Losses in steam turbines can be mainly classified into two types. They are:
- Internal losses (which occur within the flow of steam) and
- External losses (which occur outside the turbine casing).
Internal Losses in Steam Turbines:
The internal losses in steam turbines may be enumerated as follows:
- Steam, before entering the turbine, passes through these being accompanied by pressure losses. Steam gets throttled adiabatically with constant enthalpy.
- However, the enthalpy drop in the turbine decreases yielding less specific output. Thus, some available energy of steam is lost due to the irreversible process of throttling. The pressure drop varies from 3 to 5 % of the inlet steam pressure Po.
- Friction losses in nozzle are due to the growth of boundary layer and the formation of eddies in the wake, apart from the frictional resistance of walls, which varies with the height and length of passage.
- Losses are higher in a turbulent boundary layer than in a laminar one.
- In reaction turbine where pressure or enthalpy drop per stage is less due to lower speed, the laminar condition persists over a greater length of passage. So, the friction loss is less than the impulse stage.
- However, due to large number of stage, the total surface area exposed to flow is more, which increases the friction loss. Thus, the nozzle losses depends on nozzle size, surface roughness, nozzle length, roundness of entrance, divergence angle and space between nozzles, moisture and trailing edge.
- Impingement Losses: Steam issuing out from the nozzles meets the leading edges of the blades and energy may be lost if the entry is not smooth enough and eddies are formed.
- Frictional Losses: Steam encounters these losses in the blade passages, which depends on the roughness of the blade surface.
- Turning Losses: These occur as the steam turns in the blade passage.
- Wake Losses: These occur at blade exit, depending on its shape and tip thickness. The moving blade losses are taken care of by the blade friction coefficient (Kb = Vr2 / Vr1) representing the reduction of relative velocity of steam from Vr1 to Vr2 due to friction.
- When the turbine disc rotates in the viscous steam, there is surface friction loss due to relative motion between the disc and steam particles. Due to centrifugal force, steam flows radially outward.
- The moving disc surface exerts a drag on the steam, sets it in motion from root to tip and produces a definite circulation. Some parts of the kinetic energy of steam is lost due to this friction.
- An impulse stage operating with partial admission, or an early stage in such a turbine with nozzles provided only over a part of the blade periphery, will have blade idle during part of the revolution.
- Some portion of kinetic energy of the incoming steam is spent in clearing away the steam existing within the blade passage. These are called ‘scavenging losses‘ which together with disc friction losses are often referred to as ‘windage losses‘ in which some kinetic energy is imparted to the fluid at the expense of the kinetic energy of the blades.
- Since reaction turbines are designed for full peripheral admission, the windage losses, as well as disc friction losses can be neglected.
- Leakage of steam can occur between stages and along the shaft at inlet and exit ends of the casing.
- Diaphragm leakage takes place in both impulse and reaction stages through the radial clearance between the stationary nozzle diaphragm and the shaft or drum.
- Tip leakage occurs in reaction stages through the clearance between the outer periphery of the moving blades and the casing because of the pressure difference existing across the blades
- Shaft leakage occurs through the radial clearance between the shaft and casing at both high and low pressure ends of turbines. At the HP end, steam leaks out to the atmosphere, while at the LP end, the pressure being less than atmospheric, air leaks into the shell.
- Since the leaked steam does not work on the blades, it represents energy loss. Both diaphragm and tip leakages can be minimized by reducing the radial clearances, but it must avoid rubbing or metal-to-metal contact. The clearance may be as low as 0.5 mm.
- However, proper balancing of the rotor, both static and dynamic, is a must to avoid any such rubbing. It is necessary to use seals or packing to further reduce the leakage flow. These seals may be labyrinths, carbon rings, water or steam seals, or gland leak-off.
- To prevent shaft leakage, labyrinth seal may be used with carbon rings and gland leak-off.
- Labyrinth seals consist of a series of thin strips fixed with the casing which maintain the smallest possible clearance with the shaft. The small constructions make the steam throttled to lower pressure many times, till only a very little quantity leaks out.
- Carbon ring seals, which consists of a ring of carbon divided into segments, have the rings fit snugly to the shaft by springs so as to prevent leakage, and many may be used along with labyrinth glands in series in large turbines.
- Steam leaving the last stage of the turbine has a certain velocity which represents an amount of kinetic energy that cannot be imparted to turbine shaft and is thus wasted.
- Some energy loss takes place as steam flows from one stage to the next. The kinetic energy leaving one stage to the next. The kinetic energy leaving one stage and available to the next is given by ηCO (V22/2), where ηCO is the carry-over efficiency.
- In addition, there are some losses of energy due to wetness of steam (where the water particles are dragged along with steam at the expense of some kinetic energy of steam). If quality of steam is less than 0.88, erosion and also corrosion may takes place.
- Since the velocity of steam leaving the last stage of turbine is quite large (100-120 m/s), there will be energy losses due to friction in the exhaust hood of the turbine.
- Exhaust hoods to the condenser gradually increase in area like a diffuser and thus, there is a further decrease in velocity of steam and an increase in pressure as steam enters the condenser.
- Such hoods allow the turbine to operate down to a slightly lower pressure than that required by the condenser (depending on temperature and flow of cooling water and air extraction from its shell), thus increasing the turbine work.
External losses in steam turbines:
- There are some energy losses in the bearings and governing mechanisms which can be reduced by improving the lubrication system.
- Some energy is consumed by oil pumps. Since the turbines are adequately insulated the surface heat loss by convection and radiation is small.
Reference: Power Plant Engineering by PK Nag – Page 487