From New Orleans to the Netherlands, the world is full of places threatened by flooding. Water pumps are used in the fight against such disasters, and they have continued to grow more efficient due to the use of variable speed motors and the drives that control them.
The Devastating Effect on Human Life & Infrastructure
The World Health Organization estimates between 1998 and 2017, floods affected the lives of over two billion people worldwide. Besides the immediate danger to human life, flooding can devastate agricultural land (frequently located in floodplain territory) and wreak havoc on both the structural and infrastructural assets of built-up areas.
With increasingly intense downpours and rising sea levels—both observed consequences of climate change—severe flooding is expected to become more common in at least the short- and
medium-terms.
Strategies for Unpredictable Water Management
Floodwater is, by definition, unpredictable. If its arrival—on a potentially overwhelming scale—follows a long dry period, pumps that have stood for some time at a standstill need to be activated instantly, powerfully and efficiently in order to start transferring large volumes of water at continuously fluctuating flow rates. The world’s largest pumping station, outside New Orleans, can move 150,000 gallons of floodwater per second.
Different strengths of response to the flood event are achieved in the first instance by varying the number of pumps in action.
Multiple pumps are a must in flood defense systems. Even in the smallest designs, a duty pump must have a backup. To distribute work over a greater number of smaller pumps rather than fewer larger pumps is the first step toward a safer and more controllable system. The smaller the pump, the less vulnerable it is to stress from problems with floating debris,
vortices and trapped air. After this, variable speed motors are a prerequisite if pumps are to engage with the distinctive
dynamics of floodwater behavior with optimum efficiency.
The direct starting, or stopping, of a floodwater pump is likely to cause water hammer following the abrupt change in flowrate through the pipe. These hydraulic shocks shorten the life span of equipment and lead to leaks at joints and burst pipes. Using soft starters or variable frequency drives (VFDs) ensures a gentle acceleration of the pump’s rotation up to rated speed, with VFDs offering the added advantage of fully regulated motor speed thereafter.
Though desirable in principle, affecting an efficient speed regulation of floodwater pump motors can be a complex affair.For example, while it makes sense from an energy-consumption point of view for a variable speed motor to run no faster than is strictly called for, pumps and pipes that deal with large volumes of floodwater may, depending on design, be more or less vulnerable to a buildup of sediment. Systems where this is a problem may find that by running at a consistently high speed, sediment buildup is kept to a minimum, as is the corresponding mechanical stress on the motors.
It is important, too, that pumps do not overdo it. In an area like the Fens around the Great Ouse River in England, for example, water levels have to be managed to allow for navigation. A balancing act must be struck between neither underdraining nor overdraining.
What Is the Challenge for Water Pump System Designers?
The flood defense system designer must carefully work out target water levels in the environment to be protected. These
are the collection points for the data that the drives will use to determine motor speed. The higher the water rises above a target level, the faster the pump will work; the lower the water falls, the more the pump will slow down.
Beyond this, the more programmable the system, the more opportunities exist for different kinds of efficiency. Whether through a programmable logic controller (PLC) or integrated proportional-integral-derivative (PID) technology, preset values can be made to change with the calendar so seasonal variations in water level can be taken into account. Complementing real-time operations, supervisory control and data acquisitions systems (SCADA) may be used to supply information about incoming weather developments.
Advantages of Centrifugal Pumps & Variable Speed Control in Flood Defense
Centrifugal pumps are favored in flood defense systems because of their power, simplicity and relatively small size. Thanks to the centrifugal pump and fan affinity laws, there are significant energy savings to be had following motor speed reduction (motor speed cut by 25%, equating to energy consumption decreased by nearly 60%, and so on).
This is important when considering the generally high running costs of a pumping station, some of which still favor diesel engines for economic reasons (though such systems must settle for efficiency levels around only the 20% to 40% mark).
When it comes to building new (or refitting old) floodwater pumping stations, variable speed electrical systems are generally now also favored. Besides the standout dividends of controllability, efficiency and economy, they are also comparatively quiet—an important design consideration for systems in more populated environments (hence some systems’ preference for liquid cooling rather than air cooling of the motors and drives).
The future demands the marrying of more complex automation with increasingly accessible interfaces. The objective must be for more efficient systems, which, in the context of flood defense, means safer environments.