Measuring Pressure and Temperature in a Boiler Plant
Pressure and temperature are the two numbers that run a boiler plant. Local gauges let an attendant see them; transmitters send them to controls and records. Each instrument has a range where it is accurate and ways it goes wrong.
You'll learn
- Describe the common pressure and temperature instruments
- Choose between a gauge and a transmitter
- Understand thermowells and sensor types
- Know why accuracy and calibration matter
Before this: Gauge vs absolute pressure, Water level and pressure gauges
Local gauges and transmitters
A local pressure gauge with a Bourdon tube shows pressure on a dial; a bimetal or liquid-in-glass thermometer shows temperature. They need no power and are what an attendant reads on rounds.
A transmitter measures the same quantity electronically and sends a signal, usually 4–20 mA or a digital protocol, to the control system, the display panel and the data logger. Controls, alarms and trips act on transmitters, not on dials.
Temperature sensors
| Sensor | Range and character | Typical use |
|---|---|---|
| Bimetal dial thermometer | Local reading, modest accuracy | Feed tank, pipework, local checks |
| RTD (Pt100) | Accurate, stable, up to several hundred °C | Feedwater, steam, process control |
| Thermocouple | Wide range, very high temperatures | Flue gas, furnace, superheater |
Sensors in pressurized lines sit in thermowells: closed tubes welded or screwed into the pipe, so the sensor can be removed without opening the line. A thermowell slows response a little and must be strong enough for the flow and pressure.
Accuracy, range and calibration
Instruments are most accurate in the middle of their range. A pressure gauge should normally read in the middle third of its scale at working pressure: one that sits near the top is stressed and near its limit; one that sits near the bottom gives poor resolution.
Error in pressure becomes error in temperature for saturated steam. Near 1 kg/cm²(g), saturation temperature changes by about 15.8 °C per kg/cm², so a gauge reading 0.2 kg/cm² wrong misstates steam temperature by about 3.2 °C — significant for a process held to a few degrees.
- Calibrate transmitters and test gauges against reference instruments at set intervals and after any overrange.
- Protect pressure instruments from steam with siphons and from pulsation with snubbers.
- Put thermocouples where the gas is representative: not in a corner of a duct where it stratifies.
- Record calibrations; an uncalibrated trip instrument is an untested trip.
Key takeaways
- Local gauges for rounds; transmitters for controls, alarms and records.
- Bimetal for local temperature; RTDs for accuracy; thermocouples for high temperatures.
- Read gauges in the middle third of their scale; protect them with siphons and snubbers.
- For saturated steam, pressure error is temperature error; calibrate regularly.