------------+------------------------------------+

Exhaust Gas Economizers (Waste Heat Recovery)

When a ship's main diesel engine runs at normal sea speed, exhaust gases entering the economizer after the turbocharger may typically be between $280^\circ\text{C}$ and $350^\circ\text{C}$. Discharging these gases directly into the atmosphere represents immense wasted thermal energy.

An Exhaust Gas Economizer (often referred to as an exhaust gas boiler) is installed in the engine exhaust gas funnel casing. It consists of multiple banks of steel tubes (often fitted with external spiral steel fins or pins to increase surface area) connected to the auxiliary boiler steam-and-water circulation system:

• Boiler water is continuously circulated by a boiler circulating pump from the auxiliary boiler steam drum through the economizer tube banks and returned as a steam-water mixture.
• While steaming at sea, the economizer generates sufficient steam ($7\text{--}8\text{ bar}$) to satisfy all shipboard heating demands without burning any fuel in the oil-fired auxiliary boiler burner.

                         EXHAUST GAS ECONOMIZER SYSTEM

GAS PATH:   Main engine -> Turbocharger -> Economizer gas casing -> Funnel
                                           (Hot gas outside tubes)
                                                    |
                                              Heat transferred
                                                    v
WATER PATH: Auxiliary boiler drum -> Circulating pump -> Economizer tubes
                     ^                                       |
                     +------- Steam-water mixture return ----+
                     |
                Steam to ship's services

Gas and boiler water remain separated by the tube walls.

Soot Blowers and Economizer Fires

Main engine exhaust gas contains unburned carbon, lubricating oil residues, and sulfur compounds that deposit on economizer finned tubes as sticky soot. Soot insulates the tubes, reducing heat transfer and steam production; increasing deposits also increase the fire risk.

More critically, accumulated soot represents a severe fire hazard:

• Soot Blowers: Motorized or manual rotating lances fitted with high-pressure steam nozzles (or compressed air nozzles) are installed within the tube banks. GP Ratings or watch engineers operate soot blowers daily or twice daily while the engine is running at steady sea load. High-pressure steam jets sweep across the tubes, dislodging soot deposits into the exhaust gas stream.
• Soot Fires: If soot blowing is neglected, a high engine load or scavenge fire can ignite the dry soot deposit, causing an economizer soot fire. If temperatures exceed $1000^\circ\text{C}$, the steel tubes themselves can react with steam or water in an autocatalytic iron-water reaction ($3Fe + 4H_2O \to Fe_3O_4 + 4H_2$), producing explosive hydrogen gas and completely incinerating the economizer bundle.

2. Statutory Boiler Mountings and Safety Systems

Marine steam boilers require protective mountings under SOLAS and class rules. The following drawing shows a typical auxiliary boiler arrangement:

                     TYPICAL AUXILIARY BOILER MOUNTINGS  

                   [Air Vent Valve]     [Dual Safety Valves]
                           |                    |
                           +----------+---------+
                                      |
         =============================+=============================
        /                 STEAM DRUM / STEAM SPACE                  \
       |                                                             |
       |  [Main Steam Stop Valve]          [Pressure Gauge & Cock]   |
       |  (SDNR Valve to Steam Manifold)                             |
       |                                                             |
       |  [Surface Blowdown Valve]         [Two Independent Water]   |
       |  (Scum Valve for Oil/Foam)        [Level Gauge Glasses  ]   |
       |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  [With Remote Blowdowns]   |
       |       NORMAL WATER LEVEL                                    |
       |  ===============================                            |
       |                                                             |
       |  [Feed Check Valves]              [Low Water Level Alarm &] |
       |  (Main & Auxiliary SDNR)          [Burner Cut-Out Sensors ] |
        \                                                           /
         =============================+=============================
                                      |
                           [Bottom Blowdown Valve]
                           (Drains Sludge & Sediment)

Mandatory Boiler Mountings

1. Safety Valves (Dual Independent):
   • SOLAS normally requires at least two safety valves of adequate capacity; the Administration may approve one where adequate overpressure protection is provided. Valves connect directly to the steam space without intervening shut-off valves. Their combined capacity must discharge the maximum steam output within the approved accumulation limit.
   • Adjustment is protected by locked or sealed covers to prevent unauthorized changes to the spring setting.
   • External easing gear allows controlled lifting from a safe position. Test it at the pressure and interval specified by the maker and planned maintenance system; a weekly shipboard routine is not itself a SOLAS interval.
2. Water Level Indication:
   • For a boiler essential to ship safety that maintains a specified water level, SOLAS requires two means of level indication, at least one a direct-reading gauge glass. Two separate glasses are a common arrangement.
   • Protected by toughened plate glass shields to protect watchkeepers if a tubular glass bursts.
   • Fitted with upper steam isolation cock, lower water isolation cock, and a bottom drain cock.
3. Main Steam Stop Valve:
   • Heavy-duty screw-down non-return (SDNR) globe valve isolating the boiler from the ship's steam distribution header.
4. Feed Check Valves (Main and Auxiliary):
   • Two independent feed water supply lines, each terminating in a screw-down non-return valve mounted on the boiler shell below the water line. The non-return valve prevents hot pressurized boiler water from surging back into feed pumps if a pump fails.
5. Surface Blowdown Valve (Scum Valve):
   • Connected to an internal scum pan mounted just below the normal working water level. Opened periodically to skim off floating lubricating oil, foam, and suspended organic matter.
6. Bottom Blowdown Valve:
   • Connected to the lowest point of the boiler shell/water drum. Opened to purge heavy accumulated sludge, precipitated scale salts, and lower boiler water Total Dissolved Solids (TDS).
7. Air Vent Cock:
   • Mounted at the highest point of the steam space. Opened when filling boiler with water, opened when raising steam (to allow displaced air to escape until pure steam blows free), and opened when taking boiler out of service (to prevent vacuum collapse when cooling).
8. Low-Water Level Fuel Shut-Off (Burner Management System):
   • Dual independent float-switch or electrode-type sensors. If water drops to the lowest permissible water level ($LWL$), the burner management system shuts the fuel oil supply solenoid valve and sounds an alarm. The burner control system follows its approved shutdown sequence.

The Gauge Glass Blow-Through Procedure

Verifying true water level is the most vital watchkeeping duty for GP Ratings in the engine room. Blocked steam or water passages cause false water level indications, leading to disastrous dry boiler tube burnout. The statutory blow-through procedure is carried out as follows:

                  GAUGE GLASS BLOW-THROUGH TESTING PROCEDURE

     Step 1: Close water cock, open drain cock. Steam blows violently through.
             (PROVES STEAM COCK & STEAM PASSAGE ARE CLEAR)

     Step 2: Close steam cock, open water cock. Water blows violently through drain.
             (PROVES WATER COCK & WATER PASSAGE ARE CLEAR)

     Step 3: Close drain cock, open steam cock.
             (Water rushes rapidly back into glass and stabilizes at true level)

3. Boiler Feed Water Treatment & Chemistry Control

Water inside a marine boiler is subject to extreme temperatures and pressures that accelerate chemical reactions. Using raw seawater, shore tap water, or unconditioned distillate will rapidly destroy a marine boiler through three mechanisms:

1. Scale Formation: Dissolved calcium and magnesium salts precipitate onto hot furnace and tube surfaces, forming insulating rock-hard scale. A $1\text{ mm}$ layer of scale causes local tube overheating, blistering, and explosive rupture.
2. Corrosion and Oxygen Pitting: Dissolved oxygen ($O_2$) and carbon dioxide ($CO_2$) in feed water cause rapid localized pitting corrosion, drilling pinholes through steel tubes in weeks.
3. Caustic Embrittlement and Priming: Excessive alkalinity makes steel brittle along crystalline grain boundaries. High dissolved solids or oil contamination causes violent surface foaming, carrying liquid water over into steam lines (priming/carryover) and causing destructive hydraulic hammer in steam machinery.