Getting up to speed on pump control

Published:  05 September, 2013

Vacon, VSD, variable speed drive, pipes, pipework
Pump-specific variable-speed drives — Stephen Takhar.

Apart from the obvious energy-saving benefits, variable-speed drives for pumps offer lots of other desirable features. Stephen Takhar of Vacon takes up the subject.

Almost every building services installation has pumps, and provided that they’re centrifugal pumps and not reciprocating pumps they offer opportunities for making big energy savings by fitting variable-speed drives.

A major opportunity for saving energy comes from the pumps having undoubtedly been sized to deal with worst-case conditions so, in reality, it will be possible to run them at reduced speed almost all of the time without any adverse effects on the performance of the system in which they’re installed. Reducing the speed of the pump by as little as 10% will cut its energy consumption by around 25%

There are also other benefits. With a VSD fitted, it’s easy to arrange for controlled starting and stopping of the pumps, which greatly reduces the risk of water hammer and cavitation. As a result, wear and tear on the pump and other parts of the installation is significantly reduced.

But what is the best type of VSD to use with a pump? The answer is simple — choose a VSD from a range that been designed for pump applications.

Let’s see why this is a better option than just buying a general-purpose industrial VSD.

Unsurprisingly, one of the key benefits of pump-specific VSDs is that they are easy to use in pump applications, with the best types offering many dedicated pump and flow control functions.

These might be expected to include not one but two proportional-integral controllers to facilitate single- or dual-loop control of variables such as pressure, flow or temperature. Other features can include frost protection and a sleep boost function to boost pressure when a pump is about to stop due to low demand and extend the time before restarting. Other possibilities include soft filling for pipes and dry-pump protection. Finally, if a pump appears to be jammed. an auto-cleaning/auto-ragging feature carefully rotates it in reverse to remove the obstruction.

Vacon, VSD, variable speed drive, pipes, pipework
26 Vacon drives are used in the London Hilton Metropole to control the pumps in the chilled water and LTHW systems.

Some pump-specific VSDs offer special functions for applications where multiple pumps are used instead of one large pump. One option is for each pump to have its own VSD, with the VSDs communicating via a fieldbus link. When the capacity of the lead pump is exceeded, the others are started and ramped up to speed as needed to meet demand. To equalise wear on the pumps, the lead duty is automatically rotated.

Another option is for the lead pump to call for help, just as it does in with the first option, but with all of the pumps running at the same speed, in effect creating one large pump. This arrangement has the benefit of eliminating the need for anti-backflow valves. There are energy-saving benefits, as multiple pumps running at part speed use less energy than a single pump at full speed.

Pump-specific VSDs should also be easy to integrate with building-management systems as well as with control systems for other flow-related applications, such as water and wastewater management. This means they should be expected to have good network and fieldbus support, particularly for widely used systems like Ethernet IP and Profibus IO, ideally with simple, inexpensive options for extending this support to include other systems.

Smaller features can also be very useful, such as an on-board real-time clock that allows easy implementation of time-dependent functions, like changing pressure reference values during the weekend when building occupation patterns are different.

Finally, let’s look at drive efficiency — after all, one of the main reasons for fitting VSDs is to reduce energy usage, so it makes good sense to fit the most efficient drives. Fortunately, the latest pump-specific drives are outstanding in this respect, since they incorporate controlled cooling fans and low-loss film capacitors instead of conventional electrolytic types. As a result, losses are reduced by more than 15% compared with VSDs that use older technology.

Even if you’ve fitted VSDs to all of the fans in your building systems to improve energy efficiency, it’s perfectly possible that there is still some ‘low-hanging fruit’ to pick in the form of pumps that are equally suitable for VSD control. So take another look at your plant and seek out those pump opportunities. To maximise your benefits, however, do be careful to choose the right VSDs.

Stephen Takhar is managing director of Vacon.

Vacon, VSD, variable speed drive, pipes, pipework
Variable-speed drives that are designed for pump applications, such as the Vacon 100 range, offer useful features compared to a general-purpose industrial VSD.

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