Steam turbine
Speed measurement / alarm
In a coal-fired power plant, steam is produced by pumping highly purified water through boiler tubes. The water is heated to a high temperature and pressure, and then flows through several orifices into the steam turbine. Once past the orifice, the water pressure drops and it immediately flashes into superheated steam, which is directed toward the turbine blades at high velocity. Due to the high velocity of the superheated steam, the turbine blades inside the steam turbine start to rotate and turn the turbine shaft, which is connected to the generator through reduction gears, and thereby ultimately generates electricity.
Valve position, pressure and temperature measurements are all necessary to ensure stability in this process. A solution for this application is a setup with a potentiometer for valve position measurement, a 2-wire style pressure transmitter and a 4-wire RTD for precise temperature measurements. All 3 signals are connected to individual PR 4114 universal transmitters, which output 0...10 VDC signals to the PLC. The 4510 display / programming front allows local readout of both input and output signal and the process calibration function, found in the advanced menus, provides very easy and precise calibration of the open and closed valve position.
Application:
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What measurements are critical for steam turbine monitoring and how does signal conditioning help?
Steam turbine monitoring requires accurate measurement of bearing temperature (journal and thrust bearings), steam inlet/exhaust temperature, casing expansion, vibration, and rotor speed. Signal conditioners from PR Electronics convert and isolate these diverse sensor signals — RTDs for temperature, proximity probes for vibration and displacement, and magnetic pickups for speed — ensuring clean data reaches the turbine control system. The PR 4116 Universal Transmitter handles multiple input types, while the PR 4225 converts speed sensor frequency outputs to analog signals.
How is turbine speed measurement performed using PR Electronics frequency converters?
Steam turbine speed is measured using magnetic pickups or proximity probes that generate a frequency signal proportional to rotational speed. The PR 4225 Frequency/Pulse to Analog Converter accepts these frequency inputs (NAMUR, NPN, PNP, Tacho, TTL) and converts them to a proportional 4-20 mA output for the turbine control system. For example, a turbine running at 3000 RPM produces a frequency from the toothed wheel pickup that the PR 4225 linearizes and converts to a calibrated 4-20 mA signal representing the full speed range. This analog output integrates directly with the governor and overspeed protection systems.
What SIL-rated signal conditioning is available for steam turbine overspeed protection?
Turbine overspeed protection is a critical safety function — failure to trip the turbine on overspeed can result in catastrophic mechanical disintegration. The PR 9202A SIL-rated Pulse Isolator accepts magnetic pickup speed signals from the hazardous area and provides isolated outputs for the overspeed protection system with SIL 2/3 certification per IEC 61508. For temperature-based trips (e.g., bearing overtemperature shutdown), the PR 9116A Universal Converter provides SIL 2 certified signal conditioning with an integrated relay alarm output.
