SIL 2 – Continuous Duty Torque Measurement for Plant Protection

Modified continuous duty torquemeter provides SIL2 protection for process-critical systems.

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.  Normally used to inform the end user of turbomachinery train location specific degradation, the basic condition monitoring Torquetronic Continuous Duty system is a useful plant tool for performance trending and diagnostics.

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

In June 2016 we were approached by our customer, Total Refinery Antwerp, who was an existing user of a torquemeter to accurately measure shaft horsepower, the turbomachinery in this case being a steam turbine driving a compressor on a naptha cracker to produce ethane. The torquemeter was primarily used to ensure the steam turbine did not over-drive the compressor, and to produce trend data for maintenance scheduling. With the introduction of a new compressor utilising a higher efficiency design, there was an increased potential for the existing steam turbine to over-drive the compressor; potentially damaging the system and increasing the safety risk to both plant and personnel.

The 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 Integrity 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).

Proof of concept study was carried out using a small torquemeter installed in our R&D department, providing data showing optimal alignment, separation and signal strengths.  This concept was witnessed and approved by the customer.  SIL investigation was carried out and documents assessed by an external body, to the satisfaction of the third party SIL assessment company who then issued a certificate of compliance for the SIL 2 level.  Torquemeters provided all equipment and documents, and the installation took place in line with customers required shutdown period.

“We needed to ensure safe running of the steam turbine/compressor and the torquemeter reading was perfectly placed to do this.  To be acceptable on our site, a SIL 2 functional safety system was required.  By Torquemeters providing a prototype of the development product and proving the concept accuracy and control, we were happy to move ahead with the complete project.  Since the measurement technology is based on designs successfully in use since 1974, reliability was not a concern.  Delivery was required for plant shutdown scheduled for March 2017 and the system was successfully installed on time.” – Total’s Project Leader

To use shaft horsepower as a control function on steam turbines, the Torquemeter TorSil product is unique.  Speed of reaction to enable the control system to operate without the need for complicated calculations involving head pressure, water/steam quality, flow and temperature is simplified using a standard torquemeter and is made Functionally Safe using the 2oo3 voting TorSil variant.  From concept to delivery in 18 months demonstrates how our R&D expertise and engineering systems can provide a new safety critical product, on-time and on budget.

In addition to providing normal Condition Monitoring data on shaft horsepower (trending, washing cycle needs, direction of fouling information), the customer has now an automated system with SIL 2 which provides a system needing no user intervention, other than that required for SIL 2 (a functional test every 5 years).  This releases plant manhours for other duties and ensures continued, safe plant operation.

Key features of the installation includes;

  • SIL2 protection rating
  • No rotating electronics
  • Ability to compensate for changes in environmental conditions
  • High degree of reliability and accuracy
  • Long service intervals and low maintenance requirements
  • Optimisation of turbine/compressor performance during commissioning

Long-term condition monitoring