Faster tripping: Speed lowers energy, boosts arc-flash safety at Shell refinery
Vaasa, Finland — Shell Canada is realizing improved operator safety at its Sarnia-based petrochemical refinery with an electrical arc protection system. The
system uses light and current sensing and protects the continuous-processing plant against costly nuisance trips.
The upgrade, prompted by recent refurbishments, involved the increase in capacity of overhead electricity lines that feed the refinery, potentially taking the incident energy level at the air-insulated switchgear substation to more than 100 calories per square centimeter. Such a level would demand use of cumbersome personal protective equipment (PPE) and is above levels prescribed by Shell’s electrical safety policies in the area.
Shell Canada follows the NFPA 70E standard and, wherever possible, avoids live work on equipment with incident energy levels above 40 calories; live work is prohibited on equipment with energy levels beyond 100 calories. If levels are between 40 and 100 calories, then equipment should be scheduled for service during plant shutdowns. If not, live work can only be sanctioned following detailed hazard analysis that account for equipment age, maintenance history, access, and ability to perform required tasks wearing 100 calorie PPE.
The company sought a solution to optimize operator safety and minimize nuisance trips because the plant operates continuously, protected by a fail-safe, emergency shutdown system. Nuisance trips can result in financial loss, as they can initiate a sequence of controlled shutdowns that halt production, necessitating time-consuming controlled start-up.
Use of specialized arc light sensing technology meant that the system would trip within 7 milliseconds, providing faster, more efficient protection than stand-alone over-current relays or busbar differential devices, and reducing incident energy levels. Current sensing capability enabled safeguarding against nuisance trips that any single-sensing system might experience.
The system can be programmed to sense electric arc conditions using simultaneous light and current data, or light sensor information only, and operates less than a quarter of the time of traditional alternative technologies.
“The Vamp system has given us a simple add-on for the switchgear at this plant that minimizes risk in the event of an arc flash, while also positively contributing to plant reliability”, said Phil Lasek, electrical reliability specialist at Shell Canada. “The system’s modularity also gives me welcome flexibility for future expansion and improvements.”
Lasek verified the system before installation with the aid of a high-voltage engineering consultant. The engineers could not verify the tripping speed to the finest accuracy, as they had no practical way of generating a true arc flash and fault simultaneously. But they devised a triggered flash bulb system and simulated fault current arrangement that could measure the response speed with a resolution that proved incident energy will not exceed 35 calories/cm2. Lasek believes that actual incident energy level would be considerably less, but to ensure operator safety he nevertheless protects the staff for the 35 level — using the next grade higher PPE equipment, rated to 42 calories. The
multi-zone capability of Vamp 221
minimizes ongoing costs, and Lasek recently extended the system to protect a secondary busbar in facility substations.
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