Gas turbine fuel control has become more important than ever. Whether due to emissions mandates, maintaining efficiency, plant profitability or increasing the lifespan of a facility, operators and owners are looking for ways to achieve a tighter rein on fuel usage. Here are the top five reasons why now is a good time to upgrade your fuel system.
- Aging parts are unreliable. There are a great many gas turbines that have been in operation for ten, twenty, thirty or more years. Machines that have been in heavy use are likely to have worn fuel valves, unreliable shutoffs, or systems that aren’t responsive enough to operator instructions. But even peaking turbines that have seen infrequent use can suffer badly due to faulty fuel control systems or components.
As these units age, component wear can lead to problems in starting up the machine as well as unscheduled outages and maintenance personnel continually having to babysit the fuel controls.
In the past fuel control valves have been upgraded along with control system upgrades. Many times, the existing (old) fuel manifold and shut-off valves have been reused. As these components age, they may stick or hang up during a start causing unscheduled maintenance. Worst case: In an emergency will they close quickly and reliably? The shut-off valve and associated check valves, vent valves and actuators need to be replaced after years of service.
Additionally, there are many turbines with fuel control systems that are either poorly supported or have been made obsolete by the OEM. There comes a point when it becomes too much of a burden to continually scramble to find components or keep aging controls running.
- Lack of speed of response. Today, speed is everything. Operators demand turbomachinery that is going to start up rapidly on demand, and shut down instantly should problems arise. The fuel control system must match these expectations. This means valves that provide the precise fuel mix and fuel shutoffs that do the job rapidly and reliably.
Yet fuel controls don’t always measure up. In some cases, aging facilities have installed manual ball valves to keep their fuel control systems running. There is no place for such technology in a modern gas turbine plant. Even standard fuel control systems may not be good enough for current conditions. Response times of half a second to a second (and as much as two seconds for a slow motor) are commonplace. Such systems are operating at risk.
What is required is pneumatically actuated valves with response times of 200 milliseconds or less. Only then, can the operator have peace of mind that shutoffs will occur fast enough to avoid damage. Furthermore, pneumatic systems are available that only need 1 amp at 24 volts DC.
Fuel metering valves must also be able to function at the right pace. Gains are set and adjusted to allow the valve to work in coordination with the control system. Fuel control valves must offer real-time flow control to dispense the right amount of fuel at the right time, as requested by the turbomachinery control system. This has everything to do with achieving performance objectives, while avoiding the possibility of falling out of emissions compliance.
- Skills shortage. Not so long ago, plants operated with large teams of skilled operations and maintenance personnel. But the demand to do a lot more with far fewer personnel has drastically shifted the skills picture in facilities. Couple that with an aging workforce and a steady parade of retirements and there is no place for finicky or unreliable turbine systems.
Fuel controls must not be the weak link. Instead of being composed of multiple components cobbled together over the years that require constant attention, they must be unified into one comprehensive, automated fuel metering, shutoff and control system. Ideally, such pre-assembled systems would include low power actuation in order to eliminate the need for additional high-powered electrical lines to be fed into them to operate motor-driven valves.
Such systems ease the burden on less-experienced plant personnel by offering them an engineered solution that is tested, integrated and can be dropped in place. Veteran staff may be able to purchase the various components and competently tie everything together—if they have the time. But the generation entering the workforce demands fit-and-forget, leak-free systems. They want to spend their time on the big picture—overall performance, plant economics and emissions compliance—not in the routine of adjusting valves, checking for leaks and troubleshooting fuel controls issues.
- Meeting the latest standards. Standards are continually evolving. Safety standards have changed markedly over the last twenty years. These days SIL 2 or 3 safety levels are the norm for new valves. Fast shutoff of fuel to the gas turbine is another aspect of safety. The flow of gas must cease immediately as dictated by plant control systems or whenever there is a loss of pilot pressure.
Older systems lack the safety capabilities of the latest fuel control systems, including redundant pneumatic shutoff valves, and a block-and-bleed architecture to prevent leaks. Additionally, they offer pilot valves. These feature a spring-loaded piston, activated by system pressure, to open the valve. When pressure is released, for any reason, the valve closes at once. This safety feature is an important fail-safe mechanism. Those relying on all-electric systems, however, can experience shutdown delays in the event of a power outage.
Many standards and regulations apply to the fuel train. Older fuel controls often are found lacking when it comes to complying with some of these rules. For example, they may struggle to remain in emissions compliance because their control systems do not have the flexibility to respond in timely fashion to shifts in fuel heating value. By contrast, the latest systems are available with complete fuel control assemblies fully certified and tested to meet the latest requirements.
- Poor economics. Inefficient fuel systems can have a significant negative impact on plant performance and profitability. Older systems often lack the degree of responsiveness necessary to maintain a healthy balance sheet.
Component or system unreliability opens the door to unexpected shutdowns. Investment in modern end-to-end fuel controls reduces operational risk and increases the likelihood of a more predictable economic outcome.
The Gas Fuel Shutoff and Control System from Continental Controls is a complete fuel train inclusive of fuel metering, redundant pneumatic shutoff valves that meet safety and other industry standards, pilot valves, flow measurement, NOx feedback, fuel heating-value measurement, and gas leakage elimination via a coaxial valve. This integrated system includes a controller with sensors for temperature, pressure and other parameters. All components are pre-assembled, welded, tested, certified and delivered on one manifold.
For more information, visit www.continentalcontrols.com.
Rick Fisher, VP, Continental Controls Corp, rfisher@continentalcontrols.com
