A three-cell, multi-burner fired heater in a large refinery was experiencing unnecessary interruptions in service due to failures of the existing instrumentation and control systems. The heater’s programmable logic controller (PLC) based Emergency Shutdown (ESD) System was experiencing nuisance trips and alarms caused by failed instrumentation end devices (switches and solenoids) and communication failures within the existing multi PLC network.
The ignition system’s fuel to air mixture was controlled by long mechanical linkages between the burner’s fuel gas control valves and the combustion air control valves of each cell that had a long history of poor performance. Change in operating conditions, charge rate, or fuel gas quality required manual readjustment of the burners due to changes in flame pattern. Occasionally, the process was affected because operating parameters went out of specification, affecting product yield and quality.
The company sought a solution to increase heater and ESD system reliability, improve performance and address the inefficiencies of the equipment.
M S Benbow and Associates (MSB) was part of the team assigned to study and identify ways to increase heater and ESD system reliability and develop a safety instrumented system (SIS) to meet the client’s requirements. MSB performed front-end engineering and developed a high quality (+/- 30%) total erected cost estimate. The plan would upgrade the performance of the SIS to meet the client’s Safety Integrity Level (SIL) target and would include process piping and instrumentation upgrades.
MSB calculated the SIL level of the safety instrumented functions (SIF) associated with the heater based on existing process and instrument configurations. The effort included participating in HAZOP studies of the heaters, verifying the process and instrumentation diagrams and defining and documenting the SIS, based on ANSI/ISA S84.01 standards.
The scope of work also included identifying methods to improve performance such as the addition of a new triple redundant ESD system and new bridgewall pressure transmitters to detect flue gas pressure. Other identified areas for upgrade included: enhancing basic firing control operations utilizing new TDC/DCS fuel gas control valves, O2 control, replacement of existing O2 analyzers, and replacing the existing fuel gas knock out drum to eliminate liquid entrainment to the burners.
In keeping with the client’s commitment to improved efficiency and safety, MSB designed a plan to retrofit the heater instrumentation and control systems that met the client’s needs, as well as applicable OSHA, ISA and NFPA standards. MSB recommended the following process configurations that would meet the client’s SIL targets and heater reliability and availability needs:
- Replace (3) existing stack analyzers with analyzers capable of measuring O2 and combustibles (CO) to improve the controlled combustion air control system.
- Modify the existing fuel gas piping and shutdown valve configuration in an effort to reduce the total number of valves and increase the reliability of the shutdown system and availability of the heater. The new configuration would reduce the number of fuel gas shutdown valves from one per burner (22 total) to two per cell (6 total), which would allow both a shutdown on a cell basis as well as a full heater shutdown.
- Replace (11) existing PLCs with a new Triple Modular Redundant (TMR) system.
- Replace outdated flame scanners with UV self-checking capabilities.
- Install triplicated transmitters where needed to meet the SIL level of specific and install new transmitters to replace existing ESD switches.
The client accepted MSB’s recommendations and is in the process of appropriating the necessary capital project funding.