Block and process flow diagrams

A block flow diagram is a high-level visual that shows the major steps or unit operations of a process and the main material flows between them, and you should use it when you need a simple, top-level picture of a process for PSM planning, PHAs, training, or emergency response. See the Nonmandatory guidance in 1910.119AppB for examples and the primary PSM standard at 1910.119 for why simple diagrams support compliance.

A block flow diagram shows only major process steps and material flow paths, while a simplified process flow diagram includes more process detail such as major equipment types, major streams, and key control points; use the block diagram for big-picture orientation and the simplified PFD when you need more operational or safety-related context. Both are described as allowable nonmandatory tools in 1910.119AppB and serve complementary roles under the PSM standard at 1910.119.

No — block flow diagrams and simplified process flow diagrams are nonmandatory tools, but they are useful and recommended to help meet PSM requirements such as process understanding, PHAs, and training. The diagrams are explicitly described as nonmandatory in 1910.119AppB, while the PSM standard 1910.119 still requires employers to document and understand covered processes.

A block flow diagram should at minimum show the major unit operations (blocks), the major material and product streams entering and leaving those blocks, storage/holding areas, and the overall flow direction. This is consistent with the guidance in 1910.119AppB and helps meet the PSM standard's expectation that employers understand and document the process under 1910.119.

A simplified PFD should include major equipment items (reactors, heat exchangers, columns), major streams and their directions, and important control or isolation points, but it should not attempt to replace a Piping and Instrumentation Diagram (P&ID) that shows piping, valves, instrumentation, and detailed control logic. Appendix B explains this tiered approach in 1910.119AppB and the PSM standard at 1910.119 requires more detailed documentation where necessary (for example, for safe operations and PHAs).

No — a block flow diagram cannot replace P&IDs when detailed information is required for safe operation, maintenance, or hazard analysis; block and simplified flow diagrams are supplementary, nonmandatory tools. Appendix B clarifies the use of nonmandatory diagrams in 1910.119AppB while the PSM standard at 1910.119 requires employers to have the process information needed for safe operations (which often includes P&IDs).

Block and simplified PFDs should be used to give PHA teams a clear, consistent view of the process topology and major flow paths so they can identify hazards, scenarios, and interfaces between units. Appendix B recommends using these diagrams to orient and inform PHA work in 1910.119AppB, and the PSM requirement to perform PHAs is in 1910.119(e).

A qualified team — typically process engineers, operations, maintenance, safety staff, and others knowledgeable about the process — should prepare and review these diagrams to ensure accuracy and usefulness for PSM activities. Appendix B suggests using knowledgeable personnel for diagram preparation in 1910.119AppB, which supports the competency expectations in the PSM standard at 1910.119.

You must update diagrams whenever changes to the process occur that affect the accuracy of the diagrams, and you should treat those updates as part of your Management of Change (MOC) procedure. The PSM standard requires MOC controls at 1910.119(l) and Appendix B recommends keeping diagrams current in 1910.119AppB.

Yes — include major utilities and support systems on simplified PFDs when they interact with the process or can affect process safety, because those systems can create or mitigate hazards. Appendix B notes showing support systems where relevant in 1910.119AppB, and the broader PSM requirements in 1910.119 expect employers to consider all aspects that impact process safety.

You should show storage locations, connection points, and the general magnitude or role of hazardous chemical inventories on your diagrams so the PSM team can assess whether thresholds are exceeded for coverage or aggregation. Appendix B supports indicating storage and material flows in 1910.119AppB, and the PSM flammable gas aggregation interpretation explains how aggregated stored quantities can affect PSM coverage in https://www.osha.gov/laws-regs/standardinterpretations/2024-06-06.

Yes — these diagrams are effective training and emergency-response tools because they give operators and responders a clear overview of process layout, flow paths, and key control points. Appendix B points out their value for orientation in 1910.119AppB, and the PSM training requirement is in 1910.119(k).

Yes, you may simplify or omit sensitive details for security, but the diagrams still must provide enough information for safe operation, PHAs, and emergency planning; keep more detailed controlled copies available to authorized personnel. Appendix B notes the nonmandatory nature and flexibility of simplified diagrams in 1910.119AppB, while the PSM standard at 1910.119 requires adequate process information for safety purposes.

Yes — use a clear, consistent legend and simple symbols to represent unit operations, storage, pumps, columns, and flow directions so readers can quickly interpret the diagram; Appendix B illustrates example diagrams and implies the need for clarity in 1910.119AppB. Consistent symbols improve the usefulness of diagrams for PHAs and training under 1910.119.

Represent each process or building as separate blocks or sub-diagrams and clearly show interconnections and on-site movement paths so reviewers can identify where hazardous materials may be co-located or transferred. Appendix B supports using block-level views to show site relationships in 1910.119AppB, and the PSM aggregation discussion in https://www.osha.gov/laws-regs/standardinterpretations/2024-06-06 clarifies why on-site co-location matters for coverage.

Show vendor-supplied or packaged units as discrete blocks labeled with their function and any key connections (inlet, outlet, utilities); note that operational or storage quantities tied to those skids count toward on-site process inventory. Appendix B suggests representing major units and interfaces in 1910.119AppB, and the PSM standard 1910.119 plus the aggregation interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2024-06-06 reinforce why such representation is important.

Yes — simplified diagrams are especially useful for quick contractor orientation and job briefings because they provide an accessible overview of process boundaries, hazardous streams, and key safety features. Appendix B highlights their use for clarity in 1910.119AppB, and the PSM training provisions at 1910.119(k) require effective training and information for affected personnel.

Keep piping, valves, and instrument-level details on formal P&IDs and use simplified PFDs only to show major equipment, main process streams, and critical isolation points; do not rely on simplified diagrams for detailed maintenance work. Appendix B explains that simplified diagrams are high-level tools in 1910.119AppB, while the PSM standard 1910.119 expects detailed technical information where needed for safe operation.

Color-coding is not required but is a recommended good practice because it makes hazards easier to see and speeds recognition during PHAs, training, and emergencies; Appendix B supports using clear visual cues on nonmandatory diagrams in 1910.119AppB, and the PSM standard 1910.119 expects clear communication of process hazards.

Validate diagrams by field verification (walkdowns), cross-checking with process engineers and operators, and updating them through your MOC process when discrepancies are found. Appendix B recommends using diagrams to support accurate process understanding in 1910.119AppB, and MOC and documentation accuracy are required under the PSM standard at 1910.119(l).