THE OPERATION AND PERFORMANCE OF C0NTl:NUOUS CENTRIFUGALS

Factors affecting the capacity of continuous centrifugals handling "C" massecuite are examined after the completion of several test runs on various machines at different factories. Feed pipe sizing, water addition and positioning, feed distribution, scaling of screens, etc., are discussed and recommendations made. Introduction For several years continuous centrifugals (typically BMA K850) operating on C massecuites in South Africa in the range of f 9 000 P at 50°C have been rated at 1 200 kg/h (+ 30 ft3/h) and over 200 machines have been installed on this basis. The discrepancy between local throughputs and those reported overseas were attributed to the supposedly exceptional viscosity and stickiness of SA massecuites. However a recent visit by Lamusse and FitzGeraldl to Australia and Mauritius dispelled all illusions on this score as the viscosity of South African C massecuites lie within reported viscosity ranges from both these countries. As a result of this visit, the SMRI carried out an intensive investigation into the poor throughputs of local machines and the main points of investigation and the results thereof are summarized in this paper. Installations examined and types of machines Work was carried out at the following factories and the machine types are listed under each factory. FIGURE 1 Detail of modification t o BMA K850. * Present address: Illovo Sugar Millers Ltd. Darnall: Standard BMA K850 and a modified BMA K850. Modifications consisted of a speed increase from 2 200 rpm to 2 500 rpm and two rows of 5 mm holes (a total of 46) drilled into the basket. (Fig. I ) (this modified centrifugal is referred to as BMA No. 10 in the text table). One Fives Lille FC 1 000. This Fives Lille FC 1 000, was modified at a later stage by a speed increase from 1 900 rpm to 2 100 rpm. Tongaat : BMA K850. Sezela: BMA K850. Melville: Western States CC-IV-(34" x 34") basket. The K850 and FC 1000 are centre feed machines with solid baskets and the CC-IV-34 is a side feed type with perforated basket (Fig. 2). Investigational aspects During the course of the experiments the following points were investigated: 1. Flow of massecuite to the centrifugal in relation to (a) pipe and valve sizing, (b) viscosity, (c) height of massecuite above the delivery valve. 2. Massecuite distribution in the centrifugal. 3. Conditioning of massecuite. 4. Effect of spraying water and steam in the basket. 5. Molasses drainage and scaling of screens. 6. Capacity of continuous centrifugals. Results and discussion In order to compare the results of the different tests and reduce possible analytical and calculation errors, all the sampling and measurement procedures were standardised, and all the analyses were carried out at the SMRI. A Brookfield Viscometer Model RVF was used for viscosity measurements. Flow of massecuite to the centrifugals Early investigations showed that in most cases when viscosities were high, the limitation in throughput was caused by the inability to supply massecuite to the machine at the required rate. Fig. 3 shows the effect of increasing valve opening on massecuite throughput, at different viscosity levels, through a standard 100 mm diameter iris valve. These tests were carried out at Sezela. One run was completed at Darnall. Proceedings of The South Afvican Sugar Technologists' Association-April 1974 FIGURE 2 Side feed machine with perforated basket (CC-IV-34 Western States). Capacity is about the same for massecuite of different viscosities at 25 % valve open but variations in flow rate increase with wider valve opening. Grain size and concentration may also affect the flow properties of the massecuite although these were not measured. These two factors together with differences in head probably account for differences in flow rate at 25% valve opening. The viscosities plotted were not high and throughputs increased with valve opening. These results must be compared with the following graphs in which flow rates reach a definite plateau and do not increase significantly despite further valve opening. This effect of higher levels of viscosity vs. valve opening on throughput is illustrated in Fig. 4. Line A is for a medium viscosity (9 000 P) while B is for a 12 000 P massecuite. The offset in line B at 40% valve opening and the scatter of points at 100% opening on line A indicate the effects of variation in head of massecuite and hence pipe friction on the feed rate. These tests were carried out on K850 machines with standard 100 mm iris valves. To overcome this restriction a 100 mm valve was replaced by a 150 mm valve and in addition a lubrication "rod" was fitted into the feed pipe. This rod (a small-bore stainless steel pipe) enters the feed pipe above the iris valve and curves down along the pipe centre line through the centre of the iris valve to just clear of the flange. Four holes 0,5 mm are drilled into the pipe wall and the end of the pipe is sealed. A general view of the feed pipe is shown in Fig. 5. The increase in capacity due to larger valve size alone is shown by curve C. The capacity increase for a more viscous massecuite (15 000 P at 60°C) at 100 % valve opening was significant, the flow increasing from 700 kg/h to 1 700 kg/h. Later in the same test at 100% valve open, water was used for rod lubrication and the capacity increased from 1 700 kg/h to 2 150 kg/h. These points are labelled D, E and F in Fig. 4. Another observation made was that there were large fluctuations in viscosity from day to day, sometimes even during the same day, and continuous observation and adjustments of feed to the centrifugals is therefore a necessity. (Refer Table I) 26 ' Proceedings of The South African Sugar Technologists' Association-April 1974 FIGURE 40/ Western States machine. From a mechanical engineering point of view, ,centre feed is preferable and ../ centrifugals fed through the centre are less liable to vibration but the main disadvantage is uneven distribution of the massecuite. The distributing unit consists of a cylindrical distribution cup with several distributing pins (4 or 8) and an acceleration cone. This cone is bolted to the distributing cup by vertical . pins in the case of the BMA and six impellers for the FC 1000. Even under the best cond.itions, spread of massecuite distribution looks like a series of fingers spreading from the bottom to the top of the basket (Fig. 6). It was found that the finger effect on to the screens was mainly caused by erratic surges in massecuite from the accelerating cone which either strike the screen directly or hit the top part of the screen clamping cup. P*' ,' ,,' It appears that these localised surges or lobes of masseDL .," cuite that speed up the basket without being purged -= Viscosity 5400 @ 600C SZ are related to the number of bolts that support the