Claims:We claim:
1) A means provided on a rotor of a turbine for relieving thermal stresses comprising a recess (100) circumferentially formed on the rotor of the turbine at control stage in the direction of steam flow path preceding a groove of first stage blades (G1), said recess (100) depth is made equal to the depth of the groove of first stage blades (G1) to facilitate in faster heat transfer to said groove of first stage blades (G1).

2) A means provided on a rotor of a turbine for relieving thermal stresses as claimed in claim 1, wherein said recess (100) is smooth contoured and kept as broad as possible to minimize concentration of stresses and avoid loss of utilization of fluid.

3) A means provided on a rotor of a turbine for relieving thermal stresses as claimed in claim 1, wherein said recess (100) facilitates in thermal expansion of said rotor of said turbine thereby avoiding building up of stresses on the rotor.

4) A means provided on a rotor of a turbine for relieving thermal stresses as claimed in claim 1, wherein said recess (100) facilitates in distribution of stress between said groove of first stage blades (G1) and the recess (100).

5) A means provided on a rotor of a turbine for relieving thermal stresses as claimed in claim 1, wherein said recess (100) facilitates in less rate of change of temperature to said groove of first stage blades (G1) from said recess (100) and ultimately resulting in the development of less thermal stresses on the groove of first stage blades (G1).

6) A means provided on a rotor of a turbine for relieving thermal stresses as claimed in claim 1, wherein said recess (100) facilitates in prolonging the operative life of the rotor.
, Description:FIELD OF THE INVENTION:
The present invention relates to the field of reduction of thermal stresses on a rotor of a turbo machine. Particularly, the present invention relates to a provision of a means on a rotor of a turbine for relieving thermal stresses on highly stressed portions of the rotor.

BACKGROUND OF THE INVENTION:
Turbines and its components especially like rotors are subjected to high thermal stresses due to high temperature gradients created during startup and shut down cycles of the turbines. Generally, Startup cycles are of three types:
1) Cold Startup: Shut down time > 72 hours & Casing flange temperature < 100 degrees.
2) Hot Startup: Shut down time < 2 hours & Casing flange temperature > 300 degrees.
3) Warm Startup: Shut down time < 12 hours & Casing flange temperature-100 degrees to 300 degrees.
Due to daily start up and shut down cycles, repeated application of cyclic loads on a rotor of a turbine and concentration of peak stress at portions of inlet of steam (i.e., at control stage, initial stages of HP module etc.,), the rotor is subjected to high thermal stresses resulting in the initiation of cracks at the groove of the first stage blades which ultimately results in the reduction of operative life of the rotor.

The thumb rule in Thermodynamics & Heat Transfer is that thermal stress is directly proportional to rate of change of temperature.

Figure 1(a) illustrates a cross-sectional view of a rotor of a turbine.

Figure 1(b) illustrates an enlarged view of portion A of figure 1(a).

The portion ‘A’ highlighted in the figure 1(a) is the region of the rotor which is subjected to high thermal stresses due to inlet of superheated steam at control stage of the turbine. Generally speaking, heat transfer in steam turbines happens through convection and conduction and grooves are pockets of high stress concentration. But, heat transfer to the groove of first stage blades primarily happens through conduction only as the groove accommodates a plurality of first stage blades.

So, for example, if the temperature at control stage of the rotor is 500 degrees centigrade because of heat transfer from steam through convection and is generally indicated by a reference numeral T1, the temperature at the groove of the first stage blades would be approximately 200 degrees centigrade for a given period of 2 hours because of conduction and is generally indicated by a reference numeral T2.

The effective rate of change of temperature at the groove of the first stage blades is 300 degrees centigrade for a given period of 2 hours.

Several techniques have been disclosed in the prior art to reduce thermal stresses on different components operable at high temperature gradients.

US Patent no. 8684663 filed on September 29, 2010 titled “Steam Turbine with Relief Groove on the Rotor” discloses a relief groove disposed in opposite direction to the inlet steam flow on a rotor of a turbine wherein the relief groove primarily acts as a thrust balancing mechanism. The relief groove is of bulb shaped circumferential groove located on the rotor in opposite direction of steam expansion to facilitate in minimizing loss of steam and maximizing turbine performance. So, only leakage flow can enter the relief groove and the reason for providing the bulb shaped structure to the relief groove is to avoid concentration of stresses due to sharp edges. The relief groove is provided with a cover at top of its opening narrowing the passage of entry of leakage steam in to the relief groove thereby reducing or eliminating the effect of vortex flows inside the relief groove. The relief groove is also provided with a mechanism for cooling which supplies cooling steam to the relief groove through a passage and thereby reduces the excessive heating of the rotor. The granted patent is silent and did not describe fully and particularly the method of reduction of thermal stresses on the groove of the first stage blades just by provision of the relief groove in opposite direction of the steam flow.

US Patent no. 2987143 filed on October 16, 1958 titled “Reduced Thermally Surface Stressed Brake” discloses a brake member provided with a plurality of bulb shaped grooves formed intermittently in a circular fashion on surface of the brake member, wherein the plurality of bulb shaped grooves provides space for thermal expansion thereby reducing thermal stresses on the brake member.

Therefore, there is felt a need for provision of a means on a rotor of a turbine to overcome the drawbacks of the prior art thereby reducing thermal stresses on the groove of the first stage blades and ultimately prolonging the operative life of the rotor.

OBJECTS OF THE INVENTION:
An object of the present invention is to provide a simple means on a rotor of a turbine for reducing thermal stresses.

Another object of the present invention is to provide a means on a rotor of a turbine without loss of utilization of steam and having no effect on performance of the turbine.

One more object of the present invention is to provide a means on a rotor of a turbine to facilitate in free thermal expansion.

Still another object of the present invention is to relieve thermal stress at highly stress concentrated portions on a rotor of a turbine.

Further another object of the present invention is to reduce rate of change of temperature at highly stressed portions on a rotor of a turbine.

Still one more object of the present invention is to prolong operative life of a rotor of a turbine.

SUMMARY OF THE INVENTION:
In accordance with the present invention a means on a rotor of a turbine for relieving thermal stresses is provided, the means comprising a recess (100) circumferentially formed on the rotor of the turbine at control stage in the direction of steam flow path preceding a groove of first stage blades (G1), the recess (100) depth is made equal to the depth of the groove of first stage blades (G1) to facilitate in faster heat transfer to the groove of first stage blades (G1).

Typically, the recess (100) is smooth contoured and kept as broad as possible to minimize concentration of stresses and avoid loss of utilization of fluid.

Typically, the recess (100) facilitates in thermal expansion of the rotor of the turbine thereby avoiding building up of stresses on the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS:
The invention will now be described with reference to the accompanying drawings in which:

Figure 1(a) illustrates a cross-sectional view of a rotor of a turbine according to the prior art;

Figure 1(b) illustrates an enlarged view of portion A of figure 1(a);

Figure 2(a) illustrates a cross-sectional view of a rotor of a turbine in accordance with the present invention; and

Figure 2(b) illustrates an enlarged view of portion B of figure 2(a).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
A preferred embodiment will now be described in detail with reference to accompanying drawings. The preferred embodiment does not limit the scope and ambit of the invention. The description provided is purely by way of example and illustration.

Figure 2(a) illustrates a cross-sectional view of a rotor of a turbine in accordance with the present invention.

Figure 2(b) illustrates an enlarged view of portion B of figure 2(a).

In accordance with the present invention, there is provided a means on a rotor of a turbine comprising a recess 100 circumferentially formed on a rotor of a turbine at control stage in the direction of steam flow path preceding a groove of first stage blades G1. The recess 100 is smooth contoured and kept as broad as possible to avoid/reduce concentration of stress. The recess 100 facilitates in stress distribution between the groove of first stage blades G1 and the recess 100 and also helps in thermal expansion.

Importantly, for example, if the temperature at control stage of the rotor is 500 degrees centigrade because of heat transfer from steam through convection and is generally indicated by a reference numeral T1, the temperature at the groove of the first stage blades G1 would be approximately 400 degrees centigrade for a given period of 2 hours because of conduction and is generally indicated by a reference numeral T3.

The effective rate of change of temperature at the groove of the first stage blades is 100 degrees centigrade for a given period of 2 hours which is 200 degrees less than the prior art ultimately resulting in less thermal stress at the groove of the first stage blades G1.
The design requirement herein is that the depth of the recess 100 should be equal to the depth of the groove of the first stage blades G1 such that conduction of heat from the recess 100 happens faster to the groove of the first stage blades G1 because of less material which results in less effective rate of change of temperature and ultimately resulting in the development of less thermal stresses on the groove of the first stage blades G1.

The provision of groove may result in loss of utilization of fluid and concentration of stresses whereas provision of recess 100 results in smooth flow of fluid without much concentration of stresses.

TECHNICAL ADVANCEMENTS:
A means provided on a rotor of a turbine for relieving thermal stresses has several technical advantages including but not limited to the realization of :
• a simple means on a rotor of a turbine for reducing thermal stresses ;
• a means on a rotor of a turbine without loss of utilization of steam and having no effect on performance of the turbine ;
• a means on a rotor of a turbine to facilitate in free thermal expansion ;
• a means to relieve thermal stress at highly stress concentrated portions on a rotor of a turbine ;
• a means to reduce rate of change of temperature at highly stressed portions on a rotor of a turbine ; and
• a means to prolong operative life of a rotor of a turbine.

Although the invention has been described herein above with reference to the embodiments of the invention, the invention is not limited to the embodiments described herein above. It is to be understood that modifications and variations of the embodiments can be made without departing from the spirit and scope of the invention.