API stands for American Petroleum Institute.  Oil Refineries have a lot of rotating machinery, including turbines, so they have taken the lead in developing standards for turbine protection.  The 4th edition of the standard is dated 12/2007.  The publication is copyrighted and sold by API through various standards outlets.  The document is 100 pages.  Only a couple of pages directly relate to the overspeed protection function.  The standard deals with a number of protection concepts, but the main focus is on vibration.  We have summarized the overspeed requirements and included some quotes directly related to overspeed protection.

The API Standards are Recommendations, not Codes or Laws.  The standards are to assist in design of systems that will meet various competing objectives.  Standards are drafted by committees, so there will often be conflicting or ambiguous statements due to the nature of the standards formulation process.

API Standard 670, 4th Edition, 12/2000
Machinery Protection Systems

Summary of Electronic Overspeed Protection System Minimum Requirements
see also [API 670 Quotes Directly Related to Overspeed Protection]

• The electronic overspeed system must meet the following requirements.
  • The system must be based on three independent measuring circuits and two-out-of-three voting logic.
  • The system must sense an overspeed event and change the state of the output relays within 40 milliseconds.
  • An overspeed condition sensed by any one circuit must initiate an alarm.
  • An overspeed condition sensed by two out of the three circuits must initiate a shutdown.
  • Failure of a speed sensor, power supply, or logic device in two out of the three circuits must initiate a shutdown.
  • All settings incorporated in the overspeed circuits must be field changeable and must be protected through controlled access.
  • The speed sensors used as inputs to the electronic overspeed detection system must not be shared with any other system.
  • A peak hold feature with controlled access reset must be provided to indicate the maximum speed reached since the last reset.
  • Activation of on-line testing functions must be permitted through controlled access.
  • The electronic overspeed trip system should be powered by an uninterruptable power source.
  • Electro-Hydraulic Solenoid Valves
    • The turbine shall be provided with two separate electro-hydraulic, solenoid-operated valves located in the trip system.  Unless otherwise specified, the solenoid valves must be de-energized-to-trip.
    • The following requirements must be considered in the system design.
      • Solenoids can draw significantly high currents.  Interposing relays may be used when the current requirements of the solenoids exceed the current rating of the relay in the OSP system.
  • Trip Valve/Combined Trip and Throttle Valve
    • A separate independent trip valve or combined trip and throttle valve, as specified by the purchaser, shall be provided for each steam inlet.
    • The following requirements must be considered in the system design.
      • Trip valve(s) operate in the shut (tripped) or fully open position only.  In addition to the functions provided by trip valve(s), combined trip and throttle valve(s) provide intermediate valve positioning.
      • Non-return valves are normally mounted directly to steam turbine extraction connections or as close as possible to the turbine to avoid trapping large volumes of steam, which can keep the turbine operating when extraction valves do not fully close.
      • Location of non-return valves in piping below the turbine requires that low-point drain provisions be furnished to eliminate water from the extraction line before start-up and to eliminate the accumulation of water during operation with no extraction flow.
      • Location of non-return valves in piping below the oil console level may result in drainage problems.  Alternative actuator methods may be required.
      • Even when the emergency trip valve is closed, a turbine exhausting to sub-atmospheric pressure may leak enough steam to prevent the turbine and driven equipment from coming to a complete stop. A vacuum breaker will admit air to the exhaust casing, increase exhaust pressure, and reduce coast-down time.  For Turbines exhausting to a common condensing system, air admission may not be feasible and a more positive-emergency trip valve(s) may be required.