Conveying line pressure drop

Pipeline Routing and Bend Design. Direction changes should be kept to an absolute minimum in order to keep the number of bends that are considered to be the major attrition sources as low as possible. This helps also to reduce the conveying line pressure drop and with it the gas expansion effect. 

The discharge limit of the blow tank used was about 26 tonne/h with the 53 mm bore pipeline and approximately 50tonne/h with the 81 mm bore pipeline. Within this capability of the conveying facility, however, tests were carried out with conveying line pressure drop values of well over two bar and the materials were all capable of being conveyed at solids loading ratios of well over one hundred. A minimum of fifty individual tests were undertaken with every material/pipeline bore combination, in order to draw the various families of curves required. 

The pressure drop required to convey the air alone through the pipeline provides a datum for the conveying system. Only when the conveying line pressure drop exceeds this datum value will any material be conveyed, but then the greater the excess over this datum pressure drop the greater will be the material flow rate. This is illustrated in Figure 4.40, with the zero material flow rate line being the air only pressure drop for the pipeline. 

The above relationship holds equally for negative pressure conveying systems. Because of the natural limit on conveying line pressure drop available, however, the situation is limited to short distance conveying. 

Fig. 7. Phase or state diagram for horizontal conveying where represents a particular mass flow rate, line AB corresponds to the pressure drop for air alone flowing in the system, Gq = 0, and ( is the minimum pressure line where saturation occurs. Other points ate explained in text.

Source Constance, J.D. Calculating Pressure Drops in Pnematic Conveying Lines, Chenicd Engineering, Mar. 15, 1965, p. 200. 

The pressure drop inside the draft tube is more complicated because it involves acceleration of solid particles from essentially zero vertical velocity. However, the model for calculating the pressure drop in vertical pneumatic conveying lines suggested by Yang (1977) can be applied. The acceleration length can be calculated from numerical integration of the following equation. 

If the conveying velocity is a critical parameter, then the designer of a pneumatic conveying system should also consider the expansion effect caused by the pressure drop along the conveying line. The resulting increase of the gas velocity can be accounted for by increasing the pipe diameter. 

Data obtained with cement and analysed in terms of an equivalent length of straight horizontal pipeline are presented in Fig. 13. This is for 90° bends having a bend diameter D, to pipe bore d, ratio of 24 1 in 53 mm bore pipeline in horizontal plane. Almost identical results were obtained when a similar analysis was carried out for the conveying of barytes. A simple correlation in terms of the conveying line inlet air velocity was not expected, but it was not possible to determine any effect of the position of the bends in the pipeline. Data obtained with barytes and analysed in terms of a pressure drop across a bend are presented in Fig. 14. Once again, almost identical results were obtained for the conveying of cement. These data are for the same bends reported in Fig. 13. 

Knowlton, T. M and Bachovchin, D. M. The determination of gas-solids pressure drop and choking velocity as a function of gas density in a vertical pneumatic conveying line, International Conference on Fluidization, Pacific Grove, June 15-20, p. 253 (1975). 

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