Under Stress


Alstom’s G24 MXL2 gas turbine

By Russell Ray, Managing Editor

Low natural gas prices, stricter environmental regulations for coal plants, a surge in intermittent sources of renewable power and the growth of demand-response programs have led power producers to run their gas-fired plants longer and harder.

As a result, capacity factors for gas-fired plants have skyrocketed, from as little as 10 percent to more than 60 percent in some cases. This increasing reliance on gas-fired generation has created a need for comprehensive strategies to operate and maintain these increasingly vital assets. Enhancing the reliability and extending the life of these assets is paramount as power producers turn to gas to supply a larger share of base load capacity.

In a new report from Black & Veatch, natural gas-fired generation is projected to grow 3.1 percent a year through 2038. At that rate, more than 340,000 MW of gas-fired capacity will be added to the U.S. grid by 2038. By then, gas-fired combined cycle power plants will account for 50.5 percent of U.S. power production, up from 25 percent in 2014.

“What the power sector needs are long-lived assets that can be managed and maintained effectively,” said Revis James, director of Generation Research & Development at the Electric Power Research Institute. “That’s a technology challenge that can be met by smarter maintenance practices.”

Many of the gas plants now providing base load capacity were designed as peaking plants. They weren’t designed to withstand the wear and tear associated with the sustained operation of a base load facility. What’s more, the maintenance programs for these facilities weren’t developed to support the level of sustained operational reliability required for base load generation. As a result, power producers are revamping their maintenance plans for critical systems and components of combined cycle gas plants. Maintenance plans that reflect the actual operation of gas turbine generators will help the industry avoid expensive repairs and overhauls.

An effective O&M strategy is an integral part of a high-performing gas power plant. Enacting a successful strategy can lead to reduced maintenance costs and increased revenues. Over time, turbines will degrade in performance, but an effective O&M regimen can translate to a substantial increase in production.  

The Tennessee Valley Authority, which has bought or built five combined cycle gas plants since 2007, is devising a comprehensive operation and maintenance plan for its fleet of gas-fired units. Just last month, the utility announced plans to build a $1 billion combined cycle gas-fired power plant in western Kentucky to replace two coal-fired plants, a trend that shows no signs of slowing.

“We expect to have a comprehensive maintenance basis we can use in our work controls process to prioritize actions, so we can start making sure we have the right preventive maintenance tasks set up to ensure that we maintain our high level of reliability as we continue to run facilities harder and harder,” said Bill Morrison, vice president of Generation Engineering for TVA.

O&M Technology

In addition to improved efficiency and lower emission rates, combined cycle power plants can start and stop quickly and operate at varying levels of output. This flexibility is a big advantage for power producers and grid managers who must accommodate growing amounts of wind and solar power. The rapid response times and variable outputs of combined cycle plants can offset the fluctuations in renewable power.

However, frequent changes in output can damage critical components and lead to serious and costly downtime without an effective maintenance plan, James said.

“That combined cycle system doesn’t respond as well to frequent changes in output,” James said. “We’ve observed over time some material damage and equipment issues. You can have cracking damage. These are things that can be mitigated through operational strategies.”

Today’s modern-day power systems are operating at higher temperatures and higher efficiencies, but further improvements can be achieved through advanced technologies in metallurgy, coatings, cooling and aerodynamic turbine design, James said.

O&M Practices

In this pursuit of maximum efficiency, much research is being performed on innovative construction materials that can handle higher temperatures, including ceramics. To cut emissions even further, gas turbines are now being tested and operated to run at ultra-cool temperatures. These aspects of design must be considered in the installation and ongoing maintenance of the turbine. The installation of a new gas plant can take anywhere from 12 to 24 months, with simple cycles typically taking 12 months and combined cycle plants closer to 24 months.

Combined cycle O&M should be performed to tune for the optimization of acoustic dynamics, emissions and performance to accommodate changing environmental conditions such as temperature, pressure and humidity. Another key to ongoing maintenance of a large-frame gas turbine is keeping the compressor clean. Airborne contaminants can be ingested into the turbine compressor, causing fouling which could lead to pitting and blade corrosion damage. Two options for keeping the compressor clean are an on-line water wash and an off-line water wash. During an online water wash, the unit does not have to shut down. But washing the unit while it’s still online is not as effective as an offline water wash because only the first few compressor stages are cleaned. An offline water wash can restore between 2 MW and 5 MW, or more, depending on the degree of fouling.

Another maintenance practice important for large-frame turbines are regular borescope inspections. For this visual inspection, a tiny camera is used to inspect internal components such as compressed air inlets, turbine blades and seals. Borescope inspection of engines can be used to prevent unnecessary maintenance, which can become extremely costly for large turbines.

As more power producers enter more contracts for additional supplies of natural gas, it is becoming increasingly important to monitor the quality of the gas burned for generation. Too much condensation in the pipes can lead to excessive moisture in the gas, which can hurt efficiency, damage the turbine and cause costly repairs and shutdowns. The increased use of hydraulic fracturing in the production of gas from shale can be problematic for power generators.

“You want to make sure you get what you pay for,” said Greg Gowaski of Mitchell Instruments Inc. “It’s more important now than ever to know what the gas quality is. You need to be more aware of the quality of the gas you’re using to protect your equipment. You want efficient, high btu-content gas.”

That’s why measuring and monitoring the hydrocarbon dew point in a plant’s gas supplies is becoming more important, Gowaski said.

Excessive hydrocarbon content can damage the compressor, burner and generator. Servicing and shutdown costs for such damage can range from $200,000 to $4million per occurrence, Gowaski said. “Any condensation in that natural gas or in those pipes can cause shutdowns,” he said. “When these are present, you don’t always know it. You need to monitor that. There about four different technologies out there that are being used.”

The shift to more gas-fired generation will continue, even in the face of higher gas prices and increasing regulation of U.S. gas production.  The transition will continue because of ever increasing regulation of coal-fired plants, a proliferation of state standards for the production of renewable power, and a generous supply of natural gas from shale. Gas-fired plants equipped with combined cycle gas turbine (CCGT) technology compliment wind and solar power because they can start and stop quickly, and are thus capable of offsetting the fluctuations in renewable power. This is especially valuable in California, where utilities and grid managers struggle to maintain a balanced load amid a growing source of intermittent electricity.

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