Quotes from API 670 Standard for Overspeed Protection of Rotating Machinery (Steam Turbines)
Updated 4/2008

MACHINERY PROTECTION SYSTEMS / From API 670 Page 1

1.1 Scope

This standard covers the minimum requirements for a machinery protection system measuring radial shaft vibration, casing vibration, shaft axial position, shaft rotational speed, piston rod drop, phase reference, overspeed, and critical machinery temperatures (such as bearing metal and motor windings). It covers requirements for hardware (transducer and monitor systems), installation, documentation, and testing.

From API 670 Page 17

5.4.1.8.4  The relay control circuits for all overspeed channels shall be normally energized.

From API 670 Page 23

5.4.8.4  The electronic overspeed detection system shall satisfy the following requirements:

1. The system shall be based on three independent measuring circuits and two-out-of-three voting logic.
2. Unless otherwise specified. the system shall sense an overspeed event and change the state of its output relays within 40 milliseconds when provided with a minimum input signal frequency of 300 Hz. Response time must consider complete system dynamics (see note) as outlined in ASME PTC 20.21965 Section 7.

Note: 40 millisecond response time may not be adequate in all cases to keep the rotor speed from exceeding the maximum allowed for the machine. Give consideration to the following:

1.  The electronic overspeed detection system is only one part of the total overspeed protection system. Total system response time is affected by, but not limited to, the rotor acceleration rate, the electronic overspeed detection system, the trip valve(s), the electrohydraulic solenoid valves, the entrained potential energy downstream of the trip valve(s) and in the machine, and (where applicable) the extraction check valve(s).

2. To achieve proper electronic overspeed detection system response time, a minimum number of events per unit time is required. This is dependent on the method of speed sensing employed and could. for example. be affected by the number of teeth on the speed sensing surface. the tooth profile, and the shaft rotational speed (refer to Appendix J).

3. The use of intrinsic safety barriers to meet hazardous area classification requirements may introduce signal delays that preclude the system from meeting acceptable response time criteria. Care should be taken to consider these effects when designing the electronic overspeed detection system and choosing components. Alternative methods should be considered as required to meet the area classification requirements.

3. An overspeed condition sensed by any one circuit shall initiate an alarm.
4. An overspeed condition sensed by two out of three circuits shall initiate a shutdown.
5. Failure of a speed sensor. power supply, or logic device in any circuit shall initiate an alarm only.
6. Failure of a speed sensor power supply or logic device in two out of three circuits shall initiate a shutdown.
7. Items c, d, e, and f shall require manual reset.
8. All settings incorporated in the overspeed circuits shall be field changeable and shall be protected through controlled access.
9. Each overspeed circuit shall accept inputs from a frequency generator for verifying the trip speed setting.
10. Each overspeed circuit shall provide an output for speed readout.
11. The speed sensors used as inputs to the electronic overspeed detection system shall not be shared with any other system.
12. A peak hold feature with controlled access reset shall be provided to indicate the maximum speed reached since last reset.
    Note: Depending on system design. it may be necessary to reset the peak hold feature after testing to ensure that maximum rotor speed reached during an actual overspeed event is captured.
13. Activation of online testing functions shall only be permitted through controlled access.
14. The system shall be provided with fully redundant power supplies in accordance with 5.4.1.7.i.
    Note: These power supplies should be energized by the purchaser's independent and uninterruptible instrument branch power circuits.
15. The electronic overspeed detection system shall accept speed sensor inputs from either magnetic speed sensors or proximity probes (see 6.1.6). Unless otherwise specified. the inputs shall be configured to accept passive magnetic speed sensors.

6.1.6  Electronic Overspeed Detection System Speed Sensors / From API 670 Page 32

6.1.6.1  Three separate speed sensors that are not shared with any other system shall be provided for the electronic overspeed detection system.

6.1.6.2  Unless otherwise specified, speed sensors used as inputs to the electronic overspeed detection system shall be passive magnetic speed sensors (see 5.1.5).

Notes:

1.  While passive magnetic speed sensors are often employed for speed sensing, they may not allow low shaft speeds (typically below 250 rpm) to be measured, even when a multi-toothed wheel is employed. Externally-powered sensors (both active magnetic speed sensors and proximity probes) are capable of providing a signal down to shaft speeds of 1 rpm or lower and represent a better choice for these applications.

2.  For applications involving overspeed sensing, powered sensors have inherent advantages over passive magnetic speed sensors and should be considered because they allow the electronic overspeed detection system to assess the integrity of its inputs more fully. They enable self-checking and circuit fault diagnostic capabilities (such as sensor gap within acceptable range or sensor and field wiring deterioration).

3.  Proximity probes can be gapped further from the speed sensing surface than active or passive magnetic speed sensors and are therefore less likely to nib and fail during abnormal rotor vibration conditions (such as encroaching on a second critical speed during an overspeed condition) when radial vibration amplitudes at the speed sensing surface location may be large.

6.1.6.3  Mounting requirements and electrical conduit protection for speed sensors shall be identical to that required for radial shaft vibration probes (see 6.2.1.1).

6.1.6.4  A multi-toothed surface for speed sensing shall be provided integral with, or positively attached, or locked, to the driver shaft. This surface may be shared by other speed sensors, but shall not be used as a gear for driving other mechanical components. Refer to Appendix J for typical details of this multi-toothed surface.