Prevention of Overfill for Atmospheric Storage Tanks

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Prevention of Overfill for Atmospheric Storage Tanks

Author: Phillip J. Smith, Ph.D., Instrumentation Staff Engineer II 

The 5th edition of ANSI/API 2530 Overfill Prevention for Storage Tanks in Petroleum Facilities was published in September 2020.  The revision was informed by regulatory concerns over several incidents of gross overfilling leading to serious consequences affecting employees and nearby communities.  All incidents can be traced to loss of operational control of storage tanks, chiefly during filling activities, and are largely due to some combination of inadequate planning, inadequate supervision, and inadequate equipment.  Implementation of the standard requires adjustment of user operating practices.  


Organization of the overfill prevention standard was changed to emphasize management practices that should be in place to properly control storage tank operations.  The information in the standard is very similar to previous editions; however, much information has been relocated.  At first glance, organization of the standard seems relatively generic. 

  1. Scope 
  2. References 
  3. Terms and Definitions 
  4. Overfill Prevention System 
  5. Equipment Systems used for Overfill Prevention 
  6. Appendices: 
    1. Management Systems
    2. Risk Assessment
    3. Automated Overfill Prevention Systems
    4. Liquid Level Instrumentation Considerations
    5. Determining Tank Capacity and Levels of Concern (LOCs) 

However, the reader is cautioned not to skip the first three sections and rush ahead to Overfill Prevention System and Equipment. That’s because the scope provides a list of tank services that are not in scope of API 2350. Exclusions are: 

  • Tanks that do not contain Class I or Class II liquids as defined by NFPA 30 
  • Tanks less than 1320 US gallons (5000 liters), unless connected to a transporter or marine delivery system 
  • Tanks covered by PEI RP 600 
  • Tanks filled exclusively from wheeled vehicles (i.e., trucks or railroad tank cars), where the fill rate is less than 630 bbl/hr (440 gpm) (100 m3/hour) 
  • Dedicated pipeline relief tanks 
  • Tanks storing LPG and LNG 

The only normative reference is IEC 61511, which applies if safety instrumented systems (SIS) are selected for use to mitigate the overfill scenario.  The critical high (CH) level of concern and overfill definitions, although simple, are fundamental.  These should be used to keep the overfill prevention management processes focused. 

Overfill Prevention System (OPS): 

Whether manual or automatic, an OPS in accordance with API 2350 consists of the following elements: 

  1. Management System 
  2. Risk Assessment System 
  3. Defining Operational Parameters 
  4. Requirements for Procedures 
  5. Equipment Supporting OPS 

Note that IEC 61511, ISA TR84.00.08, and Annex C are only normative if equipment covered by these standards is selected for use. 

Changes in the 5th Edition:  

The major changes in API 2350 are: 

  1. A management system is required 
  2. Risk assessments are required 
  3. Automatic overfill prevention systems (AOPSs) are required, if necessary 

This has significant impact on tank operators since these requirements are new and require action to be taken.


The organization of API 2350 requires the user to define each of the system elements.  Once established, these could be used as a basis for implementing a continuous improvement process. 

  1. Management System is defined within the standard as an organized collection of policies and procedures.  Do not overthink it.  If you already have policies and procedures, they are good enough to start from.  If you do not, start with the basics.   
    1. How does your company receive products? 
      1. Plan for receipts 
      2. Communicate with the supplier during delivery 
      3. Line up, monitor, and restore facility systems 
    2. Who does these things?
      1. Is there a required procedure?
      2. Is there special equipment or materials they must use?
    3. Do not overdo it.  The management system should evolve and adapt to circumstances; excessive analysis just wastes time.  
    4. A management system should include additional processes as necessary to: 
      1. Measure results and identify results that should be corrected 
      2. Maintain adequate equipment, procedure, and staff performance 
  2. Risk Assessment System 
    1. Although now a requirement in the API 2350, methods for performing risk assessments and criteria for acceptable risk are not prescribed. 
    2. An inadequately engineered AOPS can be harmful to equipment and piping systems utilized to fill storage tanks. 
    3. To establish compliance with API 2350, E2G believes the following procedure is adequate.
      1. Identify storage tanks that are subject to the API requirements. 
      2. Determine instrumentation the tank should have as specified in the tank’s design documentation. 
      3. Inspect the tank.  Determine whether the instrumentation is as specified in the tank’s design documentation.  Observe the general condition of the instrumentation and tank for obvious signs of degradation that could impact instrument function or tank integrity. 
      4. Verify functional condition of instrumentation. 
      5. Review tank operating procedures:   
        1. Determine whether filling procedures are consistent with the tank’s instrumentation 
        2. Determine level of supervision provided by procedure 
        3. Verify operational compliance with procedure supervision requirements 
      6. Determine tank classification based on verified instrument functions and operational supervision.  
      7. Assess consequences of credible overfill scenarios. 
      8. Determine risk targets applicable to consequences.   
      9. Assess likelihood of consequences. 
        1. Assess adequacy of procedure, instrumentation, and equipment 
        2. Assess likelihood of procedure, instrumentation, and equipment failure 
        3. Assess exposure 
      10. Evaluate risk and compare to applicable target. 
      11. Resolve safety gap, if indicated. 
    4. Recognized and Generally Accepted Good Engineering Practices (RAGAGEPs) should be applied at each step.   
      1. Some steps are straightforward and common sense is sufficient to meet the RAGAGEP standard.   
      2. Some steps are subject to published industry standards.  For these steps, identification and implementation of applicable industry standards is necessary to meet the RAGAGEP standard. 
    5. E2G considers the 11-step procedure to be unique in its ability to communicate complex technical requirements to wider audiences.  E2G offers the 11-step procedure as explained above, free of charge, to promote consistency and standardization of storage tank overfill risk assessment activities.  Companies and others who choose to adopt the 11-step procedure will be responsible for identifying and implementing proper RAGAGEP.  
Resolving Gaps:   

To resolve safety gaps, API 2350 suggests improving instrumentation and supervision of tank filling activities.  However, where these measures are not adequate, other measures including AOPS are required.  Functional safety standards for the process industries suggest good engineering practice is to apply instrumented safety functions after any other practicable measures have been exhausted.  In some locations, authorities have established mandates for the installation of AOPS regardless of other safeguards.  

Automatic Overfill Prevention Systems:   

An AOPS is an engineered collection of equipment that includes:  

  1. Sensors 
  2. Logic solvers (a.k.a. controllers)
  3. Valves and switches (a.k.a. relays, contactors, and circuit breakers) 

To perform its function safely without human operator assistance, API 2350 suggests several aspects to consider when designing automated functions for overfill prevention including: 

  1. Means for stopping flow into the tank 
  2. Upset of supplier process and equipment in the event of activation 
  3. Tank and filling system characteristics 
  4. Setpoints and response characteristics of the automated function 
  5. Necessary functional reliability and integrity 

API 2350 suggests an organization of devices into sensor, logic-solver, and final element subsystems consistent with IEC 61511 practice.  However, API 2350 does not specify that AOPS must comply with IEC 61511.  API 2350 suggests use of Annex C or IEC 61511.  

In locations where AOPSs are specified by regulation, E2G still recommends performing risk assessments to identify whether a system conforming to Appendix C is adequate. 


EP 12-9-1 of the Equity Engineering Practices (EEPs) requires protection against tank overfill. The requirements for SIS are addressed in EP 12-10-1.  Otherwise, the API standard is adequate and should be consulted if there are concerns or questions concerning detailed requirements. 


E2G welcomes inquiries about and requests for assistance with API 2350 implementation, management system evaluations, risk assessments, and other tank instrumentation concerns as needed.  For more information, please fill in the form below:

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