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Comment PDF Operations & Maintenance

Maintenance: Pursuing Improvements in Rotating Machinery

By Kirill Grebinnyk and Bob Krusemark, Sulzer |

The variety of rotating equipment used in chemical manufacturing creates the need for diverse maintenance methods. Using new techniques can greatly improve the reliability and performance of all types of rotating machines

The processes that are required to manufacture chemicals on an industrial scale rely on a large variety of rotating equipment to deliver the end product. From steam turbines and electric motors to compressors, expanders and pumps, every machine has a vital role to play. Keeping the manufacturing process running requires dedicated maintenance teams and expert knowledge to deliver reliability and performance.

Installed pieces of rotating equipment are operated for numerous years — decades in many cases — and at some point, will begin showing signs of age. In some cases, the original equipment manufacturer (OEM) may no longer support the older models or may even have ceased to exist. In other situations, the lead time for a major overhaul may not match the expectations of the plant owner. Whatever the case, modern design and manufacturing processes can be applied to time-served equipment and deliver cost-effective improvements.

 

Managing power and efficiency

From onsite generators to the numerous electric motors, reliable operation of electrical assets within a manufacturing plant in the chemical process industries (CPI) is vital. Unexpected failures can cause considerable inconvenience and lost production, which will impact revenue. So, once again, a preventative maintenance program can provide significant benefits by reducing downtime and improving operational efficiency.

The use of vibration analysis, combined with thermal imaging, which is used to identify imminent bearing failure, poor electrical connections and the imbalance of phase loadings, can produce an accurate indication of the overall status of the equipment. Additional testing of the electrical winding, especially partial-discharge analysis above 6,000 V, can also provide very useful information on the overall condition of the equipment, as well as an indication of its remaining lifetime. For larger electrical assets, a repair will usually be more cost-effective than replacement, while still offering the opportunity to improve on the original design.

Continuous advancements are being made in insulation technology, and they have allowed the same insulating properties to be delivered by a thinner layer of material than in the past. Using thinner insulation, engineers must apply good design practices to keep voltage stresses to acceptable levels, which helps to extend the operating life of the motor. When the insulation is optimized, the percentage of copper in the winding can be increased, which, in turn, can reduce operating temperatures or improve output, as well as energy efficiency.

Using the information gathered from the preventative maintenance routines, it is possible to plan repairs so that they coincide with planned shutdowns or outages. Working with independent repair specialists, new windings can be designed, manufactured and tested ahead of time, minimizing project duration and delivering longterm reliability.

Optimizing electrical performance

In the case of electrical assets, such as generators and motors, monitoring the condition of the stator and the insulation on the windings provides vital information about the future reliability of the machine. Electrical insulation degrades over time, and the speed of its deterioration is affected by the operating temperatures and stresses that it is subject to.

Situations of voltage imbalance, over- and under-voltage, voltage disturbance and operating temperature all play a role, so a proactive approach can offer considerable benefits. Optimizing supply characteristics can extend the service life of the equipment, while using advanced techniques, such as partial-discharge analysis on high-voltage machines, can provide vital information about the condition of the stator.

Since the breakdown of electrical-equipment insulation is a very gradual process, implementing a condition-monitoring program is an ideal method for optimizing asset performance. Data from resistance temperature detectors (RTDs), metering devices and partial-discharge couplers enable the operator of the machine to extend its service life to the optimal point where the windings need to be replaced. This “inside information” helps to avoid unexpected failures and allows the coil replacement to be scheduled along with other planned maintenance activities, minimizing disruption (Figure 1).

maintenance

FIGURE 1. Electrical testing of new coils ensures optimum performance

The refurbishment of stators and rotors as part of a planned maintenance scheme will provide a considerable extension to service life and, in many cases, it can also achieve improved efficiency. Enhanced design capabilities, production techniques and insulation technologies enable modern coils to have an increased copper content, compared to the original parts, which reduces heat buildup and improves energy efficiency.

Minimizing running costs

The operation of some equipment, such as pumps and compressors, demands considerable energy. It is commonly accepted that 95% of the annual running costs can be attributed to energy expenditure for these equipment classes. By improving the performance and efficiency of these machines, the running costs can also be reduced.

Rotating equipment that is in direct contact with the process media and under constant environmental attack presents a major challenge. It is possible, however, to reduce degradation to a minimum by selecting the correct protective coatings. Generally, turbines, compressors and pumps are all subject to a variety of environmental conditions that contribute to corrosion, erosion, fouling and various other process-related issues that will require maintenance intervention.

Many metals form oxide layers that adhere to and passivate the equipment surface to prevent further corrosion, but the change in the physical characteristics of the equipment surface significantly increases the frictional properties and thus decreases aerodynamic efficiencies. So, there is a balance to be struck between protection and efficiency, which is where advanced coatings can help.

 

Understanding the issues

The first step is to understand the operating environment of the machinery. From there, the sources of degradation can be classified, and specific coating systems can be used to increase efficiencies, lengthen the interval between scheduled maintenance and reduce unplanned outages.

In the harshest of operating conditions, solid or liquid particles can pass through the equipment, causing erosion. Erosion that results in moderate to severe material loss can change aerodynamic efficiencies significantly. If left unattended, erosion can even affect the strength of the critical components and lead to premature wear of a blade or vane, as well as failure in service.

In operating conditions where both erosion and corrosion are present, the latter can be the primary source of attack that initially weakens the substrate surface. This chemical reaction between the component surface and the fluid passing through the turbomachine’s flow path removes any passivation and leads to an increased rate of erosion.

 

Anti-fouling protection

Fouling in turbines and compressors can be caused by the presence of small particles in the ingested air streams, process gases or steam. Fouling can cause an increase in the surface roughness of the critical flow-path components, leading to a reduction in efficiency. This is a particularly serious issue in processes where sticky hydrocarbon aerosols are constantly present.

In situations where these circumstances exist, and their effect needs to be minimized, specialized coatings can provide improved anti-fouling protection. These can be applied to both stationary and rotating blades, as well as diaphragms, guide vanes, rotors and impellers. The exact composition of these coatings can be tailored to specific applications and will include an aluminum base coat for corrosion protection, as well as an inorganic sealer and a specialized non-stick final layer.

Typically, anti-fouling coatings will have a thickness between 75 and 125 micrometers and incorporate polytetrafluoroethylene (PTFE), which gives excellent chemical resistance in low to medium temperatures with a maximum operating temperature of 550°F (290°C). PTFE provides excellent protection from chemical attack by substances with a pH between 3 and 9, as well as resistance to many solvents and fuels. In applications that require protection beyond these limits, more specialized coatings can be created to provide enhanced defense against fouling.

Extending durability

Specialized coatings can be metallic, ceramic or any combination of the two that may be required to meet a broad range of physical criteria. Aside from protecting against fouling or corrosion, they can also provide protection against wear, high temperatures or direct chemical attack, as well as providing a substitute for chromium, which can become hazardous under certain circumstances.

In each situation, expert assessment and diagnosis can lead to a prompt proposal for a solution that will significantly improve the durability of the affected components. In many of the scenarios described so far, the use of specialized coatings can deliver a considerable extension in service life, as well as a more energy-efficient application.

Legacy equipment can benefit from the application of modern coatings, since some of the older materials may not have adequate properties to withstand the modern-day operating conditions. Minimizing oxidation and corrosion rates is an important step in reducing downtime and expenses for maintenance and therefore improving productivity and the asset’s return on investment.

This is most commonly achieved with coatings containing aluminides that are applied by a variety of thermal-spray or surface-deposition techniques. By using an aluminum base coat, corrosion-resistant coatings are designed to be conductive and provide cathodic protection against corrosion. The aluminum protects the less-active base metal, sacrificing itself by enabling electron flow from the aluminum to the base substrate, which becomes negatively polarized and therefore protected against corrosion.

Solid and liquid particle erosion or fretting wear can be minimized by using coatings that increase the hardness of the protected material surface. Stellite (cobalt-chromium alloy), chrome-carbide and tungsten carbide are examples of hard-face coatings. These exhibit hardness in the 50–60 HRC range on the Rockwell Scale, which is based on a material’s indentation hardness.

 

Power train maintenance

Some processes within the CPI, such as ethylene production, are particularly demanding in terms of power, and will often have several dedicated processing lines that include steam or gas turbines driving large compressors. These pieces of equipment require regular inspections and periods of planned maintenance to keep them operating reliably and efficiently.

Equipment such as large compressors will often have spare rotors that can be quickly exchanged during a planned maintenance period to minimize any downtime. This allows the rotor that has been in service to be repaired, and if necessary, upgraded, before being returned to operation during the next planned outage.

Compressors are subjected to some major mechanical and thermal stresses, which can affect the reliability of the equipment, especially as it ages. However, modern materials and design techniques can be used to return components to their original specifications or implement significant improvements in terms of efficiency and reliability.

maintenance

FIGURE 2. Experienced engineers are key to delivering effective repairs, and often these tasks require highly specialized equipment or facilities

Inspect and test

The process of inspecting and repairing compressor rotors requires considerable expertise, as well as access to highly specialized facilities to complete any significant repairs (Figure 2). Using a range of non-destructive testing (NDT) procedures, such as wet-magnetic-particle inspection and liquid penetrant inspection, it is possible to identify any defects in the components and to propose the most effective method of repair.

From this point, basic components, such as shaft-seal sleeves, split rings and stepped impeller keys, can be easily manufactured, but specialized tasks, such as improving the impeller geometry, will require greater expertise and advanced engineering. In addition to that, processes such as high-velocity oxygen fuel (HVOF) spray systems can be used to improve or restore a component’s dimensions and save significant expense by reconditioning existing components instead of replacing them altogether.

In situations where a component has failed, it is still possible to reverse-engineer it to manufacture a new part — even the major rotating or stationary components, such as impellers, inlet guide vanes or diaphragms. Independent turbomachinery maintenance providers offer the proper skills and resources to save a damaged shaft down to the chemical composition and mechanical properties of its base material by weld-repairing it and machining to the dimensions of the original part.

 

Compressor refurbishment

Used extensively in the chemical industry, large-scale compressors perform a particularly challenging role that places considerable stress on the component parts. Regular maintenance inspections and planned repair schedules are essential for continued reliable operation, especially for the rotors.

Eventually, the time will come when, due to wear, an impeller cannot pass an NDT procedure or becomes damaged, and new parts will be required. In the past, the lead time on these components could be considerable, but today, using laser scanning, three-dimensional computer-aided design (3-D CAD) and other numerical simulations that predict the stresses and flow conditions to which the component is exposed, it is possible to create high-quality precision parts with minimal delays.

In fact, it is often possible to refine the impeller design together with changes to the vane geometry and deliver a more efficient gas-path design. Combined with improvements in materials and manufacturing technologies, it is possible to deliver considerable benefits to older compressors.

For example, modern impeller components can be manufactured from a solid piece of a low alloy or stainless steel using five-axis milling (Figure 3) or electrical discharge machining (EDM). If required, a new operating curve can be produced to illustrate the improvements in performance and efficiency. This information can be used to highlight the cost-effectiveness of the project, as well as the return on investment.

maintenance

FIGURE 3. Five-axis milling is used for precision manufacturing of pumping impellers

Turbine renovation

Large-scale assets, such as compressors, require reliable and efficient power sources, often represented by steam turbines. However, these machines are also operating in quite challenging conditions and suffer from similar damage mechanisms to the ones described previously. Clearly, as with any vital asset, they need to be monitored by a series of vibration sensors as part of a preventative maintenance routine.

By building a longterm performance trend that includes all operational aspects of the turbine, it is possible to identify the most appropriate time to carry out planned repairs, optimizing time-in-service and avoiding unplanned outages. Again, this type of forward planning minimizes downtime by ensuring all the parts and resources required are available for an efficient and effective maintenance project.

 

Stress corrosion cracking

Older steam turbines are prone to stress corrosion cracking, which can be caused by a number of factors, and getting to the root cause can take considerable effort. However, investing the time in failure analysis pays off in the long run, as the performance and reliability of the turbine will be assured and the findings may also be useful for similar pieces of equipment.

Identifying the cause in each case requires considerable expertise in material science. In-depth examinations using scanning electron microscopes and other specialized equipment are often required to establish the root cause of any cracking. In some cases, the mechanical properties of the base material may have been out of specification, while in others, a combination of deteriorating mechanisms may create a situation that enables the progression of stress corrosion cracking.

When the time comes for turbine refurbishment, it is essential to engage expert engineers with the skills and equipment required to identify the best repair practices, manufacture new parts and rebalance the entire assembly. None of these tasks are simple and all must be completed to the highest standard in order to achieve a durable and robust repair.

 

Perfecting pump performance

Like a great deal of modern machinery, pumps are required to operate with minimal downtime and with extended maintenance intervals. Pumps in the CPI have to cope with a wide variety of media in many applications, which means that there can be numerous types and designs of pumps within one plant.

At the center of a pump’s design is the impeller, which provides an increase in the fluid head. As a result, impellers are subjected to the most damage, which can have a huge impact on efficiency, productivity and running costs. Therefore, it is very important to be able to source new impellers when they are required and, if necessary, take advantage of the latest improvements in design and materials technology to increase reliability.

As discussed with compressor components, creating replacement parts has become a much quicker process than in years past, with reverse engineering being used to design and manufacture improved components (Figure 4). Improvements to the flow geometry combined with the latest manufacturing techniques, such as using five-axis milling or rapid prototyping techniques to print casting molds, can reduce the lead time required to procure the replacement parts and deliver significant improvements in performance and reliability.

maintenance

FIGURE 4. High-quality, precision parts are required for refurbishment of pumps and compressors

Balancing issues

One of the most crucial stages of the repair process is the balancing of the rotating components. Rotors operate at high speeds and with small clearances in the bearings, as well as between the rotating and stationary parts, so precision balancing is essential for continued reliable operation.

Low-speed balancing is carried out as components are rebuilt, having been refurbished or remanufactured. For multi-component rotors, such as those used in turbines and compressors, the rotor is balanced at each stage of reassembly, before finally passing a rigorous quality-control inspection.

One of the last steps is to carry out an at-speed balance test, the parameters of which are normally agreed upon with the user beforehand. This requires a dedicated balancing bunker, equipped with specialized pedestals and frequently operated in a vacuum to reduce the run time and energy used during the test (Figure 5). Such specialized equipment is only operated by OEMs and experienced independent maintenance providers.

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FIGURE 5. At-speed balancing facilities ensure smooth operation in service

The at-speed balance test involves the entire rotor assembly running at maximum continuous operating speed (MCOS) and beyond, while being monitored for vibration through special bearing pedestals, which are equipped with accelerators or velometers and a set of proximity probes. A rotor has to be low-speed balanced, typically in the range from 500 to 800 rpm, before proceeding to balance it at the operating speed. The American Petroleum Institute (API), in its respective standards for turbomachines, including API 610, API 612, API 617, defines MCOS as 105% of the highest specified design speed of the rotor assembly, and that the trip speed should be 110% of the MCOS.

Throughout the high-speed balancing process, vibration levels are monitored, and state-of-the-art software can be used to determine the optimum size and position of any balance weights that need to be added. This procedure also relieves residual stresses introduced during the repair process and ensures continued smooth operation of the rotor while in service.

 

Additive manufacturing

Taking advantage of new technologies, such as additive manufacturing (also called 3-D printing), can deliver fast and cost-effective prototyping for design assessments, as well as one-off molds and cores for parts manufacturing. This enables pump components to be recreated with a geometry that exactly matches the application, which may have changed since the initial commissioning.

Combined with the use of specialist coatings, pump remanufacturing has the potential to deliver considerable improvements on legacy designs. Using modern materials and manufacturing techniques, it is possible to create assets that are much better suited to their applications, with improved efficiency and durability.

The final analysis

The logistics and planning for refurbishing any equipment should not be underestimated. From the field-service crews involved in the removal and re-installation, to parts remanufacturing and at-speed balancing, coordinating all of the resources for a single repair project takes considerable expertise.

By implementing and following a preventative maintenance program, it is possible to make significant improvements in performance, reliability and efficiency of critical plant equipment. At the same time, utilizing expert knowledge and state-of-the-art analysis techniques, it is possible to understand and address any equipment design issues or material deficiencies.

Edited by Mary Page Bailey

Authors

Kirill Grebinnyk is a lead engineer at Sulzer’s Rotating Equipment Services (RES) division working in the La Porte Service Center in LaPorte, Tex. (Email: kyrylo.grebinnyk@sulzer.com). With more than 10 years of experience in fluid dynamics, finite-element methods and design of turbomachinery, he is in charge of Sulzer’s program for aerodynamic rerates and upgrades of turbomachinery. Grebinnyk’s area of professional expertise includes failure analysis, structural and modal analysis and testing of rotating equipment. He is a licensed Professional Engineer in Texas and holds a M.S. in mechanical engineering.

 

Bob Krusemark currently serves as a manager and technical resource for Sulzer Electro Mechanical Services in Houston (Email: bob.krusemark@sulzer.com). He graduated from Texas A & M University in 1977 with a B.S. in industrial distribution. Prior to joining Sulzer, he spent 16 years with GE Motors and another 17 years with Westinghouse and Teco Westinghouse. He is a co-author of two previous IEEE papers “Recommendations for Design and Protection of Induction Motors for Extruder Applications” and “Induction vs. Synchronous Motors — Different Perspectives End User/Motor Manufacturer.” He has over 30 years of experience working with induction, wound rotor, d.c. and synchronous machines in the petrochemicals, pulp-and-paper, mining, cement and utilities industries.

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