What this sector has to measure
A power station is a heat balance with instruments attached. Almost everything measured there exists to establish how much energy went in, how much came out and where the difference went, which is why steam flow, feedwater flow, pressure and temperature are measured with more care than the same quantities would receive in a general process plant.
The sector also packs very different conditions into one plot. A boiler sees high temperature and pressure together with a water chemistry that has to be controlled to protect the metal. A turbine hall needs vibration, speed and metal-temperature measurement that will survive a fast transient. A cooling water circuit deals with large pipes, dirty water and a requirement for a reliable flow figure rather than a precise one. No single specification covers all three, and applying one across a site is how instruments end up either over-specified or unsuitable.
Steam is the measurement that most often surprises buyers. It is hot, it is compressible and it changes state, so a volumetric reading means nothing without the line pressure and the temperature behind it, and that compensation is part of the measurement rather than a refinement added afterwards. Establishing who performs it - the transmitter, a flow computer or the control system - is worth settling before the instrument is chosen.
Where the instruments sit
Fuel and combustion
Fuel handling, feed rate, combustion air and flue gas. The measurements here determine both the efficiency figure and the emission record, so they are frequently tied to a reporting obligation as well as to control.
Boiler, steam and feedwater
Water chemistry, steam flow, drum level, pressure and temperature. The chemistry and the steam measurement are both protective, because they protect the boiler and the turbine from damage rather than optimising the process.
Turbine and generator
Speed, vibration, bearing and metal temperature and power output. Several of these are protective rather than controlling, and they are expected to survive a fast transient without being damaged by it.
Cooling, condensate and balance of plant
Cooling water flow, condenser performance, condensate and auxiliary systems. Large pipes and aggressive water mean the flow technology is chosen for reliability and for fitting the pipe rather than for the tightest accuracy.
Instrumentation used in this sector
| Measurement | What is measured in this sector | Instruments commonly specified |
| Flow | Steam, feedwater, condensate, fuel, cooling water and flue gas | Vortex and differential-pressure installations for steam with pressure and temperature compensation, electromagnetic or ultrasonic meters for water, Coriolis where a mass figure is needed, and differential-pressure or thermal measurement for gas |
| Pressure | Drum and superheater pressure, steam header pressure, condenser vacuum and differential across filters and strainers | Pressure and differential transmitters rated for the temperature at the tapping, frequently with impulse lines or seals, and vacuum-rated instruments for the condenser |
| Temperature | Superheated and reheated steam, boiler and turbine metal, bearing temperature, and the temperature used to compensate steam flow | Thermocouples for the hot and fast duties, RTDs where a stable and traceable reading is wanted, and surface or embedded measurement for metal temperature |
| Level | Boiler drum, deaerator, condenser hotwell, fuel and ash handling | Differential-pressure and hydrostatic transmitters with the density compensated, plus independent switches or a redundant channel where drum level protection is required |
| Analysis | Boiler and feedwater chemistry, dissolved oxygen, conductivity and pH, plus flue gas oxygen and carbon monoxide | In-line water chemistry analysers with sample conditioning, and combustion analysers mounted where the sample is representative and the analyser remains accessible |
| Vibration and speed | Turbine, pump and fan vibration, rotating speed and over-speed protection | Proximity and accelerometer based vibration measurement, and speed sensors arranged so that protection remains independent of the control system |
Which of these is appropriate depends on the unit, its duty and the standard the installation follows. The protective measurements in this sector normally carry their own redundancy requirement.
How to specify for a power station
- Steam compensation, stated rather than assumed. A volumetric steam figure without the line pressure and temperature behind it is not the number the efficiency calculation uses, so say whether the transmitter, a flow computer or the control system performs the compensation.
- The condition at the tapping rather than the nominal condition. High-temperature steam and condensate are different measurement problems with the same nominal medium, and the impulse lines or seals follow from the temperature at the tapping point.
- Whether the measurement is protective. Drum level, over-speed and vibration protection usually require independence from the control system and sometimes redundancy, and that requirement comes from the plant standard rather than from the instrument.
- Water chemistry as a protective duty. Conductivity, pH and dissolved oxygen are measured to protect the boiler and the turbine, so the specification follows from the water treatment regime rather than from the process.
- Vacuum measurement, which appears only in this sector and needs an instrument rated for it rather than a standard pressure transmitter used outside its intended duty.
- Large pipes in cooling water service, where the choice is frequently driven by the pipe and by whether the line can be opened rather than by accuracy.
- Emission reporting obligations, which fix the method, the analyser and the record for the flue gas measurements and belong in the enquiry before the instrument is selected.
- Site conditions: ambient temperature, dust, vibration and the area classification around fuel handling and generator cooling, all of which affect the enclosure selection rather than the sensor.
Standards and approvals that may apply
- Pressure equipment and boiler standards applied by the project, which govern the pressure boundary and the materials rather than the electronics.
- Emission monitoring obligations, where a measured value is reported to an authority and the method is fixed rather than chosen.
- Hazardous-area certification schemes for fuel handling areas and for generator cooling, which the installation may be required to carry.
- Functional-safety requirements for protective measurements such as drum level and over-speed, which normally come from the plant standard rather than from the instrument.
- We describe what an installation may have to meet. We do not claim that any approval is held for a product unless the corresponding document has been supplied, and where it has not, the product page says so.
Applications in this sector
