Overspeed protection systems play a key role in ensuring the safe operation of machinery in power plants and other critical industrial applications. In the past, ODSs were predominantly mechanical devices but nowadays, most overspeed systems are electronic in nature.
While an ODS can be integrated in the control system of a machine, it is strongly recommended that an independent ODS is implemented as a dedicated standalone system. This is because a completely separate (“segregated”) electronic overspeed protection system provides an isolated layer of protection with fast response times, while being compliant with important industry certifications and standards (SIL safety and API 670).
In general, there are two architectures for an electronic ODS: rack-based or DIN-rail mounted.
Rack-based systems are the most common and often include non-safety related functionality (for example, additional outputs and/or alarm/status information) in addition to the core overspeed detection and protection function. More recently, scalable DIN-rail mounted systems that offer a more modular and versatile solution have become available. These DIN-rail systems are fundamentally simple and highly robust, designed for ease of use and reliable operation with a longer proof-test interval. As such, they offer a compelling and cost-effective alternative to traditional rack-based systems.
The core idea of overspeed detection and protection is quite a simple one and should be implemented as simply, reliably and robustly as possible, while still meeting the requirements of machinery protection and safety standards such as API 670 and IEC 61508. Unnecessary complexity in overspeed systems is often created due to feature creep – the addition of secondary, non-essential functions. DIN-rail mounted overspeed systems are typically designed to focus primarily on safety related functionality.
The cost of rack-based solutions tends to be high due to their more complex hardware and non-safety related functionality. This also results in increased installation and maintenance costs which are not financially justified compared to DIN-rail mounted solutions. Therefore, total cost of ownership (TCO) for a DIN-rail overspeed system is considerably lower.
Scalable DIN-rail overspeed systems are modular in nature, so inventory management becomes a lot easier with a dramatically reduced number of spare parts/modules required to cover both large and small installations.
The same DIN-rail overspeed modules can be used for either simple SIL 2 1oo1 or 2oo2 solution architectures or the more demanding SIL 3 2oo3 architectures required by critical rotating machinery. It is also worth noting that DIN-rail overspeed modules are often SIL certified “by design” with all its associated advantages, compared to many rack-based systems which are SIL certified using the “proven-in-use” approach.
Being modular in nature, scalable DIN-rail overspeed solutions have only one system (module, software and documentation) that needs to be learned and maintained. This avoids complex maintenance schedules and the necessity for deep technical expertise across multiple systems. Significantly, it also reduces the possibility of human error – essential in a safety-critical system.
By concentrating on core safety related functionality, scalable DIN-rail overspeed solutions are inherently simpler and more reliable, which directly results in a lower amount of downtime. Importantly, their excellent reliability also results in improved safety integrity levels and longer proof test intervals, which makes the system testing and verification of those systems a lot easier compared to more complex rack-based systems.