Driven by both economics and regulations, power plant operators are continually looking for new ways to improve the uptime and reliability of their units. It is a multi-front battle, and now more units are designed for combined-cycle or co-gen applications to recover heat that would be wasted otherwise. Turbine manufacturers also are building combustion and steam turbine rotors out of lighter materials and with tighter tolerances.
Heat Recovery Steam Generator (HRSG) manufacturers are doing their part with advanced designs that help them deal with the stresses of frequent cycling. Vogt has developed methods to retain heat in the boiler in order to minimize thermal stress and reduce startup time. Rather than the two hours plus it would take to go from a cold start to a full load, for example, keeping the water hot cuts the time in half. Alstom, on the other hand, has a once-through design that eliminates the high pressure drum, allowing for a 25 minute cold start, and also a design called the OCC (Optimized for Cycling and Constructability) Horizontal HRSG – which uses single row harps and thin-walled header pipes.
Then there are the steps that are taken on-site to improve the operations of existing equipment. Older turbines are being retrofitted with retractable and brush seals that close off even more of the heat loss. Updating plant control systems can optimize air/fuel ratios and improve heat output. But just upgrading the control software won’t do the job if field equipment, and in particular actuators, can’t match the speed and accuracy of the software.
Controlling combined-cycle inputs
Combined-cycle plants offer greater power output for the same amount of fuel expenditure and emissions, but require greater control of the inputs to achieve the desired results. Optimizing a combined-cycle plant requires tuning the gas turbine, not just for its own efficiency, but also to maximize the stability and efficiency of the HRSG. There are 4 inputs that matter: fuel and air for the combustion turbine; exhaust gas and feedwater for the HRSG. The inlet guide vanes (IGVs) control the inputs for both the CT and the HRSG. First, the IGVs regulate the airflow into the combustion turbine.
The amount of airflow then determines the temperature of the exhaust gas entering the HRSG and, as a result, the boiler temperature.
The IGVs’ setpoint is determined based on the CT exhaust temperature, the compressor discharge pressure and the turbine speed. The IGV modulates during acceleration of the turbine, the loading and unloading of the generator and the deceleration of the gas turbine. In so doing, it maintains the proper flows and pressures in the compressor, minimizes the pressure drop across the fuel nozzles and maintains a high exhaust temperature, even at low loads.
With both simple- and combined-cycle plants, the CT exhaust temperature directly relates to turbine efficiency. While running a simple-cycle plant, or a combined-cycle unit in simple mode, exhaust heat simply means wasted fuel and should be minimized. With a combined-cycle plant, the opposite applies. Optimum operation consists of maximizing the exhaust temperature and maintaining that level at all operating loads, since the exhaust temperature determines the HRSG boiler temperature and temperature instability causes thermal fatigue. Unstable steam temperature also means lower than optimal temperature setpoints, highly active spray valves and unnecessary throttling of feedwater valves.
The limitations of air and oil
Controlling both the CT and HRSG efficiency, therefore, starts with precision control of the combustion turbine IGVs in order to regulate the exhaust temperature leading into the HRSG boiler.
Such control is critical to quickly bring units on-line without risking thermal damage, particularly when the units are being cycled daily. Traditionally, operators have had two options: pneumatic actuators and hydraulic actuators. Both technologies, however, have their drawbacks – they are difficult to calibrate, do not have high repeatability and can even lead to unplanned outages or plant derates.
