TL;DR
A gradual heat rate increase with no load change is almost always silica or sodium deposition on turbine blades. Check steam chemistry and compare blade path pressure ratios to baseline.
What you might see
- heat rate rising over a period of weeks or months
- turbine stage efficiency below expected value
- blade path pressure ratio deviating from baseline
- steam quality analysis showing silica or sodium above limits
Likely causes
Silica carry-over from the boiler depositing in the high-pressure blade path as steam expands
Sodium or chloride contamination from condenser leaks or poor boiler water chemistry
Reduced steam purity during load transients that exceed the boiler's separation capability
Long intervals between steam blows during start-up allowing loosely-bonded deposits to accumulate
Required tools
- Steam chemistry lab data (silica, sodium, conductivity)
- DCS blade path pressure ratio trend
- Turbine heat rate calculation spreadsheet or DCS heat rate block
Safety first
- High-pressure steam sampling requires trained chemistry personnel and proper sample cooler and quench equipment. Do not sample a live steam line without isolation hardware.
- Turbine blade inspections require full unit shutdown and proper confined-space entry procedures for the turbine casing.
Procedure
- 1
Pull weekly steam purity data from the chemistry lab and plot silica and sodium trends against the turbine manufacturer's steam purity limits.[1]
- 2
Review the blade path pressure trend across stages. A rising pressure ratio in a stage indicates mass flow restriction from deposits.
- 3
Compare current turbine heat rate to the performance baseline at the same load, temperature, and pressure conditions.
- 4
If steam chemistry shows silica or sodium above limits, investigate the boiler drum level control, separator performance, and condenser tube integrity.
Warning: Steam chemistry sampling on a live high-pressure system requires appropriate PPE and isolation procedures. Follow your plant chemistry sampling procedure. - 5
If deposits are confirmed, evaluate whether an online turbine wash (if the design permits) or an offline water soak can remove them.
- 6
Plan a turbine outage for blade inspection and mechanical cleaning if deposits are severe or the wash is ineffective.
Sources
GE Vernova GE D / H Class Steam Turbine general technical documentation, GE Vernova
Steam turbine blade path deposits, steam purity limits, general power generation references (general)
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