Thursday, 24 May 2012

Problems caused by incorrect warming through

The main object of warming through is to ensure straightness of the rotor.To do this a negligible temperature gradient must exist throughout the rotor. There is a tendency for the rotor to hog where the steam is introduced( that is to say the rotor bends due to temperature gradient rather than sagging under gravitational forces) with the rotor steam is introduced. Hence the rotor must be rotated.
The graph below indicates the importance of this.
graph of delection against speed
The line is the out of balance force due to centrifugal force equal to the mass of the rotor.
Hence, the offset at 3000rpm to cause an out of balance equivalent to the mass of the rotor is 0.102 mm

testing of the engines after shut down ahead and astern should be taken as part of the warming through process. Close watch of the relevant nozzle box temperatures is a good indication of the condition of the turbine.
Second object of warming through is to prevent distortion of the casing. Rotation of the rotor churns up the steam and provides adequate mixing. With underslung condensers the temperature gradient is virtually unavoidable, hence separate condensers are better.
The third objective is to prevent thermal stresses caused by the temperature gradient in thick materials such as at the bolt flanges. Vertical slots are often provided to help alleviate this problem, this distortion can also lead to non concentricity of the casing
Distortion of casing
This is particularly prevalent in open cylinder designs such as axial plane or double casings.
Heat transfer rate is at its greatest where the steam is condensing on the surface of the casing. This in turn is governed by the inlet pressure of the warming through steam. Hence, warming through in steps providing adequate period to stabilise the temperature at each step.
Complete warming through cannot occur until nearly at full power , hence, warming through much above atmospheric saturation temperature is pointless.
Also as part of the LP turbine runs at lower temperature, warming above 100oC is unnecessary. Protracted warming through periods are unnecessary. A temperature of 82oC at the LP inlet belt in 30 mins is acceptable
Vibration caused by an out of balance of the rotor may be alleviated by running for a short period at reduced engine speed followed by a slow increase in speed.

1 comment:

  1. I am the author of most of the work on your website against which you are making financial gain. If you do not gain permission or remove it I will take further action. My site is non-commercial, I note you have placed content from which is a commercial site, you are in breach of his copyright and as such ne may not be as generous in allowing you time to remove his work before taking further action.