Front and Center at ASME/IGTI Turbo Expo

HOT TOPICS AT IGTI INCLUDED EXTENDING TURBOMACHINERY LIFECYCLES, MAINTENANCE, SUPERALLOYS, WET GAS COMPRESSION, BOIL-OFF GAS, LNG, SUBSEA COMPRESSION AND TURBINES RUNNING ON FUEL OIL

Dusseldorf, Germany was the June 2014 venue of Turbo Expo under the auspices of ASME’s International Gas Turbine Institute (IGTI). Mike Ireland, Managing Director, ASME Research & Technology Development, introduced the keynote session by calling Turbo Expo the “flagship conference for ASME,” and as such, this year’s event enjoyed the highestever attendance at over 3,000, up from 2,400 in 2013. A further record was set with 1,229 papers presented in 329 sessions. The exhibit featured 118 vendors.

The keynotes focused on turbine lifecycles. We tend to hear about turbine problems, catastrophic failures, and the teething issues on the latest and greatest machines. This can leave a false impression on turbine reliability and lifespan. But the reality is that many gas turbines (GTs) remain in operation for an awful long time. Charles Soothill, Senior Vice President of Technology, Alstom Switzerland, highlighted the very first GT, a 4 MW model installed in Neuchatel, Switzerland in 1939 that stayed in service until 1997.

“With more renewables, which are being given priority onto the grid, gas plants are running less and so we are seeing even greater lifecycle cost pressure,” said Soothill. “Further, more startups create lifecycle fatigue damage.”

Per his estimates, GT lifecycle costs amount to twice equipment “first costs.” (The sum of the initial expenditures involved in capitalizing a property; includes items such as transportation, installation, preparation for service, as well as other related costs.) Steam turbines, on the other hand, have similar lifecycle to first costs. Soothill also pointed to Alstom’s work with additive manufacturing techniques, such as selective laser melting as a means of replacing the casting of blades.

This theme of total lifecycle costs continued throughout the keynotes. Pratyush Nag, Director of Engineering, Product Line Manager, Siemens, began his talk by summarizing the prediction for the worldwide power-generation mix. Gas moving from a current 23% share of the energy mix will reach 25% by 2030, he said. That number, though, is made to look worse than it is due to the fact that there will be a 50% rise in electricity demand during that period.

Meanwhile coal will fall from 40% to 35% and oil from 4% to 2%. Fossil fuel marketshare will decline from 67% to 62%, which will be mainly compensated by renewables, expected to soar from 4% to 13%. However, gas generation power plants are still expected to be the mainstay to compensate for the volatility in renewable power generation.

Nag stated how users are evaluating overall lifecycle costs that do not just cover power output, but include reliability and availability on the grid. This is exerting an influence on warranty terms. “To reduce lifecycle costs, we have to make progress in system integration, clearance controls, coatings and process simulations, and maintenance services,” said Nag.

Siemens has developed thermal barrier coatings (TBCs) and sensors to better monitor hot-gas-path components and thereby lower lifecycle costs. In addition, Hydraulic Clearance Optimization (HCO) shifts the rotor against the flow direction to optimize turbine clearances during steady state operation.

The axial shift of the rotor is performed automatically by hydraulic pistons behind the compressor thrust bearing. A gain in power and efficiency due to the conical shape of the turbine is higher than the potential losses on the compressor side. Nag said it produces a power boost of several MW on some Siemens machines, and that over 100 units already have it in operation.

Karsten Muhlenfeld, Director Engineering, Rolls-Royce, rounded out the keynotes by discussing aero engines. He stated that the money is no longer in the sale of engines. OEMs, therefore, have to be up to the challenge in a market filled with an abundance of aftermarket challengers. “While it is important to build new engines, it is more important to service them well,” said Muhlenfeld. “More than 50% of our total turnover is from servicing engines.”

Compressor advances

Subsea compression earned the opening presentation on the first day. Dirk Bueche, Man Diesel & Turbo, explained subsea’s thermal challenges compared with traditional compression. Modeling, he said, is essential for such things as assessing thermomechanical stresses or the potential of hydrate formations in the motor-compressor and process pipes.

Man Diesel & Turbo opted for a comparatively slow-running, multi-stage radial compressor with an integrated electric motor in a single hermetically sealed casing. Motor cooling is achieved by direct extraction of process gas from the compressor, which is fed back into the process resulting in a quasiopen- loop cooling. By choosing active magnetic bearings an oil-system is avoided. Guiding principles are operational safety, reliability, and remote operability (p. 18).

Bueche was followed by Matteo Bertoneri of GE Oil & Gas, who covered the testing of a single-stage centrifugal compressor under wet gas conditions (Figure 2). Testing at a Southwest Research Institute (SwRI) facility is being done to manage liquid content up to 5% liquid volume fraction (LVF) and 40% in liquid volume mass (LVM). In addition, work is being done to verify aerodynamic and thermodynamic performance, vibration, rotordynamics, erosion and axial thrust.

“The objectives are to bring about subsea compression simplification, remove or reduce size of the scrubber, provide a smaller footprint and increase overall reliability,” said Bertoneri.

This compressor, not originally designed for wet gas, had labyrinth shaft end seals with continuous air purge, as well as tilt pad bearings to support the rotor. Several layers of paint were applied to show the extent of erosion during testing. The conclusion of the tests: the centrifugal compressor could operate with a significant amount of liquid content both in steady state and transient conditions.

Boil-off gas

Mustapha Chaker, LNG Engineering Specialist at Bechtel Corporation, discussed applications for boil off gas (BOG) compressors in LNG plants. As heat leaks into the cryogenic tanks from outside, LNG boils. LNG, after all, exists at a temperature of approximately -162°C. If the vapor is not drawn off by the BOG compressor, the pressure will increase to the point where venting is required. The compressor helps to keep tank pressure under control.

But at the same time, it is not optimum to re-liquefy the gas constantly. Therefore, Chaker discussed ways to minimize BOG, such as optimization of the design of the compressor and other LNG components, as well as altering process conditions to reduce BOG. “If heat loss from the tanks, cool-down line, and loading line is observed, it is seen that each contributes approximately to one-third of the total heat loss.” (Table 1).

Chaker recommended that BOG compressors should be selected for lower head and high capacity. They can either be speedcontrolled or use Variable Inlet guide Vanes (VIGVs). Depending on the scenario, reciprocating, screw or centrifugal compressors can work, though the latter are usually used with VIGVs.

“As LNG plant size grows, BOG management becomes more complex,” added Chaker. “You begin to see more problems with hydraulics, turndown and transient behavior in the compressor.”

Superalloys

Paul Lowden, Director of Operations at Luburdi Turbine Services, introduced superalloys (SAs), which he defined as the substrate materials that make the turbine hot section function despite operating in intense heat and stress.

From a thermodynamics standpoint, SAs are in what is known as a metastable condition. Diamond, for example, is a metastable form of graphite. What this means in terms of turbine materials is that they change over time by diffusion as the atoms rearrange themselves. “The hotter it gets, the more rapidly they change,” said Lowden.

When loads are applied, these materials can suffer from various types of deformation. Elastic deformation is where the metal stretches reversibly, while in plastic deformation, the crystal lattice planes slide over each other resulting in permanent deformation. “Creep is time dependent plastic deformation,” said Lowden.

Alloys are created in an attempt to combat these issues by mixing two or more elements. There can be substitutional alloying elements which replace a nickel atom in the crystal lattice, interstitial alloys alloying elements where additional particles go in between the gaps in the base metal’s atomic structure, and finally compounds which create a new crystal arrangement.

"Compounds have dramatically different properties to their forming elements and can add significantly more strength,” said Lowden.

The structure of most metals is made up of individual crystals or grains having different lattice orientations. Both the size of the grains and the properties of the grain boundaries between grains are the weak links and exert influence over mechanical properties.

Cobalt- (CO) and nickel- based (Ni) superalloys provide a unique combination of oxidation resistance and strength which is why they are often deployed in turbines. Other materials such as Molybdenum (Mo) have higher temperature strength, but are not oxidation resistant, while aluminum (Al) offers oxidation resistance but is very weak at high temperatures.

"Superalloys can have 10 or 15 elements,” said Lowden.

Chromium (Cr), he said, is good for oxidation resistance, while Mo and tungsten (W) add strength. Al and titanium (Ti) help harden. Carbon and other elements can also be added to strengthen grain boundaries. Due to these factors, SA ingredients are selected to take advantage of their various strengths and minimize the impact of their weaknesses.

Lowden also mentioned techniques such as solid solution strengthening where material such as tungsten with a large atomic diameter is subjected to a Co or Ni solvent. Precipitation hardening can also be used, for example, with Ni3Al being used in nickel-based SAs.

"Grain boundaries can be strengthened by the precipitation of hard carbides,” said Lowden. "You can lock convoluted grain boundaries together by controlling the heat treatment.” While a fine grain is best for toughness and fatigue resistance, there are other strategies. Single-crystal casting, of course, eliminates grain boundaries and directionally solidified blades have high creep strength by controlling their columnar orientation.

As for oxidation, Lowden said this was less of a concern in the metallurgy of SAs, these days. Typically, protective coatings such as MCrAlYs or aluminides are the preferred approach to providing oxidation resistance.

GT maintenance

Ron Natole, owner of Natole Turbine Enterprises, delivered a primer on GT maintenance. Beginning with an overview of the GT market, he pointed out that there are about 30 models and over 12,000 turbines,10MW or larger, in operation worldwide.

Following a major blackout in the U.S in the sixties, over 3,000 peakers were installed; many have run fewer than ten hours per year. Over many decades, they may only have accumulated a few thousand fired hours, he said. Despite being 40 years old, they may not yet have had their first major overhaul.

“Many of these rusty old peaking turbines are being repaired so they can demonstrate black-start operation to serve in the East Coast capacity market,” said Natole.

The point: inspection and maintenance intervals depend on the GT’s application — base load, intermediate or peaking — and if maintenance is being done on the combustion area, hot gas path or other components. The combustion section, said Natole, is inspected and overhauled most often as it is the hottest part.

Further maintenance factors are load cycle, water or steam injection, environmental factors, prevailing maintenance practices, fuel type and starting frequency. A GT running on distillate rather than natural gas, for example, means shorter maintenance intervals, while frequent starts and stops might require adding maintenance intervals. “You might have to set your interval at every 3,000 hours rather than every 12,000 hours,” said Natole (Figure 3).

Over time, as components have become more complex and firing temperatures have risen from 1,500°F to as high as 2,600°F, different materials, cooling holes, and greater geometric designs have shifted maintenance practices. “Those using an earlier class of turbine shouldn’t transfer their maintenance practices to newer F-class models,” said Natole. “There are many special coatings and alloys, and more rigid inspection time frames required for newer materials. Further, a lot of these parts are not repairable due to the number of cooling holes and their overall structure.”

There is an ongoing debate about the virtues of the latest brand of turbine controls versus older analog systems (Turbomachinery International Nov/Dec 2013), Natole’s take is that today’s control systems are much better, but things are happening so fast that alarms are sometimes too late to save the turbine. He cautioned users not to become too dependent on alarms. Even if something has not reached an alarm point, one can see it trending towards trouble, such as a cooling problem.

He ended by advising attendees to take a close look at who they are trusting with repairs. “Repair shops are not created equal,” said Natole. “Even those that have a good reputation, may not have experience in a specific model of engine or technology.”

Changing fuels

“Fuel type influences price, quality, availability, reliability and emissions,” said Andrew Bromley of Turbotect, during his seminar on GT fuels. “Distillate fuels are twice or three times the price of residual grade fuel oil.”

Each fuel has unique chemical and physical properties that can influence performance, he said. They may, for example, have an abundance of trace metal contaminants that can cause hot gas path corrosion. He stressed that the fuel selected must be appropriate to the GT model and the intended application.

For example, combined cycle plants using heavy fuel oil must consider that power degradation will inevitably occur due to ash deposition, and periodic shut downs will be needed for GT washing and ash removal. He also noted that heavy fuels are normally not allowed for advanced GTs or aeroderivatives. Older, lower firing temperature machines are usually better for such fuels.

But then again, it all depends on the unique properties of the fuel. For example, Arabian Super Light (ASL) is a light density, low viscosity crude oil that is now being produced in Saudi Arabia. ASL contains virtually no heavy metal contaminants, such as vanadium, so ash deposits are not created during combustion.

For this reason, it is being successfully used as back-up fuel on gas-fired F-class gas turbines in Saudi Arabia. However, because ASL has low viscosity, the use of a lubricity improvement fuel additive is recommended for reliable operation of lubrication-sensitive components such as flow dividers.

Natural gas is the fuel of choice if it is available. Pricing wise, it costs around a quarter of the price of crude oil and about a third the price of residual oil as regards $ per MMBtu (Figure 4), but prices vary widely from region to region and according to availability.

Figure 4: Fuel cost comparison ($ per MMBtu)

A shortage of natural gas can drive plants towards alternate fuel sources, such as crude oil or fuel oil. Many projects in Iraq, Bangladesh, Sri Lanka, Pakistan and Morocco use heavy fuel oil due to natural gas shortages or because of disruptions to the gas transmission network. India, on the other hand, uses naphtha, a plentiful byproduct of its many refineries. Crude oil is also widely used in Saudi Arabia which contains the bulk of the world’s 450 crude oil- and fuel oil-fired turbines.

Bromley believes the number of heavy fuel oil projects is inversely proportional to the number of GT sales. Those deciding to look at this area are advised to pay close attention to the contents of the fuel which can vary widely. Crude oil, for example, can contain water, salts, trace metals, asphaltenes and solids. And with residual fuel oil, all contaminants in the crude feedstock are concentrated.

“Any trace metals in fuels will cause ash in the GT,” said Bromley, “and this can lead to high temperature corrosion if appropriate fuel treatment procedures are not applied.” Operational problems can also occur if incompatible fuel oils are blended — for instance coming from different refineries. Asphaltenes can precipitate, causing sludge and plugging of fuel filters and flow dividers.