Complementary Differential Expansion Measurements with the SKF VM600 Protection System

As described in Application Note CM3074 “Straight Differential Expansion Measurements with the VM600”, the VM600 Machinery Protection System is specifically designed to perform critical measurements that are used in the control of large steam turbine generator trains. One of the most important measurements is that of differential expansion.

Differential expansion monitoring measures the change in axial clearances between the machine rotor and stationary casing caused by thermal changes inherent in most machines. The primary purpose of a differential expansion monitor is to guard against axial rub between rotating and stationary parts, the consequences of which can be catastrophic.

There are many configurations for measuring differential expansion. This Application Note discusses the common sensor configuration of complementary differential expansion or the “shaft collar method” as called by the VM600 software. The sensor orientation is considered, together with the appropriate MPC-4 Machinery Protection Card configuration example.