Ching-Po Liu

Abstract

An improved procedure is developed to reduce the thermal shock influence in a steam turbine due to a cold start operation. Thermal shock, a term used to describe the sudden imposition of a large heat flux and temperature gradient on a material surface, is a potential damage mechanism if its intensity is sufficiently high. For the case of a turbine cold start, the thermal shock intensity depends on the film coefficient and the initial temperature difference between the steam and the rotor surface. Reduction of thermal shock intensity can be achieved in several ways, such as lengthening the transit time. lowering the film coefficient, and reducing the temperature difference between the steam and the rotor surface. One of the most efficient procedures employed to control the thermal shock influence is to increase the heating steps and warm up time. A case studied in this project, the normal two-step heating operation causes a very high compressive stress, 437 MPA, on the platform during the first heating step. Sometimes its magnitude is two or three times greater than the compressive stresses during second heating step. In this two-step heating operation the first heating temperature was 204C and the total operating time for the first heating step was 75 seconds. An improved procedure, a three-step heating operation, was developed to make the stresses caused by each heating step uniform. The first heating step of the two-step heating operation was substituted by the first two heating steps of the three-step heating operation. The maximum thermal stress as a result of the improved operation was 358 MPA and the total operating time of the first two steps was 22.5 minutes. Under the 22.5 minutes operating time the optimum temperature for the first heating step was 162C, followed by another temperature increment to 204C. Another study was conducted to study the effects of thermal stresses on different film coefficients (heat transfer coefficients). Under the same operating conditions the stress did not change significantly for the film coefficients from 50% to 150% of the normal operating value of 10,000 W/m-K. The thermal behavior obtained in this project will be useful for a turbine designer to ensure that no propagating crack would be formed within the life-time of the design machine. It is obvious that the longer the warm up time, the lower the thermal shock will be. From an economic point of view, longer warm up times are undesirable. A turbine designer needs to consider both these aspects before arriving at a strategy for the cold start.

Library of Congress Subject Headings

Steam-turbines; Thermal stresses; Shock (Mechanics)

Publication Date

1988

Document Type

Thesis

Department, Program, or Center

Mechanical Engineering (KGCOE)

Advisor

Names Illegible

Comments

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works. Physical copy available through RIT's The Wallace Library at: TJ737.L58 1988

Campus

RIT – Main Campus

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