Dummy piston arrangement on Parsons Turbines&Double Flow Turbines

Dummy piston arrangement on Parsons Turbines

In parsons reaction turbines there is always an end thrust due to the steam at inlet being higher than the exhaust. This leads to high thrust bearing loading. The dummy piston arrangement is a wheel or drum integral to the rotor. Forces are balanced by the drum offering a greater surface area to the low pressure balancing steam than to the HP steam.Note the drawing above is not to scale. A labyrinth arrangement is fitted to seal the drum.

Double Flow Turbines

These are found mainly on large LP turbines. Here steam enters mid rotor and passes axially towards both ends. The advantages are;

• End thrust is balanced removing need for dummy pistons or cylinders on reaction turbines . Reduces the size of the thrust on impulse-reaction turbines
• As steam flow is split the final stages blade height and angle is reduced allowing for increased efficiency and reduced centrifugal stress. Greater power per unit size may be absorbed.

The main disadvantage of this system is increase rotor length leading to increased risk of sagging

Blade Sealing

May be end or tip tightening

End Tightening
This is seen particularly on reaction turbines. It requires accurate positioning of the turbine rotor and is normally associated with lengthy warm up perios during which the position of the rotor is carefully monitored. Operational limitations on rapid power changes may be in place. The author has seen this system in use on very large but compact turbo alternators which required a warm up period consisting of increaseing the rotor speed in stages over one hour

Tip Tightening
Clearance is governed by maximum blade centrifugal stretch