Two-thirds of fuel used in manufacturing escapes as waste heat and, for many companies, this waste heat has become a way to reduce expenses and become more energy efficient. Many firms have taken to using waste heat to power (WHP) systems to recapture as much of that lost energy as possible. They have turned to a few reliable and some newer technologies to cogenerate power while their business performs its primary purpose, while others use waste heat to generate power.
Combined heat and power (CHP) systems are one way to reclaim fuel and heat lost in the industrial process. The options for combined heat to power are microturbines, steam turbines, gas turbines, fuel cells and reciprocating engines. For WHP CHP systems, the various cycle systems are Kalina, organic Rankine (ORC), and steam Rankine (SRC).
However, some of the more proven technologies are steam turbines or heat recovery steam generators among cogenerative CHP systems and the SRC or SRC system among WHP systems. There are other systems available to add WHP, mostly in the research and development phase. Steam turbines and SRC systems are of some of the most widely used and well-honed technology for heat to power systems. They are known for their adaptability and reliability. Each system is different, and each has its benefits and detriments.
Firms use steam turbines to create energy under CHP programs from under 100 kilowatts (kW) to more than 250 megawatts (MW). Durability and reliability are its strongest features. The implementation is typically cost-effective, and the biggest detriment to a steam turbine is that efficiency declines as power output is reduced.
Steam Rankine Cycle
Companies that have waste heat at temperatures more than 800 F are best suited to using SRC systems. For economical use of an SRC system, plants need to produce waste heat of a high enough temperature. Some organizations—like those processing products such as petroleum coke—produce well over 2,000 F of waste heat.
The maintenance of these ancillary systems is just as important as the primary system’s maintenance. In fact, by ensuring proper adherence to preventative maintenance, these supplementary WHP systems can allow businesses to continue to benefit from the power generation boon that waste heat gives them without overly taxing budgets.
Neglect of preventative maintenance of these WHP systems can create myriad problems for the steam system causing unnecessary system downtime and loss of the energy advantage that WHP systems bring. Typical failures can be severe, but are often easy to diagnose.
Typical Failures of Steam Turbine WHP Systems
Steam turbines are comprised of multiple systems of rotating and stationary blading. While the purpose of the stationary blading (nozzles, vanes, partitions and stationary blading) is to direct the flow of the passing steam, the purpose of rotating blading is to convert the steam energy into mechanical energy by way of speed and torque. These systems work together as part of the entire steam turbine system. The purpose of preventative maintenance of these systems is to work to avoid the most common and often devastating failures that are typical of these systems when neglected.
The most frequent and severe failure mechanisms of the steam turbine WHP system are based primarily on the blading functions. The seven most common are: corrosion, creep, erosion, fatigue, foreign or domestic object damage, stress corrosion cracking (SCC) and thermal fatigue. These can often be mitigated by regular equipment inspection following a maintenance schedule appropriate for the facility.
Of steam WHP systems, the most common failure events were blade and bucket failures. Those failures typically are the fault of stress corrosion cracking, foreign object damage and erosion. Before becoming a catastrophic failure, most issues would have been detectable during various system checks during regularly scheduled maintenance. SCC can lead to cracks in the blades due to corrosive materials, where erosion damage is created by fine debris or unwanted moisture.
The lube and bearing systems have the highest frequency of failures across all sizes of steam turbines. Often, these failures occur due to contaminants—including dirt, moisture and foaming. Filters can easily remove some of these impurities, but the removal of water from oil-based lubrication systems involves the addition of water separators and oil purifiers. This failure can be easily prevented with obedience to a quality maintenance schedule that includes checking the lubrication systems.
Performing maintenance checks and repairs on a regularly scheduled basis can be the difference between having major downtime from preventable failures and only having downtime when required by the maintenance schedule. Most major failures begin in stages where simple testing and checks would diagnose the start of what later became a major repair.
Steam turbines require inspection daily or multiple times weekly for oil and steam leaks. This is because moisture and lubrication loss failures can not only be some of the hardest to detect but also cause some of the most damaging problems that are hard to detect until they have become a problem.
Weekly testing is an important part of a steam turbine system cycle. Testing emergency backup and auxiliary lube oil pumps is imperative to ensure proper operation. The main lube oil take and low-pressure alarms for the oil, as well as the simulated over speed trip, also require weekly testing. There is also a need for weekly cycling of main stem stop and throttle valves, control valves and extraction valves. In addition, it is important to monitor trends weekly to let maintenance professionals know what may need further inspection to ensure proper operation.
Monthly, if not sooner, end users should sample and analyze lube oil and hydraulic fluid as well as cycle the valves. This ensures that contaminants, particulates and water have not infiltrated the lube oil and hydraulic fluid. During monthly fluid checks and valve cycling, check any systems that are deferred based on their continued reliability.
Yearly, there should be visual and functional testing of all valves, cams, rollers, rack and pinions, bearings, seals, lubrication systems, drain systems and other pertinent valves and devices for wear, damage or leaks. Furthermore, these systems, servomotors, and all instrumentation require inspection for any kind of thermal, vibration, or mechanical distress. There are many yearly checks and tests, but all are important.
However, a company can postpone some tests for up to three years if there is a primary system that is electronic and has an operating system (OS) test switch. If the system is not electronic or does not have an OS test switch, annual testing is the best practice.
This is not an exhaustive list. Each application and system will have its maintenance schedule based on either manufacturer, governmental or personal recommendations.
For minor outages and major overhauls, they can come between every two and 12 years depending on the history of problems, how extensive the previous tests were and the age of the equipment.
Overall, the most important factor in ensuring a lower frequency of outages is wise upkeep of preventative maintenance. Preventative maintenance is the difference between a major overhaul every three years and a major overhaul every 12 years. With minor changes in daily routine and a quick response to minor repairs, maintenance of steam waste heat to power and combined heat to power systems can be just as beneficial as the additional power they supply.