Chemical Technology April 2016

PUMPS & VALVES

Conclusions As we can see from the calculations, we have an inherent problem with the head loss in the suction line. The suction line loss of 8,4 m is far too large and has to be reduced to about 1,5 m. To do this we can increase the size of the suction hoses, but this may dredge far too large an amount of water and gravel to handle. The Diamond River Hunters then decided to dig the diamondiferous gravel out of the stream with a front end loader and to dump the material into a hopper from where it was sluiced into the ‘trommel’. The small pump necessary to raise clean water to the hopper is achievable. The Klondike Gold miners use this technique, but the earthmoving equipment is forever breaking down and spares are hard to come by. The moral of the story, for us all to note, is to spend far more time planning a venture and designing the infrastruc- ture before heading out on site where rectifying engineering problems is far more difficult than being able to redesign on a computer in the office! References 1 “© MultiChoice Africa (Pty) Ltd. All rights reserved”. 2 The hydraulic Transport of Coal and Solid Materials in Pipes – R. Durand, E. Condolios –LaboratoireDauphinois d’Hydraulique, 1952 3 Durand, R. (1953). Basic relationships of the transportation of solids in pipes - experimental research. Proc. Minnesota Interna- tional Hydraulics Convention, Minneapolis, Minnesota, pp. 89-103 4 Slurry Transport using Centrifugal Pumps – K.C. Wilson, G.R. Ad- die, A. Sellgren, R. Clift – Blackie Academic & Professional, 1996 5 Slurry Pump Handbook – 2009, Fifth Edition, Electronic Version, February 2009, © 2009, Weir Slurry Group, Inc. All Rights Reserved

from the slurry supply surface level to the centre line of the pump m. = Head loss of inlet in metres of slurry = Loss of head inmetres of slurry in the suction pipe = at 3 100 m above sea level, pressure is 6,1 m of water =6,1/1,14 = 5,4 m slurry =2,19 ft water is 0,67 m water at ambient temperature which is 0,67/1,14 = 0,59m of slurry

H H H

fs

atm

H

vap

Z

= 5 m

s

H

=0,5 x v 2 /2 g = 0,5 x 2,2^2/2 x 9,81 = 0,12 m

i

of slurry H fs

=8,4 m of slurry

NPSH a = 5,4 – 1,92 - 5 – 0,12 – 8,4 = - 10 m which is seriously negative, so there is no suction head available to allow pumping to take place. If the pump was at sea level then the equation would be as below:

NPSH

a = 11,9 – 1,92 - 5 – 0,12 – 8,4 = - 3,54 m , even at sea level there is not sufficient available

NPSH. NPSH r

= this can be read off the pump curve and is always positive.

So operating at sea level will not make any difference to the available NPSH as calculated above. Since the operation is at 3 100 m above sea level, a reduc- tion in the suction height to 1 m will make

NPSH

a =5,4 – 1,92 - 1 – 0,12 – 8,4 = - 6 m which is still less than the NPSHr and cavitation will occur.

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Chemical Technology • April 2016