Motor & ID Fan System for Main Crude Unit at Refinery
Engineering Dynamics Incorporated (EDI) performed a field test of an induced draft (ID) fan system at a refinery that was experiencing failures of couplings, flexible disc style. The fan is part of an atmospheric furnace that heats approximately 152,000 barrels of crude per day. The ID fan is driven by a 350 HP induction motor. The motor speed is controlled by a variable frequency drive (VFD) from 0 to 1200 RPM. The trouble began when the motor was changed out for one of similar electrical performance, but of different physical size.
The failure of the original flexible disc coupling consisted of a crack in the spacer, which appeared to originate at a bolt hole. Initially, plant maintenance was blamed for possibly over tightening the coupling bolts. The 45 degree angle of the crack in the coupling spacer is a typical indication of high torsional vibration.
To quantify the transmitted and dynamic torque in the coupling, a TorqueTrak 10K telemetry system from Binsfeld Engineering was used with a Vishay strain gage. The waterfall plot below shows a torsional natural frequency (TNF) of the system near 58 Hz, which was being excited by 1× electrical frequency of the VFD. This resulted in high dynamic torque in the coupling when operating the fan at 1000 - 1200 RPM.