Daniel P. Duffy 2017-01-17 17:27:44
Every turbine differs in kind (steam or gas), power output, and size (from a small emergency backup generator to behemoth hydro eclectic dam turbines), operations (simple cycle or combined cycle), customers (local or general power grid), manufacturer, location, installation, etc. But the one factor they all have in common is the need for maintenance. Though each turbine’s maintenance procedures and requirements will vary with its size and complexity, adhering to these maintenance standards is necessary for their continued operation. The questions then becomes: What are the crucial maintenance procedures for today’s turbines? What best practices do manufacturers recommend? How can regular maintenance prevent downtime and reduce expenses? Maintenance vs. Repair Non-operators often get maintenance and repair confused. They are two different and opposed things. The first is to be planned for and encouraged, so that the second can be avoided completely. Maintenance occurs at regular intervals during the turbine’s operational cycle. It’s a cost of doing business that is factored into the overall operating budget and included in the power station’s balance sheet. As a proactive exercise, maintenance actually saves money. Repair, on the other hand, is never necessary unless lack of sufficient maintenance makes it so. In any case, it is never desirable. It is unscheduled and causes operational shut downs, scheduling delays, and unbudgeted expenses. Repair has a ripple effect as resources are suddenly delayed, denied, or reallocated to meet the emergency, a cumulative effect that badly hurts the bottom line. Repair is reactive and unanticipated. The very definition of “penny wise and pound foolish” is the neglect of maintenance in the foolish attempt to trim costs that inevitably results in a major budget-busting repair event. The operational versatility of a gas turbine presents unique maintenance challenges. Wear and tear may differ significantly with operational application. In addition to electrical power generation, these applications include aviation, processing plants, oil and gas, and small industrial applications. Furthermore, this turbo machinery is scalable and ranges in size from major power plants servicing large-area electrical power grids, to small backup emergency generators. Each kind of application will require similar kinds of maintenance, but the frequency and cost of each effort will vary considerably. Gas Turbines vs. Steam Turbines vs. Combined Cycle Technology For power generation applications, there are basically two main types of turbines: steam turbines and gas turbines. The older of the two, steam turbines have been around since the start of the industrial age. Steam turbine blades rotate when they are impacted by high-pressure steam. The steam is generated in a water boiler that is heated by a fossil fuel (typically coal), which flashes the water to steam, which escapes via a pipeline to the turbine. There is a wide variety of steam turbines from small units (generating 0.5 to 2.0 MW at 150 to 400 psi pressure and 500 to 750°F), to large turbines (up to 100 MW at 600 to 900 psi and 750 to 900°F), and large utility grid units (at more than 200 MW and over 5,000 psi over 1,300°F). Once the steam passes through the turbine, driving its fan blades and turning its shaft to generate electrical power, the steam escapes and is condensed back into liquid and returned to the boiler for reheating. The best analogy for a gas turbine is an airplane jet engine. A gas turbine compresses air in a compressor module, combines the compressed air with fuel (usually natural gas), and then ignites the mixture. The expanding torrent of hot gas passes through the turbine and like hot steam in a steam turbine, impacts the propeller blades. This again rotates the drive shaft and generates electricity. In power generating applications, the gas turbine shaft is either directly coupled to the generator shaft or indirectly by means of a gear box. A gear box is necessary to allow for variance between 60 or 50 hertz applications (though the use of a gear box will result in minor energy losses, approximately 2% of output). The hot gasses are much hotter than steam (ranging in temperature from 2,200 to 2,900°F) having greater energy per unit volume. To prevent the rapid decomposition of the turbine blades, they are cooled with air flows from small openings. This airflow creates a protective film on the blades’ surfaces to prevent direct contact with the hot gasses. So, a complete gas generator consists of the following components connected by a drive shaft: compressor module, fuel injection ports, combustor unit, and the actual turbine. In either case, turbines are machines that transform heat energy (indirectly from a boiler or directly in a combustion chamber) into kinetic energy (in the form of high-pressure gas moving at high flow velocities), and then transform kinetic energy (from the rotating drive shaft) into electrical power from an attached generator. Steam turbines run on the Rankine Cycle, while gas turbines operate on the Brayton Cycle. Though producing the same electrical power output, they vary greatly in application, operating efficiency, and type of fuel used. The efficiency of turbines and their power-to-weight ratios increase with size, which explains why they are not used to power small vehicles instead of internal combustion engines. At the very large end are turbines utilized by massive hydroelectric power dams. However, this article will focus on the midrange power generators and their maintenance requirements. The gas turbine has many advantages over the steam turbine in terms of flexibility. A gas turbine is simple to use and starts up quickly (without the time needed to heat up the boiler of a steam turbine). This long build up time for a steam generator means it can only provide the same flexibility as a gas turbine at the cost of low efficiency. Gas turbines are often built in a modular fashion, which allows for the removal, inspection, and maintenance or repair of individual components. This configuration also makes for easy replacement with substitute modules for a quick return to service. However, since the gas turbines operate at higher temperatures, their blades are often made from exotic alloys. And there is the problem of what to do with the excessive heat after the hot gasses leave the turbine. A combined cycle generator takes this problem and turns it to an advantage. By utilizing combined cycle technology, a gas turbine unit can generate 50% more electricity from heat—energy that would normally be wasted. In a combined cycle system, the combustion turbine generators work together with heat recovery steam generators and steam turbines. The hot gasses, normally vented out from the gas turbine, are captured by a heat exchanger that uses the exhaust heat to generate steam for the second cycle of power operation. The heated steam is then used to drive a steam turbine, and then condensed back into water and fed back into the heat exchanger unit to be recycled again as steam. Monitoring vs. Maintenance Management The first step in any maintenance program is a system of continuous and regular monitoring of the system’s performance. This allows for self-diagnosis in almost real time as to actual or potential problems as they arise. For electrical power turbines of all types, a large number of operational parameters will need monitoring. Starting with operational speed (as measured by RPM), an operator can keep track of the turbine’s electrical load or horsepower. Measuring pressures and operating temperatures at both the inlet and downstream from the turbine (and other turbines in combined cycle systems) will provide indications of changes to operating efficiency. Watching for significant expansions in turbine casings and blades will indicate the presence of material strains that could possibly lead to physical failures. Temperatures and pressures are also monitored at other locations such as sealing steam, condensate pump discharge, exhaust vents, and pipes. In addition to its temperature and pressure, the purity of the water and steam should also be monitored for the presence of impurities. Extraction pipeline thermocouples can be used to monitor the turbine for the presence of water induction. Water induction is the entrance of liquid that could damage turbines. This water can come from external sources, inlet steam that has not been completely vaporized, or condensed steam that has accumulated within the turbine shell and on the blades. Water induction is a frequent occurrence and can cause serious indirect corrosion or even water hammer within the turbine. The thermocouples are probes set inside the turbine casing or pipe that is being monitored. A paired heater and thermocouple are installed at the tip of the probe with the heater set at a hotter temperature than the incoming steam. Should water induction occur (the water would be much cooler than the heater), the probe’s temperature loses heat and an alarm sounds. Physically, the turbine’s operator must keep watch on its operation fluids (lube oil for the rotating shaft and hydraulic fluid). Changes in the pressure and temperature of these fluids would possibly indicate leaks and the potential for frictional failures or overheating. An indirect measurement of the operating fluids can be obtained by measuring the pressures and temperatures of the cooling water supply. This cooling water is used to radiate excess heat generated by these working fluids, a waste heat that is usually radiated away and is not typically reused like the exhaust heat. In addition to pressure and temperature readings, the vibrations induced by turbine operation can be indicative of trouble. The gearbox used to switch the frequency of the turbine from 50 Hz to 60 Hz can often grind and vibrate in a manner that suggest possible problems. The turbine’s bearing supports (foundations, bolts, struts, etc.) can vibrate in an eccentric manner. These are measured by shaft proximity probes located at applicable bearing locations. Measuring the bearing structure’s metal temperatures may indicate structural fatigue. Not only should the main support structures of the turbine itself be monitored, but the smaller bearing locations (like the pinions of the gear box) should also be watched. But monitoring is just the first step. Continuous, regular, and planned maintenance is a necessity for proper turbine performance. This maintenance plan should be guided and, if need be, modified according to the information provided by the monitoring systems. Implementation of this plan is the responsibility of the operator’s management system and associated organization. Documentation is at the heart of these procedures. Tracking, recording, and communicating the results of equipment inspections with follow up scheduling of maintenance tasks is managed via this documentation system. How the documentation system is maintained is almost irrelevant. Whether its sophisticated computer software or hand-written notes, what matters is that the management system is complete, accurate, and timely with clear lines of communication and responsibility. The system will also need to manage any outsourced tasks performed by contractors and ensure the on-time delivery of necessary spare parts from appropriate suppliers. Maintenance documentation should include plans for power outages that may result from turbine downtime during repairs. Also included in the documentation are equipment user manuals provided by manufacturers and suppliers. Each facility requires lock out or tag out procedures for the proper shut down and de-energizing of critical power supply and steam feed elements. Health and safety plans would be required to cover emergencies and how to respond to them. Turbine Failure Mechanisms: What Could Go Wrong? Anything with moving parts will wear out in time. But certain components of a turbine system are more critical than others, and are subject to higher risk. Quick failure can occur from running the turbine at faster-than-designed rotational speeds and resultant overheating. Impact and wear and tear caused by water induction also rates high on the list of things to avoid. Leakage of operating fluids (lube oil and hydraulic fluids) can result in frictional wear and tear, while long-term damage can result from contact with corrosive steam. Stuck valves can be unpredictable and result in immediate damage. The turbine component most vulnerable to wear and tear are the impellor blades as they receive the most normal wear and tear from intentional steam and hot gas contact, temperature induced stress and strain (especially gas turbines), impact cracking and chipping from foreign objects, or some combination of these failure mechanisms. Blade failure represents the majority of actual turbine failure occurrences and is the most common turbine failure mode. Inspection and maintenance of the condition of the turbine blades is essential to prevent failure of the entire turbine. These failure modes are shown in more detail in table 1. Associated moving parts are subject to similar failure mechanisms. These parts include rotors and shafts, shells and turbine casings, blade rings and diaphragms, and turbine discs. Though blading failure is perhaps the most common, another frequently occurring turbine failure involves the loss of operational fluids. The leakage of lube oil and hydraulic fluids can result in turbine and generator rubs, wearing out of the drive shaft and shot bearings. The most severe failures result from operating a turbine at higher-than-designed speeds, though this happens more often in smaller turbines than massive hydro or grid power turbines. Maintenance Procedures and Schedules By “continuous turbine maintenance,” the industry means exactly that. Maintenance and inspections are nonstop and never ending. Each day, the operator should inspect the turbine for escaping steam and leaking operational fluids. On a weekly basis, vibration readings are taken for the gear box and support structures. Also necessary each week is the testing of fluid pumps, fluid pressure alarms, and the trip switch that would prevent operating at too high a speed. Weekly tests should also include simulations of high-speed operations. The operating fluids should be sampled and tested monthly with throttle and control valves cycled to test their performance and prevent sticking. Each year testing and visual inspections should be performed for all operational valves, rollers, bearings, rack, and pinion linkages, and structural supports, seals, and filters. All instrumentation and controls should be inspected and tested annually as should gear boxes. Long term, the operator should plan and schedule minor outage for every two to four years, and major overhauls at three-year intervals. The minor outages involve more in-depth visual inspections with a borescope for corrosion and mechanical damage. These internal inspections should also be performed for all operational valves, alignment of the gear box and drive shaft, foundations, and anchoring or support structures. Major overhauls of the entire turbine system should be performed with a special emphasis on areas that have experienced failures in the past and items with a high-risk factor. Major overhauls are often performed on a case-by-case basis depending on the type of turbine, its application, and manufacturer’s guidelines. Major Service Suppliers Capstone Turbine is a major supplier of advanced microturbine technology backed by an extensive field support plan. Their Factory Protection Plan (FPP) minimizes downtime and fixes maintenance costs. It features all-in-cost protection support for any product-related issue in the field for up to nine years. These services are provided by a nationwide fleet of over 1,200 Authorized Service Providers (ASPs). The Capstone Service Network (CSN) monitors, records, and provides key data on operation and performance. Parts are provided by a network of authorized distributors with the parts inventory necessary to maximize field support and minimize parts delivery wait times. Regional factory service centers provide technical support, as well as parts and field services. Elliott Group offers a turnkey approach to routine maintenance, installation and startup assistance, planned outages, and complex repairs. These services include complete tooling, project planning, and scheduling. While in the field, in addition to standard maintenance and installation, they provide diagnosis and problem analysis, technical advice, and repair recommendations. They also provide services to other OEMs manufacturing multiple types of turbo machinery. Specifically, they repair and remanufacture all blades, impellers, shafts, seals, and bearings. Field welding and CNC machining services are also provided, including specialized parts manufacture if needed. PW Power Systems, Inc. (PWPS) is a world leader in developing and manufacturing energy solutions for power generation, offering products for industrial gas turbines. Through its world-class partnerships, PWPS provides complete project management, overhaul, and reconditioning of heavy rotating equipment to support planned and unplanned maintenance activities with specific concentration on the high-temperature “F” class industrial machines. The service teams of project managers, technical directors, shop personnel, and consulting engineers enable PWPS to provide true turnkey outage management by utilizing the latest in component repair technology licensed from Pratt & Whitney. For over 40 years, Solar Turbines has provided overhaul services for over 15,000 gas turbines and over 6,000 gas compressors. This extensive and complicated product line requires their in-house expertise for maintenance and repairs. Going beyond just repair work, Solar Turbine can restage gas compressors to increase efficiency and optimize turbine output speed. Their standard line of services includes startup and commissioning, process support for electrical and mechanical controls, and in-situ sub assembly replacement. DE Daniel P. Duffy, P.E., writes on topics of energy and the environment.
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