Energy-Tech Magazine

The causes of rotating machinery vibration problems are not always obvious. The effects of pipe strain and lubrication issues for example, can be difficult to identify. This article describes an example of one of these cases.

This case study pertains to a steam turbine that drives a 1 MW generator through a gearbox. The turbine consists of a single Curtis stage, which is comprised of two wheels. The turbine operates at 5,025 rpm and is coupled to a gearbox that reduces the speed to 1,787 rpm. The steam turbine inlet conditions are 650 psig and 700°F, and the exhaust pressure is 265 psig. The steam turbine is integrated into a hydrogen production plant and the exhaust steam is exported to a customer via a pipeline. At low hydrogen plant rates there is not enough steam produced to run the steam turbine.

The steam turbine was commissioned in 1999, but was used intermittently based on available steam flow. A picture of the turbine train is shown in Figure 1.

The turbine is fitted with sleeve-type journal bearings, tilting pad type thrust bearings, and inlet

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end and exhaust end “X” and “Y” shaft radial proximity type vibration probes. The gearbox is a double helical type with sleeve-type radial bearings and a single pinion shaft proximity type vibration probe. There are no thrust bearings in the gearbox and a common lubrication system provides lube oil to the turbine, gearbox and generator. The main oil pump is driven from the bullgear shaft and is mounted above the oil reservoir. There also is a motor driven auxiliary oil pump that is installed on top of the oil reservoir. The turbine-to-gearbox and gearbox-to-generator couplings are both flexible disc-type couplings. The overall arrangement is shown in Figure 2. The steam turbine speed is controlled by a Woodward 505 digital governor system.  

2008 failure
In October 2008, the steam turbine shut down on low oil pressure. There were numerous low oil pressure alarms and high gearbox radial vibration alarms that preceded the shutdown. A gearbox inspection revealed that the gearing was heavily worn, so the gearbox was removed and re-built with new gearing, new seals and repaired bearings. There were significant steam leaks from the steam turbine seals and since there was a history of high turbine vibration prior to the gearbox failure, it was decided to overhaul the steam turbine before re-starting it. However, the manual block valve upstream of the turbine leaked by and the turbine could not be isolated from process steam. Since there was a scheduled plant outage in 2009, plant personnel decided to wait until the plant outage to overhaul the turbine and then re-start it. In the interim, the generator was removed and overhauled at an outside shop.

2009 rebuild
In October 2009 the hydrogen plant was shut down for 2 weeks for routine maintenance. During the outage, the steam turbine was disassembled and inspected. As expected, there was radial and thrust bearing damage and shaft seal damage. The turbine rotor also was bowed.

As a result, the turbine was re-built with new bearings and new carbon ring shaft seals. A new turbine shaft also was manufactured, and the old wheels were cleaned and installed on the new shaft. The assembled rotor was dynamically balanced in two planes in accordance with API 617 criteria. A residual unbalance check also was performed in accordance with API 617.

During the outage the steam turbine trip/throttle valve was overhauled, the gland steam condenser was overhauled and the steam turbine inlet manual block valve plug and seat were replaced. The lube oil also was replaced, the system was flushed, and all the instruments were calibrated and the controls were function tested.

2009 start-up
In November 2009 the steam turbine was re-started following the normal start-up procedure. In this procedure, the Woodward controller ramps the turbine to full speed at a pre-defined rate.