Chemical Equipment Update
a function of water temperature and pressure so that stresses are kept to a minimum.
Long periods on stand-by
During heat maintenance or stand-by operation (eg in multiboiler operation, if the secondary boiler is not required), the boiler cannot give off any steam. Depending on the control concept, either the steam removal valve is closed or the secondary boiler is run at a lower pressure than the prevailing network pressure. The burners only come on sporadically in this operating mode, in order to balance out losses from heat conduction and radiation. If this state is maintained over any length of time (>3 days), temperature layering starts to occur in the boiler. If boilers that are kept warm in this way are then switched to normal operation, the high operating pressure (hot upper area) makes it seem as if a boiler is available immediately. The boiler control will then apply a high burner load to this boiler in a very short time if there is a corresponding requirement. As a result of the temperature layers in the boiler, extreme thermal stresses will then occur. This can be prevented by the installation of heat
maintenance coils in the floor of the boiler. Steam heating of this heating coil is from underneath, which means harmful temperature layering in the boiler are reliably avoided. To ensure that this solution can be implemented, however, a multiboiler system or a reliable supply of external steam is essential. With marked load changes, ie if load changes occur quickly,
resulting in marked pressure fluctuations, unfavourable flow states can occur in the boiler. The steam bubble formation that is necessary to remove heat from the heat surfaces can slow down, or lead to many smaller bubbles joining together to form larger steam bubbles, which do not separate immediately from the heated surfaces and thus encourage local overheating. For this reason, special measures are required for boiler systems which supply consumers with extremely fluctuating loads, in order to limit the pressure fluctuations in the boiler independently of the
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consumer units. This can be achieved, for example, by:
❒ Increased boiler safeguarding on the pressure side and integration of a reducer station between the boiler and the consumers.
❒ Integration of a steam store for load peaks. ❒ A pressure maintenance device after the boiler, with a regulated steam removal valve, in order to protect the boiler from an excessive pressure loss.
Summary
The avoided causes of boiler problems listed here show that this is a very complex topic. It stretches from planning through construction and setting to the operation of the boiler systems. A conclusive discussion of all the relevant problems is not possible within this framework. Because of the material complexity of steam boiler systems, it is vital that the following is noted:
❒ Steam boiler systems should only be planned by trained, experienced specialists, since many possible sources of problems can be avoided at this early stage.
❒ The quality of the boilers, burners and boiler system components used is of decisive importance if the system is to run smoothly and without breakdowns.
❒ The correct installation of the system requires a competent plant engineer with a knowledge of the interplay between the various boiler house components.
❒ Operation and support by the operating staff are extremely important and have a decisive effect on the working life of the steam boiler system.
❒ A maintenance and teleservicing contract with the boiler manufacturer is always a major advantage. ❒
Enter 16 or ✔ at
www.engineerlive.com/ece
Dipl-Ing Paul Köberlein is with Loos International is based in Gunzenhausen, Germany.
www.loos.de
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www.engineerlive.com
Fig. 4. Diagram of heat removal at the heating surfaces under load through the formation of blisters. Fig. 5. Diagram of the natural
circulation within the boiler.
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