MORE SAFETY FOR EMERGENCY DIESEL ENGINES FOR THE BELGIUM NUCLEAR POWER PLANTS

摘要

Each nuclear plant in Belgium is equipped with a series of U(ltimate)P(ower)S(upply) units, also called emergency power units. These consist of generators driven by multi-cylinder (typically 18) diesel engines, which are marine derivatives. Unlike marine applications, the steady-state load does not produce pulsating torques. However, these diesel engines are designed to start upon short notice following a blackout and reach full power within a few seconds to guarantee the availability or all safety valves and ventilators. Such sharp and quasi-cold starts, periodically performed to guarantee the UPS availability, may spell utter failures of the crankshaft, as demonstrated by a fatigue failure observed on the fillets connecting the crankpin to the web faces. The fillet cracks initiate in bending mode and then progress in torsion mode to excessive transient torques arising in the power train during successive starts. Aware of the potential risk and conforming to the Belgian nuclear safety rules, the plant operator of Doel sponsored the development of a nondestructive technique enabling the inspection of each fillet for cracks without first removing each piston rod from its crankpin. As a result, Laborelec developed a specific eddy-current probe which avoids fully dismantling the engine, as is done during ten-yearly overhauls with dye-checks for cracks. Inspecting crankshaft fillet integrity with this least obtrusive technique requires 24 hours per engine. It can thus be performed more frequently to prevent total crankshaft failures in time and monitor the engine fatigue caused following the mandatory monthly start-up tests. This promising technique may also find marine applications. Measuring the transient torque arising between the engine and the generator showed that this reached very high values shortly after starting the engine and injecting fuel at full throttle to teach full power within seconds. The pulsating torque of the 18-cylinders engine occurring 9 times per crankshaft rev excited a low-frequency torsion resonance of the power train further enhanced by the backlash clearance of a elastomer block flexible coupling. Best solution is to resort to a clearance-free tire-type elastomer flexible coupling. Elastomer hysteretic characteristics contribute to damping torsion modes, which appear at frequencies too low for the engine internal damper to be of any use.