MEASUREMENT OF SEDIMENT-LADEN FLOWS IN PIPES

Abstract

The technology of long distance pipe line transpor tation of solids is now approaching the realm of a mature art. Engineering problems related to pipeline conveyance of sediments are numerous? however, this study has been limited to the aspect of measurement of sediment-laden flow through pipes with the conventional measuring instruments namely, orificemeter, venturimeter and bendmeter. Modern electronic equipment is available for the measurement of sediment-laden flow in pipes? but often this equipment is very expensive and some of these are still in the developmental stage. The measurement of single phase flew with standard meters viz. orificemeter, venturimeter and bendmeter has been thoroughly investigated and their installation arrangements duly specified. However, the effect of solids on the performance of these meters has not been fully studied. Thus the primary objective of the study is to study ttie effect of sediment, fluid and flow characteristics on discharge coefficients of bendmeter, venturimeter and orifice meter. In particular the effect of sediment size and relative density, and its concentration are studied. The influence of flow direction on the discharge characteristics of the orifice meter is also studied. iii The discharge coefficient of pressure differential meters such as these, for sediment-laden flows can be derived from energy equation and equation of continuity for water and sediment. The -analytical expressions have been arrived at for all the three meters assuming uniform distribution of sediment across the cross section and that additional energy is requir ed to maintain the sediment in suspension. The experiments for this study were carried out in a 10.16 cm diameter recirculatory flow system. Three flow meters namely venturimeter, bendmeter and concentric circular orificemeter were studied. Concentric circular orificemeters were used to study the effects of flow direction. Four DQ/D (the ratio of orifice diameter to the pipeline diameter) values namely 0.8, 0.7, 0.5 and 0.4 were used, whereas the geometry of the venturimeter and bendmeter was not altered. The venturimeter was of 10.16 cm x 4.85 cm and the bendmeter had a radius of bend of 15.08 cm (about 1.5D). Three sizes of uniform sand (relative density 2.65) 120 - 150 W, 150 - 180 M&t 180 - 212 m and coal (relative density 2.04) 300 - 355 W constituted the sediment, whereas clear water was the carrier medium. The first few experiments in each series were made with clear water flowing through the recirculatory system. Following these clear water runs sediment was added to the clear water and runs taken at constant concentration and varying discharges. In all the runs it was ensured that there iv was no deposition in the pipe invert. The average concentra tion and the discharge were measured volumetric ally under steady state conditions. The volumetric concentration of the sediment was varied from 5000 - 70,000 ppm. Experimental results were checked with analytical results. While analytical results gave C/C0 values less than unity, experimental results indicate the opposite trend. Hence data have been analysed from dimensional considerations. The principal findings of the present study are as follows : (i) The discharge coefficient of a bendmeter for sedimentladen flows has been found to be independent of the Reynolds number UD/v. It is dependent on wd/v(Particle Reynolds number), and Xthe absolute volumetric concen tration of sediment. The ratio C/CQ increases with increase in ad/v and X. The relations proposed give the coefficient to an accuracy of + 0.5 percent. (ii) The discharge coefficient of an orificemeter is dependent on Ud/v. "d/v. * ^d D</D- GraPhical relationships amongst these parameters has been developed with the help of which the discharge co efficient of an orificemeter can be predicted to an acxMiracy of ±1.5 percent. The discharge coefficient is higher for vertically downward flow and is lower for vertically upward flow than for horizontal flow. In all the cases the orifice discharge coefficients increase with increase of sediment concentration and increase of wd/v. (iii) The discharge coefficient of a venturimeter has also shown dependence on the same parameters as used in the analysis of the data on orificemeters. The proposed relations yield the coefficient to an accuracy of ± 2 percent for the data collected at Roorkee and elsewhere. It has also been seen that the coefficient of a venturimeter is less than that for clear water flow when u/« is less than 60. At higher values of U/« the coefficient is greater than that for clear water. (iv) The work of Hsu et al. concerning sediment distribu tion across a diameter has been extended using the data collected in this study.