What is the ideal discharge rate for Cement / Fly Ash into a Concrete Mixer Truck and how to control it?

September 6, 2022

Gravity discharge cement weigh batchers are the most common in the industry, but can be impacted by weather, plant air supply, mix design, free fall distance, and batcher design.  For a typical 200 YPH dry batch plant, the discharge rate should be 110 lbs. per second.  Physically a butterfly valve is used to control the flow, while being operated by the outputs (Open/Close Pulse) of an Automated Batch Controller.  The Batch Controller compares the actual discharge rate Vs the targeted rate within a pre-determined cycle window, and makes open/close adjustments as needed to match the targeted rate.  Ideally the initial opening of the valve should hit the target rate, with adjustments every 3-4 seconds as needed.  The adjustments should be minimal so as to not over correct and/or cause the controller to close the valve, only to have it re-open in full.

If the flow of the cement is random and un-controllable double check the following;
Proper aeration (dry air @ 15 psi) to the cement weigh batcher.
Weather impact – paint the inside of the cement weigh batcher to seal the steel pores from outside moisture.
Confirm Weigh Batcher Dust Vent is operational and properly sized.

A good butterfly valve is well balanced and does not restrict the actuation of the valves disk. The improper mounting of a butterfly valve between two steel flanges can restrict the actuation.  In addition too much air CFM to the actuator can over-throw the disk of which flow controls might need to be added.  In order to determine the proper size of a valve, calculate the valve opening area and multiple by 2 lbs. / square inch / second.

As always if you have any additional questions please don’t hesitate to call us at 1.800.434.3462.