Turbomachinery overspeed protection is not just about safely shutting down a system when it is being stressed beyond its recommended limits. It begins with a thorough understanding of the physical design limits of each turbine system and ends with the design of a complete control system capable of preventing a catastrophic overspeed event.
The primary control system is the first line of defense against overspeed, typically referred to as Normal Overspeed (NOS) protection. It is an essential element in ensuring safety of the machinery. The primary controller regulates machine speed and load output by controlling the fuel delivery that powers it.
Furthermore, it regulates the amount of energy input to the rotating machinery that drives speed and load output for either electrical power generation or mechanical work performed through pumps, compressors and blowers. As increasing power output is required for higher loads on driven machinery such as generators or compressors, the risk of machine overspeed upon sudden loss of load increases proportionally.
Theoretically, the NOS protection system should prevent any overspeed events. But, in reality the system cannot always catch overspeed events before they happen. The Emergency Overspeed (EOS) protection system (Figure 3) is an extra layer of protection that operates independently from the primary control system. It acts as a second line of defense and has the independent ability to determine if the machinery is headed for catastrophic overspeed.
EOS protection systems for older turbomachinery are typically mechanical emergency governors, more commonly known as mechanical bolts, installed directly on the main turbine shaft. More modern control systems incorporate various forms of electronic overspeed protection systems. These emergency systems are designed to respond when NOS protection systems fail to prevent the equipment from operating at rotational speeds exceeding about 110% allowed in maximum design.
Although primary control and emergency overspeed protection systems must work independently, they also must be designed to work together effectively as a total protection system.
(You can read the rest of this article in the March April Edition of Turbomachinery International)