A section-by-section velocity map makes local conditions visible at every bore change. It connects inlet flow to actual velocity at local pressure and temperature, including the post-step point that a single inlet-to-outlet calculation can miss.
For a positive-pressure roots blower project, the map connects the selected inlet airflow to the actual velocity experienced after each transition. It also reveals conditions that a single full-load calculation can miss.
Define the Operating Cases First
At minimum, prepare velocity maps for normal production, maximum credible production, minimum approved production and line purge. If the system conveys several materials or routes, each approved recipe needs its own case or a documented bounding case.
The inputs should include blower-inlet volume at stated pressure and temperature, total gas mass flow, feeder leakage, pipe internal diameters, expected static pressure at section boundaries and gas temperature. Using one unqualified m³/h value throughout the line will produce incorrect actual velocities.
Divide the Route at Physical and Pressure Changes
Create a calculation node at the feeder outlet, every bore transition, major elevation change, branch or valve, and receiver entry. Long constant-bore runs may require intermediate nodes when pressure falls substantially along the section.
For each node, determine the actual gas volume at local absolute pressure and temperature, then relate it to the internal flow area. The map should show the velocity immediately before and after a transition because the post-step value is often the local minimum.
Use a record like this for each operating case; fill it from the local pressure model and validated material limits rather than copying values from another line.
Section-by-Section Velocity Map Worksheet
| Section / node | Local inputs and result | Acceptance check |
| Feeder outlet | Bore ___; Pabs ___; T ___; Qactual ___; v ___ | Validated v range ___ to ___ |
| Before enlargement | Bore ___; Pabs ___; T ___; v ___ | Local minimum / maximum ___ |
| After enlargement | New bore ___; Pabs ___; T ___; v ___; Lredevelop ___ | Stable transport; redevelopment ___ |
| Receiver entry | v ___; T ___; filter ΔP ___ | Product, wear and filter limits __ |
Apply Material-Specific Boundaries
The lower boundary should come from representative test data or defensible operating experience with the actual material grade. It is not a universal handbook velocity. Add a project-specific margin for changes in moisture, fines, feeder consistency and inlet-air density.
The upper boundary can be governed by bend wear, particle degradation, noise, receiver entry, filter loading or a process-specific quality limit. Record which mechanism sets the boundary in each section. This prevents the design team from treating “more air” as risk-free.
Examine Transitions as Local Disturbances
An enlargement reduces gas velocity abruptly and disturbs solids distribution. The transition geometry, angle and orientation influence how material enters the larger section. A smooth fabricated expansion may reduce disturbance, but it cannot compensate for an excessive bore ratio or inadequate gas flow.
The straight distance after the transition should allow gas-solid flow to redevelop before another bend or enlargement. Where plant space forces components together, representative testing or conservative operating limits may be necessary.
Check Turndown and Purge Separately
At reduced material rate, operators may lower blower speed to reduce energy and product velocity. The post-step sections must still remain above their verified transport limit. The control recipe should therefore be based on the most restrictive section, not just the feeder inlet.
Purging can create another low-velocity case. As the line empties, solids-related pressure decreases and downstream gas expansion can be smaller. The largest pipe section may then receive less actual velocity than it experienced during loaded conveying. The purge case in the largest bore therefore deserves its own calculation.
Use the Map for Blower and Control Selection
The accepted map defines the required inlet airflow range. Its pressure model defines the corresponding normal and maximum differential pressure. These paired points are compared with model-specific roots blower data, including permitted speed, absorbed power, motor capacity and discharge temperature.
The velocity map should show local minimum and maximum conditions for every approved operating case. Keep calculated values, material limits and control actions together so each pipe section can be checked.
Glossary
Bounding case: Operating condition selected to represent the most restrictive credible combination of variables.
Calculation node: Defined pipeline location at which pressure, temperature, area and actual velocity are evaluated.
Local actual volume: Gas volume flow at the pressure and temperature of a particular pipeline position.
Post-step velocity: Gas velocity immediately downstream of a pipe enlargement at local pressure and temperature.
Section boundary: Point separating pipeline regions with different bore, pressure behavior or equipment.
Velocity map: Section-by-section record of calculated gas velocity across specified operating conditions.
Velocity margin: Difference between operating velocity and the validated minimum needed for stable transport.