Ten Key Steps for Reliable Lean Phase Pneumatic Conveying

by Lyn Bates

1. Do not use sloping pipe runs between 200 and 800 inclination. The re-fluxation, (run-back and re-conveying of the same material), will require the pipe to re-transfer the product slipping back on top of its normal load, and the effect is accumulative. At best this will significantly increase the conveying burden, but more than likely it will tend to accumulate to block the lower bend.

2. Allow a ‘reasonable’ horizontal conveying length, (15 to 20 pipe diameters), before the first bend, to allow the bulk material to accelerate to a stable conveying speed and reduce the sectional loading of the conveying pipe. Material slowed by the bend occupies a greater proportion of the cross sectional area of the pipe and thereby offers more obstruction to the air flow, which increases the pressure drop along the system.

3. Do not fit bends close together on the pipe run for reasons similar to the above.

4. Consider stepping the pipeline on long runs. Air expands with pressure drop, so velocities inevitably increase along a constant pipe run. Higher material velocities increase wear on bends and product degradation.

5. ‘More air’ can be ‘Less transfer capacity’ in dilute phase systems. Larger solids and gas frictional losses caused by higher gas velocities can absorb more energy than the extra input of energy. There is an optimum gas flow rate for a given lean phase flow system. Check with a specialist for the correct solids/gas ratio balance and top performance.

6. Product damage and wear at bends is very material dependent. Blind Tee's usually have much merit, but cause a greater pressure drop than long radius bends. Use dense phase pneumatic systems for minimum particle attrition and lowest pipe wear, except for some very fragile flake particulates, such as tea, which can be delicately transported by a 'weak' lean phase system because they are easily blown along at low air velocities.

7. Rotary valves do leak and also pass air differentials through the returning empty pockets. They also tend to fill with product on one side of the valve as the empty pockets are rotated to present a space for material to flow in. Back flow of gas and biases intake causes many feed problems. Make sure that the valve is properly vented and that the feed channel is of good form for flow across the whole cross section. A short inlet standpipe is useful for duties such as a cyclone outlet or a feed into a high-pressure line.

8. Check that cyclones have an unobstructed outlet. If there is intermittent or periodic discharge, allow clear buffer capacity to avoid interfering with the cyclone operation.

9. Allow for the pressure drop on the supply line to the blower. Include an allowance for any filter, silencer, acoustic box or compressor enclosure and extended supply run, as may be appropriate. Be sure to compensate for the temperature increase due to gas compression.

10. Provide adequate instrumentation This is essential to know what is going on and to facilitate a proper investigation of any problems that may arise.

Remember - If you have to knock the pipes, it's not working properly. Fix it, before it gets worse, because it will and invariably at the worst possible time.


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