Part 1: Gas Turbine Operation - Basic Operational Concepts
Summary: This episode is Part 1 of a discussion on Gas Turbine Operation with my guest, Robert Hawkins. In Part 1 we cover concepts related to the basic operation of Gas Turbines, which are important to understand for effective troubleshooting when issues arise. In Parts 2 and 3 we will cover common Operational Issues and how to troubleshoot these in more detail.
- Factors affecting power output for a Gas Turbine
- Different Gas Turbine configurations and their impact on troubleshooting
- Inlet Guide Vane (IGV) and Inlet Bleed Heat (IBH)
- Explanation of Gas Turbine max load factors
- Exhaust spread and relationship to combustion issues
Guest Bio: Robert is a Lead Technical Support Specialist for Nexus Controls Remote Diagnostic Service team and has over 30 years of experience with steam and gas turbines, mechanical, and control systems. His past roles include Product Services Engineer, Instructor, and Field Engineer.
This article summarizes part 1 of our podcast series on Gas Turbine Operation. It focuses on basic operational concepts for gas turbines including factors affecting power output, different configurations and their impact on troubleshooting, Inlet Guide Vane (IGV) and Inlet Bleed Heat (IBH), max load factors, and exhaust spread relationship to combustion issues. The guest expert for this discussion was Robert Hawkins. Robert is a Lead Technical Support Specialist for Nexus Controls Remote Diagnostic Service team and has over 30 years of experience with steam and gas turbines, mechanical, and control systems. His past roles include Product Services Engineer, Instructor, and Field Engineer. So let's get to it...
Why is it important to understand gas turbine operational concepts?
Gas turbines are sophisticated machines. It’s important to understand the basic operational concepts of a gas turbine so that:
- Operational performance can be optimized
- Troubleshooting (start-ups, shut-downs, recovery), in a complex machine and high stakes environment, can be handled safely and expediently, for example, response times in a matter of minutes and restarts in a couple of hours.
Gas turbine operational concepts
A gas turbine is made up of three major mechanical components:
- Axial flow air compressor (the ‘compressor’)
- Combustor (where the fuel and air mix and burn to produce the hot compressed gas that is the working fluid)
- Turbo-expander (where the hot compressed gas / working fluid expands as it produces the shaft torque)
Gas turbines work on mass flow, using a combination of air mass from the compressor and fuel mass. The fuel enters the combustor and mixes with the air, burns and becomes hot, pressurized gas. That compressed air and hot gas then gets expanded through the turboexpander which generates the shaft torque that drives the given equipment. The shaft energy produced will, for example, power a generator that then produces electricity.
The basic premise of gas turbine operation is that power is a function of air mass and fuel mass. The primary characteristics of this mass flow are, temperature, and pressure. On a simple level:
- Power = f (air mass + fuel mass)
- Mass = f (volume, temperature, pressure)
Because volume is set by the compressor / turbine casing flow path, it is not typically a factor of variation in unit performance
To optimize power, both air and fuel pressure and temperature must be monitored, the job of the control system. If the variables are not at proper levels, then the control system must first try to correct them and, if unsuccessful, it will work to protect the turbine.
Air mass is fed by the compressor. Primary characteristics that affect the compressor include air temperature, cleanliness, and mechanical condition. Secondarily, air filters can be the cause of flow restrictions and the extraction systems off of the compressor can be a source of reduced air flow as well as leaks on the system.
Fuel mass is primarily controlled by the fuel supply pressure and gas energy content as measured by fuel analysis. Secondary factors include valve operations on the unit and the combustion hardware including the components that feed fuel into the unit and allow it to burn properly.
Types of gas turbines
There are three types of gas turbine designs:
Simple cycle unit: In this configuration, the gas turbine is using air and fuel to power a generator or gas compressor.
Combined cycle unit: A combined cycle gas turbine powers a generator and also utilizes the heat of the exhaust gas, to drive a second heat engine (typically a boiler into a steam turbine). Thus, a single energy source is used to drive multiple turbines. On the graphic area marked A above, the exhaust would then be directed into a heat recovery boiler rather than direct to atmosphere.
Cogeneration unit: Cogeneration or CHP (combined heat and power) uses a gas turbine to produce power and uses that power to produce heat for other processes in the plant. This is similar to the combined cycle arrangement but the exhaust heat is not used to drive a steam turbine. For example, the heat or thermal energy produced can be used to heat steam for dryers in paper mills, or steam-heating in a housing project.
Gas Turbine Control
The inherently more complex and sophisticated operation of combined cycle and cogeneration gas turbines creates higher-order operational challenges. Complications arise from the need to adjust both air and fuel to simultaneously maximize power production (load control) AND maximize exhaust temperature. This requires a dynamic, on-going balance between air and fuel. The ‘air’ portion of this relationship is, relatively, free; it costs energy (fuel) to compress but is the largest component of the mass flow. To be cost-efficient, a simple cycle gas turbine will use as much air as practical for the process with required generated load being the only concern for fuel consumption. A combine cycle or CHP unit will have the dual concerns of fuel consumption for load generation AND exhaust temperature. For that reason, a combine cycle or CHP unit will optimize air flow to maximize exhaust temperature and fuel consumption for load.
Components such as inlet guide vanes (IGVs), and extraction valves such as inlet bleed heat (IBH) are used concurrently to continually optimize operational parameters.
Inlet Guide Vanes (IGVs) are adjustable nozzles at the front end of the compressor to control the overall admission of air into the compressor. IGVs cut back on air flow at the same time fuel is reduced to maintain maximum exhaust temperature during reduced load operation and will open up to keep exhaust temperatures below maximum allowable limits. IGVs modulate the airflow into the gas turbine and are either less open or more open for partial load operation points, however, IGVs are never closed.
Extraction valves are open or closed based on the specific point in the start-up process for proper compressor flow characteristics. Once a turbine is up and running, extraction valves are closed.
Inlet bleed heat (IBH) takes air off of the compressor and recycles it back to the inlet. The purpose, in simplified terms, is to reduce air volume into the combustor to enable proper combustion. IBH is another way to modulate and control overall air into the combustors. Keep in mind this is a simplified explanation of a highly sophisticated combustion process and system.
The bottom line is that troubleshooting these systems requires a deep understanding of concepts and equipment involved to achieve and optimize control.
Troubleshooting: Air Considerations
Gas turbines do not operate at a set max load. Due to the nature of gas turbine design, temperatures insight the gas turbine must be managed. That control of temperatures determines maximum load for a given set of machine conditions (compressor mechanical condition, inlet air temperature / humidity, fuel energy content, etc.)
Air Mass, Fuel Mass. When a machine is under power, the first priority is to understand whether or not conditions have changed to cause under-power operation. For example: Air coming into the compressor may be cold and cold air is denser than warm air. Thus, during cold weather conditions, the compressor is pushing a lot of air into the combustor. This mass, combined with the fuel mass, creates a lot of power since power = f (air mass + fuel mass). Likewise, it holds that in summer when air is hot, air is less dense, so the combustor works less efficiently. To minimize seasonal variations, system inlet bleed heat baselines a unit at a design condition to better manage output. Air mass has a significant impact on power = air mass + fuel mass.
Maximum internal temperature. Gas turbine control is based on managing the maximum temperature inside the turbine, as established by material limits, such as the type of metal / steel that is used inside the gas turbine. “Firing temperature” is the term used for the temperature of the hot, compressed combustion gas entering the turbo-expander. In order to determine the inlet air temperature, the variable exhaust temperature is measured. Gas laws are then used to calculate the that inlet temperature or firing temperature. The exhaust temperature varies based on dynamic discharge pressure from the compressor, so the relationship between temperature and pressure is used to properly control and maintain the equipment. Thus, complexity stems from optimizing two dynamic and independent variables simultaneously.
Air quality. Air quality conditions also have impact on gas turbine performance. Specifically: Are inlet filters clean and thus not restricting air flow? Are IGVs in proper position? The control system checks for these factors and, if not at proper levels, triggers alarms to alert operators. Operators must check the alarm queue for an indication of what might be going on.
If all is good with the inlet filters and IGVs, the next questions become: Is air leaking away from system (via an improperly closed extraction valve for example)? Is the compressor dirty or damaged?
Instrumentation, data and control systems help create an understanding of these important indicators and operating parameters.
Troubleshooting: Fuel Considerations
Fuel-side Exhaust temperature. Combustors properly mix the air and fuel to meet regulatory requirements on limiting NOX (nitrous oxide) and CO (carbon monoxide) gas emissions. Fuel is burned in a particular way to meet these requirements. Measuring emissions can indicate if fuel is not burning properly. When this occurs, an understanding of the exhaust temperatures at the end of the machine help to troubleshoot what is happening.
Why does this work? Conditions in the combustor are fairly relatable to a specific area of the exhaust. It may seem that air flowing through the “mix master” of spinning blades creates a combination of the air and hot gas flows. However, there is very little mixing of the flow as it comes out of a particular combustor and goes through the turbine. Although combined, the fuel gas streams remain adjacent to each other as they flow through and into the exhaust duct. Thus, by intelligently measuring and analyzing the exhaust temperatures, informed assumptions can be made about what is happening inside the combustor. Exhaust temperatures are vital to understanding and troubleshooting. The control unit monitors and provides this information to operators.
Exhaust temperature spreads. Monitoring exhaust temperature trends is also key. In general, if the temperature spread is increasing, it can indicate an issue with instrumentation or combustion hardware. The control unit provides this information to operators.
Fuel quality: Temperature and pressure are key operating parameters. Pre-heating the gas fuel to manage the gas condition is important to manage the fuel quality because treatment of gas temperature and pressure are needed for proper fuel combustion.
NEXT UP - In part 2 of our three part podcast series on gas turbine operations we'll cover common sources of troubleshooting such as turbine start-up and shut-down, vibrational issues, and other auxiliary systems that are sometimes overlooked as root causes of operational issues.
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