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and carbon dioxide emissions. Diesel engines have a high thermal efficiency, which leads to low carbon dioxide emissions. The main problem of diesel engines is the emission of nitrogen oxides (NOx) and particulate matter (PM). These two pollutants are traded against each other in many aspects of engine design. Very high temperatures in the combustion chamber reduce PM emissions but, on the other hand, produce higher levels of nitric oxide (NO). By lowering the peak temperatures in the

PM (g/kWh)

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Example of SUMEBore™ coating equipment for remanufacturing of medium-speed diesel cylinder liners.

combustion chamber, the amount of NO produced is reduced, but the likelihood of PM formation increases.

An additional challenge facing the diesel car is the quality of diesel fuel. Sulfur in the fuel causes the exhaust to smell, is incompatible with the necessary exhaust aftertreatment technology, and can cause corrosion on the cylinder liner and the piston. The use of low-grade fuels can lead to serious maintenance problems because of their high sulfur content. The challenge is more pronounced in marine diesel engines, as the allowed sulfur content in the fuel is much higher than in fuel for on-highway applications.

The Sulzer Metco SUMEBore™ materials toolbox for cylinder bore coatings.

Legislation forces the pace

In recent years, legislation concerning the emissions of diesel passenger cars has become increasingly restrictive, especially for NOx and PM. This trend is predicted to continue in all different engine sectors as shown in . One way the car industry is reacting to these requirements is by pushing the use of lightweight components, replacing steel or cast iron. The substitution of cast-iron engine blocks is an important part of this development. Even diesel engines— which continue to gain a substantial increase in market share in Europe—are now being cast in aluminum, although diesel engines run at up to three times the pressure of gasoline engines. Progress in aluminum alloy development and new casting techniques lead to improved material properties that enable aluminum to meet the requirements. The ability to produce high-quality aluminum engine blocks opens a large window of possibilities, especially bearing in mind that nearly 50% of all new cars in Europe have diesel engines. Apart from reduced emissions, this growth is accompanied by requirements such as lower fuel consumption, larger power output and torque, and, especially in passenger cars, more compact engines due to space limitations.

Weight reduction

The highest weight reduction on a crank case of a passenger car can be achieved by completely replacing the cast iron through aluminum, also omitting any cast-in cast-iron liners. This task can be solved by applying a hypereutectic high- silicon-containing aluminum alloy, which is expensive, is difficult to cast, and requires special honing techniques or by using a cheaper aluminum alloy with good castability and putting a thermally sprayed, protective coating directly on the aluminum cylinder wall. This will minimize the necessary bore-to-bore distance and will lead to a very compact, lightweight design of the crank case. Here, the Sulzer Metco SUMEBore coating solution comes into play and offers a low-cost, mature technology that has proven its reliability and its effectiveness. More than three million cylinder bores have been coated over the past five to six years, mainly in diesel engines of passenger cars but also gasoline engines of highperformance streetcars and a variety of race engines from F1, GP2, NASCAR, and DTM to LeMans LMP1 diesel, MotoGP, and so on.

Wear in the cylinder

A thermally sprayed coating on the cylinder wall is exposed to a variety of different wear mechanisms at work in a diesel engine, such as scuffing and scoring, corrosion, carbon deposits on piston and combustion chamber, abrasion by soot and high ash content, and so on. Diesel fuel contains sulfur. Even low-sulfur diesel fuel contains 300 to 400ppm of sulfur. When the fuel is burned in the engine, this sulfur is converted to sulfur oxide (SOx). In addition, one of the major products of the combustion of hydrocarbon fuels is water vapor. Therefore, the exhaust stream contains NOx, SOx, and water vapor. In the past, the presence of these substances was not problematic because the exhaust gases remained extremely hot, and these components were exhausted in a gaseous state. However, when the engine is equipped with an exhaust gas recirculation (EGR) system to reduce NOx, the exhaust gas is mixed with cooler intake air and recirculated through the engine. The water

Coating of a cylinder bore of a diesel engine with the F210 APS gun.

vapor condenses and reacts with the NOx and SOx components to form a mist of nitric and sulfuric acids. This phenomenon is further intensified when the EGR stream is cooled before it is returned to the engine—cooled EGR. This can lead to significant corrosion damage in the cylinder in very short time as can be seen in .

The SUMEBore™ coating solution

Sulzer Metco offers a complete solution package to address such challenges in the cylinder liners. The package consists of a production system, which is highly stan-

Metallographic section of a corrosion resistant MMC coating for heavy-duty diesel applications (as sprayed).

Coating

Cast-iron liner

200 μm

dardized for this specific market segment, tailored coating materials, and extensive know-how for the industrialization based on more than ten years of experience with the SUMEBore technology in different markets. The market segments covered range from small engine blocks (passenger cars), to high-speed diesel engines/liners (trucks), up to mediumand low-speed diesel engine liners with bore diameters exceeding 500mm. An example of SUMEBore equipment designed to address the market for the remanufacturing of medium-speed diesel engine liners is shown in . Sulzer Metco has developed a full range of ther- mal-spray coating materials that can be applied on cylinder walls, all of them exhibiting low friction and offering protection of the surface against scuffing, corrosion attack, and abrasive wear. The materials toolbox is shown in . The materials can be applied to steel, cast iron, aluminum, or magnesium surfaces. These coating materials are used as powders, offering the highest flexibility of customizing the compositions to customer needs at the lowest cost. The powders are alloyed metal powders or metal matrix composites (MMC) blended from metal and ceramic powders or pure ceramic compositions. The most promising compositions—currently widely applied in the heavy-duty diesel market—are MMCs with a ceramic content of more than 30% by weight.

Materials and equipment

The materials are applied using an air plasma spray process (APS). There are a number of different plasma guns to best cope with the full range of the cylinder bore sizes. They ensure fast deposition of the coatings, high deposition rates, and a very stable process. The guns with the highest performance can spray powder in excess of 500g/min. The F210 plasma gun for the coating of small cylinder bores is shown in . A typical metallographic cross section of a corrosion-resis- tant and highly abrasion-resistant MMC coating on a cast-iron liner of a heavyduty diesel engine is shown in . Figure

shows a large liner of a diesel locomotive with a 317.5mm diameter bore after coating application using a high perform-

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Large bore liner (317.5 mm diameter) after coating.

ance plasma gun and a feed rate of 500g/min.

Sulzer Metco has the knowledge of the complete process from premachining, through washing, surface activation, plasma coating, and smooth-surface finishing by diamond honing (mirror finishing). In order to be able to offer the complete process, Sulzer Metco has entered into partnerships with suppliers of all necessary equipment that Sulzer Metco does not produce in house.

Sulzer Metco, with its global presence, is able to supply a customized, industrialscale SUMEBore solution package to almost any part of the world, including the necessary process support and maintenance on site.

Peter Ernst

Sulzer Metco AG

Rigackerstrasse 16 5610 Wohlen Switzerland

Phone +41 56 618 83 39 peter.ernst@sulzer.com

Pump arrangement for eight-effect evaporator.

of high-solid liquids and fiber suspensions in the pulp and paper and general industries. This experience and knowhow ensures that Sulzer pumps and process equipment also operate efficiently and reliably in biofuel applications.

Biodiesel is produced by a transesterification reaction from natural oil and fat to fatty acid methyl ester (FAME). Transesterification is the process of exchanging the alkoxy group of an ester compound by another alcohol. For this process, Sulzer Chemtech offers basic engineering and key equipment like packings and internals, static mixers, and heat exchangers. Using Sulzer technology, this process is flexible and can operate with varying amounts of methanol and water in the raw biodiesel. Both are unwanted pollutants and need to be removed from the feedstock. Sulzer guarantees efficient separation of biodiesel with a minimum methanol fraction. Worldwide over 60 biodiesel refineries rely on Sulzer packings, internals, and trays. Over 120 first-generation bioethanol plants have been supplied by Sulzer Chemtech worldwide. In the United States Sulzer Chemtech has a market share of over 60%.

Developing non-food sources

Second-generation biofuel manufacturing processes are in pilot and demonstration plant phases. Research into their production process is focused on developing technologies that can convert cellulosic biomass into transportation fuels. Cellulosic biomass includes agricultural or forestry wastes, such as corn stalks or wood chips, or energy crops like fastgrowing trees and grasses. Using cellulosic biomass as a source of new transportation fuels has obvious advantages, but these materials have different chemi-

cal structural bonds than food-based crops do, and these bonds are more difficult to break down. However, due to the availability of the feedstock, these second-generation fuels will play an important role in diversifying the world's energy sources and in curbing greenhouse gas emissions.

New processes

For the production of ethanol from cellulosic feedstock, two main processes are under evaluation: biochemical a and thermochemical b. In the first—the biochemical process—biomass is broken down to sugars using enzymatic and/or chemical pretreatment processes and then converted to ethanol via fermentation. This process is similar to the production of first-generation ethanol because cellulosic biomass contains sugars as well. However, these sugars are much harder to release than those in starchy biomass are, and some are difficult to ferment. A secondary product of this process is lignin, which can be burned to produce heat and power or can be converted to other fuels and by-products.

In the second, biofuels (and other bioproducts) can also be produced thermochemically from any form of carbon containing biomass. In this approach, feedstock is gasified at high temperature to convert biomass into syngas (CO and H2) which is then converted through various synthesis processes into bioethanol or biodiesel. This method is particularly important because as much as one third of cellulosic biomass—the lignin-rich parts—cannot be easily converted biochemically. At present, the total energy input needed for the production process may still be high, but in some cases, the biomass feedstock can provide most of the energy. Net CO2 emissions from

TRANSPORTATION

Ahlstar up

CPT

MBN

ZPP

MCE pumping, steam, and chemical mixing systems

In addition to pumps, Sulzer also offers equipment and other products that are key for these processes.

The separation columns in ethanol plants operate in extremely fouling conditions. Clean tray (top) and tray in the beer mash tower.

Sulzer V-Grid™ trays are the industry standard for bioethanol columns. V-Grid trays are highly resistant to the buildup of solids from the fermented liquid being fed into the distillation train. This quality extends the runtime and improves the economics of the plant.

Due to its unique geometry, Sulzer highcapacity MellapakPlus™ structured packing has proven very successful in reducing the capital and operating costs of ethanol plants. The packing is used in the distillation train and also in air pollution control.

ligno-cellulosic ethanol can therefore be almost 70% lower than from gasoline or first-generation bioethanol. This value could reach 100% if electricity co-generat- ed using the by-product lignin displaced electricity from gasor coal-fired power plants. Another very interesting thermochemical conversion process is based on pyrolysis and very intensively studied by several parties. The thermochemical biomass conversion process is complex, and it uses components, configurations, and operating conditions that are similar to petroleum refining. Consequentially, companies from the oil and gas field are also active in the development of advanced biofuels.

Experience from oil and gas

With their mature technologies in the oil and gas as well as the hydrocarbon processing industries, Sulzer Pumps, Sulzer Chemtech, and Sulzer Metco are well positioned to advance second-genera- tion biorefinery processes. Additionally, Sulzer Pumps’ R&D center for pumping and mixing technology in Karhula, Finland, has its roots in the pulp and paper process. The Finnish engineers focus on treatment and handling of highconsistency stock. This task requires pumps that are able to handle slurries and fluids with a high content of solids as well as mixers and agitators, which could be used in the fermenter tank. This specialized knowledge will support the development of fuel from feedstock such as algae, wood chips, or switchgrass, as each second-generation biorefinery will have need of 80–100 pumps, several mixers and agitators, as well as additional process equipment . Handling the ligno-cellulosic feedstock for ethanol production requires corrosion-resistant and wear-resistant materials, a field in which

Sulzer Metco has expertise. Sulzer Chemtech develops chemical processes in collaboration with clients and is already involved in pilot-plant activities regarding advanced biofuels . During its commercialization, a chemical process has to be transferred from the laboratory to industrial dimensions. Sulzer Chemtech has successfully scaled up processes for first-generation ethanol and other industries. It is now also ready to engage in the scale-up of cellulosic ethanol plants.

Strong growth expected

In view of the climate change and the need to reduce CO2 emissions, lawmakers all over the world support the development of these new biofuels. The European Union has announced a plan endorsing a mandatory 10% minimum target to be achieved by all member states for the share of biofuels in transport petrol and diesel consumption by 2020, which is subject to production being sustainable and second-generation biofuels becoming commercially available. In the USA, the Department of Energy (DOE) offers financial awards to evolve new technologies for developing cellulosic ethanol, and the government set the goal of annual production of 80 billion liters of advanced biofuels by 2022. Sulzer technology will support such initiatives and the development of environment-friendly technologies in pilot and demonstration plants and eventually in future commercial installations.

“If God had wanted animals to have wheels, he would first have built roads,”stated the American biologist Richard McCourt. It is true that the availability of roads or rails is almost essential to make efficient use of wheels; the harder and flatter the underlying surface, the less resistance is encountered during rotation. To find examples of rolling locomotion in nature, it is therefore necessary to look in areas with firm, flat surfaces such as savannas or deserts.

Rolling dung balls

This type of terrain is home to a creature that knows the secret of rolling: the dung beetles. They mold the dung excreted by herbivorous mammals into spherical balls, which it quickly rolls away using its hind legs and hides in an underground burrow. This dung serves as the beetle's own personal food supply or is used to rear young. The dung balls molded by dung beetles can easily reach a height of five centimeter—three times higher and 20 times heavier than the beetle itself. The efficiency of rolling transportation is clearly illustrated by the fact that the dung beetle can move its heavy load at the rapid rate of 20cm/s.

In 1990, the biologist Joh Henschel discovered a living wheel in the sand dunes of the Namib Desert in southwest Africa: Carparachne aureoflava, the wheel spider. The vast sand dunes are its habitat; it hides in burrows that extend diagonally into the dunes to a depth of up to 50cm. It is not easy to dig the narrow tunnels in the shifting sand but the spider succeeds by continuously lining the tunnel wall with silk to stabilize it during the burrowing process. To protect itself

Carparachne aureoflava, the wheel spider.

against predators, it seals the entrance to the burrow with a combination of sand and silk. It emerges at night to prey on insects for food.

As fast as a Ferrari

Wasps from the Pompilidae family are the deadly enemy of the wheel spider. The female spider-wasps search the dunes tirelessly for wheel spiders. If they discover a hatch constructed of silk, they break in. The spider in the narrow burrow defends itself using its front legs or rips the supportive silk amour from the walls if necessary. If the spider succeeds in repelling the first attack, the wasp becomes more determined and digs a crater into the sloping dune above the spider's hiding place. It will dig for hours in order to reach its prey in the moving sand. To mine a 15cm deep crater, it must remove 5kg of sand from the dune using just its legs—this is equivalent to 80000 times its bodyweight.

© dirkr, 2009 Used under license from Shutterstock.com

If the wasp succeeds in breaking into the spider's burrow, the spider makes a final bid for survival. It rushes past the predator to the edge of the crater, sprints part of the way down the dune, and suddenly flips onto its side, curling up each of its eight legs at the third leg joint to transform itself into a wheel. The spider then cartwheels down the dune on the rim of its leg joints, gathering increasing speed. According to Joh Henschel's measurements, it rotates up to 44 times per second—the same number of revolutions as the wheels of a Ferrari travelling at a speed of 300 km/h. The wheel spider can thus easily travel approximately 10m in 10s and will only uncurl its legs after a distance up to 100m, when it reaches the foot of the dune. The wasp appears to lose sight of the rolling spider and, despite being able to fly, is unable to relocate its prey.

Herbert Cerutti

A large steam turbine for a power generation facility undergoing a lowspeed balance at the workshop. This lowspeed balance stand has a capacity for rotating elements to 50 tons at 500 rpm.

All rotors are part of a machinery train, be it a generator, gearbox, compressor, or other mechanical assembly. The presence of an unbalance in any rotating component in the assembly may cause the entire train to vibrate. This induced vibration, in turn, may cause excessive wear in bearings, bushings, shafts, spindles, gears as well as the civil structures. The vibrations set up alternating stresses in structural supports and housings, which may eventually lead to total failure. Not surprisingly, the primary reason given by most of our clients for balancing a rotating element at the maximum running speed has little to do with the technical aspects of balancing or modal analysis. Rather, it is

reliability, availability, profitability, and efficacy of the business unit that is weighed to reduce the overall risks inherent in the operation and maintenance of high-speed rotating equipment.

Low-cost insurance

An at-speed balance of the rotor, as a part of the overhaul, is considered low-cost insurance. Shop balancing, by contrast, is usually undertaken at low speed— generally below 1500 rpm—mostly due to the perceived initial cost and time advantages compared to the efforts required to balance a rotating element at high speeds. Low-speed balance stands come in a variety of types and sizes

depending on the parts being handled and the measuring system being used. Most people are familiar with the balance machine used in a tire repair shop, where a lead weight is attached to the rim to compensate for an unbalance. A balanced tire gives the driver a comfortable ride on the road throughout the speed range of the vehicle. For most low-speed, rigid-shaft rotors with no adverse operational history, a low-speed balance in the workshop is usually sufficient . These rotors typically run at speeds that do not excite the first criticalspeed band . Most of the rotors in this rigid class operate below the influence of the second critical-speed band, some-

times called the first flexural mode, where the shaft takes the shape of a sinusoidal waveform, with identifiable nodal points outside of the radial journal bearing centerline, as is shown below

.

Mandatory for high-speed rotors

At-speed balancing is necessary for some rotors running below the second criticalspeed band and is considered mandatory for any rotor that operates within the influence of, or above, the second criticalspeed band. Some of these high-speed rotors also require additional tuning of the low-speed residual unbalance to allow the rotor to come through the first critical-speed band without excessive deflection of the centerline. Excessive shaft deflections during the start-up and shutdown cycles will open up the clearances of the shaft seals, can damage the bearings, and may include severe rubbing of the rotating element surfaces to the stationary parts of the casing. While all of the original equipment machinery manufacturers have their own at-speed balancing facilities, they are dedicated to processing the new rotors for their various product lines and providing a small number of aftermarket services for clients. Very few of these specialized balancing bunkers are available to the independent repair market at a competitive price. With the order books for new equipment at historically high levels, it is now harder than ever to schedule an opening at the OEM balance facilities that coincides with the clients’ overhaul cycle.

Extensive experience

Sulzer Turbo Services overhauls many rotating elements where the client has requested an at-speed balance as a part of the repair work scope or where the operational history suggests a benefit . It also engages in modernization work where seal and bearing design improvements can be demonstrated in the bunker before being placed into service. To better serve its clients, Sulzer Turbo Services operates two at-speed balancing facilities, including one of the largest and most advanced bunkers in the United States at its workshop near Houston , Texas, and a second facility in Winterthur, Switzerland. Both facilities undergo constant revisions and upgrades to keep their capabilities in step with advances in the industry. These two specialized balance bunkers provide reliable and timely service for all six of Sulzer Turbo Services’ strategically placed regional repair facilities, as well as to other OEMs and customers directly. Both are equipped with advanced electronics and diagnostics to provide state-of-the-art troubleshooting capabilities.

Generator fields

One of the areas we have developed specifically for generator fields is a procedure that allows us to complete shortedturn detection and analysis work using an air-gap flux probe that senses changes in the radial flux density as the rotor surfaces passes the probe. The resulting waveform highlights the presence of one

or more shorted turns by amplifying the minute variations in the flux slopes testing at the operational speed of the field. For this procedure, the field is operated at its normal running speed with normal excitation. This procedure identifies problems that can affect the proper operation and the formation of hot spots in the generator field that normally would not be seen until the unit was taken into operation. In many cases, the discovery of these issues at the job site would require a forced outage to make the required corrections.

Research and development platform

Because of the unique nature of the facilities, the bunkers are also utilized by our research and development departments of Sulzer Turbo Services, Sulzer Metco Surface Coatings, and our independent research arm Sulzer Innotec to develop and operationally test various sealing system configuration designs. Tests include measurements of the durability of various abradable coating systems applied to segmented seal rings for use in the next generation of steam turbines and industrial gas turbine engines.

Some elements that affect rotor balance

Rotors are built up in a variety of ways to accommodate the process gas or fluid, the stress levels acting on the parts, and the materials available for construction. Some examples are listed below:

First critical mode

30

20

Axial location (inch)

Second critical mode

30

20

Axial location (inch)

PANORAMA

•Solid forgings for steam turbines and hot gas expanders

•Forged shafts with shrunk-on disks, or impellers, only

•Forged shafts with all of the sleeves, disks, and other trim parts shrunk on

•Forged disks or packs of disks bolted together with one central or many peripheral tie bolts using a Hirth or Curvic type clutch fit to transmit torque between disks and to the shaft stub ends

•Welded cylindrical assemblies—these are usually found in very large steam turbines or high-speed axial-compres- sor rotors where the shaft body is

High-speed steam turbine, typical of the type requiring high-speed balancing.

made of hollow cylinders seam welded together by lasers to reduce the weight of the rotor assembly

• Lastly, a mix of any of the methods above

The excitation of vibration modes is then further complicated by differences in materials, material quality, rotor size, and stiffness. Any unbalance located along the rotor that coincides with deflection in the mode shape will amplify the deflection at those positions. Unless corrected, the unbalance creates a vibrating force on the bearings. The movement of individual disks or groups of disks, along the shaft or off centerline during

the operation cycle of the machine can become problematic, especially if the shift of the mass encroaches on an area where the deflection is amplified.

Quality assurance process

When a new rotor is manufactured, the OEM often balances the rotor at speed as part of the quality assurance process. The initial balance work provides the owner with a benchmark rotor signature to compare with any subsequent inspections. It is also important to consider items attached to the shaft, such as coupling hubs and thrust disks, as they can affect the balance and the response of the rotor. Progressive stacking and balancing of a rotor can provide a reasonable level of low-speed balance, and it is an important tool to ensure that the rotor is built with a low level of vibration. This method has been used by Sulzer Turbo Services many times to fix a rotor with a poor vibration history. Progressive stacking and balancing does not assure a smoothly running, completely assembled rotor because it does not account for the vibrations caused by system excitations as it is accelerated through the critical speed bands and up to its operating speed.

During the service inspection of a rotor, it is always worthwhile to check the weights added to correct the rotor’s inherent unbalance. The objective of any balance process should be to use the minimal number of weights to correct any unbalances in the rotor.

Running above the first critical speed

Many high-speed flexible rotors run above the first critical speed. With such rotors, it is also important to correct the unbalance response at the first critical speed for two reasons:

•The unbalance forces and resulting shaft deflections may be so violent that the rotor will not pass the first critical speed, or it may do so only after wiping out the shaft seals. Even minimal contact between the shaft and the seals will open the seal clearance values and decrease the efficiency of the unit. This also creates localized heating of the shaft, which can amplify the deflection at the first critical speed and may result in a permanent shaft bend.