(9b) SIL2 Torquemeter for Functional Safety Process Control of Steam Turbine Driven Compressor

Use of a torquemeter to accurately measure shaft horsepower independent of fuel, pressure, temperature, and flow variations is known to many oil and gas plant turbomachinery users worldwide. Used to inform the end user of turbomachinery train location specific degredation, the basic condition monitoring torquemeter system is a useful plant tool for performance trending and diagnostics. The large fuel use of steam turbines in ethylene manufacture makes plant efficiency extremely important to bottom line considerations.

For end users of torquemeter devices, it has always been possible for them to be used as process control devices, since accurate delivered power measurement helps run close to performance limits, maximising plant throughput and therefore revenue. The single measurement aspect of the torquemeter system does not provide for a functionally safe system in process control terms.

This paper outlines the successful development of a process system, using novel technology, which can be applied as a retrofit or new build system.

Summary

For one ethylene plant operated by Total, their existing torquemeter was successfully monitoring shaft horsepower and was set to react to an increase in demand from the compressor by ensuring the steam turbine power delivery was within acceptable limits for the compressor rotor rating. Any potential over run in power from the steam turbine was prevented by linking the torquemeter reading to the steam demand valve, which closed on the first sign of potential over torque. Since this process loop was safety critical, after a customer functional safety review a requirement was identified for a two out of three (2oo3) redundant voting system of torque measurement, so that in the unlikely event of a torquemeter failure, the manufacturing process could continue safely. The Safety Intergity Level required for plant compliance from the customer was SIL 2. Accuracy during normal running was to be maintained at +/-1% of reading at normal torque and operating speed.

Single failure mode (plant still running) had to maintain accuracy at +/-5% of reading at normal torque and speed. To move from a single reading torquemeter to a 2oo3 voting functional safety torquemeter required the integration of three pairs of measuring coils, detecting three independent values, which could be processed in three independent SIL Modules to provide 3 completely independent values for comparison by the customers data acquisition system and associated Emergency Shutdown system (ESD). This paper will explain how proof of concept study was carried out using a small R&D developed torquemeter; how the recorded data showing optimal alignment, seperation and signal strengths led to the final design; how the end-user was involved in all stages of the development; and how the final design was installed. It will also summarise some of the SIL concepts and work carried out to achieve the required level 2 product.