Selecting what the industry calls “value strategies” is now the name of the game in gas-turbine maintenance. On the other hand, common sense may be the best guide to optimum operation and cost control.

By Penelope B. Grenoble

	Two Hours of Downtime

Dan Levin likes to think of gas turbines as “really reliable” pieces of equipment that in general are relatively simple to operate. A comfortable enough statement until Levin, who is vice president of marketing and sales for the Energy Customer Service Business at Rolls-Royce, adds the caveat: As long as they’re responsibly maintained.

“Maintenance is relatively simple,” says Levin, “but it has to be followed. When it’s not, the results can be very costly.”

Mark Basford, vice president of sales and marketing for the Wood Group Pratt and Whitney, expands on the thought: “Operators need to become familiar with the operation and maintenance instructions that come with their unit and any service letters or bulletins from the OEM [original equipment manufacturer]. They need to pay attention to what the instructions tell them and do their best within operating limits to follow those instructions.”

Points well taken, especially considering that advancing technology is affecting the why and how of turbine maintenance and the fact that the maintenance equation has become complicated by the increased use of turbines to peak shave and, in the power generation industry, an emerging dominance of investment priorities.

“In this deregulated world,” says Joe Daleo, vice president of DWD Turbines Ltd., “everyone is looking for a competitive edge. While maintenance costs may be one of the few things you can control, the way maintenance is done continues to evolve. Technology and the volume of repairs have increased to the point that everybody is rethinking how this all goes together. We’re seeing a lot of automation, for example, which is occurring because the OEMs want to service a good chunk of their fleets, and to do this, they have to establish very efficient processes. Plus parts are becoming more difficult to repair.”

Likewise, changes in operation also appear to be affecting maintenance. “We’re providing operators with help meeting their service intervals in this new market,” says Mike Bradley, field service manager at Turbine Resources Unlimited Inc., which specializes in the independent power market. “Our clients are having a difficult time with cycled, intermittent operation. Once upon a time we knew that with five or 50 starts a year these machines would go to 48,000 hours. Now we're putting 15 to 300 starts a year on them.”

Basford supports this take on changing circumstances in the power industry. “A lot of the units we support are old peakers. They’re 20 to 30, in some cases 40 years old, and because they only run intermittently, companies pay less attention to them than they probably should. Units that are run baseload suffer from a different problem, which is that people who sell power insist they not be taken offline unless it’s for a scheduled outage.”

Ron Cox, director of marketing for the Turbine Generator Business Unit at Alstom Power Inc., has a similar view. “A lot of operators of peaking gas turbines lose sight of their obligation to replace parts or overhaul the engine. I think it’s a case where the units were originally bought to operate 300 to 500 hours a year, and they now they’re running 1,000 to 1,500 hours a year. And all of a sudden there isn’t money in the budget for the kind of maintenance that’s needed.”

And as both Levin and Cooke point out, a good chunk of people currently entering the power generation business are more interested in investment, as opposed to operational challenges. For this segment of the market, Levin suggest it’s better for the OEM to provide maintenance right out of the gate. In fact, he estimates as much as 80% of the power generation market opts for long-term maintenance contracts when purchasing new equipment. “It’s a way of sharing risk. We offer a TotalCare agreement, which develops a partnership between the OEM and the end-user wherein we both share the goal of a highly available plant that performs to its maximum capability.”

Photo: Rolls-Royce

Far from being static devices, turbines are subject to mechanical and thermal forces and must be maintained accordingly.

“One of the reasons people like long-term service agreements [LTSAs],” says Cooke, “is that they provide the certainty of fixed maintenance, which the banks like. Insurance companies also like these contracts. Large fleet operators, however, don’t. They know they’re losing millions of dollars every year for the sake of certainty and would rather pay for what they need. Another reason for OEM service contracts is that the very latest engines don’t have much of a track record, and an operator may feel for the first several years that only the engine manufacturers have sufficient familiarity with this new equipment.”

As Cox sees it, LTSAs (or at minimum some kind of parts guarantee agreement), provide a productive option at different levels of operations. “Institutional users who are in a turbine size of less than 4 megawatts and in continually running mode are far better off with some sort of long-term agreement that helps them out of the warranty period and takes them through several cycles. Middle-range operations in the range of 5 to 100 megawatts are best off with either an independent service provider [IPS] or OEM. Utilities with large fleets that are putting out as much as 1,000 megawatts may also outsource their maintenance because time is so critical.

“Typically on new gas turbines, there’s about a three- to five-year period where these parts come up to full lifetime production. So if you’re buying a serial number one engine, you’re a far better off with an agreement with an OEM, which will constantly be coming back with upgraded parts. With a mature engine, it’s a different story—although those parts still will have to be upgraded over time.”

LTSAs are typically based on a fixed annual cost, are inclusive, and the operator is locked in for six to 10 years. For some operators, this translates to lack of flexibility.

“It’s been my experience that operators have been dissatisfied with the LTSA once they operate under it,” says Basford. “This is because they don’t always get what they thought they were going to. As an example, one operator I know of had an LTSA that guaranteed he would have the spare parts to support his outages. He had a breakdown that occurred early in the contract, with the result that he had to wait 45 days for the equipment he needed. Because the breakdown occurred early into the contract, the guaranteed parts weren’t available.

“In cases like this you're stuck between a rock and a hard place because if someone else supplies the parts to keep the unit running, the OEM won’t warrant the work. On the other hand, maintenance is very complex, and a LTSA is one way people try to avoid having capital spares on the shelf. And lot of customers want the assurance of working with the OEM because of the risk-sharing.”

Another liability of LTSAs is that they can limit customers from taking advantage of parts or service that may be more appropriate to their operations. And although the OEMs stress their more expanded knowledge base, it goes without saying that their fees are likely to be higher. Developing partnerships with one more ISPs can compensate for some of these liabilities, including the ability to purchase services, parts, and equipment at an operator’s discretion and flexibility in adapting to individual operations. The fact is OEMs have partnered with aftermarket operations and service each other’s equipment through these partnerships. In addition, OEMS are manufacturing parts for competing equipment.

Given these emerging trends, Cox cautions operators who opt to go out on the open market about “gray market” parts. “There are three levels of parts out there: OEM parts that we or any of the OEMs provide; non-OEM parts, which are new parts manufactured under different patents or using improved cooling and coating designs to improve an engine’s operating life and performance; and gray market parts, which may be early versions of an OEM part when it first came out that may lack subsequent lifetime or performance improvements. Gray market parts can also be found as new or used parts that were reconditioned.”

Another potential disadvantage of going outside the OEM relationship is that smaller operators and investors often don’t have the technical staff needed to assess and manage their relationships with ISPs.

Cooke also suggests that operators who elect to go out on their own should carefully monitor their LTSA as it winds down. “Once the OEM or service provider recognizes you won’t be renewing the contract, you have to be on the lookout for limited-life maintenance practices that could mean a part may not be repairable in the future. You can do a very thorough refurbishment and bring a component back to as-new condition, in which case it’s got a long life ahead of it. Or you can do a partial repair and make it cosmetically pass inspection. But it won’t last nearly as long.”

On the other hand, Bradley points out why his customers in the independent power generating market want to take advantage of a value partnership with an ISP such as Turbine Resources Unlimited. “After a period of four or five years, if nothing has changed, you don’t need the OEM. But if change your operating mode, it can be hard on components, and the issues change as the equipment ages. Some customers think components can be repaired forever, but in some applications, longevity between the service intervals is just not going to be there on a repaired component.”

For operations without technical staffs there is also the option of bringing in a consultant. “Operators don’t get a lot of information from OEMs,” says Daleo, “and they have to make multimillion dollar decisions based on very limited data. We can provide some of the hard data they need by cutting up parts and destructive testing to determine what's wrong. This might also help operators in an LTSA convince their OEM to bend the rules a bit and allow them to use parts or coatings from another supplier.”

Whoever’s doing the maintenance, Levin recommends operators get to know their machines. “Walk around, check that fluid levels and pressures are correct, that filters aren’t clogged and there aren’t any leaks developing. If you take very good care of those basics—making sure air filters are intact, that oil filters are changed at the right intervals, and the fuel-treatment skid is working properly and providing the right fuel inlet temperature—the machine should go from one maintenance cycle to another uneventfully. Cox agrees. “Fuel and air quality are very important. If you look after both, you’ve probably covered 80% of your problems.”

Levin thinks fuel systems are one aspect of preventative maintenance that are often neglected. “Gas turbines can be very sensitive to the dew point of fuel. If you’re getting liquid in the fuel, you’re going to see different types of secondary damage, anything from quick erosion of combustion systems, the hot sections of the turbines, even stress on the casings of the machine. These can occur when filters don’t get replaced, the fuel is not at the right temperature or liquids get into the fuel system.

“If the air filter and inlet housing aren’t properly maintained, they can develop leaks, which pull unfiltered air from the atmosphere. In locations where there is a lot of abrasion in the air, salt from the sea, for example, or sand, these contaminants can be sucked in and cause erosion and secondary damage to the compressor, resulting in costly repairs.

“Leaks in the air system are very common—because operators take off and bleed air at various points for instrumentation, cooling and inspecting the duct work. But it’s important to check, because with a leak you lose efficiency. The result is you can end up running your turbine hotter than you may have intended.

“The oil system is another system that warrants attention. Even when operators look at these pretty carefully, we still find problems, primarily from contamination. We recommend that regular oil samples be drawn, depending on the way the machine is being used. If the machine is running full time, you’ll probably want to pull samples monthly. We send the oil samples out to a lab to make sure the lubricating components are correct and the oil hasn’t started to break down. In a nutshell, what you want to check for is the oil level, the oil pressures at different points of the engine, and the oil quality itself.

“Apart from these checks and inspections, we’re always doing structural and package inspections, looking for leaks and things that have gotten loose, especially with the structural members. With units that start and stop quite frequently, you get an opportunity to look at those things, and because they go through so many thermal cycles, there’s always the chance for things to become loose.

“These machines also require washing, and depending on where they’re operating, the washing cycle can be extremely critical. These are ‘breathing machines,’ and when they’re located next to a freeway or an industrial plant, they’re going to suck in pollutants such as ash and diesel exhaust. As these accumulate on the compressor, the engine loses efficiency, and as it loses efficiency, it begins to run hotter and hotter. The higher temperature, of course, causes thermal stress, and if a machine gets dirty enough, it’s not uncommon that a compressor can surge on the engine, causing mechanical damage. This includes damage to seals, which causes greater internal leakage and ultimately, higher repair bills.

“So depending on where the equipment is located, we would recommend monitoring these machines closely, recognizing when you’ve got the symptoms of a dirty compressor, and then taking the engine down and soak-washing it, or doing online washing if that’s what the OEM recommends.”

If things are as straightforward as Levin describes, what can go wrong? Basford thinks missteps or mistakes in judgment are typically cost driven. “If the manufacturer recommends you do a hot section inspection at x thousands of hours, and you open the unit up and see that everything looks like it’s in great shape, you may decide to add another 2,000 hours before you do the service. If you’re lucky, there’s no huge problem when you open it up the next time. If not, a piece may crack and break off as a result of thermal distress and move downstream, where it damages the turbine. To do good preventative maintenance, you have to start with a baseline and then use future inspections to project failure rather than operate to failure.

“On-condition maintenance is okay as long as you follow good engineering practices. It’s when you push the envelope. Instead of doubling the time between maintenance, increase it by a percentage of time and make sure that every time you look at the components, no degradation is noted. On the other hand, on-condition maintenance could also mean that the next time, instead of doubling it, you may need to cut it in half because there’s been sufficient degradation to warrant an earlier inspection.”

Turbine Resources Vice President Bill Howard agrees that operators need to know their machines. “You can know that after a certain amount of engine hours, under a certain type of fuel, you’re going to see degradation, which means you need to inspect for that. In the long term you save money by spending money on maintenance. Maybe you have the engine apart and the nozzle doesn’t look so bad, but 24,000 hours later, the nozzle is scrap. What you should have done when you inspected the unit was have it repaired and thermal barrier coated.”

“There’s a risk factor in misinterpreting on-condition maintenance,” says Cox. “The term came out of aviation, where the machines would be inspected and the maintenance performed. Typically, major maintenance intervals are planned according to the number of starts and operating hours and at a period when the owner can afford to have a machine down. Based on operating experience, there are a number of algorithms we’ve developed that basically tell us within 3% to 5% the lifetime and condition of those parts. Because quite frankly, if you have a machine under long-term agreement, as a supplier you want to maximize the lifetime of those parts. You don’t want to be taking them out early. Nor do you want to take them out too late because then you can’t recondition them. So there’s a lot of planning and a lot of engineering that goes into this.”

“Pulling a unit on condition rather than by time interval allows customers to get more use out of the engine,” says Levin, “which means better utilization of their assets. But this requires that you have good inspection techniques. You have to know exactly what you’re looking for and where problems are likely to occur. Some customers want to run hotter or at higher power for certain periods of time, but they will sacrifice because the units will require an overhaul sooner. On the other hand, some customers might prefer, or may not need, to run at full power and will scale back. This can increase the time between overhauls 20% or more.”

“Operators can certainly extend the service interval,” says Cooke, “but they know they’re doing that at some risk because the parts might not be repairable. Usually they do this in an informed way—maybe a delayed schedule may just work better in terms of manpower availability.”

And how about doing the work yourself? “It might be worth the benefit to a smaller independent power producer with one or two units to work with someone who has the tooling and spare parts and manpower to support them,” says Basford. “But with a larger organization that has a lot of the same equipment, it would probably be wise for them to do some of the work in-house.”

“It’s an absolute fact,” says Levin, “people need to be well trained to maintain these units. And it’s becoming more and more difficult to find skilled people. One thing we provide with our systems is a predictive maintenance monitoring system, which we consider a critical element in watching equipment and keeping track of and logging all the alarms and all the operating trends so we’re able to identify problems before they occur—and as long in advance of when they might occur as possible. Lack of knowledge is a primary barrier to good maintenance, which means education and training are very important elements.”

“People think turbines are static devices,” says Cox. “They’re not. They’re mechanical: They undergo thermal and mechanical forces, and they have to be maintained as such.”

Penelope B. Grenoble specializes in topics related to technology and the environment.

DE - January/February 2008