CPD PROGRAMME | PARALLEL PUMPS standby arrangement. So installing two pumps in parallel with one pump selected to give full design flow and a second to provide a complete backup is common in many building projects, regardless of their sensitivity to failure. It is typical, in this dutystandby installation, to alternate the active duty pump on a timed basis to even out the wear on both pumps. In the event of one of the pumps failing, the standby pump takes over the duty. The pumps are each likely to have a pressure-flow characteristic similar to that shown in Figure 1, where the system curve represents the characteristic of the pipework system. The pressure drop through the system, Pdesign kPa (or head in metres water) is determined as part of the design process for the pipework system at a flowrate, Qdesign Ls-1, that is required to meet the design loads. The system curve is then produced for a range of flowrates using the relationship that P Q2. The pump curve is a function of the pump geometry and would be provided by the manufacturer. The operating point indicates the flowrate (and system pressure drop) if a specific pump, operating at a particular pump speed, is installed in the system. Ideally, a pump should be selected so that its operating efficiency is high when the pump is operating in the system. When the speed of a pump is reduced, the shape of the pump efficiency curve will typically maintain approximate correspondence with the shape of the pump characteristic, but manufacturers data will be able to offer more accurate detail. To prolong pump life, a pump should be selected so that it does not operate consistently towards the left end of the pumpcurve, where the flow is very low and the pressure is high. If two identical pumps were running at the same time and same speed in parallel (such as in Figure 2), the flowrate will be twice that of one pump operating on its own particular generated pressure. (By comparison, if the two pumps were operated in series, then the flowrate would remain the same as for one pump, but the generated pressure would double.) If this parallel pump arrangement is used so that both pumps concurrently share the flow, then each of the pumps would be smaller than the equivalent duty-standby arrangement as, at full flow, each one would be responsible for delivering just half the design flowrate. These pumps are likely to be physically smaller than the duty-standby pumps that they replace, so willbe lower-cost units and have less embodied carbon. If one of the pumps fails, leading to just Combined efficiency curve Pressure (kPa) Combined operating point Flow through Total flow each pump -1 Flowrate (L.s ) Figure 2: Two pumps operating in parallel arrangement Only one pump operating in parallel arrangement Pressure (kPa) 80% 100% Flowrate (L.s-1) Figure 3: Single pump operating in parallel arrangement (one having failed) one of the parallel pumps operating, it might be assumed, erroneously, that the single pump operation would deliver 50% design flow. However, considering the operating point in Figure 3, due to the shape of the system characteristic and the particular pump characteristic, this example would still be able to deliver 80% of design flow; for comparison with more traditional descriptions, this is known as having 80% redundancy. At most times, commercial HVAC systems operate at part load. For example, considering monitored installations in central London, the demand exceeds 80% for less than 3% of the operating hours. So if one pump were to fail (in a dualduty pump application), it would only potentially impact on building operation ifit coincided with that 3% of the occupied period. In any case, there would still be80% of maximum design flow so not a complete system failure even if it was to fail in the peak 3% hours. For a chilled water system, the cooling coil output characteristic is such that 95% output would be delivered to the air at 80% water flowrate, although the dehumidification at the coil will be less than design, as the mean coil temperature will be higher. Heating coils have a similar, and slightly greater, proportional heat output at 80% flow. In practice, there are likely to be considerations that increase this redundancy factor so reducing the risk. For example, building chilled water systems are likely to have a safety factor added to the system design pressure drop (around 1015%), and when variable speed pumps are selected, they often have excess speed capacity that could be used when one of the pumps fails or is taken out of service. Together, these factors may increase the redundancy by 5% and so reduce the potential hours when failure of a pump may impact the building operation. In the case of the London office example, it would bring this down to approximately 1.5% of occupied hours. 36 May 2019 www.cibsejournal.com CIBSE May19 Supp pp35-38 CPD v2.indd 36 26/04/2019 15:37