Steam turbine CONDITION?MONITORING
Timely PCM averts second turbine failure at Corus steelworks1 December 2009
Swift action to install a condition monitoring system prevented the failure of a second steam turbine at the Corus steelworks in Scunthorpe, UK
The central power station on the Corus steelmaking site in Scunthorpe, UK, provides electricity for the site by burning exhaust process gases – waste gas from the BOS (basic oxygen steelmaking) plant and blast furnace gas, mainly nitrogen, oxygen, sulphur dioxide, carbon monoxide and carbon dioxide. This is on a grand scale – there are six steam turbines generating up to 53 MW of energy for the facility. The balance of demand is drawn from the grid.
It was only when turbine No. 5 was taken off-line for routine maintenance that serious cracks and the loss of a rotor blade were discovered (Figures 1, 2).
The discovery gave rise to suspicions about an identical turbogenerator at the plant, so to ensure its continued safe operation the maintenance team decided to conduct immediately a HAZOP (hazard and operability analysis) study on turbine No. 4. A protective cover was put in place immediately, but it was clear that early failure detection was also required.
‘We needed a quick response vibration monitoring unit to provide protection not only for the equipment but for our personnel’ says maintenance engineer Josh Mortimer. ‘Having seen how successful the Corus PCM [plant condition monitoring] team had been at Teesside and in other areas of the Scunthorpe plant, in preventing plant failures, I was keen to identify a system that was quick and easy to install and that was also easy for our operators to understand. The FAG DTECT X1 unit seemed ideal for our requirements.’ ‘Teesside’ here refers to the installation of a similar system at the company’s beam mill on Teesside as a result of which plant engineers had been able to detect the need for, and manage, an internal redesign to the hot saws gearbox after it became apparent that there were inherent design faults within the gearbox itself that were causing regular bearing failures.
It was decided to install the DTECT X1 system (Figure 3) on the exciter end bearing and the turbine end bearing of turbine No. 4. Shortly afterwards the online vibration monitoring device detected a problem with one of the rotor blades. ‘Our attempts to protect this generator paid off’, says Chris Smith, Scunthorpe’s PCM engineer. ‘By picking up the blade failure early on, we were able to prevent the failure of the whole turbine. In lost generation alone, this machine is worth £35 000 a day. Minimal damage and a quick return to service is obviously the preferred option.’
Wolfgang Hahn, manufacturing manager for the central power station at Corus Scunthorpe confirms the company’s intention to expand the scope of the PCM department. ‘PCM has been working well for us. We get a good service from the department and have saved the company a lot of money’, he confirms. ‘We are therefore moving towards a full condition-based monitoring system. Energy prices remain high so it is in our own interests to reduce the need to draw from the grid by maintaining reliable equipment in our own plant.’
The FAG DTECT X1 is at the economy end of FAG’s range of condition monitoring equipment although it is claimed to have characteristics that would normally only be found on more expensive systems. It allows early detection of damage by selective frequency vibration monitoring based on individually adjustable frequency bands. Using this method, specified machine parts can be monitored. The signal collected by the sensor is broken down into its frequency components by means of fast Fourier transformation.
The system is flexible and can be matched to the requirements of the application. The base device is available as a 2 or 8 channel system with an external multiplexer, but all conventional acceleration sensors, speed sensors and proximity probes can be attached. Process values such as speed, temperature, load, torque and pressure can be recorded.