Purge velocity in the largest section can be lower than the velocity reached during loaded conveying. Check that low-flow condition separately, because successful production does not prove that the line will clear after feed stops.
This creates a specific risk: the largest downstream bore may have adequate velocity during production but insufficient velocity during the final clearing period. A purge sequence should therefore be verified as its own operating case, not copied from the loaded design point.
Define What “Clear” Means
The purge objective depends on the process. A routine stop may require removal of loose material so the next start is reliable. A product change may require a stricter residual-mass limit. Maintenance or hygienic service can demand visual inspection and documented cleaning beyond pneumatic purging.
The project should state the acceptance criterion before timing the cycle. Possible evidence includes receiver weight stabilization, no additional solids discharge over a defined interval, pressure reaching a repeatable empty-line value or inspection of accessible points. A fixed number of seconds without evidence is not a defensible universal criterion.
Build a Separate Purge Velocity Map
Use the expected purge airflow, local absolute pressure, gas temperature and every internal bore to calculate the section conditions. Account for feeder leakage paths, valves and any air that is diverted away from the main route.
The map should follow the changing pressure as solids leave the line. It should pay particular attention immediately after each diameter increase and through the final large-bore section. The lowest velocity may move during the cycle rather than remaining at one fixed point.
Coordinate the Feeder and Blower Sequence
A typical positive-pressure sequence stops material feed while maintaining the required conveying airflow long enough for the final solids to reach the receiver. The precise order depends on feeder type, isolation valves, branch arrangement and material behaviour.
Where a variable-frequency drive is used, speed should not be reduced automatically when the feeder stops. The purge recipe may require the normal conveying airflow or another validated setting. Any temporary speed command must remain within the roots blower’s approved operating range and should not exceed receiver or filter capacity.
Include the Receiver and Filter
All purge air eventually reaches the receiver. The filter must handle the instantaneous actual volume at its operating pressure and temperature. If a branch closes or a receiver valve changes position during the sequence, differential pressure can rise rapidly.
Monitor receiver pressure and filter differential pressure during validation. A line can appear to clear while the filter becomes the next constraint. The purge should not rely on continuous operation of the blower relief valve; relief is abnormal overpressure protection, not routine airflow control.
Plan for Abnormal Stops
Loss of power, a feeder trip or high-pressure shutdown can leave material distributed through several bores. Restart logic should not assume that the normal purge was completed. The operating procedure should define safe depressurization, inspection and a controlled recovery sequence based on the material hazard and system design.
If pressure rises unexpectedly while purging, continuing full airflow may compact material or increase stored energy. The control system should stop feed immediately and apply project-specific alarm and shutdown logic.
Validate and Trend the Cycle
Commissioning should test the most difficult approved product and route. Record blower airflow, differential pressure, speed, motor current, discharge temperature, filter differential pressure, purge duration and material recovered. Repeat the test to establish a baseline.
Longer purge time or failure to return to the established empty-line pressure can indicate deposits, filter loading, leakage or a changing material condition. Trend these values rather than extending the timer indefinitely.
Check purge performance at the largest bore under the lowest credible airflow and pressure case. Record its acceptance limit and sequence separately from loaded conveying so a successful production run does not mask a clearing risk.
Glossary
Clearance criterion: Measurable condition used to declare that residual material has been removed sufficiently.
Empty-line pressure: Repeatable differential pressure of the cleared route at a stated airflow and configuration.
Largest-bore section: Downstream pipeline stage with the greatest internal flow area.
Purge recipe: Approved blower, valve and feeder sequence used to clear a specific material and route.
Purge velocity map: Section analysis of gas velocity during the changing pressure conditions of line clearing.
Recovery sequence: Controlled procedure for clearing or restarting after an abnormal incomplete purge.
Residual material: Solids remaining in pipeline or components after normal conveying stops.