Measuring Boiler Water Level
Water level is the most safety-critical measurement on a boiler. It is shown directly by gauge glasses and measured for control and protection by floats, conductivity probes and differential-pressure transmitters. Each works differently and each can be fooled.
You'll learn
- Describe the main level measurement methods
- Know which are used for control and which for protection
- Understand the density correction for DP level transmitters
- Keep level instruments trustworthy
Before this: Water level and pressure gauges, Water level control and blowdown practice
Ways to see the level
- Gauge glass: shows the level directly. The reference everything else is checked against.
- Float switch or float transmitter: a float in a chamber beside the boiler rises and falls with the level.
- Conductivity probe: water conducts electricity and steam does not, so a probe tip covered by water completes a circuit. Several probes at different heights give switching points.
- Differential-pressure (DP) transmitter: measures the weight of the water column and calculates level from it.
Control and protection
Level is used for two different jobs. Control keeps the level at its set point by adjusting feed; protection shuts the boiler down if the level falls too low. Good practice keeps them independent, so that a failed control sensor cannot also disable the protection: separate probes or chambers, and two independent low-water cut-outs on many boilers.
Floats and probes give switching points; DP transmitters and capacitance or guided-wave radar probes give a continuous level for modulating control.
Density, and how level instruments are fooled
A DP transmitter measures the weight of the water, but hot boiler water is lighter than cold. Saturated water at 10 kg/cm²(g) has a density of about 883 kg/m³, against about 996 kg/m³ for cold water. Calibrated with cold-water density and left uncorrected, the transmitter reads level wrongly by an amount related to that 13% density difference. Drum level transmitters are therefore density-compensated, using the steam pressure.
- Sludge and scale in float chambers jam floats; blow chambers down routinely.
- Deposits on conductivity probes make them read water when there is none; clean and test them.
- Swell and shrink make every level instrument read a temporary false level during load changes.
- A choked gauge glass connection makes the reference itself wrong; blow it down before trusting it.
Key takeaways
- Gauge glasses show level; floats, probes and DP transmitters measure it for control and protection.
- Keep control and protection independent; use two low-water cut-outs where required.
- Hot water is about 13% lighter than cold at 10 kg/cm²(g): DP level transmitters must be density-compensated.
- Blow down chambers and clean probes; check against the gauge glass.