Using the T-S Diagram for Practical Steam Turbine Troubleshooting

Most engineers working with steam turbines have seen the Rankine cycle and the temperature–entropy (T-S) diagram. Let's explore how to use it as a practical diagnostic tool.

In day-to-day operations, attention tends to focus on vibration, bearing temperatures, valve performance, or control stability. These are important, but they do not explain whether the turbine is converting steam energy into work efficiently. For that, the thermodynamic path matters.

The T-S diagram is not an academic exercise. It is a structured way of understanding whether a steam turbine is losing efficiency, whether the problem is internal or external, and whether intervention is justified.

The Expansion Path Defines the Work Output

The useful work produced by a steam turbine depends on the enthalpy drop between inlet and exhaust. The larger the enthalpy drop, the more mechanical work is available to drive a generator or compressor.

On a T-S diagram, turbine expansion appears as a downward line from high temperature and pressure to lower pressure and temperature. In an ideal case, this expansion would be isentropic, meaning entropy remains constant. Graphically, that is a vertical line as seen in the Rankine cycle in the Figure below between points 1 and 2s.

In reality, expansion is never perfectly isentropic. Entropy increases due to internal losses. The actual expansion line therefore tilts to the right, and is that between points 1 and 2 in the Rankine cycle in the Figure below. The difference between the ideal vertical line and the real expansion path represents lost work. That loss directly affects output and heat rate.

When operators observe reduced power at constant steam flow, or when heat rate begins to drift upward, the first question should not be “what failed mechanically?” but rather: has the thermodynamic expansion path changed?

The Rankine Cycle on a T-s diagram

Real vs Isentropic: What Efficiency Drift Looks Like

Every turbine is delivered with an expected isentropic efficiency. This expresses how close the real expansion is to the ideal case. Over time, that efficiency changes. The T-S diagram provides a structured way to understand why.

If the real expansion path shifts further to the right compared to baseline conditions, entropy at the turbine exit (point 2) increases. That means more energy is leaving the turbine unused. The enthalpy at the turbine exit rises, and useful work decreases.

In operational terms, this can be linked to:

• Internal inefficiencies
• Flow path degradation
• Leakage increases
• Changes in steam conditions

Importantly, vibration levels may remain acceptable while thermodynamic efficiency is degrading. This is why a thermodynamic view complements mechanical monitoring rather than replacing it.

Reheat and Multi-Stage Turbines

In reheat turbines, steam first expands through the high-pressure (HP) section, then returns to the boiler for reheating before entering the intermediate-pressure (IP) section. This reheating restores enthalpy, even though pressure is reduced in the process.

On the T-S diagram, this appears as a drop during HP expansion, followed by a rise during reheat, then a further drop through IP and LP expansion.

Rankine Cycle with Re-heat on a T-s Diagram (2 turbine stages)

If the reheat temperature decreases due to boiler or reheater limitations (point 3 on the Figure above), the enthalpy entering the second turbine stage is lower. The total available enthalpy drop across the machine decreases. Output falls — even if the turbine itself is mechanically healthy.

Similarly, if condenser backpressure increases (point 4), the final point of expansion shifts upward on the diagram. The expansion line shortens, reducing enthalpy drop. In this case, the turbine may be blamed for reduced output, when the root cause lies in condenser performance.

The T-S diagram forces the correct diagnostic sequence:

1. First confirm inlet steam conditions.
2. Then confirm reheat temperature, if applicable.
3. Then confirm exhaust pressure.

Only after these are verified should internal turbine efficiency be evaluated. Without this thermodynamic structure, troubleshooting becomes reactive and unfocused.

Using the T-S Diagram as a Monitoring Framework

In daily operations, engineers rarely calculate full thermodynamic state points. However, the conceptual framework of the T-S diagram can guide data interpretation.

When output drifts or heat rate increases, a structured thermodynamic review should consider:

• Has inlet pressure changed?
• Has inlet temperature changed?
• Has reheat temperature shifted?
• Has exhaust pressure increased?
• Has steam flow changed?

Even small deviations in these parameters alter the expansion path and therefore the enthalpy drop.

This structured approach prevents unnecessary mechanical intervention. It also strengthens communication between operations, maintenance, and boiler teams by providing a shared thermodynamic language.

Procurement and Performance Guarantees

For procurement engineers, understanding the T-S diagram has contractual implications.

Performance guarantees are typically defined relative to isentropic efficiency at specified steam conditions. If correction curves for off-design conditions are unclear, disputes arise when plant conditions deviate from design assumptions.

Specifying clear reference conditions for:

• Inlet pressure and temperature
• Reheat conditions
• Exhaust pressure
• Flow rate

Ensures that performance verification remains transparent.

A Practical Conclusion

The T-S diagram is not purely theoretical. It is a decision framework. It clarifies whether performance loss originates in the boiler, reheater, condenser, or turbine itself. It reveals efficiency drift before vibration alarms appear. It distinguishes thermodynamic degradation from mechanical damage.

For operations and maintenance engineers, this structured view reduces unnecessary outages. For procurement engineers, it strengthens contract clarity. And for the entire plant team, it replaces reactive troubleshooting with disciplined analysis.