A typical power plant steam turbine will have steam entering the HP turbine at 180 bar (2610 psi), 540⁰C (1,000⁰F). Steam from the HP turbine is then returned to the boiler where it is reheated to 540⁰C (1,000⁰F) at approximately 45 bar (652 psi). The reheated steam is returned to the IP turbine and is exhausted to the LP turbine at around 10 bar (145 psi), 180⁰C (356⁰F). Finally, the LP turbine will exhaust to the condenser, which is held at around 720mm Hg (-0.95 bar/-13.77 psi) vacuum.

Summary:

• HP Turbine Inlet – 180 bar (2610 psi), 540⁰C (1,000⁰F).
• IP Turbine Inlet – 45 bar (652 psi), 540⁰C (1,000⁰F).
• LP Turbine Inlet –10 bar (145 psi), 180⁰C (356⁰F).
• Condenser Inlet – 720mm Hg (-0.95 bar/-13.77 psi) vacuum.

Condenser vacuum levels are dependent on effective condenser cooling. Poor cooling can have a considerable impact on the overall plant efficiency. For example, a typical loss of vacuum from 720mm Hg (-0.95 bar/-13.77 psi) to 702mm Hg (-0.935 bar/-13.59 psi) equates to roughly a 3% power loss.

Typical Power Plant Steam Turbine and Generator

A typical marine steam turbine will operate at 65 bar (943 psi) and 515⁰C (959⁰F) at the HP turbine inlet. Steam is exhausted to the LP turbine at around 6 bar (87 psi) at 165⁰C (329⁰F). Steam is then exhausted from the LP turbine into the condenser at 720mm Hg (-0.95 bar/-13.77 psi) vacuum and very low steam temperatures. Note that due to the low temperatures of the LP turbine exhaust steam, the steam would be in a liquid state if exposed to atmospheric pressure.

A simple Curtis Wheel driving a high-speed centrifugal pump will operate at 60 bar (870 psi) or much less on de-superheated steam. Exhaust steam may be 3.5 bar (50 psi) and 150 ⁰C (302⁰F) (typical of a Coffin type feedwater pump turbine).

Turbine Bleeds

The term ‘bleed’ refers to the relatively small volumes of steam that are ‘bled’ from the steam system at various points within the system. Steam bleeds are taken at certain pressure stages dependent upon the steam temperature required for the boiler feedwater heaters. A typical turbine may bleed at 19 bar (275 psi), 9 bar (130 psi), 6 bar (87 psi) and 0.5 bar (7 psi), depending upon the feedwater heating arrangement. Non-return valves are used to control the flow through steam bleeds.

Water Ingress

Water ingress into a steam turbine is a major source of concern as it does considerable damage to turbine components. Typical components damaged due to water ingress include the fixed blades, moving blades, and diaphragm labyrinth seals.

It is critical that turbines are very carefully warmed through when starting. A methodical and slow warm-up procedure not only allows for differential expansion (axial and radial growth of the turbine due to its increased temperature), but also ensures there is no water in the steam lines. Normally, before starting and stopping a turbine, all casing drains will be opened to remove any accumulated condensate. In the steam inlet pipe there is also a steam separator (normally, but not always) which separates water droplets from the steam flow by causing a rapid change of direction of travel; the rapid change in direction causes moisture droplets to coalesce on the interior separator body surfaces and then drain out of the separator base.

Water content within the steam lines can be monitored by fitting thermocouples to the top and bottom of the steam lines/pipes. Water will be detected due to the temperature difference between it and the steam (the water will have a lower temperature).

Tip – If a thermocouple does detect water, it is often too late to prevent it entering the turbine unless the measuring points are located far away from the turbine itself.

Water Damaged Turbine Moving Blades