Subsynchronous Vibration In A Large Water Flood Pump

Abstract

In 1973 Aramco embarked on a large water flood projects for the Ghawar oil reservoir. After approximately 1 year in service three of the 16 X 16 X 20 2-stage DVMF pumps developed a severe shaft vibration of over 20 mils at a subsynchronous frequency. Testing of one unit with both vibration and pressure pulsation instrumentation indicated the problem was caused by a hydraulic excitation of a shaft critical speed. This paper describes the unique solution to the problem by modifying the suction and discharge piping from abrupt bell shaped reducing/expansion section to long tapers with a 3 to 1 length to diameter ratio. This piping change eliminated the vibration instability on all three machines without resorting to machinery redesigns. The modification was extended to the three additional units in the system and as a bonus, the life of impellers, which had suffered cavitation and vane cracking damage, was improved by a factor of three.