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Accounting accurately for the losses that arise from pipe fittings, such as valves, becomes a significant and
important issue, given the current emphasis on energy reducing technologies. Efficient design is only
possible if reliable loss coefficient data are available. For paste slurries which are in use in many industrial
settings, this situation is exacerbated by the necessity to use less water, resulting in laminar flow designs
becoming more common. Against this is the prevalent design reality that whilst reliable loss coefficient data
are available for turbulent Newtonian flow, this is simply not the case for laminar flow. The objective of this
paper is to show that fittings losses become much more prominent in laminar flow design. Work done by the
authors for the measurement of loss coefficient data on diaphragm valves is reviewed and updated. This
work, which is particularly important for paste flow applications, is then used to illustrate the importance of
laminar flow loss coefficient data in the context of a practical design example. Pumping power requirement
inaccuracies of the order of 50% are illustrated. The literature available to the paste piping designer
contains a direct contradiction, and further research on this important practical issue is called for.
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