Pushing water uphill

Edited by the Engineeringtalk editorial team Sep 14, 2001

Cascading Toshiba variable speed drives are providing precision pressure control in the latest mains water booster pumps being built by Allan Aqua-Systems.

Cascading Toshiba variable speed drives are providing precision pressure control in the latest mains water booster pumps being built by Allan Aqua-Systems.

All buildings more than a few storeys tall require the mains pressure to be boosted, and this has to be matched to the flow demand, which varies considerably throughout the day.

Allan Aqua-Systems has been building booster sets for 17 years, with the oldest only just coming to the end of their design life.

Booster set design constantly develops, with the addition of drives, such as the Toshiba units provided to Allan Aqua-Systems by TM Automation, being the latest innovation.

Each set comprises an array of pumps, usually vertical multistage units, with the number in use increasing with the instantaneous water demand.

When Allan Aqua-Systems started in the business, the pumps all used fixed speed drives.

This meant that quite often the booster was oversupplying for the demand, which because of the cubic relation between power and flow lead to considerable amounts of electrical power being wastefully consumed.

"In those days nobody worried about energy efficiency.

The builders did not want to increase the capital cost of construction by installing sophisticated controls and the buildings1 owners simply passed the running costs onto the tenants," recalls Ian Burke of Allan Aqua-Systems.

"Careful selection of variable speed drives, pumps and hydraulic accumulators can result in considerable energy savings.

"But there has been a complete turnaround as people have become more environmentally aware and wiser about occupancy costs.

We saw the very start of this trend and began to explore the use of variable speed drives to optimise electrical power consumption." A typical Allan Aqua-Systems booster comprises three pumps, but the company has also built units with seven or more.

All the pumps have to be managed so that they operate as an efficient and effective unit.

Specialist drive engineers from TM Automation have helped develop the simplest possible control architecture, for ease of commissioning and so that reliability and availability would never be compromised through the projected 20 year working life.

They also helped identify those drives from the Toshiba range that are specifically rated for pump duties.

Each Toshiba drive is programmed to recognise when it is approaching a defined relatively high set point speed, and sends a signal to a solid-state micro-controller, which then gets the next pump/drive unit ready to switch on and share the load.

TM Automation1s Robert Garrick, offers some technical input: "If we look first at flow in the water mains, demand dictates flow rate and pressure, height of lift defines the power in kilowatts.

With unregulated pump drives there is virtually no control, so little stability of pressure from the taps and cisterns.

Our drives flatten out the pressure curve and ensure stability.

"Now let1s look at the electrical load.

In the past, when you started a big pump direct on line, you created a massive harmonic spike in the power supply.

That is not acceptable these days, so we use the drives in a soft-starting mode and get the pumps running smoothly.

"We are also able to develop profiles of how the pumps share the load depending on requirements.

For instance, we don1t have one pump working flat out and another barely ticking over for long periods, although that is the preferred set up for short periods.

The controller constantly optimises the booster against actual demand." Now, as Allan Aqua-Systems is beginning to replace the boosters it installed when it was a young start-up company, Ian Burke is delighted to see how his new sets are reducing electrical energy consumption, enhancing power quality and accurately matching water demand and supply.