California’s power industry was in the news most of last year, as the state’s ageing infrastructure struggled to balance a rapidly escalating consumer demand for electricity and the unforeseen effects of deregulation. With today’s razor-thin allowance for capital improvements at public utilities, few plants can afford to replace gantry cranes that are past their prime. On the other hand, no nuclear facility can continue to maintain its equipment without them.
Last autumn the San Onofre Nuclear Generating Station (SONGS), majority-owned and operated by Southern California Edison (SCE), was facing a 1 April deadline to begin staging operations for a planned 20 May refuelling and maintenance outage at the Unit 2 reactor. Doing refuelling outages in the spring and autumn allows the units to be available to meet the peak demands of the summer cooling season. SONGS has two operating reactors (Units 2 and 3 – Unit 1 is being decommissioned) and is refuelled every 24 months. The goal is never to have both units down simultaneously.
The scope of work involves removing spent nuclear fuel and installing new fuel, along with several other tasks that cannot be performed when the unit is operating, such as valve, pump, pipe and instrumentation repair. It is also the only time crews can open the turbines and perform inspection, repair and upgrade work. Gantry cranes lift each piece of equipment that must be disassembled – cover panels for the generator, the generator itself and the pumps. The rotor alone weighs 200 US ton (181t). All of these maintenance projects depend on the availability of SONGS’ two 27-year-old, 225 ton (204t) Paceco gantry cranes.
The cranes are routinely inspected under contract by Crane Pro Services, with scheduled maintenance performed by SONGS maintenance crews. Crane Pro’s on-site technician had alerted management that the performance of the gantries was declining and suggested that modernisation with new component-based technology would markedly improve crane performance and reliability.
Looking ahead to pave the way for the May outage, SONGS maintenance executives commissioned Crane Pro’s nuclear power specialist Jim Knisley to carry out a detailed engineering survey to evaluate the gantry cranes’ condition.
According to SONGS project supervisor, Bob Hunt, the previous shutdown in August 2001 had taken longer than anticipated. ‘Our cranes were ageing, and mechanical and electrical problems were increasing. Our turbine gantries are used 24/7 during refuel outages. During each outage we would incur more and more gantry down time. We had just recovered from a longer than usual shutdown but we had a fair amount of time before our next outage. We saw this as an opportune time to perform the survey.’
Results of the survey were available in November 2001 and, as expected, the news was not good. The cranes had not been operating smoothly for some time – they were noisy and were suffering an epidemic of ground-down wheel flanges. This prevented the gantry from moving freely as wheels encountered cement instead of rails. The study showed that the wheels had worn seriously out of diameter, outside accepted standards (Crane Manufacturers Association of America). As a result, the gantry was moving unevenly on different sized wheels, causing the crane to wrack. After reviewing the data, the SONGS engineering department de-rated the cranes to 25 ton (23t), certainly not adequate to perform the tasks of the upcoming shutdown.
Clearly at least one of the cranes had to be renovated prior to the 1 April deadline to begin staging for the May outage. ‘Our immediate concern was the financial impact of crane breakdowns during the shutdown, prolonging our time away from the grid,’ recalls Hunt. ‘We knew that if we did nothing the risk was too high to bear. The loss of revenue is approximately a million dollars every day that one unit is offline.’
Crane Pro’s survey revealed all of the expected mechanical problems. However, it also shed light on hidden electrical issues along with opportunities for improvement. According to Hunt, this was the catalyst that pushed the project out of the realm of routine repair into complete modernisation.
‘Along with preventing unscheduled downtime during the outage, our long-term goal was to ensure reliable crane performance for the anticipated 20-year remaining life of the plant and Crane Pro showed us how we could accomplish that goal,’ says Hunt. ‘At the time, we were uncertain if we could do the modernisation before the next shutdown. Our options were to fix the mechanicals immediately, and do the electrical modernisation later after the shutdown, or do the entire job now. Did we have time? To find out, we had to move fast, develop a specification and send it out for bids.’
Crane Pro Services claims that it was the only one of the four bidders able to assure SONGS that all the modernisation work could be completed before the 1 April deadline, at a cost of $1.2m for the first crane. ‘Actually, some of our competitors said it couldn’t be done within the three-month window available,’ says district manager Mike Mitchell. ‘We received the order on 14 December, and immediately pulled in experts from all over the country – not just hands, but technicians specialising in power and heavy maintenance cranes. We recruited Crane Pro people from the Gulf Coast, the Great Lakes, the Rio Grande area and the Alleghenies, along with five technicians from our own district. Internally, we revenue-share with other districts, who receive credit for the portion of the project their technician handles.’
According to Bob Hunt, what they were looking for was not necessarily a Cadillac but something safe, reliable and durable. ‘Once we decided to go forward with modernisation rather than repair, we saw an opportunity to improve performance with new features installed by workers that know cranes. The new features had to be safe, enhance work efficiency and increase accurate performance of the crane without eating up the schedule,’ says Hunt.
In addition to installing 16 new tool steel gantry wheels and eight new Nord gantry reducers, the electrical modernisation plan involved replacing the festoon system and replacing the DC motors and controls powering six critical crane functions: main hoist; auxiliary hoist; main trolley; access trolley; access hoist; and gantry drive. In their place were installed new Marathon AC motors rated for continuous duty. The DC drives were replaced by Drivecon AC variable frequency drives.
‘We helped the customer understand and agree why it wasn’t a good idea to put good money back into outdated technology,’ says Jim Corns, Crane Pro’s project manager and front-line liaison with Southern California Edison. ‘AC motors are more reliable, easier to maintain and ensure better parts availability. More important, however, is the infinite range of control delivered by variable frequency drives. In addition to improving load-handling performance, variable frequency drives help protect equipment from unnecessary wear, and prolong equipment life.’
Cab controls were replaced with modern Drivecon alternatives, and a radio control was fitted that allows the crane to be alternatively controlled from the ground by one of two required safety spotters, reducing the operating crew needed from three to two.
According to Bob Hunt, deciding how far to take the modernisation effort was a shared process. ‘We make electricity, Crane Pro offers crane solutions. We don’t trespass on each other’s areas of expertise. We did voice some of our own ideas, such as wireless controls. However, Crane Pro broadened our mental picture and offered us technologies we were unaware of. We ended up with a menu of options to consider. We ran the safety, cost and time equation, and then made our decisions.’
One new feature chosen off the menu was an XY position indicator for the gantry and trolley. In the past the operator could not be sure when he was centred over the load. Now a digital readout in the cab provides X and Y axis coordinates when the operator makes a pick. To put the item back he can use the axis coordinates to position the trolley and gantry precisely where they were before. When these features are needed, the operator runs the crane from the cab.
Crane Pro also recommended that SONGS install programmable limit switches. In the future, virtual walls can be programmed in to stop crane operation with a load on the hook in certain areas. Using limit switches to define a critical load path prevents expensive mistakes in 15 to 20 different areas where dropping a load could cause millions of dollars in damage – such as over the turbine or steam generator. This function is the responsibility of administrative personnel watching over the operator.
Getting back to the original problem, the 16 new gantry wheels had their own technological edge. A unique design of the tool steel wheel incorporates two different Brinell hardness ratings for maximum wheel life. A special treatment on the rolling surface and the flanges improves wear and lowers the drag coefficient, making the crane roll more easily. A slightly softer metal in the interior structure allows the wheel to absorb vibration, making it less prone to cracking.
The first crane was successfully load tested on 21 March, one day ahead of schedule. On a sunny California morning with most of the employees and upper management watching, the crane lifted eight 34 ton bags of water to universal applause. ‘It was a remarkable team effort,’ says Jim Joy, California Edison’s maintenance project manager.
According to Bob Hunt, Edison’s experience on the revamp project dovetails with the SONGS commitment to provide an uninterrupted electricity service to the public. ‘Partnering relationships are very important, and it’s amazing how much can be done when people work together,’ says Hunt. ‘Running an operation like this without developing and utilising partnerships like ours with Crane Pro raises our level of effort and challenges our focus. We want to concentrate on just one thing here which is making safe and reliable energy.’
Identical work is scheduled to be done by Crane Pro Services on SONGS Unit 3 gantry in August this year.
