Coal, Gas

Keeping the Lights On

Issue 10 and Volume 118.

After initial receipt of actuator into repair facility technician breaks it down, logs in the parts and readies for refurbishment
After initial receipt of actuator into repair facility technician breaks it down, logs in the parts and readies for refurbishment

By Steve Beddick and Brian Sims, Moog Inc

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 present demanding motion control challenges. Motion control is the key to machine performance, safety and ultimately the ability to supply power to customers.

Power plants 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. Downtime of even one plant can put stress on the electricity network. If plants exceed or underperform declared output, it can mean a fine from regulators. So, power plant operators look for ways to keep turbines running at peak performance.

Why is motion control important?

In the turbine system, the fuel gas control valve on the actuator is the primary interface between a complex control system and the mechanical portion of the plant. From a safety perspective, it’s 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 compromises the turbine and causes severe damage in the power plant.

The motion control system affects:

  • The power output of the turbine and the revenue stream,
  • Energy efficiency that equates to profitability as 80 percent of the cost of running plants is fuel,
  • Level of emissions ,which is both a cost and compliance issue, since improperly calibrated equipment could cause a plant to exceed its emission allowance, and
  • Consistent power output, which prevents penalties for unstable energy production levels.

The challenge for power plants is keeping their motion control systems operating at a high level in spite of around-the-clock operations in rugged environments. Plant operators with finely tuned repair and maintenance programs will help their plants minimize downtime and ensure optimized performance of actuators and servo valves.

Preventative maintenance programs for planned outages as well as special retrofit kits and exchange programs are the kind of options that plant operators should put in place to keep plants running efficiently.

But a well-constructed repair program isn’t always enough. Instead, power plant operations & maintenance teams should focus on evaluating, repairing and testing their equipment. And if that equipment is made in a foreign country, then identifying local country service centers and as a field service. If possible, plant O&M teams should identify the equipment’s OEM to conduct repairs because they generally know the requirements of the application and often work with the turbine OEMs to design products that can withstand tens of thousands of cycles per year.

Technician cleans, repairs and reassembles each actuator
Technician cleans, repairs and reassembles each actuator

No Quick Fix for Union Power’s Maintenance

Entegra Power Group’s Union Power Station near El Dorado, Arkansas, which went online in 2004, is one of the largest combined cycle power plants in the United States. The station is comprised of eight (8) GE Combustion Turbines (CT) and four (4) GE Steam Turbines (ST). The equipment is arranged in “power blocks” consisting of (2) CTs and (1) ST; each block generates 550 MW of electricity. The plant’s total output is 2.2 GW. The plant normally operates in cyclic/peaking modes, which have a different maintenance schedule than a continuous mode.

In 2012, Union Power Station began planning for upcoming inspections. The plant’s managers had 21 days for the inspection of each combustion turbine and 18 days per steam turbine. Managers had to refurbish fuel gas and steam control valve assemblies during these outages. Extended outages like these, which plant managers need to refurbish this type of equipment, occur only once every few years. To maintain reliability, the plant had to return the equipment to as-new condition. A quick fix to meet the schedule was not an option.

Union Power wanted OEM repairs and upgrades, and the timeframe to perform the repairs within the inspection window left no margin for error. If a 550-MW block of power was offline even for a single day, everyone knew the lost revenue would be significant.

Audit Sets the Stage

Union Power approached its local distributor, AirDraulics, to determine the best solution. Before the planned outage, AirDraulics audited the plant by checking model and serial numbers for equipment to determine the physical layout, hazardous area ratings and actuator and process valve geometries. The audit confirmed that the fuel gas systems on all (8) CT were identical and half of the CT units were also equipped with Moog Inlet Guide Vane Actuators (IGV). The steam valve configurations differed as two blocks had 24″ valves and the other two had 18″ valves.

With this information, Moog service technicians proposed a service plan that included a combination of spares, repairs and exchange units to meet Union Power’s inspection schedules.

Technician working on finished actuator on test bench to assure it meets or exceeds original warranty requirements before sending it back to customer
Technician working on finished actuator on test bench to assure it meets or exceeds original warranty requirements before sending it back to customer

Putting Spares, Repairs and Exchanges to Work

For the combustion turbine units, Union Power purchased one spare set of actuators. These served dual roles as a rotable set during planned outages and emergency spares during forced outages. For each block, technicians used Union Power’s spares and a set of its exchange units complete with process valves. At the end of the preventative maintenance outage, technicians returned Union Power’s spare set to the site and replaced the Moog set in the exchange pool inventory.

The plant managers and service technicians determined that the steam turbines actuators and valves could be repaired within the allotted time, too. Union Power coordinated the repair and removed the steam process valves from the actuators and sent the actuators to Moog for repair. After completing the actuator repairs, technicians conducted a final acceptance test, and sent the actuators to Union Power Station with a two-year warranty. Union Power reassembled the valve to the actuator, installed the assembly and tested it for commissioning.

Union Power has completed maintenance outages on three of its four blocks on schedule and commissioned the units without incident. Union Power has set the final block’s maintenance outage for this fall.

Authors

Brian Sims is the services manager for Moog Inc. in Tewkesbury, U.K. He has nearly 37 years of experience in engineering and aftermarket support across all markets.

Steve Beddick is the aftermarket sales manager for Moog Inc. in East Aurora, N.Y. He has nearly 30 years of experience in engineering and aftermarket support for rotating equipment used in power generation.

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