THE ESTIMATED ASSESSMENT OF THE IMPACT OF OPERATIONAL STRESS ON THE CRACKING OF THE DISKS OF ROTORS OF GAS PUMPING UNITS GTK-10-4

About 600 GTK-10-4 gas-turbine units are currently in operation on trunk gas pipelines in Russia. Data analysis and results of over 200 surveys of the technical state of assemblies and parts of GTK-10-4 gas-turbine units with different running time leads to the conclusion that the elements limiting the service life of GTK-10-4 are disks of high pressure turbines (HPT), which are subject to operational cracking.

The HPT disks are made of heat-resistant steel of martensite class EP428. The rated flue gas temperature before the HPT turbine is 780°С.

Calculation of stresses at the rated rotation speed was performed under a program to solve the problem with taking into account the creep of the materials of the disk and rotor blades in the plane stress axisymmetric condition by the finite element method. A stationary temperature field was used alternating in the radial direction and alternating in the radial direction, and averaged over the wall thickness. The elastic stress state at the initial moment of time without regard to plastic deformation at stresses above the yield limit as well as a technique taking into account small deformations and small displacements were considered. Plastic crumpling of contact surfaces is followed by intense plastic flow due to creep of the material. This crumpling is accompanied by redistribution of loads from the blades between all the pairs of teeth of the shank till the onset of the steady state.

The calculations of stress-strain state enable to estimate the possibility of occurrence of cracks in fir-tree grooves of disks and the lifetime of disks to cracking at operating temperatures above the rated one.

It is shown that overheating of the metal of the drive crown by 35 – 70°C above the rated value causes a multifold decrease of service life and may lead to formation of cracks on contact surfaces of teeth and necks of the inter-groove crown by the running time significantly shorter than that assigned by the manufacturer.

At operation, even short-term deviations of the gas temperature before the turbine above the rated value should be avoided. It is recommended to monitor the serviceability of the system for cooling the disk surface with cooling air and an anti-surge device. 

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