Sugar mill boilers generate stearn by boiling water in the furnace and convection bank
tubes. They depend on adequate flows of steam and water circulating through the tubes
to provide the required steam output at specified temperature and pressure without
overheating the tubes. Natural circulation boilers, which are exclusively used by
Australian raw sugar factories, are driven by steam and water density differences. Such
boilers can be difficult to design well with the consequence that circulation problems
may occur under normal operating conditions or by operating the boiler above MCR.
SRI has developed a boiler circulation analysis model, which allows the flow rates and
steam quality in individual wall circuits and tubes to be predicted. The paper
summarises the development of the model and describes its application to solve several
operational problems. After the SRI circulation model was developed, it was tested on
the decommissioned main boiler at Qunaba Mill using maintenance records that had
documented tube-overheating problems and a circulation analysis carried out by a
consultant to investigate the problem. The SRI model predicted most of the overheating
and the re::.ults compared well with those of the consultant. It was shown that detailed
heat flux distribution produced by the FURNACE code plays an important role in
precise predictions of overheating problems. Overheating related problems were
investigated for the Fairymead No. 1 boiler (roof tube creep) and the Farleigh No.4
boiler (deposition problems in the convection bank tubes and mud drum). In both cases,
the model predictions were consistent with the problems encountered. The solution
modelled for the Fairymead boiler, involving rerouting the supply pipes to the frontwall
roof circuit, indicated that the problem would be eliminated. The modifications to the
boiler were made, and after two seasons, there have been no more signs of tube
overheating. A final solution to the Farleigh boiler was not so clear cut, but the model
predicted a small improvement by removing the mud drum baffle. The bat11e was
removed and after one season the amount of deposition was considerably reduced. The
SRI circulation model is also an important tool for investigating boiler uprating. This is
to ensure that the boiler is not driven beyond the self compensating regime where
increasing the steam output may reduce the circulation. An uprating investigation was
carried out for the Pleystowe No.2 boiler. It was found that increasing the steam output
from 160 t/h to 220 tlh would alter the relative distribution of flows between different
circuits but it would not result in any of the tubes being put at risk of overheating.