Keeping the Lights On

Combined Cycle Gas Turbine Plants seek "as-new" performance of actuators and servo valves when making updates and repairs
Motion control plays a key role in ensuring consistent power output and efficient operations
The right repair services are critical in ensuring uptime of turbines during parts replacement

Many of the world’s most efficient power plants are now Combined Cycle Gas Turbine Plants (CCGT) incorporating both gas and steam turbines. Gas and steam turbines represent particularly demanding motion control applications because motion control is the key to machine performance, safety and ultimately the ability to supply power to households around the world. The application uses rugged inlet guide vane actuators and fuel gas control valves to provide the correct ratio of air and fuel in varying loads and ambient environments to ensure optimum efficiencies and minimize emissions. From a safety perspective, preventing leakage is also critical to reduce risk of fire and lower insurance costs.

In many countries, there are energy shortages so power plants must run around the clock and any downtime can affect the comfort of the population—and potentially the country's economy, too. In some places, old power stations, especially coal-fired plants are being shut down so it is vital to keep those plants that are energy efficient operating at maximum output. In today’s environment, downtime of even a single plant can put a stress on the existing electricity network. In addition, if the power plants exceed or underperform the output that has been declared, it could be subject to a fine from the authorities. Consequently, all power plants are concerned with maximizing uptime for greater energy output and looking for ways to keep the turbines running reliably at peak performance.

Why is motion control so important in Combined Cycle Gas Turbine Plants?

In the turbine system, the fuel gas control valve on the actuator is the predominant interface between a very complex control system and the mechanical portion of the plant. From a safety perspective, it is vital to ensure a rapid closing time for the main valve actuator in the case of an emergency shutdown to avoid potentially destroying the turbine. If there is an excess of steam or fuel in the inlet, this also compromises the turbine and causes severe damage in the power plant.

The motion control system affects the following:

The power output of the turbine and thus the revenue stream
Energy efficiency that equates to profitability as 80% of the cost of running these plants is fuel
Level of emissions which is both a cost and compliance issue as improperly calibrated equipment could cause a plant to exceed its emission allowance
Uptime and consistent power output which prevent penalties for unstable energy production levels.

Keeping critical systems operating

Moog Gas Turbine Actuator

Moog provides actuators and servo valves to most OEMs of gas and steam turbine equipment including working with the designers of new turbines to develop the specifications and even the performance expectations of the equipment. Moog fuel metering actuators and fuel gas control servo valves are known for rapid closing times, leading-edge failsafe functioning and the highest dynamics. In addition, all hardware can be precisely adjusted according to the load required from the network to ensure efficiency. In many ways the most critical operations of the plant will benefit from the performance of the Moog actuator and valve – including conserving fuel, preventing pollution and providing the declared power output.

The challenge for power plants is keeping their critical motion control systems operating at the same high level as the original equipment in spite of 24/7 operations in rugged environments. In addition, to Moog Global Support repair and maintenance services, Moog offers special services to help power plants minimize downtime and ensure optimized performance. From expedited services for downtime to preventative maintenance programs for planned outages to special retrofit kits and exchange programs, there are numerous options to keep plants running efficiently and to maximize output.

Moog Steam Turbine Actuator

The specialized Moog Power Generation Actuator Service is a complete evaluation, repair and testing service that can be offered in local country service centers and as field service. This is a more comprehensive service package than just a repair. As the original equipment supplier, Moog knows the motion control requirements of the application and has worked with the turbine OEMs to design a product that can withstand 30,000 cycles per year. These cycles result in a million micro motions of the actuator which can represent increased wear over time. In addition, Moog Global Support Servo Valve Repairs include several repair levels for fuel gas control valves from an initial evaluation test to a high level repair which could include a full strip down, ultrasonic cleaning, replacement or repair of a torque motor and replacement of the bushing and spool assembly. As with the actuator repairs, Moog technicians have the expertise to ensure quality repair using original parts and Moog has global facilities for quick turnarounds.

From the availability of product specifications and original drawings to service technician training (often at the turbine OEM in Japan or other countries), Moog has invested in ensuring the valve and actuator will perform as “as-new” or better. Achieving the “better” standard is accomplished by incorporating product upgrades when possible in the repair service including a proprietary surface finishing on rods and seals on some actuator models for improved longevity.

Moog technicians positioning actuator for disassembly

Below are two case studies of CCGT power plants in the United Kingdom and United States that worked with Moog to obtain “as-new” or better motion control performance on turbines while avoiding expensive downtime.

Case Study: Great Yarmouth Power Station

Great Yarmouth Power Station

Background

Great Yarmouth Power Station is a Combined Cycle Gas Turbine (CCGT) owned and operated by RWE Npower Plc located on the United Kingdom's east coast. In 2012, the station started experiencing problems with a major low-pressure steam control valve, controlled by a servo actuator originally made by Moog in the USA. The control valve was making a lot of noise, with the bearings being the prime suspects.

The engineers at the power station made an enquiry to Moog about replacement bearings. Moog engineers suspected that the symptoms potentially pointed to something more than just the bearing and sought to ensure “as-new” performance due to the critical nature of this application.

The challenge

The customer provided details about the actuator to the service technicians at Moog’s Tewkesbury (UK) office. Moog sent an extract of a report describing the full extent of a Moog service. The depth of information supplied in the report, including value-added audio visual content encouraged engineers at the power station to opt for Moog's service offering.

Refurbished Process Valve
under test

Moog Service Report
for Great Yarmouth

It turns out that the "on-receipt" test confirmed the worst fears of Moog’s engineers: it was more than just the bearings at fault. In fact, the list was longer than initially thought and replacement parts included the piston and bearing assembly, the crank-arm, bushes/bearings, connecting pins, and all of the soft seals.

Moog 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 Moog and submitted to the customer along with a revised cost and repair schedule. Despite this ongoing repair work, the plant needed to continue operations.

The Moog solution

Moog offered a special expedited Actuator Service from its facility at Tewkesbury that included a full on-receipt test, a complete strip-down investigation, a review of all parts, replacement of the faulty parts, a rebuild to its original specification and test to ensure the original Moog specification.

Providing this special service also had some challenges. Not only did Moog engineers in Tewkesbury need original parts shipped in from the USA, but they also had to match the service window with planned downtime at the power station. Fortunately, Moog’s global network ensured they could get spares shipped overnight. The engineers then reassembled the actuator with new parts and tested it to its original specification – confirming that all faults had been rectified.

From order confirmation on the 8th of November, the repair took just ten days, with the unit leaving Tewkesbury on the 18th of November.

The result

During the outage, Matt Keen, Moog Systems and Projects Manager, visited the power station to review the installed equipment, and was able to highlight other service issues. So impressed were the engineers at the power station that they have since asked Moog to undertake additional proactive maintenance on another actuator and the associated process valve.