If the steam from the exhaust boiler exceeds the steam demand onboard, a steam dump valve automatically dumps the surplus steam to a dump condenser.
http://www.steamesteem.com/index.html?steam-boilers/boilers-...

In one exemplary embodiment, the heat rejection capability of a main ACC 22 is supplemented by an auxiliary condenser 24, such as an air-cooled steam dump condenser that can be used during plant startup and steam bypass operation to supplement the heat rejection capability of the ACC 22, thereby reducing steam turbine back pressure.
https://www.google.com/patents/US7367177

A dump condenser is required when the steam consumption needs to be drastically reduced in a very short period of time and when the boiler cannot follow this variation.
http://www.bronswerk.com/en/Dumpcondensation-System/PT21/

At inert gas firing the master burner's load is limited to operate between 50% and 100%. A steam dump controller should automatically take over the steam pressure control and by means of a control valve dump the excess steam to a steam dump condenser.
http://www.steamesteem.com/index.html?steam-boilers/control-...

But in the event that, for example, a process stops operating, a dump condenser must be able to respond with lightning speed in order to absorb the heat.
bronswerk.com
Falls jedoch z. B. ein Prozess plötzlich stoppt, muss ein Niederschlagskondensator blitzschnell dazu in der Lage sein, die Wärme zu absorbieren.
bronswerk.com

This requires an air-cooled dump condenser that can immediately switch from a standstill situation to full capacity.
bronswerk.com
Dazu ist ein luftgekühlter Kondensator erforderlich, der sofort vom Stillstand auf volle Leistung schalten kann.
bronswerk.com
http://www.linguee.de/englisch-deutsch/uebersetzung/dump con...