Pushing machines to the limit: Transitioning to digital overspeed protection
Every process and/or utility turbine—whether it is fossil, hydro, nuclear, or alternative fuel driven—is designed to withstand the strain of rotating speeds up to a specified limit. Engineers involved need to understand rotating machinery designs and limits, conduct thorough analysis, and test for protection from overspeed catastrophes.
Operating machinery within designated limits leads to optimal performance and useful machinery lifecycle results. Running an engine too fast can lead to compromised employee safety, system failures, damaged systems, and unplanned downtime. Oil and gas and power industries recognize the destructive potential and related economic losses of overspeed events. Over the past five years, these industries have shifted their approach to managing this concern by adopting solutions that can help eliminate potential risk factors and improve the overall safety and efficiency of plant operations.
Primary risk is overspeed
An overspeed event occurs when the system operates above the maximum allowable continuous designed rotating turbine speed. The consequences of running the turbine beyond its allowable continuous design speeds varies by turbine type and model and depends on several factors, including the duration of the overspeed event and the speed attained.
Many turbine manufacturers typically design their rotors to be able to withstand up to 120% of the maximum allowable continuous operating speed. When a turbine operates beyond the designated operational speed limit, the stress on the machine may exceed the strength of the mechanical connection between the turbine blades and the rotor hub, resulting in operational failure and the potential release of blades from the rotor. With some engines, even a momentary overspeed event can result in greatly reduced engine life or even catastrophic failure. The Electric Power Research Institute (EPRI) estimates that the destruction of a large steam turbine, combined with the value of the lost power generation, can cost a plant more than $100 million (R. Torok, 2006, EPRI).
To prevent machinery from operating beyond the recommended limits, there are a few lines of defense. The first is the primary turbine control system, and the second is the failsafe, such as a mechanical emergency overspeed governor more commonly referred to as an “overspeed trip bolt.”
Typically, there are two types of primary control systems responsible for the first line of overspeed protection on turbine applications, mechanical hydraulic control (MHC) and digital electrical hydraulic control (DEHC).
The MHC turbine, which was the industry standard for decades, bleeds the emergency trip oil when an overspeed event occurs to reduce turbine speed. Most turbines with a MHC system have had one hydraulic oil system, where the same hydraulic oil was used for turbine bearing lubrication and the turbine control oil systems. Due to this combined use and the inherent introduction of particulate and water contaminants into the oil, these systems ultimately operated with poor quality oil, which in turn had negative consequences on primary and emergency overspeed control system elements. This has led to numerous instances where overspeed protection has failed to function properly over time in MHC applications.
On the other hand, DEHCs employ the use of sensors, electronic hardware, and customized software that allow operators to identify potential problems before they even occur. With DEHCs, plant operators can test their overspeed systems and controls, offline and online, without putting additional wear and tear on the system. These tests can be designed to occur at lower revolutions per minute (RPMs) speed and without the risk of shutting down the entire plant.
While a transition from MHC to DEHC may initially disrupt operations, the conversion eventually reduces scheduled maintenance and unplanned downtime, saving time and money. The transition period is also an opportunity to provide upgrades to the overspeed trip bolt and primary controls that fuel industry advancements.
Mechanical to digital benefits
In light of the benefits of adopting a DEHC approach, many oil companies have even instituted mandates to switch their operations from mechanical overspeed trip bolts to digital overspeed solutions. The move, fueled by plant operators seeking out more reliable solutions than their existing methods, allows operators to run their machinery and plants for longer periods of time between planned outages and helps them avoid unplanned outages. Planned outages typically occur every 4 to 10 years, depending on the industry and application.
Plant operators also need to conduct routine safety inspections and test overspeed controls without worrying about the risks associated with unplanned overspeed events. According to the EPRI, the insurance industry estimates that 50% of catastrophic overspeed events occur when the mechanical overspeed protection system is being tested (R. Torok, 2006, EPRI). New digital control installations and retrofits enable plant operators to virtually test their systems without physically pushing the machine to the limit.
Companies that elect to be members of the American Petroleum Institute (API) must comply with Standard 612 for steam turbines and Standard 670 for machinery protection systems. These standards mandate that the primary and emergency overspeed control systems must be electronic (rather than mechanical) and fully independent from each other. Furthermore, the emergency overspeed protection system must use two-out-of-three voting. Two-out-of-three voting permits operators to electronically simulate an overspeed event in individual channels while keeping the other two channels on-line to protect the machine. API 612 and other industry standards leave little room for continued use of mechanical overspeed protection systems in the future.
Operators still weighing the costs and possible downtime for upgrading to digital systems should consider the improved accuracy and reduced risk for costly outages with DEHCs. Mechanical overspeed systems require significantly more effort to install, maintain, and test. With digital, however, there is no need to manually test the system and risk it going into overspeed and possibly offline.
Detect, avoid overspeed
Overspeed prevention falls into two categories, detection and de-energizing the system. In the detection phase, the primary electronic control system and electronic emergency overspeed detection system independently monitor the turbine speed to ensure equipment is operating at acceptable levels. During an overspeed event, operators only have 10 milliseconds, according to the API 670 standard, to react and trip the bolt. In the oil and gas industry, the actual process of tripping the oil to stop the system from running is known as de-energizing the system. Reaction time varies depending on the mass and physical design of the rotor as well as the type of turbine. Control systems and overspeed protection system vendors differentiate themselves in the de-energizing phase.
Oil and gas and power generation companies want longer running reliability without sacrificing safety and the ability to detect latent failures in their trip system through online test capabilities. The costs to maintain mechanical systems and associated risks are higher than upgrading to digital. Downtime can cost a plant upwards of $1 million per day depending on the industry. Moreover, insurance companies and plant owners want assurances that personnel safety and equipment protection are routinely verifiable, which only digital systems can provide.
It’s important to consider the best provider to ensure operators’ plants receive the most innovative, high-quality digital control systems. Partnering with an original equipment manufacturer (OEM) that understands rotating machinery design and limits at a detailed and fundamental level and can conduct thorough analysis and testing for the unit will guarantee high-quality protection from overspeed catastrophes.
– Darren Herschberger is a product line leader for GE Measurement and Control, a division of GE Oil & Gas. Jim Blanchard is product line manager for GE Oil & Gas. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, email@example.com .
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Citations: Torok, R. (2006). “Turbine Overspeed Trip Modernization.” EPRI. Web.
Darren Herschberger is a product line leader for GE Measurement and Control, a division of GE Oil & Gas. Darren assists customers with steam turbine-excitation and plant conversions, modifications, and upgrades. Darren has more than 20 years of experience in the oil and gas industry and is an expert in engineering, sales, and technical applications. Darren is based in Longmont, Colo., where GE engineers control solutions for large steam turbines in the fossil and nuclear fleet. Darren has a master’s degree in business administration from Indiana Wesleyan University in Marion, Ind.
Jim Blanchard is a product line manager for GE Oil & Gas, a provider of advanced technologies and services across all segments of the global oil and gas industry on land, offshore, and subsea. Blanchard is responsible for providing control platforms and offerings across market segments of the GE controls retrofit business. An industry veteran with expertise in sales, applications, and commercial leadership, he has been with GE for six years and worked in the automation and controls industry with several major firms for over two decades. He holds a master of science degree from the University of Southern California.