When multiple vessels are interconnected through piping, valves, or common headers, OSHA considers them a single process for threshold quantity calculations. This connected vessels doctrine is one of the most consequential coverage determinations facilities face. Whether a valve separates two vessels, whether vessels share a common containment area, or whether they are physically close but not directly piped — each scenario has been addressed in OSHA letters of interpretation. Facilities that attempt to fragment a process into sub-TQ pieces by adding isolation valves typically find little support in these letters. OSHA's definition of process is intentionally broad.

What OSHA Has Said

Feb 15, 2017
Interconnected Tank Farm Vessels and TQ Aggregation
OSHA confirms that tank farm vessels interconnected through a common header or transfer piping must be treated as a single process. Adding isolation valves does not automatically create separate processes.
Read OSHA Letter ↗
Apr 28, 1994
Definition of Process and Aggregation of Quantities
Foundational OSHA interpretation establishing that the term process includes any activity involving an HHC and extends to connected vessels and interconnecting piping, regardless of the number of units.
Read OSHA Letter ↗
Jun 1, 1994
Physically Separated but Functionally Connected Vessels
OSHA addresses whether vessels in different facility areas but connected through piping constitute a single process. OSHA position: connectivity, not proximity, determines aggregation.
Read OSHA Letter ↗
Jul 18, 1994
Valve Isolation as a Barrier to TQ Aggregation
OSHA evaluates whether normally-closed block valves between vessels create separate processes for TQ purposes. Operational connectivity, not valve position, governs the analysis.
Read OSHA Letter ↗

⚡ Key Compliance Takeaway

If vessels can exchange HHC material under normal or reasonably foreseeable abnormal operations, aggregate their inventories for TQ purposes. Adding valves after-the-fact to fragment a process below TQ is unlikely to withstand OSHA scrutiny.