Turbomehanika successfully retrofitted the controls for two identical steam turbine trains that are parts of the Old Nitric Acid plant in Petrokemija Kutina. The trains are manufactured by Escher Wyss and commissioned back in 1968. Operating continuously since. Each train consists of a steam turbine, two process compressors and a gas expander all coupled together. The steam turbine being the main drive with the expander supplementing it by expanding the process product gasses through.
The scope of the retrofit was to replace the existing steam turbine control and the compressors antisurge protection with a new PLC based Integrated Turbine Compressor Controls (ITCC) system. Siemens SIMATIC S7-1500 PLC together with Bently Nevada Adapt 3701/55 ESD are used as the PLC hardware.
Turbomehanika have been the main contractor for the job with Arirang and Intea being subcontractors.
The Old Control System
The old control system consisted of hydraulic turbine controls and compressor antisurge protection. They were both using the same 7barg hydraulic/lube oil system.
Steam Turbine Controls had a flywheel isochronous governor operating three steam control valves actuated by a linear hydraulic servomotor. The governor and the trip unit were both integrated together at the steam turbine front pedestrian. The project spec required to use the existing trip device. Separating one from each other was quite a challenging task that required gaining a deep knowledge of the Escher Wyss control concept. The governor was cut out while the trip device and the servomotor remained a part of the current system.
Compressor Antisurge System had a control unit and the blow-off valve operated by a hydraulic servomotor. The control unit was cut out while the blow-off valve with its servomotor remained parts of the current ITCC.
All through the recent years the customer were keep experiencing issues with the antisurge controls. It would open the blow-off valve when it was not supposed to causing costly plant shutdowns. While commissioning the new controls we found that in some operating modes compressors are expected to run quite near to the surge region. The old antisurge controls were most probably not being able to deal with that operating situation.