A Case Study: Unusual Problems Discovered During The Rerate Of 2800 HP Boiler Feed Water Pumps And Turbines

Abstract

The boiler feedwater system in a world scale ethylene plant utilized horizontal multistage barrel case pumps and single stage impulse type steam turbine drivers with mechanical governors. As part of a major debottlenecking project, they were rerated to provide more flow. The entire pump inner case assemblies (including rotor and inner casing) were replaced with higher capacity elements. The steam turbines had new governor valves and nozzle rings installed to produce more power. The control scheme was modernized with an electronic governor that adjusted the pump speed to maintain a constant header pressure. The new equipment was installed in September 1994, nothing went well. The pump experienced high blade pass frequency vibrations. The solution required unique hydraulic modifications and inboard bearing bracket stiffening to eliminate a resonance. The turbines tripped mysteriously and broke governor valve stems. The cause was a valve resonance problem that were teething problems with the electronic governor control system that took time to sort out. These problems became very high profile when the newly rerated boiler feedwater pumps tripped and caused a total plant shutdown resulting in a 14 hr flare. Not only was there a major financial impact from lost production, it also prompted the ministry of the Environment to initiate a review of the company’s entire flaring history. Although it took a difficult year of effort, the pumps and turbines are now operating satisfactorily. The problems discovered are discussed, along with the analysis performed and the solutions implemented.