GREAT YARMOUTH Power Station is approximately 220 km northeast of London; it is a combined cycle gas turbine (CCGT) owned and operated by RWE npower Plc. In 2012, the station started experiencing problems with a major low-pressure steam control valve. A servo actuator made by Moog Inc., a global provider of motion control systems, controlled the valve. The valve was making a lot of noise, with the bearings being the prime suspects. When the bearings failed, the failure caused side loading and damaged the actuator’s piston.

The engineers at the power station were originally going to turn to a local third-party repair house because a quick turnaround was necessary. When the repair house was slow to respond, the engineers made an inquiry to manufacturer replacement bearings. The manufacturer, knowing that the symptoms potentially pointed to something more than just the bearing, and wishing to maintain the integrity of its servo actuator, offered a custom-designed actuator service rather than parts alone.

Making a Decision: OEM or Third-party Repair House

The manufacturer’s repair service included a full on-receipt test, a full strip-down investigation, a review of all parts, replacement of the faulty parts, a rebuild to its original specification and test to the original specification. The reports the OEM promised to deliver the station managers included photographs and video of the actuator during the investigation and subsequent rebuild and test. These reports also included a closed loop and plots for speed and frequency response of the servo valve.

The third-party repair house’s offer was a report that simply indicated a part had been “tested,” with little or no description for a power station engineer to review. While the repair is made, industry experts say that many times the third-party repair houses provide little or no information in the way of a report. And the ones that do, say power industry experts, sometimes do it only after being called by a customer who enquires about the results of a repair.

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Fig 2: An actuator from the Great Yarmouth Power Station is prepared for disassembly.

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Fig 3: A refurbished process valve undergoes testing at manufacturer’s service site.

In the case of Great Yarmouth, the station’s operators sent details about the actuator, and Moog replied with an extract of a report from a previous service. The extract helped the station’s engineers see what to expect. The information supplied in the report, including the potential for audio–visual content impressed the engineers at the power station. Managers from the station called the original manufacturer because it offered not only a diagnostic and repair service but also genuine parts supplied, tested and fitted by the manufacturer.

Investigating the Issues

Once the manufacturer began testing the servo valves, it saw that more than just the bearings were at fault; in fact, the list was longer than initially thought. The manufacturer’s repair team identified that the piston and bearing assembly, the crank arm, bushes and bearings, connecting pins and all of the soft seals were in need of overhaul.

The manufacturer confirmed that it was substantial wear to all of these components that was causing the noise and that there was also substantial leakage of hydraulic fluid. The actuator “on receipt” performance testing was subsequently videoed by the manufacturer and this was delivered to Great Yarmouth’s engineers along with a revised cost and repair schedule.

Once the power station’s engineers authorized the repairs, the actuator and servo valve maker had a lot of work to do. Not only did it have to get original parts shipped from the United States, but also had to match a very tight service window with planned downtime at the power station. Once the parts arrived, the manufacturer reassembled the actuator with new parts and tested it to its original specification – confirming that all faults had been rectified.

From order confirmation on November 8, the repair took just ten days with the unit leaving the manufacturer’s repair facility on November 18. During the outage, the manufacturer visited the power station to review the installed equipment and was able to highlight other service issues. The engineers at the power station have since asked Moog to undertake additional proactive maintenance on another actuator and the associated process valve.

According to Matt Keen, systems and projects manager for Moog Inc., many power plants run engines for years on end without doing much in the way of servo valve or actuator maintenance. Keen says some of these plants’ actuators are 1.8-meters tall and sit atop a turbine. Because actuators can be suspended on top of or even upside down relative to the turbine, adds Keen, they can be hard for maintenance managers to inspect and service.

Avoiding Unplanned Maintenance

The best way to avoid a breakdown, of course, is putting in place a maintenance schedule, say industry experts. Power station maintenance managers should enlist equipment manufacturers to make a site visit and identify equipment and its location and document the maintenance and upkeep of things like oil. For instance, a power station operator can look at control system loop closure to see if there is a mismatch between the actuator control and signal per the manufacturer’s specifications.

Great Yarmouth Power Station was able to get a level of reporting better to other service suppliers. The inclusion of photos, in-depth product information and a repair process video, which confirmed the damage, gave the engineers at the station faith in the manufacturer’s abilities.

Sam Mellor, formerly a C&I technician at Great Yarmouth Power Station, said,

– We were thankful for Moog’s co-operation regarding the overhaul of the actuator; especially in light of the notice that they were given.

The original request from the power station was for the manufacturer to supply spare parts. But the manufacturer’s policy was to supply third parties with only non-field-replaceable components. So, instead, the servo actuator maker offered a complete overhaul and service using genuine parts tested to original specifications.

When a power station wants a long-lasting repair with warranty, it is important to find manufacturers that will supply non-field-replaceable spare parts and adhere to strict quality and product-liability procedures. The best way for power stations to ensure the terms of the product warranties is to make sure industrial products are serviced and installed by qualified personnel employed by the manufacturer.

Dr. Sherif El Henaoui, Moog, 
selhenaoui@moog.com