The overwhelming majority of small-turbine-overspeed-trip devices are mechanical: They incorporate a spring-activated trip bolt that moves out and contacts a trip lever whenever a preset overspeed is reached. The overspeed trip setting is selected at the time of installation in the plant. It is then re-checked periodically, and mechanical assemblies are replaced on an as-needed basis. But therein lies the problem.
A number of factors make it necessary to check and, possibly, re-calibrate these built-in devices on a yearly basis. Needless to say, process plants are not always performing this preventive maintenance activity in timely and procedurally correct fashion. Unscheduled downtime events and serious injury, even deaths, have resulted from such oversights. The situation has not improved given, among other things, reshuffled priorities, lack of money for training, attrition of experienced personnel, and hiring freezes mandated by people who cannot even begin to comprehend all the consequences of their edicts. The list could go on and on.
Again, conscientious reliability-focused engineers or technicians will make their voices heard. Well-engineered and highly reliable electronic Overspeed Prevention (OSP) systems are available for new and retrofit applications. These systems offer end-users the security and dependability of a two-out-of-three voting system with the flexibility and ease of do-it-yourself installation and configuration.
A typical OSP system incorporates a two-out-of-three voting feature that monitors turbine speed and will initiate a trip command to prevent overspeed events. Systems of this type incorporate three identical speed modules that individually measure a frequency-input signal from a passive or active magnetic pickup sensor. A supervisory module continually monitors the three speed modules for proper operation, which helps to eliminate unnecessary downtime and increases system availability.
Years ago, we adopted OSP systems that provided users with a multilevel password function for added security. Each level provided access to higher system functions, including Peak Speed Reset, Overspeed Test Mode, Configuration Mode, and Set New Passwords. We found that the three-speed modules offered a greater level of dependability. Open-pickup detection on “passive” sensors was provided as well as “dynamic” sensor-failure detection by way of the supervisory module. The latter knew when a speed module should have received a valid frequency input. Active sensor power was provided as an output and each speed module was capable of displaying its current speed and set point. Moreover, each speed module incorporated pilot lights to indicate overspeed test, trip, power failure, and fault conditions.
My recollection is of systems that were API and ISO compliant and, thus, facilitated integration in existing systems. We looked for systems that emphasized redundancy and compliance with sound specifications. Following that thinking, today’s plants may want to find systems with self-diagnostic testing and data communication capabilities. The facility will then have a package that greatly enhances the safety, reliability, and maintenance effectiveness needed in this day and age.TRR
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX. Email him at firstname.lastname@example.org.
Tags: reliability, availability, maintenance, RAM, overspeed trip devices, small turbines, steam turbines, electronic-overspeed-prevention systems, OSP systems, personnel safety, process safety