Exploiting the technical opportunities of variable-speed pumps
Variable-speed drives can have a significant impact on overall life-cycle costs — Britta Frank.
Their ability to save energy is just one benefit of applying variable-speed drives to pumps. BRITTA FRANK explores other possibilities.Variable-speed drive (VSD) has become a familiar term, and many have embraced this technology to reduce energy consumption and keep costs down. VSDs have been extensively tried and tested with pumps and pumpsets for a number of years, but further technological improvements continue to provide benefits for the end user. Variable-speed drives are used in many kinds of building-services applications in single and in multiple configurations. They are also frequently used for water and waste water as well as in manufacturing processes. Interface
Rather than simply being a piece of equipment working on its own, one of the main developments in VSD pump technology is the ability to interface with other building-management equipment such as devices for remote interrogation and diagnosis. Measurement and control systems that can co-ordinate several pumps can also increase reliability and reduce operating costs. Modern electronic controls ensure performance is continuously matched to variations in operational conditions. In some cases, energy consumption can be reduced by up to 50%, which provides fast payback on capital cost. Sensors detect variations in demand and adjust the frequency of the supply to the pump motor as required. The variables monitored generally include pressure, differential temperature and differential pressure. When used with a single pump the most common form of control is a discharge pressure sensor. This sends a signal to the VSD which, in turn, varies the speed to hold the pressure constant. In multi-pump arrangements, if the flow variation for the majority of working time is substantial (such as six to one or, even, as low as two to one), it is worth considering using more than one pump to meet maximum flow. This approach will then improve efficiency at low demand. Sequence control
Sequence control can be used to start the pumps depending upon demand and will also execute automatic cyclical changeover of the pumps to ensure equal running times per unit. Intelligent systems can also increase output pressure to compensate for friction losses in the pipework. Multi-pump systems can utilise a single VSD to control one pump in combination with a number of fixed-speed or slave pumps. However, careful economic and hydraulic analysis of the operation is recommended if pumps are used in this way. It is important to remember that in pressure-controlled supply systems, the pumps must shut off immediately the demand ceases. Operating pumps at zero-flow conditions wastes energy, but, more seriously, this can cause high temperature within the pump which should be avoided at all costs. Ultimately, VSD systems can also have a significant impact on overall life-cycle costs. Matching pump operation to flow conditions provides smooth control with fewer stops and starts, which reduces the risk of mechanical stress and surge pressure. Wear and tear problems are also reduced, resulting in low pump maintenance and repair bills. Many of today’s VSDs have been designed to work with other equipment. Such products have been designed to remotely monitor pump and associated systems and provide a diagnosis. They have the added benefit of recommending a course of action to the customer. Reliability
Depending upon the configuration of the application, it is possible to maintain and improve the process and system reliability as well as optimising control and energy efficiency. VSDs support condition-based maintenance by means of repair and maintenance of the pump. The principle of monitoring and adjusting service periods based on use and conditions is now widely used in building services. Maintenance and energy managers are under increasing pressure to control costs, and this principle is ideal for sites without dedicated maintenance personnel to monitor equipment. Routine maintenance programmes can provide a solution, but this can lead to higher costs due to work being carried out and parts being replaced earlier than necessary. Condition-based maintenance maximises the total life of the equipment, which will ultimately result in further reductions in life-cycle cost.
Not only do variable-speed drives significantly reduce the energy consumption of pumps but they can also interface with monitoring and control systems.
Complex measurement and control systems that can co-ordinate several pumps can also increase reliability and reduce operating costs. Along with labour and supervision costs, it is possible to control operational spend by monitoring how a system is functioning and regularly measuring items such as shock-pulse signature, temperature, noise, energy input, flow rate and pressure. However, the topic specifiers and operational personnel will be most concerned with is energy cost, and many aspects require careful consideration. Energy consumption is a key issue, particularly in the EU, because of its effect on three main objectives of energy policy — safeguarding energy supplies, improving competitiveness and protecting the environment. Britta Frank is market sector manager for KSB Ltd, 2 Colton Way, Loughborough, Leics LE11 5TF.