Inspection of Vibration and Stresses on Compressor
Blades on Gas Turbine

ABSTRACT

The most important and expensive components of gas turbines are the blades. A number of unexpected and unusual compressor blade (row # 2) failures at the Jeddah plants recently prompted a rigorous scientific investigation to identify the problem source. Very specifically, the problem was to determine why blades would be fractured only after about 7826 hours of operation, instead of the average normal life expectancy of 60,000 hours. Another failure of compressor rotor blade had occurred in Feb. 1988. The blade had served for only 6680 hours in three years after mounting. Unavoidable vibrations being a possible factor, a program was initiated to investigate the dynamic characteristics of the compressor blades of the second row experimentally, theoretically and numerically, also the compressor blade stresses as developed from normal loading conditions as well as the stresses developed from vibration of the blade, which are the subject matter of this research.

In the experimental investigation a blade was mounted on the turbine rotor and excited sinusoidally. In the theoretical approach the beam theory was used, accounting for bending and torsion, as well as twist and taper of the blade. In the numerical approach, the NISA finite element package was used to provide the numerical analysis, employing an eight-nodes isoparametric element each with three degrees of freedom. The results obtained from the three methods agree reasonably well. The first fundamental frequency was found to be 127 Hz. With the turbine operating speed of 3600 rpm (60 Hz), it is observed that the fundamental frequency is only 6 % above the second harmonic.

Stress analysis of the compressor blade was based upon the finite element method. The NISA program was used for determination of the stress distribution in the x, y, z directions as well as the principal stresses. Based on the various theories of failure namely; the Maximum shear, the von Mises and the Octahedral shear stresses, were also determined. The stresses were determined from the centrifugal force and the air pressure indicate that the maximum stress occurs on the suction side at the root. Also the highest alternating tensile stress occurs on the suction side at the root of the blade. And the vibratory stress determined from the first six frequencies indicate that the normal stresses along the blade in the x, y, z directions are higher than the tensile strength of the material of the blade.