The role of commissioning in maximising pump performance
Exceptional control of the constant-temperature heating system in Bury St Edmunds Sports & Leisure Centre is provided by pumps from Biral’s low-speed range. Their flat characteristic curves provide good control without developing excess pressure in the system.
PETER NEARS explains why variable-speed pumps should be commissioned by a competent person to achieve effective operational performance, maximise energy savings and prolong life expectancy.The latest designs of pump with built-in inverter drives need to be correctly commissioned because there is a wide choice of how to operate and control these new pumps. Biral’s P range of pumps, for example, have built-in inverter control linked to advanced permanent-magnet technology. This tried-and-tested technology is applied to pumps up to 65 mm bore, giving flow rates suitable for boilers up to 1000 kW with a difference between flow and return temperatures of 20 K. As is common with all glandless circulators, the motor shaft must be horizontal and particular care must be paid to the position of the terminal box. Only qualified people should be allowed to wire and commission such pumps as a residual current inside the terminal box can be present for up to 10 minutes after the power has been disconnected. If a variable-speed pump is over-sized, the computer chip in its drive cannot see small changes in system resistance when the pump is working at the end of its curve. The pump will then always run at maximum capacity and maximum power consumption. A pump running at maximum power consumption will always run excessively hot, causing failure of the motor windings. The message from many pump manufacturers is that a variable-speed drive on a pump will solve all problems — but this is only applicable if the pump is correctly sized.
To maximise the benefits of variable-speed pumps, areas B, C and D of the characteristic curve are no-go zones. Operating in area A provides maximum potential for variable flow rates and energy savings. Pump selection
The chart shows the operating range for achieving good selection for variable speed pumps. The ideal design duty is in band A of the chart, which provides maximum potential for variable flow rates and energy savings. A pump in band B would be better replaced by a smaller pump because the energy savings achieved by a pump operating in this area as the flow changes would be small. A pump in band C should be reconsidered as a fixed-speed pump because savings offered by varying its speed will be negligible. Specifying a fixed-speed pump enables the capital cost to be reduced. Pumps that are oversized appear in band D. The system curve in this area is too flat to detect changes in system resistance, and the pump will run at maximum output, with no potential for energy saving. The operating speed for night setback or frost protection is shown on the chart as curve min 1. There are two basic levels of control for variable-sped pumps — control by proportional pressure or constant pressure. Proportional control is suitable for most systems. Constant-pressure control only makes sense where the pump is fitted in a primary loop circuit, such as a low resistance boiler, where pressure changes are minimal but flow rates may need to be varied to match modular boiler systems. In proportional-pressure control mode it is necessary to know what the consultant’s maximum design figures are to enable the pump to be set to the maximum design head or resistance. If this information is not available, the pump should be set at minimum speed and gradually run up until the index circuit is satisfied and the correct temperature difference across the boiler or system is achieved. It should then be left in this position to allow thermostatic radiator valves or zone controls to control the resistance in the system and allow the pump to vary the flow rate and generated head accordingly, saving up to 70% of the maximum power consumption of the pump. Precise pump selection, using multi-speed or variable-speed drives, is a vital factor in determining the life expectancy of the pump. A one-size-fits-all approach will not give all the benefits one might expect. Peter Nears is Product Manager for Biral Pumps at Hamworthy Heating, Fleets Corner, Poole, Dorset BH17 0HH.