SIL-3 MPU’S

SIL-3 MPU’S are single-channel variable-reluctance (VR) magnetic pickup speed sensors designed for use in SIL-3, SIL-2, or SIL-1 (safety integrity level) rated turbine or engine safety systems. Drake Controls supplies, configures, and services these SIL-3 MPU’S as your single authorized Woodward source.

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Applications

These speed sensors are designed for use in SIL-3, SIL-2, or SIL-1 (safety integrity level) rated turbine or engine safety systems. When applied with a SIL- rated logic solver and final element, users can easily create an overspeed safety instrumented function (SIF) which meets the required safety integrity level for the specific application.

As these variable reluctance (VR) speed sensors do not utilize active components, they have a very low failure rate, making them ideal for use in safety systems requiring long system life and low probability of failure on demand (PFD) values.

Rotating machinery requires speed sensing for rotor control, monitoring, and safety. Speed sensing is accomplished using a variety of technologies. Variable-reluctance-based MPUs (magnetic pickups) are commonly selected because of their simplicity, reliability, and low cost. MPUs are passive probes in the sense that there are no active signal conditioning electronics in the probe. VR (passive) sensors also do not require an external supply, simplifying the overall system and increasing system reliability values.

MPUs produce a voltage output when ferrous material passes through the magnetic field at the end of the pickup (pole piece). Since most turbines and engines have large gears made of ferrous material (usually iron or steel), magnetic pickups can usually be installed without adding attachments to a gear or shaft. Non-ferrous materials, such as aluminum, brass, and some stainless steels, will not excite a magnetic speed sensor.

To simplify application and installation concerns, these sensors have been validated to be directly compatible with Woodward’s ProTech ® and MicroNet™ safety logic solvers when used in SIL-3 or SIL-2 certified safety systems, including:

  • ProTech-GII—Overspeed Detection System (SIL-3 certified)
  • ProTech-SX—Simplex Safety Logic Solver (SIL-2 certified)
  • ProTechTPS—Safety Logic Solver including Overspeed Detection (SIL-3 certified)
  • MSM—MicroNet Safety Logic Solver (SIL-3 certified)

For industrial applications, it is recommended that standard API670 be referenced, as it includes application guidelines for speed sensors applied on rotating machinery, including steam turbines, gas turbines, turbo expanders, and variable-frequency drives. Compliance to functional safety standards IEC61508, IEC61511, and ISA-84.00.01 is increasingly required in new and retrofit systems. Safety-certified passive speed sensor are a good choice in these systems because they are Type A devices (passive), take up less of the safety allocation, and have a lower PFD values than active speed sensors.

Description

These sensors are single-channel, variable-reluctance speed sensors consisting of a single inductive coil and a permanent magnet. A ferrous gear tooth passing the sensor pole piece changes the magnetic field strength, inducing an AC voltage. The frequency of the output signal is proportional to the velocity of the gear tooth passing the sensor’s pole piece. The amplitude of the signal depends on the following:

  • Angular speed
  • Air gap between sensor face (pole piece) and target (gear tooth)
  • Target (gear tooth) geometry
  • Target (gear tooth) ferrous material properties
  • Load impedance of the electrical circuit used to sense output signal

Depending on the application, these sensors can be ordered with imperial or metric threads and different thread lengths (determined by part number).

Sensed minimum and maximum frequency is dependent on the application (toothed gear, air gap, overspeed detection system (ODS), trigger level, etc.). VR sensors are not designed to measure low frequencies. The sensor’s voltage level is low at lower frequencies, which results in a signal voltage level that may be too low to be detected by some overspeed detection systems. Depending on the ODS, it is usually possible to measure frequencies up to about 40 kHz.

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