Book - School of Science and Technology

210 Pumps and other auxiliary equipmentthe temperature range 10±180 C which is encountered in practice, the flow rate inlitre/s ˆ 0:238 kW/K.To limit a circulating pump to this exact duty would mean that the water flow throughthe many parallel circuits and, in effect, each terminal fitting, must be precisely thecalculated quantity. This is obviously impracticable bearing in mind that the pipeworksystem will be built up on site from commercially available materials under less thanideal working conditions and may, in any event, deviate slightly from the design. Inconsequence, it is usual to make the best calculation possible at the design stage and, thishaving been done, to add a margin to the calculated pump volume requirement. The sizeof the margin, which will vary between 10 and 20%, depends upon the complexity of thesystem arrangement: for either a simple single-pipe layout or for a reversed returnsystem, 10% would be adequate. There is a difference of opinion among designers asto the validity of adding some similar margin to the calculated pressure loss: experiencesuggests that such an addition should be made only after careful study of the pumpcharacteristics.For larger systems it is usual to provide a duplicate pump for each circuit, in order toprovide some insurance against failure. The two pumps are piped to the system in paralleland each is fitted with isolating valves on the suction and delivery connections. Wherepumps are provided in duplicate for a high temperature system, it is good practiceto arrange for a small circulation to be maintained through the stand-by pump so thatit may be brought into service at the system working temperature, thus avoiding thermalshock.It used to be thought good practice to provide a non-return valve on the deliveryconnection from each pump so that change-over could be a simple matter of electricalswitching, without making use of the isolating valves. Such arrangements resulted, however,in the pumps and the valves being totally neglected, there being no need for the plantoperator to visit them. In consequence, non-return valves may be considered to be anunnecessary and inappropriate provision, except in the case of packaged twin pump sets,as Figure 8.8, where a single flap is integral to the construction. The particular merit ofproviding duplicate pumps in this way is that the combination is very compact: some suchdual sets are made with pumps of different sizes in order to provide for day/night orwinter/summer duties.Flap valvePlanElevationFigure 8.8 Centrifugal pumps. Dual vertical spindle type