Dual Pressure (double evaporation) Boilers

The main reason for the adoption of this design of boilers is to allow use of modern high efficiency watertube boilers witghout fear of damage through contamination by cargo or fuel oils.
The basic design consists of a D-Type boiler design upon which is mounted a Steam/Steam generator drum. The steam generated by the main boiler heats water in the Steam/Steam generator which produces steam requirements.

The primary drum is initially filled with high quality feed water and suitably dosed. Make up is limited to small amounts due to leakage therefore the feed pump may be of simple design. An example could be a steam or air driven reciprocating pump. The chemical treatment is simple with little requirement for addition or blowdown.
The above design shows the fitting of a superheater. These are normally only fitted where the generated steam will be required to power turbine operated machinery most typically an alternator.

Secondary drum.

The U-tube heating elements are passed through the manhole dorr and expanded into headers welded into the dished end of the drum. The tubes are well supported. A manhole may be fitted at the lower part of the shell allows access to the heating elements. The drum is generally mounted integral, supports are attached to the structure of the primary boiler. The secondary drum also acts as a steam receiver for the exhaust gas boiler.Typical pressures are 63 bar for the primary circuit and 23.5 for the secondary.

The author has not sailed with pressures anywhere near this with this design. Primary pressures of 35bar and secondary pressures nearer 15 bar have proven sufficient even to drive an alternator. Of note is that these designs are obviously more expensive than a normal single steam drum plant even taking into account the improved efficiency. They are therefore generally associated with larger motor powered plant with large waste heat units capable of supplying all requirements including an alternator. However the author has sailed on this plant on a 20,000 tonne product tanker.

Where these boilers are installed in Motorships a "simmering coil" may be fitted. This is located in the primary drum and is supplied from the exhaust economiser to keep both circuits warm thereby preventing any possible damage due to lay-up.
Mountings are those typically found on any boiler with low level water alarms and low/low level shut off on both boilers. The accumulation of pressure test for the safety valves fitted to the secondary drum are calculated with the primary boiler firing at maximum rate generating maximum heating steam supply.

Typical circuit incorporating Dual Pressure Boiler

Under port conditions the main boiler is fired to providing heating steam for the secondary drum. From this steam is supplied for tank heating or to a turbo-alternator via a superheater.
When the vessel is underway the main boiler may stop firing. A waste heat circulating pump passes water from the secondary drum via the waste heat unit back to the drum. The steam produced is again available for tank heating and powering a turbo-alternator.
Cross over valves are fitted for Harbour and sea-duty conditions.