Drives in Flood Defence Systems

Aug 17, 2022 | 5 Minute Read

From New Orleans to the Netherlands, the world is full of places threatened by flood. And the age-old weapon in the fight against disaster - the water pump - continues to get more efficient thanks to the use of variable speed motors and the drives that control them.

The World Health Organisation estimates that, 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

And 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.

Pre-pump technology was largely confined to the construction of earthworks, embankments, dams and diversionary canals. Then the steam engine allowed the Victorians in the East of England to operate powerful pumps so that below-sea-level land could be drained well enough for farming. Steam power gave way in the twentieth century to the might of diesel and then - from the 1970s - to electricity.

What are the challenges that face a motorised flood pump? And why are variable speed motors an especially suitable response to them?

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 (the world’s largest pumping station, outside New Orleans, can move 150,000 gallons of floodwater per second) at continuously fluctuating flow rates.

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 defence 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 is the first step towards a safer and more controllable system. The smaller the pump, moreover, 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 behaviour 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 lifespan 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, effecting 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 build-up of sediment. Systems where this is a problem may find that, by running at a consistently high speed, sediment build-up is kept to a minimum - as, then, is the corresponding mechanical stress on the motors.

It is important, too, that pumps don’t overdo it. In an area like the Fens around the Great Ouse river, for example, water levels have to be managed to allow for navigation (leisure boats today where it was once commercial waterway traffic). A balancing act, in other words, must be struck between neither underdraining nor overdraining.

The flood defence system designer must therefore 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 PLC or integrated proportional-integral-derivative (PID) technology, pre-set values can be made to change with the calendar so that seasonal variations in water level can be taken into account. And, complementing real-time operations, supervisory control and data acquisitions systems (SCADA) may be used to supply information about incoming weather developments.

Centrifugal pumps are favoured in flood defence systems because of their power, simplicity and relatively small size. There are thus, thanks to the centrifugal pump and fan affinity laws, significant energy savings to be made 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 favour diesel engines for economic reasons (though such systems must settle for efficiency levels around only the 20-40% mark).

And so, when it comes to building new (or refitting old) floodwater pumping stations, variable speed electrical systems are generally now also favoured. Besides the standout dividends of controllability, efficiency and economy, they are also comparatively quiet - an important design consideration for systems in more populated environments (and hence some systems’ preference for liquid cooling rather than air cooling of the motors and drives).

As for the future, all motorised systems are getting smarter, marrying more complex automation with increasingly accessible interfaces. With, in particular, the emergence of phone app connectivitythe possibilities for motor control, monitoring and maintenance are diversifying and evolving towards a goal of ever more efficient systems; which, in the context of flood defence, means ever safer environments.

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