Marianne Williams, Emerson, USA, explains the significant benefits that can be achieved through the remote partial proof-testing of level measurement instruments in an overfill prevention system.
T
he overfilling of process vessels and storage tanks containing hazardous materials has long been a leading cause of serious incidents in the refining, gas processing and petrochemical industry. The consequences of a product spill can be catastrophic, including injuries or fatalities to personnel, significant damage to assets, harm to the surrounding environment, and damage to an organisation’s reputation. The best way to deal with a potential incident is to prevent it from happening in the first place, and best practice for minimising the risk of overfills involves employing several independent layers of protection. The critical first line of defence is the basic process control
system (BPCS) that monitors and controls the production processes. If the BPCS is functioning correctly, there is no need for the other layers of protection to become active. An overfill prevention system (OPS) provides an independent second layer of protection. The OPS is normally dormant but operates when the BPCS fails to prevent the tank level from reaching the critical high point. The OPS stops the situation from escalating by alerting an operator, closing valves and/or shutting down pumps. A thorough safety plan must also consider the worst-case scenario, where both the BPCS and the OPS fail to stop a spill from occurring. A third layer of protection is typically provided by a dyke or concrete wall 3 39 Autumn 2021
