Two marine steam boilers' operation

By Lars Josefsson

Keeping a slave boiler pressurized in a two boiler system

In a two boiler system it’s often a problem to keep the slave boiler at operation pressure when the steam demand is low. This problem does not appear when the vessel is loading or unloading in a harbor since those operations normally need both boilers. On the other hand, when the ship is at sea and only one oil-fired boiler is used then the slave boiler tends to cool down far below the required stand by conditions.
Different methods have been used to solve this problem. Installing steam heating coils in the bottom of the boiler is one method and a sophisticated start-and-stop method for the slave boiler’s burner to keep the pressure at desired level is an other.

Those installations will be unnecessary if you happen to have an EGE, exhaust gas economizer.
Just connect the exhaust gas economizer to the slave boiler instead of the master boiler. This operation method will guarantee normal operation pressure on both boilers all the time at sea.

The method has been used in many ships and the chief engineers are satisfied with the result.

A two-boiler system has other advantages as well. It provides a fully automatic, very flexible and economic operation with maximum safety and availability ensured by the two separate units. At inert gas production, one boiler can be operated at a preset fixed output while the other will follow the load variations automatically.

When bringing a boiler on-line with another boiler, ensure that the working pressure is the same in both boilers and be sure to drain condensate from the steam line, before cracking the valve, to avoid water hammering. Let the steam line be heated up before the steam valve is fully opened.