Pipeline inlet velocity: the evaluation begins with defined process conditions, material data and equipment boundaries. A roots blower is selected by airflow and differential pressure, but pneumatic conveying stability is commonly assessed using gas velocity inside the pipe. Connecting those two quantities requires the actual volumetric flow at the pipeline location being checked. A catalogue flow stated at blower inlet, standard or normal conditions cannot be divided directly by pipe area unless the reference conditions match the local pipeline conditions.
The Core Velocity Relationship
For a circular pipe, mean superficial gas velocity is:
Here, is mean superficial gas velocity,
is actual volumetric flow at the selected pipe location and
is the actual internal pipe diameter. Consistent units are required; for example, cubic metres per second and metres produce metres per second.
The equation uses the complete empty-pipe area. It does not subtract the volume occupied by solids, and the result is not particle velocity.
Why the Inlet Value Matters
In a positive-pressure, constant-bore line discharging near atmospheric pressure, the material feed point is normally at the highest pipeline pressure. The same air mass occupies a smaller actual volume there than it does downstream, so inlet gas velocity is often the lowest velocity in the route.
Particles also enter with little forward velocity and must accelerate. If effective inlet airflow is below the material’s stable requirement, local solids concentration can rise and material may deposit near the feeder.
Convert the Airflow to Local Conditions First
Before applying the equation, identify whether the blower value is:
- Inlet actual cubic metres per hour.
- Free-air delivery.
- Normal or standard cubic metres per hour.
- Actual flow at blower discharge.
- Measured flow at a pipeline instrument.
Pressure must be absolute for gas-condition conversions, and temperature must be absolute. Site altitude affects atmospheric pressure, while compression heating changes discharge-air density. Article 031 owns the detailed pressure-temperature conversion; the important rule here is that and the calculated velocity must refer to the same local state.
Use the Real Internal Diameter
Nominal pipe size may differ from actual bore. Heavy wall, corrosion, deposits and wear liners change area. Because diameter is squared, a modest bore error can materially change the calculated velocity.
Each section with a different diameter needs its own calculation. In a stepped pipeline, velocity should be checked immediately before and after the transition as well as at the feed point and receiver.
Account for Feeder Leakage
The blower package may deliver more air than reaches the conveying line. Pressurized air leaking through a rotary airlock toward the hopper reduces effective pipeline flow. Purge, fluidization or bypass air can add flow at other points.
The calculation should therefore use a documented air balance:
- Total blower delivery at stated conditions.
- Supply-pipe losses and intentional branches.
- Estimated or measured feeder leakage.
- Auxiliary air additions.
- Credible pipeline leakage.
Check More Than One Operating Case
Velocity should be evaluated at minimum blower speed, maximum material rate, maximum filter resistance, cold start, normal production and any future duty. A line may have acceptable velocity during warm clean operation and become marginal when pressure rises or effective flow falls.
Excessive velocity is also undesirable. It can increase air friction, bend wear, particle impact and receiver-filter load. The target is a validated operating window, not the highest available blower speed.
Glossary
Actual bore: Measured or specified internal diameter available for gas and solids flow.
Air balance: Accounting of supplied, leaked, diverted and added gas flows across defined system points.
Inlet velocity: Superficial gas velocity at or near the material feed point.
Local actual flow: Gas volume per unit time at the pressure and temperature of the selected location.
Mean gas velocity: Cross-sectional average gas speed derived from actual volumetric flow and pipe area.
Pipe area: Internal cross-sectional flow area calculated from actual pipe bore.
Reference airflow: Volumetric flow value tied to specified pressure and temperature conditions.
Velocity window: Validated range between unstable low velocity and unnecessarily damaging or inefficient high velocity.