Gas turbine systems are widely used in onsite power installations for industrial plants, commercial office buildings, hospitals, shopping centers, high-rise apartments, and other distributed energy applications.
The onsite gas turbines used can range in size from 30 kW, on up to 20 MW. Gas turbines in the 30- to 500-kW range are generally referred to as microturbines; those in the 500-kW to 20-MW range are known as industrial gas turbines.Many communities are considering, researching, or implementing microgrid solutions. The underlying rationale often involves complex business, operational, and economic issues. See our FREE Special Report: Understanding Microgrids. Download it now!
This article, the first of two, deals with onsite gas turbine operations and maintenance, and focuses on the issues encountered with these industrial turbines. The second article will deal with the operations and maintenance (O&M) of microturbine systems. For part two of this interview, click here.
To bring you the most current and authoritative information on gas turbine operations and maintenance, Business Energy magazine (Formerly Distributed Energy) interviewed a leading expert on gas turbines, Nick Pozzi, manager of customer service for the Gas Turbine Division of Kawasaki Gas Turbines—Americas in Grand Rapids, MI.
In the following interview, we systematically walk through all the major components of a typical onsite gas turbine system in the 1.5-MW class (although the insights here are relevant to all sizes of gas turbines), including these:
- Air intake
- The compressor
- The combustor
- The turbine
- Air emissions
- The gearbox
- The electric generator
- Preventative and predictive maintenance
- Who should perform the maintenance (in-house versus outside experts)
Business Energy (BE): Are there any significant differences between a steam turbine and a gas turbine?
Nick Pozzi (Pozzi): Yes, indeed! It is true that there are strong similarities between a steam turbine and a gas turbine. The steam turbine uses steam to create motion in the turbine and the gas turbine uses a hot gas. Both use the energy of these fluids to turn a generator, which makes electricity. A gas turbine operates at a much higher temperature than a steam turbine (typically, at 2,000°F versus 1,200°F for a steam turbine).
Accordingly, a gas turbine must be designed to withstand these much higher temperatures. The components of the gas turbine (rotor, blades, vanes, etc.) are fabricated from much more expensive metal alloys—e.g., inconel. Further, the exhaust of a gas turbine can be used to make steam. And, when channeled into a steam turbine-generator, that steam can be used to make more electricity; or it could be used to operate a chiller to make cold water or cold air for air-conditioning a facility.
Because of the need to use these special metal alloys, a gas turbine is considerably more expensive than a steam turbine. A 1.5-MW gas turbine can cost in the neighborhood of $1.2 million—versus $750,000 for a comparable steam turbine.
BE: I noticed that your gas turbine system has a filtering unit on the air intake. Why is it necessary to filter air entering the turbine system?
Pozzi: The air has particles in it that are potentially damaging to the gas turbine system. Without an effective air-filtering system, these particles over time would reduce the turbine efficiency. Poor air filtration could cause what we call FOD—foreign object damage. Someone could also inadvertently drop a nut or bolt into the air intake duct, which could then damage compressor or turbine blades. As a specific example, we know of a case where snow found its way into the air intake and subsequently turned to ice. Some ice chunks were then drawn into the engine, caused vibrations, and, two weeks later, a turbine blade loosened up, requiring that the entire gas turbine be shut down for repair.
BE: Specifically, what particles are in the air that could potentially damage the compressor-turbine system?
Pozzi: There are many particles in ambient air, but most important are airborne salt (sodium chloride) particles, especially for sites along the coast. Such salt particles can be very damaging to a turbine. Salt is, of course, very corrosive to metals. But beyond that, it leads to a buildup of dirt on compressor and turbine blades. Once salt particles attach themselves to turbine blades, they apparently attract other particles—dirt. The net result can be a loss in the turbine’s power output of up to 15%.
BE: How do you actually filter the air?
Pozzi: We draw intake air from the ambient through a special filter called a HEPA filter, not unlike the filters used in vacuum cleaners. It is most important to use this generic type of filter, as it removes 99.87% of the particles in the air. In sum, the HEPA filter keeps the turbine cleaner, resulting in higher power output.
BE: What if the operator of a turbine system has been negligent and has not done a very good job of filtering intake air? What to do then?
Pozzi: The solution is to water wash the engine periodically. Some operators do this once a month. This water wash is done with the turbine running. As an alternative, the turbine can be washed by the so-called cold-wash method, done at low revolutions per minute (rpm) when the turbine is at purge speed (30% of speed). A purge is required on all turbines to remove any potentially explosive vapors.
The point is that, with good intake-air filtration using the HEPA filters, there is no need to wash the compressor-turbine system as often as before. The turbine system stays cleaner for a longer time, thereby avoiding a degradation of performance.
BE: What do you mean by “purging” the turbine system? Why is it done and how?
Pozzi: Consider that the turbine system has been off. And now you wish to turn it back on again. There could be explosive unburned fuel vapors still lurking inside the turbine system—in the compressor, combustor, turbine, waste-heat recovery system, etc.
To guard against the danger of explosion, it is standard practice to first purge the turbine system of these potentially explosive vapors. But one needs to purge safely. And this means one must proceed without activating the igniters (spark plugs) in the combustor (i.e., the turbine’s combustion chamber), for the sparks could trigger an explosion. The control system does this automatically during a normal startup.
The starter motor (hydraulic or VFD) is used to rotate the turbine at 6,600 RPM (revolutions per minute), turning the turbine at about 30% of its usual RPM, enough to make possible the drawing in of fresh ambient air, which will then quickly displace (or purge) any potentially explosive gases or liquids from the system. That done, it is then safe to fire up the igniters, thereby setting the turbine into operation.
Incidentally, in carrying out this operation, it is important not only to purge the turbine, but also the waste heat recovery boiler. One does not want dangerous unburned fuel vapors lingering in the waste-heat recovery boiler. The Kawasaki 5-MW turbine system has a diverter downstream from the point where hot exhaust gases emerge from the turbine. This allows the hot turbine gases to either be sent out the stack or through the waste-heat recovery boiler. By law, the operator must first purge the turbine with a minimum of six air changes before firing up the turbine.
BE: So filtering intake air then is very important? And it will greatly extend the life of a gas turbine system?
Pozzi: Yes! Changing the HEPA air filters periodically is very important. Indeed, it is one of the most important things a turbine operator can do to maintain his gas turbine system.
Nonetheless, the overall lifetime of a gas turbine system is greatly affected by the physical environment it is placed in. Is it located adjacent to a paper mill spewing sulfur compounds into the ambient? Such could lead to the formation of coatings on turbine blade surfaces or to premature bearing failures.
A gas turbine located at one site may last five years before it needs an engine change or a bearing replacement; and the same turbine at a different site may last only four years. It all depends on turbine usage and on location. Good air-filtration systems prevent damage from occurring, thereby increasing the lifetime of the equipment.
Where there are special environments—e.g., an offshore oil-drilling platform—it makes sense to increase inspection frequency. There, a gas turbine is used to drive pumps to move oil. In the air there are many salts and other contaminants. It is common practice to flare off hydrogen sulfide gas from the wells, some of which is drawn into the turbine system inlet, possibly causing premature damage to turbine blades.
BE: What sort of maintenance is required of the gas turbine itself? How often is it done and by whom?
Pozzi: We recommend a maintenance service plan to our customers who call for our service technicians to perform three quarterly inspections and an annual inspection. In the second year, we recommend doing another three quarterly inspections and an annual inspection. At that 16,000-hour point, when this second annual inspection is being done, the technician will also do what’s known as a hot-section inspection.
BE: Please expand on these quarterly inspections. What is involved in these quarterly inspections? Who does what?
Pozzi: During the quarterly inspection, the maintenance technician collects turbine system performance data—on such variables as vibration, pressures, temperatures, and outputs. The original equipment manufacturer (OEM) then analyzes the data and issues a serviceability report. This report compares the newly collected data to baseline data collected during startup of the new turbine. If we notice any deviations from the baseline, we flag the problem early and investigate it ASAP—meaning during the next available shutdown.
Such aggressive predictive maintenance helps eliminate downtime and possible expensive repair. This is one of the things we do to achieve over 98% reliability and availability (R&A). Indeed, some of the gas turbine systems that we have installed have achieved 98% R&A for the past 13 years. Preventive-maintenance and predictive-maintenance methods will vary some from one OEM to the next and the prospective turbine system buyer should inquire about an OEM’s R&A track record.
What else is done during a quarterly inspection? The service technician will check lube oil and air filters for dirt. He will check magnetic pickups for metal deposits—for significant accumulated metal pieces could be a warning of early bearing failure. Even though most magnetic pickups are alarmed, we don’t wait for the alarm to sound or the automatic shutdown to occur. Instead, we inspect each quarter for early warning signs.
BE: What about the annual gas turbine system inspection? What happens there and who does it?
Pozzi: The most important part of the annual inspection (as called for in the service agreement between OEM and customer) is the use of a borescope to examine the internals of the turbine. This borescope is very similar to the fiber-optic cable that a physician uses to examine a person’s colon. The OEM technician snakes the borescope’s fiber-optic cable inside the gas turbine so that he can inspect its internal components and take photos.
The technician is looking for cracks in the lining of the combustion chamber, erosion on turbine blade tips, loss of protective coating on blades, and signs of overheating—e.g., a blade tip may be melted off. Besides doing the borescope inspection, the OEM maintenance technician during this annual inspection also checks every alarm and shutdown device to see if it is working properly. And he also checks all major fluid levels and filters.
BE: How often does he find something wrong? What happens if he finds a damaged blade?
Pozzi: If something is not working up to specifications, we replace it. Such is part of our typical maintenance service agreement. Gas turbine systems seem to have two lifecycle periods they go through: the first six months of operation, and after year 10. I am referring her mainly to the ancillary equipment provided with the turbine package—things such as transmitters, switches, automatic valves, human machine interfaces, and data loggers (computers that store historical data).
Remember that every minute that we are down counts against our availability. Accordingly, we do everything in our power to make sure things will work correctly until the next inspection. If we think they will not, we change any suspect components.
Also bear in mind that even our scheduled downtime for the quarterly and the annual inspections, the hot-section inspection, and engine and gearbox change-out time—all this counts against our availability. Accordingly, we can’t afford any unscheduled downtime. Our customers depend upon our equipment to run successfully all the time. So we do everything possible to make that happen.
BE: The compressor is quite clearly an important subsystem of an industrial gas turbine system. What sort of maintenance does that require? Who should do what when?
Pozzi: An important component it indeed is. Yet the compressor—which of course turns on the same shaft as the turbine—is very robust and only rarely needs any maintenance attention. Don’t forget that the compressor is upstream from the combustor and, as such, is not subjected to hot combustion gases, but only to the ambient air drawn in to be compressed before it flows into the turbine’s combustor. Essentially, the only maintenance needed for the compressor is that done once every 32,000 hours (about 4 years), as part of a total shop overhaul and rebuilding of the entire turbine.
BE: You mentioned that every 16,000 hours you do a hot-section inspection. Please expand. What is a hot-section inspection? Who does it, when, how, and how long does it take?
Pozzi: A hot-section inspection is an examination of those parts of a turbine system that are exposed to the hot gases created when compressed intake air is mixed with natural gas or other fuel inside the combustor and ignited by the igniters. In a word, the hot sections are mainly the combustor (i.e., the combustion chamber) and the turbine section—and any other components exposed directly to flame or to hot combustion gases.
This hot-section inspection is done every 16,000 hours (about every two years). It is crucial that the inspection be performed by an experienced and knowledgeable turbine technician, who does the inspection onsite.
The inspection involves opening up the combustor and turbine sections; carefully examining walls, linings, turbine blades, and vanes, etc.; replacing any worn or damaged components; then reassembling and starting up the system. On a 1.5-MW turbine system, this will usually take an experienced turbine technician about three days.
BE: There is a difference, then, between a borescope inspection of the turbine and a hot-section inspection?
Pozzi: Oh yes! Most definitely! A borescope inspection is done every year—that is, every 8,000 hours of operation—as part of the annual inspection. It is a way of examining the internals of the gas turbine—by snaking a fiber-optic cable inside the turbine casing—without going to all the time and trouble of opening up the turbine and looking at its internals directly.
By contrast, we do a hot-section inspection only once every 16,000 hours (i.e., once every two years). Such an inspection is much more thorough than a borescope inspection, for it involves opening up the turbine and examining it directly. Both the service technician and the turbine owner are able to see the internals of the turbine directly and what its actual condition is.
During a hot-section inspection, the turbine technician replaces damaged turbine blades, vanes, and any other components showing wear. Upon completion of the inspection and of any needed repairs, we guarantee to the customer that the gas turbine system will be good for another 16,000 hours of operation. We give him a serviceability report indicating bearing wear and clearances, and noting any components that were replaced. Our guarantee gives the customer a comfortable feeling about the equipment and its performance.Add Distributed Energy Weekly to your Newsletter Preferences and keep up with the latest articles on distributed power, fuel cells, HVAC options, solar, smart energy systems, and LED lighting retrofits.