Blowing time is dependent on the batch size and SLR you want to convey at. Putting time in plc is not the way it is usually done. Use a low pressure switch to detect the end of conveying cycle. ■

Dear Mr Bangun,

Time is not the first option to control the cycle time of a pneumatic blow tank system.

If the set time is too short, the blow tank will not be empty and the pipeline will still be filled with non moving product.

After restarting the pneumatic conveying, you might block the pipeline by injecting more material into an already filled pipeline.

If the set time is too long, the pipeline will be purged empty and then you are loosing capacity.

If this capacity loss is acceptable, then it is OK to use a set cycle time.

The better alternative is to use the kettle pressure to check whether the kettle is empty and the pipeline is (partly) purged.

The end of the cycle time is then given by a pressure measurement, indicating a pressure of 1/3 of the continuous pressure.

The pressure measurement also serves for determining the moment of opening the discharge valve, closing the pressurizing valve and closing the discharge valve.

Also processing the pressure and conveying time can be used, according to:

actual pressure <= factor * integral (pressure * d(time))

If you want to calculate the discharge cycle, split the cycle in segments:

-pressurizing time

-initial kettle unloading time when pressure is built up.

-continuous unloading time

-rest unloading time

-purge time

-switch over time

These variables depend on kettle outlet geometry and air volume and pipe diameter as well as loading ratio.

best regards

teus ■