Preventing overfill conditions in tanks and vessels is a job for safety instrumented systems across many industries. In a Flow Control article, A Systemic Approach to Storage Tank Overfill Protection, Emerson’s Lydia Miller and AnnCharlott Enberg describe the key components in a comprehensive tank management system to meet the safety requirements for the application.
Lydia and AnnCharlott open highlighting a sobering statistic:
Insurance data shows that for all the tanks around the world, there is one overfilling incident for every 3,300 filling operation. Perhaps that sounds like a small proportion, but overfill accidents can be devastating.
To avoid overfilling:
…an overfill prevention system depends on the right combination of level measurement instruments, operator interfaces, safety controls systems and final control elements. Additionally, condition monitoring of those components helps ensure an operational safety system.
Accurate level readings for operators is critical:
It should be easy to see the contents of any tank from the control room, but this depends on having accurate and reliable level instruments for every situation.
Safety instrumented function within a safety instrumented system contain a sensor, logic solver and final control element to perform a safety function upon demand. These components in the loop must be tested and therefore:
…proof-testing is required on a regular basis, and partial proof-testing can extend the time period before a full proof test is required.
Traditionally proof tests have been performed manually. This method can be:
…error-prone and time-consuming. It can also interfere with production, causing negative financial impact.
With modern level measurement devices such as Rosemount Radar Level transmitters and Vibrating Fork switches:
Partial proof-testing of level instruments can now be accomplished easily from the control room with devices that have been designed especially for use in safety-instrumented systems (SISs). Devices that provide a high safe failure fraction can also help achieve a higher level of safety.
The overfill prevention system is above the basic process control system at the safety layer and governed by international standards such as IEC 61511 and API 2350.
Read the article for more on the importance of risk assessments in determining the automatic shutoff requirements, operator notifications and types of technologies required for use in safety applications.
