Losses in steam turbines


There are two main types of losses in a steam turbine.

(a) Internal losses (Due to flow of steam inside the turbine)

(b) External losses (Losses not concerned with steam)

The internal losses in steam turbines may be enumerated as follows:

  1. Losses due to steam Leakage

(i)  Steam leakage in regulating and main valves

(ii) Steam leakage through seals and glands

(iii) Steam leakage through spaces between nozzles and moving blades

(iv) Steam leakage through spaces between diaphragm and turbine shaft

(v) Steam leakage through space between moving blade rings and turbinecasing

  1. Nozzle friction losses

(i) Energy loss of steam before entering the nozzles

(ii) Loss due to frictional resistance in the nozzles walls

(iii) Friction between steam molecules

(iv) Growth of boundary layer in the nozzle

(v) Deflection of the flow

These losses are accounted in terms of the by the velocity coefficient,Cf.

  1. Blade friction losses

(i) Impingement Losses

(ii) Losses due to rough surface

(iii) Losses due to eddies formation

  1. Disc friction losses

It is due to the blade wheels rotation in a space full of steam. This loss is due to viscous friction at the wheel surface.

  1. Partial admission losses

There is an admission loss due to the viscous friction at the wheel surface.

  1. Gland leakage losses

Losses due to leakage of steam through the labyrinth glands.

  1. Residual velocity loss

(i) Normal velocity loss

(ii) Loss due to whirl component

(iii) Eddy losses due to non-uniformity of flow

(iv) Pressure loss through the hood

7. Carry-over loss

When steam passes from one stage to another stage through the diaphragm, energy loss takes place. It is called carry over loss. Consequently the steam energy available at the next stage will be less than that of the exit at preceding stage.

8.Losses due to erosion and corrosion

When the dryness fraction of steam is less than 0.88,turbine develops   erosion and  corrosion  simultaneouly.

9. Throttling losses in steam turbines

In this expansion without recovery of any useful work.

10. Loss due to boundary layer formation on the blade surfaces

It depends upon Reynolds number, roughness of blade surface and exit Mach number of the steam.

External losses in steam turbines

These are the losses which do not concern the loss associated with the steam.

List of external losses include

(i)  Bearings friction

(ii) Friction in sealing

(iii) Fluid friction at the outside rotating surfaces of the turbine

(iv) Energy loss in the bearings and governing mechanisms

(v) Pumping power losses