Power loss at nuclear facilities is an unfortunate reality. When power loss happens, protocols are initiated to ensure equipment operation is safely halted. In the case of nuclear facility cranes, understanding the reasons behind the power loss and the actions that should be taken enables engineers and technicians to appropriately plan and practice for a breakdown, hazardous or catastrophic event.
Steve Lawrence, Product Manager, Konecranes Nuclear Equipment and Services
Electrical power can be cut to nuclear cranes in three primary scenarios:
- An unplanned power outage, tripped circuit or generator breakdown.
- A controlled outage, as in deliberately pressing the red emergency stop button, that shuts off power to a nuclear crane in the event of a hazardous situation. Often, this action is taken when a load limit is exceeded, the crane has an unbalanced load or two-blocking, over-speeding or improper threading of the hoist rope in the drum grooves is taking place.
- An earthquake is detected, which trips a seismic switch that is rated to stay open and cut off power to the crane during a seismic event.
In a power loss or emergency situation, the key objective for ensuring nuclear crane safety is to remove power from the motor or controls and set the brakes to securely hold the load in place. Luckily, the electrical systems of most industrial cranes—including those employed in nuclear facilities—are designed to operate fail-safe.
Single failure-proof cranes key for safety
As the term implies, “fail-safe” ensures that should there be a failure, crane operation will still be safe. In the case of an industrial crane, when power failure is experienced, the crane’s electrical system returns to a non-active, safe position. At the same time, the crane controls also go to a safe, neutral position that prevents the crane from resuming motion when power returns. In addition to a fail-safe mechanism in the electrical system, the load is also secured by the crane braking system. For single failure-proof nuclear cranes, fail safe electrical and braking systems are particularly important due to the critical nature of the loads these machines carry.
Controlled descent after power loss
When there is power loss, either intentional or unintentional, the first concern should be that the nuclear crane’s braking system holds the load safely in place while preventing uncontrolled motion. But leaving a load suspended with the power off still leaves a risk for eventual failure. For that reason, the correct procedure is almost always to perform a controlled descent as early as it's safe to do so.
In the absence of power, this requires a manual braking system. While many industrial cranes employ a manual, disc braking system for such events, using such a method can cause potentially damaging friction and heat. Konecranes has found a solution for this, which is one of the safest emergency lowering methodologies in the industry.
Konecranes nuclear cranes feature an eddy current brake in the drive train. Using power from an alternator that generates its own electricity, this brake creates electronic braking resistance that provides controlled braking, limiting the descent to about 50 percent of rated speed.
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