Part 2: Gas Turbine Operation - Troubleshooting Startup, Shutdown, and Combustion
Summary: This is Part 2 of a discussion on Gas Turbine Operation with my guest, Robert Hawkins. In Part 1 we covered concepts related to the basic operation of Gas Turbines, which are important to understand for effective troubleshooting when issues arise. In Part 2 we cover issues that can occur with startup, shutdown, and combustion. In Part 3 we will cover considerations related to support systems, compressor performance and water wash, turbine performance, and vibration monitoring.
Topics Covered:
- Gas Turbine startup sequence including safety checks, purge, speed spoil, ignition
- Gas Turbine shutdown and trips including turning gear and cool down, gas vs steam
- Combustion considerations including Dry Low NOx (DLN), tuning, and liquid fuel
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.
Podcast Article
In part one of this series, we covered concepts related to the basic operation of gas turbines, which are important to understand for effective troubleshooting when issues arise. In part two, we cover common issues that can occur with startup, shutdown, and combustion. In part three, we will cover considerations related to support systems, compressor performance and water wash, turbine performance, and vibration monitoring.
Why is it important to understand operational issues with gas turbines?
Operational staff and those who work with gas turbines, need to have a thorough understanding of the way their highly sophisticated equipment works so that they can employ proper troubleshooting techniques.
Gas Turbine Startup
Gas turbine startup requires a complicated sequence of events since numerous systems are interacting at different times. Startup is one of the most common periods of unit operational issues so an understanding of this phase of operation is critical to productive troubleshooting.
With a multi-step startup process, sequencing is important to understand since several things happen at different times and the steps must occur in the proper order. The basic sequence of a gas turbine startup includes the following steps:
Safety Checks and Purge
- The unit does a security/safety check on the equipment to ensure it is ready to start, confirming that temperatures and pressures are appropriate, valves are closed or open as needed for operations, etc.
- When the machine gets the okay to start, the starting system begins turning the compressor and pushes air through the unit.
- Then, the unit is purged with three to four volumes of air through the gas turbine. This volume is the sum of the volume of the turbine casings and the exhaust system.
- The intent is to push any combustible gases out of the casing and exhaust before the next step, which is ignition.
- Depending on the size and the configuration of the exhaust system, purge time can vary significantly between configurations. Thus, purge time is not a standard duration, but rather a standard volume of air exchange.
- Some sites have a purge credit upgrade, which is often not clearly understood. A purge credit can eliminate the purging period for units that need to start as quickly as possible. For emergency start units that need to come online immediately, a purge credit can be a major benefit.
- Concurrent with the purge is a gas leak test. This test is another safety measure in which the fuel valves are checked to ensure that they seal against gas fuel pressure in order to cut off gas fuel flow as necessary. The gas leak test, as the name implies, also checks for leaking fuel valves. The gas leak test is also part of the purge credit feature. It provides reassurance that the unit is ready to start when needed.
Ignition
- Next, there's a brief period called speed spoil, when the turbine starting system cuts out and the rotor speed decreases to a level appropriate for the fuel to ignite. That level is important to ensure the correct volume of air is used to mix with the fuel. When the speed spoil concludes, the rotor is at the right speed to fire the unit. The starting system re-engages for driving the rotor, speed stabilizes and begins increasing.
- The fuel valves open up and inject fuel into the combustors at the same time the two igniters on the machine are energized. These igniters are essentially big spark plugs that throw a spark into the air / fuel mixture, igniting two chambers.
- The flame propagates around to the other combustors through crossfire tubes. Flame propagation is then confirmed by flame detectors, which are UV sensors looking for the presence of a flame by the emission of UV radiation.
Warmup
- Upon flame confirmation, the gas turbine then moves into the next sequence, called warmup. In general, this is a brief period of one minute employed to soften the thermal jump that happens inside the turbine and combustors as a result of a very intense and sudden flame. It is important to understand that this ‘warmup’ has no relationship to a waste-heat / HRSG (heat recovery steam generator) boiler if the unit is so equipped.
- When the warmup is complete, the unit accelerates to operational speed. The startup is now finished. The turbine combustion system brings the speed up to full speed no load (FSNL), which is 100% speed operation and the load equipment - or whatever the turbine is driving - is brought online to begin producing power.
Gas Turbine Shutdown
There are two ways of stopping the gas turbine. The first method, shutdown, consists of ramping down the fuel as the unit unloads. When the fuel gets to the appropriate point, the unit flames out naturally and coasts down. Universally, this is the preferred method to bring the unit offline.
The second method is called tripping the unit, where the fuel is cut off at an operational point, no matter where it is (i.e. loaded, starting up, etc.). Tripping the unit is a protective measure that removes the combustible gases from the system. The unit flames out and then coasts down to a stop. This event is hard on the gas turbine due to the rapid change in temperature. Specifically, one moment there is an extremely hot flame, at thousands of degrees, and the next moment, only ambient air is going through the system. This thermal shock is very hard on the gas turbine, so the preference is to ramp down the fuel to achieve shutdown whenever possible.
Regardless of how the unit goes offline, it must coast down and then go onto turning gear for a period of time. The unit goes through a continual slow rotation or—depending on the model of turbine—may turn a quarter rotation every couple of minutes. It’s important for the rotor to be turned for about 24 hours to cool the unit down, as measured by the wheel space temperatures. Operators must pay attention to wheel space temperatures because they display the internal temperatures of the equipment when it is shut down. Those temperatures need to be low, to ensure the rotor can be allowed to sit stationary.
In the first 24 hours after operation (approximately), while the unit is still hot, one might expect a very hot rotor to sag if it were to come to a standstill without proper cooldown (thinking that the hot metal could not support itself between bearings). This is not the case with gas turbines; their rotors are relatively light in their construction, very stiff in design and configured with short bearing spans. As a result, the rotor, if left without proper cooling, will actually bow up rather than down. This has to do with thermal differences in the unit, where the rotation on turning gear moves the hot part of the turbine rotor into the cool airflow path that's naturally moving through the casing, thereby avoiding the rotor bow by evenly distributing the dispersion of rotor heat.
In gas turbines, a bow up is undesirable as clearances will tighten between the rotating and stationary parts in the unit. If those clearances tighten up, rubs can occur if the rotor were to attempt to turn before the bow straightens itself out. When rotated, rubs cause those clearances to open up, and efficiencies would decrease the next time the unit ran because of the loss of tight clearances. After enough time has passed for satisfactory cooling, turning gear can be eliminated without any fear of the rotor bowing up.
Turning Gear Special Cases
Peaking power plants, commonly known as ‘peakers’, operate during times of high demand and therefore must be available in more of an “on demand” mode. For example, ‘peakers’ may need to start up within an hour or two of having been shut down. The concept is that if the gas turbine is on turning gear (rotating), it will not bow up. Since ‘peaker’ rotors are always inline (straight) so they will be able to start at any time. Thus, the 24-hour rotating period is a general guideline.
Consider an outage period, when it would not be ideal to wait 24 hours while the unit rotates on turning gear since lube oil pumps, electrical services, etc. must remain on during that time. During an outage the rotor is going to cool naturally and move back to a straight-line position anyway so it is not necessary to be on turning gear in this case.
Gas Turbine Combustion
Fuel
Gas turbines are referred to as gas turbines not because they run on gaseous fuel, such as natural gas. The gas is in reference to the combustion gas that expands through the turbo-expander(combustion gas is the working fluid of the turbine) just as a steam turbine uses steam as its working fluid. The majority of industrial turbines currently operate with natural gas as their primary fuel. There are units that operate on liquid fuel, but they're less common, due to emissions requirements, and usually serve as the backup system if gas fuel is not available. Because they are on standby all of the time, liquid fuel systems are prone to operational issues.
Gas turbines equipped with natural gas systems will operate Dry Low NOx (nitrogen oxide) systems. These systems significantly reduce emission gases that are of concern. This type of technology is not industrially feasible on liquid fuel systems. With liquid fuel systems, water or steam injections can be used to reduce emissions, however, they are not as effective as the gas-fueled Low NOx systems in lowering emissions. The term ‘Dry’ in DLN is to identify the process as not using a dilutant (water or steam) in the combustion emission reduction process.
DLN Systems
Dry Low NOx (DLN) systems do not use any water injections as liquid fuel systems do. With the advance of technology for combustion, equipment and processes for the Dry Low NOx DLN system minimize emissions. Dry Low NOx uses a fuel/air mixture that is air rich and fuel lean (less fuel and more air). This system uses an elaborate scheduling process and advanced hardware to significantly reduce emissions by orders of magnitude from standard combustion.
In general, there are two types of DLN systems; DLN-1 and DLN-2. They are physically and operationally different. The first, DLN-1, operates on smaller turbine systems. Because of the nature of the smaller gas turbines, the compressor is less massive in its output than the bigger machines. Thus, its’ air load versus the fuel cannot be adjusted as gradually as in DLN-2 systems. During initial operation DLN-1 uses the traditional way of burning fuel, known as diffusion flame, where fuel and air are thrown together into the fireball, producing a very hot flame (creating high NOx emissions). The DLN-1 system goes through phased sequencing, moving from diffusion flame to lean-lean combustion, and then into premix, which is a form of combustion that produces lower emissions.
DLN-2 is used on larger gas turbines and is always in premix because of the nature of the equipment and its capabilities. It works via fuel flow sequencing through different nozzle feeds. There are six different nozzles in each combustor, and they ‘open’ or ‘close’ to get the right combination of fuel to go in with the air at a particular operating point.
DLN premix is staged combustion, where the gas fuel is mixed with air outside of the combustion zone, allowing for very rapid and thorough combustion once it enters the flame zone. This rapid combustion results in a ‘cooler’ flame which is the mechanism that reduces NOx production so effectively.
Figure 1 shows a simple drawing of the DLN-1 combustor. Fuel is injected through primary and secondary fuel nozzles depending on the mode of combustion. Flame resides in primary and / or secondary zones depending on the mode. Premix (PM) mode has approximately 80% of fuel entering the primary zone without flame, mixing with the air in that area and then flowing into the secondary zone where it ignites and burns quickly and efficiently. The venturi is the physical component of this process that allows PM to occur.
Figure 2 shows cross-section of the DLN-2 combustor. The red box and green line shows the approximate positioning of the fuel nozzle inside the combustor. There are six of those nozzles positioned co-axially inside the combustor. The fireball is indicated by the red circle. Zone A (green line) is the mixing area for fuel and air prior to combustion much like the primary zone in DLN-1 in PM mode. With DLN-2 this PM mixing happens throughout the combustion process (combustor is always in premix combustion).
Tuning
In DLN combustion, there is a balance between the two emission gases CO (carbon monoxide) and NOx (nitrogen oxide) that come out of the combustion process at different points. Because these gases occur as a result of different chemical reactions in the flame, as one is lowered, the other will increase. Thus, there has to be an optimization of fuel and air so that both the CO and NOx are kept low.
Balancing / optimizing is achieved by adjusting the flame temperature to by varying the mixture of fuel. This is called tuning. Tuning is based in science, but there is also a bit of an art to it. Auto Tune is a control system that generally handles tuning, however, there are tuning specialists who also manually do equipment tuning to manage emissions. Further, balancing of the fuel air mixture can be seasonally sensitive as ambient air temperature shifts. Tuning is therefore an on-going adjustment as seasons change and as the gas turbine combustion hardware ages or is replaced.
NEXT UP in the final session of our three-part podcast series on gas turbine operations, we will cover considerations related to support systems, compressor performance and water wash, turbine performance, and vibration monitoring.