Absolute measurement of pneumatically conveyed powder using a single long throat venturi

Abstract

The principle of using a long throated venturi meter to measure the flow rate of two mixed phases is widely published, yet there is no practicable application for accurate measurement in relatively lean particulate conveying. Coal fired power stations would benefit from such a meter. Pulverised coal is conveyed pneumatically through pipes from the mill to the furnace. Control devices that ensure delivery of coal to each burner is equal require an accurate on-line measurement of the coal flowing through each line. Relative comparison of coal flow rate between similar pipelines is possible on-line, but to date no absolute on-line measure has been achieved, and weigh hopper discharge measurement for total coal flow rate is crude. Using a program developed by Azzopardi et al. [B.J. Azzopardi, S.F.C.F Teixeira, C.I. Pulford, A quasi-one-dimensional model for gas/solids flow in venturis, Powder Technology 102 (1999) 281–288.] for calculation of the pressure drop in a two phase venturi, a venturi was designed with optimised geometry to give a measurable sensitivity to coal flow rate at the relatively lean conditions encountered in a power station. The venturi was inserted into a pneumatic conveying test rig designed to simulate a coal fired power station. The program predicted the pressure drops measured in the test rig venturi with good agreement. The pressure calibration graphs thus obtained provided a possible means of calculating the air and coal flow rates. Air flow rate was accurate to within 2%. Coal flow rate accuracy was within 20%. Error analysis showed this error to be related to experiment. The program accuracy was shown to be within 7%. This accuracy merits a full scale power station trial.