The main production facility for a UK fertilizer company produces ammonium nitrate which is subject to the UK Control of Major Accident Hazards (COMAH) regulations. This organization ensures that businesses take all necessary measures to prevent major accidents involving dangerous substances such as this one and is enforced by the UK Health & Safety Executive (HSE). A review of the emergency shutdown system under IEC61508 Functional Safety Standard revealed an urgent upgrade was needed of electrical, control and instrumentation to satisfy SIL2 System designation.
One of the main areas needing attention was the critical temperature monitoring of the reactor process and the requirement for hardwired alarms on selected Safety Loops because alarms were being generated by a combination of the diverse technology being used and the alarm software of the DCS. Modern safety standards do not allow combining control and shutdown systems in the same DCS.
The Moore Industries STZ and STA were chosen by the company and HSE to provide a hardwired shutdown system meeting the requirements for SIL2 system capability. The highly accurate STZ temperature transmitter features sensor backup and failover capability along with a flexible configuration to accept a range of existing thermocouples. The STA trip alarm, which is the company’s plant standard for SIL hardwired trip alarms, features highly reliable relays and configuration software that is easy-to-use yet robust.
The STZ accurately measures redundant temperature sensors and provides measurement and sensor diagnostics. Should the primary sensor fail, the backup/failover sensor will be transmitted to the STA, Safety Trip Alarm. The STA relays trip when conditions exceed the temperature high and low limits for process safety, triggering an alarm and emergency shutdown. The STA also retransmits its temperature input back to the DCS, via its 4-20mA analog output.
- Sensor Backup/failover of STZ
- Relay trips