A Twist for Surgical Robotics

Since 2000, California-based Intuitive Surgical has dominated the world of robot-assisted surgery. Its da Vinci system has logged well over two million surgical procedures.
Now, however, a Massachusetts startup called Medrobotics has launched a system of its own that features a design that could make robots a much more common fixture in operating rooms.
Rather than been being limited to straight, “line-of-sight” procedures as in other surgical robots, the company’s Flex Robotic System enables surgeons, with full HD visualization, to guide the device through the twists and turns of the body’s pathways. Result: minimally invasive surgery can be performed on sites that were difficult or even impossible to reach before.

All the key components of the Flex Robotic System can be set up for surgery in about 10 minutes, say surgeons. The disposable Flex Drive (blue component) attaches to the Flex Base and is inserted into the patient.
“With this system, we can now reach areas, such as the top of the voice box, which were very hard to access,” says Eugene Myers, M.D., a head and neck surgeon and professor at the University of Pittsburgh Medical School. “For example, this can mean the difference between a person losing his voice or retaining it.”
So far, the Flex Robotic System has received both U.S. Food and Drug Administration approval and Europe’s CE mark for transoral procedures of the mouth and throat. In October 2016, the system also gained European clearance for colorectal surgeries, expanding the range of operations. Costing about $1 million--around half the price of the latest da Vinci model--the system offers a relatively small footprint and a reported 10-minute setup time in operating rooms.
The device is gaining recognition, including a Best of Show award in the 2016 Medical Design Excellence Awards, and a 2016 Red Herring Top 100 Award, which recognizes technologies most likely to change people’s lives. Meanwhile, the technology has attracted about $150 million in investment for Medrobotics.
“The global market for robotics and other tools for minimally invasive surgery is expected to double by 2020,” says James Jordan, chief investment officer for Pittsburgh Life Sciences Greenhouse, an early investor in Medrobotics. “But that type of increase is not likely to occur without flexible robotics technology.”
Roots at Carnegie Mellon
The Medrobotics story began at Carnegie Mellon University’s Robotics Institute, which has spawned robotics innovations ranging from devices to inspect power plants and explore space environments to autonomous tractors and self-driving cars.
In the late 1990s, Robotics Institute engineering professor Howie Choset became fascinated with what has become known as “snake robots,” highly-articulated mechanisms with many degrees of freedom. These robotic snakes can thread their way through tightly packed areas in such applications as search and rescue missions and the manufacture of giant aircraft wings.
Along with fellow CMU engineering professor Alon Wolf and heart surgeon Marco Zenati, M.D., Choset developed the first prototype of a snake robot for minimally invasive surgery.
Together, the three men founded Medrobotics in 2005. They initially targeted the device for heart surgery, and it was used successfully in a limited investigational trial on three patients in Europe in 2010. It became apparent, however, that the high cost of developing a commercial device for cardiac applications, as well as the regulatory hurdles, dictated moving to other surgical procedures for faster commercial launch.
Noting that commercializing a surgical device is “not something that a robotics professor can accomplish with grants to the university,” Choset knew it was time to hand off his invention to Medrobotics. The company not only had to raise an enormous amount of capital for development and manufacturing infrastructure, but its engineers had to refine Choset’s design, both to pass muster with regulators and to meet the clinical demands of surgeons.
“The genius of Professor Choset’s design was his revolutionary motion system, specifically the relationship of the inner and outer mechanisms of the robot that allows the articulation to occur, just like a snake,” says Samuel Straface, a Ph.D. and former scientist who serves as president and CEO of Medrobotics. “Our engineers had to transfer that know-how into a viable product for clinicians.”
Follow the Leader
The Flex Robotics System that received FDA clearance in July 2015 combines the benefits of a laparoscope – a rigid, straight-viewing device used in minimally-invasive surgery -- with an endoscope, a flexible instrument for visual diagnosis.
Much of the device’s intellectual property revolves around the disposable Flex Drive, which snaps into a reusable base that contains system motors and controls. For transoral applications, the portion of the 1.5-lb drive inserted into the patient and containing the camera measures about 17 cm long, with a diameter that ranges from 17.5 mm to a maximum of 28 mm at the “distal end” where the surgery takes place.
The drive contains an external cable for electrical connections to the distally mounted, detachable camera and LEDs, plus an internal lumen that serves as a channel for tubing that carries fluid to a lens washer for the camera. The Flex Drive also contains accessory channels on either side of the camera to accommodate miniature instruments that the surgeon inserts during an operation.
At the heart of the Flex Drive’s technology is its motion system, which features a series of articulated segments or links. The design includes both inner and outer articulated segments, made of medical-grade polymer and arranged in a concentric mechanical assembly.
The control scheme causes each of these two assemblies to become semi-rigid or flexible by adjusting the tension on cables running through the segments. The outer assembly can also be articulated by varying the tension on its cables when part of it moves beyond the distal end of the inner assembly.
Using this ‘‘follow-the-leader’’ strategy and alternating flexible and rigid states, the drive moves to the surgical site under the direct control of the surgeon. Once positioned, the device can become rigid, forming a stable surgical platform.
A key distinguishing feature of this flexible robot is that it offers multiple degrees of freedom distributed across its length. When compared to the typical human arm’s 7 degrees of freedom, the device’s more than 30 degrees of freedom may enhance the physician’s ability to access, visualize, and perform surgeries.
And unlike other robotic systems, where the surgeon sits at a console several feet away from the operating table, the physician sits or stands next to the patient and uses a joy-stick-like controller on an adjacent console to move the inserted scope to the surgical site. A touchscreen monitor shows system operation, and a video display provides real-time, 3D visualization of the surgery.
Once the robot reaches the surgical site, the surgeon manipulates two other controllers, one in each hand, that control tiny instruments inserted through the Flex Drive’s two accessory channels. For example, the instrument controlled by the left hand might grasp a piece of tissue, while the instrument controlled by the right excises tissue for a biopsy. What’s more, the surgeon gets haptic feedback throughout the procedure.
Meeting the Engineering Challenge
Bringing the Flex Robotic System to its current state of the art has required a strategy of agile engineering, notes R&D director Russell Singleton, a Ph.D. electrical engineer whose career has included many years with startups. “The approach is to get the design out as fast as you can, then do more iterations as you go along, based on the needs of the application.”
The team also features a blend of engineering disciplines. “There is no mechanical engineering department, or electrical engineering department,” notes Singleton. “We are organized by project or program, and we encourage the cross-fertilization of ideas.”
Singleton says he prefers to buy as many components as he can off the shelf. However, the unique demands of this flexible system have dictated several homegrown designs. Since the second half of 2014, when the Flex Robotic System debuted in Europe for transoral applications, company engineers have made significant changes in the design of key components.
The “chip-on-tip” camera, which snaps onto the distal end of the Flex Drive, has evolved from a disposable, 800 x 800 pixel 2-D version to the current dual 1920 x 1080-pixel, high-definition 3D model that can be detached, sterilized and reused.
With a goal of creating a much more mobile and nimble platform, engineers have also reduced the size and weight of the Flex Drive to one tenth of what it was in 2014. In addition, based on the input of surgeons who work closely with the company, engineers have created a family of miniature surgical instruments that are disposable and measure just 3.5 mm in outer diameter. Some specialized instruments for operating on the voice box have diameters as small as 1.5 mm. Moreover, engineers have had to design these instruments to be flexible, since they too must navigate through channels that bend and twist to fit the body’s anatomy.
“Medrobotics’ engineers are not only dedicated and talented, but they are used to fast turnaround,” notes Marshall Strome, M.D., a professor and chairman emeritus of the Cleveland Clinic Head and Neck Institute. “I recall cases where I asked engineers in the morning to modify an instrument, and they had a new design to show me by the end of the day.”
The engineers also must tackle the tough task of developing motion control and path planning algorithms, a particularly challenging job in snake robot systems with their multiple degrees of freedom. Singleton cites MATLAB as a useful software package to model and simulate motion and path planning. Among other key tools: SOLIDWORKS for 3D CAD modeling of components and Altium for printed circuit board design.
The distal end of the Flex Drive includes the camera, LEDs, and accessory channels through which the surgeon inserts custom-designed instruments
Ramping Up for Production
On the manufacturing side, the Flex Robotics System must run a gauntlet of tests needed for regulatory approval. The battery of tests described in the company’s 510 (k) premarket submission to the FDA in 2015 include: electrical safety testing, biocompatibility, sterilization and packaging, animal studies, and human factors testing, among other things. As production ramps up, clean rooms, both at Raynham and offsite, will be used to assemble the camera, as well as disposable components, such as the Flex Drive and surgical instruments.
Michael Gallagher, the Medrobotics’ vice president of Operations, says the Flex Robotic System requires about 3,000 components, which presents challenges in materials and resource planning. Continuing design changes trigger additional adjustments in manufacturing.
For example, the move to 3-D vision requires changes in the Flex Base, such as replacing multiple circuit boards and machining of the frame. Meanwhile, field upgrades to systems installed in hospitals have to be managed. “It’s a very fast-paced environment, like being strapped to a rocket, and you have to be agile,” says Gallagher.
To manage such tasks, Gallagher cites QAD software as an important tool in materials and resource planning. In 2017, the company plans to introduce Omnify product lifecycle management software, a package that includes quality assurance features. On the test side, Gallagher points to software packages like LabVIEW TestStand and Euresys, both used in important tests of the Flex Drive’s camera lens.
Throughout the system’s development, Medrobotics has embedded manufacturing engineers and supply chain managers alongside design engineers on project teams, says Gallagher, both to optimize components for production and test, as well as to ensure better resource planning. For example, engineers configured complex boards on camera assemblies for more efficient testing on bed-of-nails fixtures. To reduce costs and simplify production, the engineering team completely redesigned the Flex Drive, replacing custom-machined components with injection molded parts.
Growth Spurt
As Medrobotics positions its system for new surgical applications, engineers will need to make further design and manufacturing changes, including the size and shape of the Flex Drive, new types of surgical instruments, the configuration of special retractors and fixtures through which the drive enters a patient, and motion control and path planning.
Still, the company is confident that it will play a major role in the growth of surgical robots in the years ahead. Nearly every day, surgeons can be found at the Raynham facility, where four complete systems have been set up to acquaint medical personnel with the technology. Surgeons, such as Dr. Umamaheswar Duvvuri of the University of Pittsburgh School of Medicine and Dr. David Goldenberg of the Penn State College of Medicine, are demonstrating the system to interested surgeons in surgical settings.
In addition, surgeons in Europe are helping to prove the technology’s viability. In July 2016, Medrobotics released the results of clinical trials involving 80 patients with throat lesions at four hospitals in Germany and Belgium. Using the Flex Robotic System, surgeons accessed the target area in 94% of the cases. No device-related adverse events were reported
Surgeons familiar with the system point to such benefits as being able to see behind anatomical structures, such as organs, to detect hidden tumors. They add that flexible robotics also helps them perform more precise surgery, which can mean reduced need for chemo and radiation therapies in cancer patients.
Encouraged by the response from the medical community, CEO Straface expects to see several hundred hospitals worldwide using the Flex Robotic system within the next three years. Beyond transoral and colorectal applications, he sees the device being used in an expanding array of procedures, such as gynecological surgery. Eventually, he believes surgeons will choose flexible robotics for operations on the beating heart.
With an intellectual property portfolio that includes 200 patents issued or pending, Straface says his company has achieved “hard won” leadership in the field of flexible robotics for medical applications.
“Analysts are predicting a mammoth rise in the adoption of robots for surgery,” says Straface. But for that to happen, he says that a paradigm shift must occur so that surgeons “are no longer limited to line-of-sight robotic technology.”


Engineer's Guide to Corrosion: Part 3

Materials may range from mild steel in condensate/feedwater systems and waterwall tubes to high-alloy steels in superheater/reheaters and turbines. Alloying elements obviously have a strong impact on corrosion mechanisms that can affect the various steels. In Part 3 we will examine corrosion issues, and include a discussion of methods of control. Corrosion is quite a complex science, with steam generation corrosion being just one part of a huge mosaic.
Mild Carbon Steel Corrosion and Control - Feedwater
Iron is an amphoteric metal, meaning that both low and very high pH solutions will cause corrosion. Part 1 of this series outlined the basic corrosion mechanism in acid. At the temperatures common to the condensate/feedwater system and steam generator, general corrosion is minimized at a mildly basic pH.
For typical heat recovery steam generators (HRSGs) at combined-cycle plants, the recommended feedwater pH range is 9.6 to 10 (as measured at 25oC). The most common chemical used for feedwater pH control is ammonia (NH3), a weak base that generates hydroxyl ion (OH-).
Because feedwater is pure (or always should be), a common method to control ammonia feed is from specific conductivity (S.C.) readings, as in pure water a direct correlation exists between S.C. and pH, with S.C. being much easier to measure.
Beyond pH, however, another issue has arisen regarding flow accelerated corrosion (FAC). A future article will cover that topic, but it is worth mentioning here that in the condensate/feedwater systems of modern HRSGs (virtually none of which contain copper alloys) a small amount of dissolved oxygen (5 to 10 ppb in the feedwater) is required to generate the proper protective oxide surface on carbon steel. As we will review in that future article, FAC is an extremely serious issue.
From a materials aspect regarding FAC control, fabrication of FAC-susceptible components with P11/T11 or P22/T22 (1¼ and 2¼ chromium concentration, respectively) should be a standard consideration for all new HRSGs.
Yet another important subject, which requires a separate article for full discussion, is steam generator corrosion control when the unit is off-line. In these cases, oxygen is an enemy, and air in-leakage to cold units standing full of water can cause damaging corrosion.
A technology for corrosion protection that has been known for decades and is now re-emerging, is filming amine use. Amines are organic compounds with one or more ammonia groups attached. (Some small chain amines, known as neutralizing amines, are used in place of or along with ammonia as pH-conditioning agents.)
The amine groups and perhaps other functional groups on the chain attach to metals, while the hydrophilic organic chain establishes a barrier to water. In recent years, some positive results have been seen in full-scale tests of these products. Potential drawbacks still remain, including organic breakdown in high-temperature steam. Organizations such as the Electric Power Research Institute (EPRI) continue research into these products.
Boiler Water Corrosion Control
The ammonia (or perhaps neutralizing amine) used for condensate/feedwater conditioning can provide enough alkalinity to also protect the steam generator proper, as long as impurity intrusion is prevented. But condenser tube leaks will introduce a variety of contaminants to the condensate and the steam generator, including scale-forming ions and also chloride and sulfate, among others. The latter, and chloride in particular, can generate acids that quickly consume ammonia and leave the steam generator in a vulnerable condition.
So, basically from the advent of high-pressure power production, most steam generators (and here we are only examining drum-type boilers and HRSGs) are treated with an additional pH-conditioning agent. Most common has been tri-sodium phosphate (Na3PO4), which generates alkalinity as follows:
Phosphate will also react with hardness ions to prevent them from forming hard scale in the steam generator, but unfortunately, TSP has some drawbacks that limit its effectiveness. One of the most notable is its reverse solubility at high temperatures.
Tri-sodium phosphate solubility vs. temperature.
The compound cannot be dosed liberally, as most of it precipitates directly on boiler internals. Compounding this issue is that the TSP can react directly with the boiler tube material to form iron-sodium phosphate compounds. For these reasons, chemists at some plants have adopted straight caustic (NaOH) feed as the method to establish boiler water alkalinity. However, due to iron’s amphoteric nature, the caustic concentration must be limited to a maximum of 1 part-per-million (ppm).
At this point, another factor must be considered. Even in systems with good feedwater chemistry, iron oxide corrosion products still are generated and travel to the steam generator. At the high boiler temperatures ,the particulates precipitate, generally on the hot side of the tubes. Deposition sets up porous deposits, where water can penetrate the deposits through various channels.
An illustration of wick boiling.
As the water approaches the tube surface, temperatures increase. The water boils off, leaving impurities behind (wick boiling). The contaminants can concentrate to very high levels. One of the most frightening reactions possible is shown in the following equation:
As can be seen, a product of this reaction is hydrochloric acid. While HCl may cause corrosion in and of itself, the compound will concentrate under deposits, where the reaction of the acid with iron generates hydrogen, which in turn can lead to hydrogen damage of the tubes.
Under-deposit acid formation. Illustration courtesy of Ray Post, ChemTreat.
In this mechanism, atomic hydrogen penetrates into the metal where it reacts with carbon atoms in the steel to generate methane (CH4). Formation of the gaseous methane and hydrogen molecules causes cracking in the steel, greatly compromising its strength. Hydrogen damage is troublesome because it cannot be easily detected. After hydrogen damage has occurred, plant staff may replace tubes only to find that other tubes continue to rupture.
A tube failure due to hydrogen damage. Notice the thick-lipped fracture, indicative of failure with little metal loss.
For many decades, hydrogen damage has been at or near the top of the list of high-pressure steam generator corrosion mechanisms. What power plant personnel often fail to recognize is that even small condenser tube leaks (or other impurity ingress), if they are chronic, can allow repetitive under-deposit corrosion and hydrogen damage.
Under-deposit corrosion is not limited to low pH conditions. Per the wick boiling example outlined above, another compound that can concentrate within deposits is caustic. Concentrations may rise to levels many times that in the bulk boiler water. The concentrated NaOH attacks the boiler metal and its protective magnetite (Fe3O4) layer via the following reactions:
The potential for under-deposit caustic gouging is the primary reason why the free caustic concentration in the boiler water is typically limited to 1 ppm.
Key Takeaways
The reader will have observed a number of important items in this discussion, but several takeaways should be emphasized.
• The amphoteric nature of iron requires pH control within a rather narrow window.
• Deposits that accumulate within the steam generator greatly influence the corrosion potential due to their ability to allow concentration of impurities. That is why regular chemical cleaning is important, although this can be a difficult task in HRSGs with the complex evaporator tubing.
• Proper feedwater chemistry control is required not only to prevent FAC, but also to minimize carryover of particulates to the steam generator.
• Units should not be operated with condenser tube leaks unless the condensate system is equipped with a condensate polisher.
A Note on Steam Chemistry
Space did not permit a discussion about steam chemistry, but it is highly important. In fact, many of the guidelines for boiler water chemistry have developed around the prevention of impurity carryover to the steam system and turbine. The corrosion mechanisms that certain impurities can induce include pitting, stress corrosion cracking, and corrosion fatigue.
1. Comprehensive Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2013. 3002001381.
2. Buecker, B., and S. Shulder, “Critical Water/Steam Chemistry Concepts for HRSGs”; webinar for Energy-Tech University, Oct. 25-26, 2016.
3. B. Buecker, “Condenser Chemistry and Performance Monitoring: A Critical Necessity for Reliable Steam Plant Operation”; paper IWC 99-10 from the Proceedings of the 60th Annual International Water Conference, October 18-20, 1999, Pittsburgh, Penn.


Engineer's Guide to Corrosion: Part 1

Part 1 of this series provided an overview of a number of the most common corrosion mechanisms that can occur in steam generators at power and industrial plants. Here, in Part 2, we focus on the most common materials in these systems because an understanding of material constituents is needed for subsequent corrosion study.
The universal material for condensate/feedwater piping and boiler waterwall tubes is mild carbon steel, as this material often offers the best combination of strength and price. A transition from the mild steels to higher-strength alloys, including the ferritic and austenitic steels, is needed in superheaters and reheaters where temperatures are higher and the density of the cooling medium is much lower. The table outlines the chemical composition of the most common materials employed for steam generator fabrication.
Common steam generator materials. Adapted from Ref. 1
In considering materials for condenser and feedwater heater tubes, current practice calls for all-ferrous alloys, usually stainless steel. Some older power-generating units still have heaters and/or condensers with copper alloy tubes. Most common are Admiralty metal (70% copper, 29% zinc, and 1% other alloys, primarily arsenic or phosphorous), and the copper-nickel alloys, notably 90-10 and 70-30.
Turbine rotors and blades are composed of high alloy steels. Typical for turbine blades is 403 SS, which contains 12% to 13% chromium. A popular material for modern supercritical plant turbine blades is type 422, 12 Cr ferritic steel. HP turbine rotors are often ferritic steels containing 1% to 13% chromium and some nickel, molybdenum, and vanadium.
Alloying Elements in Steel
At first glance, the numerous elements that make up the steels outlined in the table may be bewildering. Most are alloying elements to improve various properties (toughness, resistance to heat, and so on) of the steel, while a few are impurities whose content must be carefully controlled.
Carbon: Carbon is the alloying element that defines steel, as opposed to plain iron. As stated in Reference 2, it is the “most important alloying element in steel.” Carbon increases the tensile and yield strengths of steel, although it also lowers the ductility and toughness. As is evident in the table, a relatively small percentage of carbon is necessary, and indeed will dissolve in steel, for alloying purposes. Above about 2% concentration, the element forms carbon nodules in the metal; these are the cast irons, whose ductility and strength decrease with increasing carbon content.
Chromium: Chromium is the element that in 12% concentration or greater imparts the “stainless” quality to stainless steels. The chromium causes steel to form a layer of chromium oxide that protects the steel from its environment (but with cautions to that chemistry as will be outlined later). However, even at lower concentrations than 12% chromium benefits also may be derived, including greater strength and toughness, and improved resistance to creep and oxidation at high temperatures. (Creep is the deformation of steel at elevated temperatures.) Also, even small amounts of chromium provide increased resistance of steel to flow-accelerated corrosion (FAC). [3] Both single-phase and two-phase FAC have plagued many steam generating systems where some failures have caused fatalities.
Nickel: Nickel improves toughness and in certain cases corrosion resistance of steel. But, it is most important when alloyed at 8% concentration or higher, as at this concentration and above, nickel causes the steel to assume the austenitic structure.
A brief discussion of the two most common steel crystalline structures will aid in understanding. When metals are fabricated from molten material, the crystalline structures that form grow into grains as the material solidifies.
Illustration of metal grains.
The body-centered cubic structure. Note the atom centered within the cell.
The face-centered cubic structure.
Grain size may vary depending on the metal or alloy and fabrication methods, for example, fast cooling, slow cooling, subsequent heat treatment. We will examine grain behavior later with regard to some special corrosion issues. For example, grain boundaries often serve as corrosion sites.
The crystalline structure of the metal within grains is also important. For the mild steels and even those that contain significant chromium, but without nickel, each unit cell of the crystal has a body-centered cubic structure, as shown.
These steels are defined as ferritic. If the steel is heated to a high temperature, this structure morphs into a face-centered cubic structure known as austenite.
In some cases, austenite offers better mechanical properties than ferrite. The addition of 8% or greater nickel to steel as an alloying element allows the steel to remain as austenite at room temperatures and below.
The table may now become more understandable with regard to the crystal structure of each of the steels. A third structure, martensite, can also be produced by special treatment, but it will not be considered here. However, note that a relatively new type of steel, the duplex alloys, are being used in some applications. These consist of a combination of ferrite and austenite. The duplex alloys offer some advantages, but at least one spectacular failure mechanism is known, in an air pollution control application, which will be outlined in a later article.
Molybdenum: Molybdenum increases steel’s strength and resistance to wear, among other benefits, but also contributes to high temperature strength.
Others: Elements like silicon and manganese can help protect steels against oxidation and problems related with residual sulfur in the steel, respectively. Phosphorus in low concentrations can increase steel hardness, but becomes troublesome in higher concentrations than those shown in the table. Other trace elements that may be beneficial, depending upon the alloy, include aluminum, boron, columbium, copper, nitrogen, niobium, and tungsten.
Some Mechanical Steel Failure Methods
The components of a high-pressure steam generator are subject to a number of mechanical stresses, including tensile, compression, and others. These often occur at attachments such as supporting infrastructure and tubing connections at headers, drums, and similar locations. Stressed locations are often greatly influenced by the high temperatures during normal operation and cyclic issues when units are reduced in load or come off-line, as has become much more common in recent years.
Several stresses will come into focus in later corrosion discussions. For example, some components that cycle frequently may fail from simple fatigue. Tube attachments are classic locations for fatigue. Corrosion fatigue, as the name implies, involves structural weakening by fatigue that then allows corrosive agents to accumulate in the weakened area, usually a small crack, and then increase the corrosion rate. Stress corrosion cracking (SCC) can occur in components; turbine blades are a prime example, in which normal operation induces stresses that allow corrosion to occur.
Another common phenomenon in steam generators is the deformation known as creep. Materials begin to lose strength at elevated temperatures; the ultimate example may be if the metal is taken to its melting point. But even at temperatures well below the melting point, the combination of heat and stress will cause metals to deform.
Very common to high-pressure steam units, and especially in superheaters and reheaters, is long-term creep. Over time, the material may deform or elongate to the point of failure. Corrosion often may play a part in long-term creep, particularly if corrosion products inhibit heat transfer. At the opposite extreme, if corrosion or some other mechanism causes a partial or full blockage of fluid flow in high-temperature water or steam-bearing tubes, short-term overheat may be the result.
Rapid tube failure. Note the thin-lipped structure at the failure.
Such dramatic failures may be corrosion/chemistry related, but also may be induced by operational upsets. In coal-fired power plants, firing a unit too rapidly is one possibility. Another, which has occurred over the years, is neglecting drum level during operation and allowing portions of waterwall tubes to go dry.
In Part 3, we will examine primary corrosion mechanisms and their control in greater detail. These issues are too-often overlooked, sometimes to the point of a catastrophic failure.
1. C. Bozzuto, Ed., Clean Combustion Technologies, Fifth Edition; Alstom, 2009, Windsor, CT.
2. J.B. Kitto and S.C. Stultz, Eds., Steam Its Generation and Use, 41st Edition; Babcock & Wilcox, 2005, Barberton, OH.
3. B. Buecker, “Flow Accelerated Corrosion – A Critical but Often Not Well Recognized Steam Generator Issue”; Engineering 360, August 2016.