All UK conventional fossil fired boilers were built in the '70s to designs a decade earlier, employing radiant and convection superheating that satisfied base load operation only. Cold starts were practicable using modest firing rates. However, aggressive hot starts were a problem owing to inadequate drainage. Using large quantities of service records, Innogy's aim was to (i) establish procedures that minimise plant damage (ii) reduce energy costs and (iii) improve start-up flexibility. It transpired for example that the precise time of opening the drains was critical and that constant admission pressure was required at the turbine. Many examples were given of on-line parameters that were available to the operator, e.g. stress increase, temperature drop, pressure, heat input, MW load. Of particular interest was the display of stress profile across a superheater outlet header, which had a direct bearing on thermal fatigue strains.Here it needed to be emphasized that although the paper dealt with hot starts only, a major issue was the logging' of the bends in individual pendant tubes of the superheaters by trapped condensate. This, by blocking steam flows, could lead to overheating of the straight section of the tubes. More important was the quenching effect on tubes and headers when the condensate suddenly flushed through. This was far in excess of the "old rule" of not permitting more than a 40 deg C differential to develop across a ligament. Poor fuel distribution between burners was an additional problem.
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