Identify what is actually redundant
Two vessels may share an inlet, discharge header, air supply, power source or controller. A failure of a shared component can stop both. Single-vessel operation is therefore a defined operating mode, not a guarantee that every system fault leaves production unaffected.
Establish a safe isolation boundary
The faulty vessel and relevant connections must be isolated according to the approved arrangement before maintenance. Residual pressure and connected product paths require attention by trained personnel. The operating vessel must not be able to unintentionally pressurize or fill equipment under maintenance. These boundaries should be visible on the approved piping and control documents.
Recalculate the production rate
A single vessel must complete filling, transfer and transitions sequentially. Its average output can be much lower than half of a twin-vessel headline rate if shared timing or continuous-feed requirements change. Evaluate the real cycle and the available upstream or downstream buffering. State the minimum production objective for fallback in the purchase specification.
Test and document the mode
Acceptance should demonstrate the authorized transition into and out of fallback, the relevant alarms and the resulting throughput. Operators need a clear instruction for deciding whether the mode is allowed for the current fault. Restoring two-vessel operation should include checks of the repaired item and interlocks rather than an unreviewed reset.
General engineering guidance. Numerical examples are illustrative; project specifications and acceptance criteria are agreed for the actual duty.