Shell Canada is a major oil and gas company in Canada. One of Shell's many technical accomplishments is the development of the boiler safety shutdown system at its Sarnia Manufacturing Centre (SMC) located in Corunna, Ontario.
In addressing issues related to the safety of fired equipment, Shell determined that an improved Instrument Protection System (IPS) was needed to safeguard Boilers 3 to 6 at SMC, against spurious shutdowns, unsafe conditions, and unwanted combustion conditions.
The existing boiler protection system relied on instruments that detected the presence of flames and that monitored combustion airflow and steam drum level. If any of these instruments generated an alarm, the corresponding boiler was locked up. This arrangement resulted in many nuisance shutdowns due to instrument failure.
Key project objectives
* The key to eliminating these nuisance shutdowns was the elimination of single-point failures.
* The IPS also had to provide the means to monitor and protect the boiler, including an automated shutdown and purge-cycle that would be activated by alarm conditions.
* A graphic interface was needed to assist the operator in performing a safe boiler start-up.
* The new IPS had to facilitate changes to field devices, allowing a field instrument to be removed from daily service - for repair or maintenance - while the boiler continued to operate safely.
The software
The system integrator, Hinz Consulting of Calgary, Alberta, recommended the GE Fanuc Genius Modular Redundancy (GMR) system, because of previous experience with GMR technology in Shell's Isomax hydrocracker process. These systems, installed at SMC in July 1993, have historically demonstrated the reliability of the GMR system.
In the oil and gas industry where high-availability systems and maintaining stringent regulations are essential, a solution provider with the right kind of experience and expertise is required. GE Fanuc Automation offers critical control solutions that have been field-tested in hundreds of installations around the world with TüV SIL 3 certification. GE Fanuc Automation works with customers as a partner to enable seamless, fault-tolerant systems to ensure continuous uptime.
The solution
To make the new system cost-efficient, two duplex GMR systems were implemented. GMR requires identical central processing units (CPUs) and application programs. One system monitors Boilers 4 and 5, while the other monitors Boilers 3 and 6.
Each GMR system consists of two Series 90-70 programmable logic controllers (PLCs), individually connected to three buses, A, B, and C. Each bus controls a Genius I/O subsystem for triple or dual redundancy of input field devices.
Each PLC receives triplex or duplex discrete or analog input signals from the three buses and independently performs a two-out-of-three (2003) or 2002 vote on each input. The outcome is placed in the I/O table used by the application program. Single-user discrete outputs are directed from each of the PLCs to Genius output blocks (wired in the H configuration group) where 2003 voting is performed on the command output state.
A second voting stage is then performed by the application program to determine whether the boiler should be shut down on the following conditions: no flame detection, insufficient combustion airflow, or low steam drum level. If shutdown is required, the automatic shutdown sequence is carried out, followed by an automatic purge cycle.
A GE Fanuc Automation Cimplicity human machine interface (HMI) software package is implemented to display realtime data and to assist the operator during start-up of the boilers. Cimplicity software communicates to the PLC through a multidrop Series 90 Protocol (SNP) network.
The benefits
The Boiler Safety Shutdown System was implemented as part of Shell's management plan to ensure the safety and integrity of the facility. GE Fanuc Automation's GMR safety solution met the customer's need for on-line maintenance, calibration, and testing of all equipment and provided the following additional benefits:
* Proven performance for diagnostics coverage (provided by the Genius I/O blocks).
* Cost effectiveness (based on typical PLC pricing).
* Ease of use and high acceptance by experienced PLC users.
* Approval by TüV.
For more information contact Daniel Coetzee, GE Fanuc Automation, 031 583 2602, daniel.coetzee@gefanuc.com
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