Chemical Technology April 2016

Chemical Technology • April 2016

PUMPS & VALVES

elevation above sea level has on the operation of centrifu-

gal pumps. Then, again, one must consider what effect/s

the transport of gravel and sand in the piping have on the

hydraulics of the system.

Solutions

In simple terms, the pump motor spins the impeller which

creates a vacuum at its centre and, depending on the loca-

tion of the end of the suction hose, above or below the centre

of the impeller, atmospheric pressure at the surface of the

pool of water creates the driving force to ‘push’ the water

through the suction hose. The static pressure created by

the column of water between the level of the pool and the

impeller, will, however, either add to the atmospheric pres-

sure or subtract from it.

Then there is the frictional loss of pressure in the suction

hose which can never be greater than the pressure induced

by the atmospheric and static pressure otherwise there will

be no flow to the centre of the impeller.

That is not the end of the road: there is still the possibility

of cavitation when the pressure in the water column falls be-

low the vapour pressure of the water at ambient temperature

and bubbles are formed. This disrupts the centrifugal action

of the pump resulting in a distinct fall-off in the efficiency of

the pump, or in no pumping at all.

Added to this is that the transport of gravel and sand clas-

sifies the liquid being pumped as a ‘slurry’ which has its own

set of rules when calculating the pressure drop in the hoses.

An excellent publication from the pump manufacturers,

Weir Slurry Group Inc, is the “Slurry Pump Handbook –

2009”, (Fifth Edition, Electronic Version, February 2009),

which defines the applications and constraints of using

these slurry pumps.

Specifics

We could consider what production rate of diamondiferous

gravel per day could be hand sorted by a small team of

sorters in a 6-hour production day. Some storage could be

considered. However, as we are not considering a homo-

geneous slurry pumping system where all the particles are