Why VSDs offer more than just energy savings

Author: 阿西布朗勃法瑞公司（ABB）

Variable speed drives (VSDs) are well known for their energy saving abilities – by driving the motor at the actual speed needed by the process, energy use can be reduced dramatically, in some cases by as much as 50 percent.

Yet the benefits of VSDs don’t stop there - modern drives offer a wealth of control and monitoring functions that can help water utilities cut maintenance costs, avoid damage to process equipment, maintain stable water flows, and reduce the risk of burst pipes.

One of the main water-specific features offered by VSDs is level control. This feature helps when pumping slower for longer, which is generally more energy efficient than pumping intensively in short bursts.

Level control allows a water tank or reservoir to be filled, or a wet well to be emptied, at a rate that matches the incoming or outgoing flow. Traditionally, this would be achieved using a PLC, along with instruments to detect levels, and float switches or other mechanical components. Level control uses the drive to ensure that the pumps are operating at the best speed to balance energy efficiency against the required flow rate, avoiding the need for any additional components.

A similar function is soft pipe filling – by providing a pump with soft-start, this feature allows pressure to build up smoothly in empty pipes. This avoids pressure peaks when pipes are momentarily empty, reducing overpressure and eliminating water hammer which can damage pipes and lead to leaks.

Keeping pumps clean

Blockages in wastewater treatment are a common problem for water utilities. It most often occurs when debris gets trapped around the pump impeller, causing it to seize up. This requires the pump to be extracted from the wet well, cleaned, and then reinstalled. It also means regular inspections, which can be expensive and require specialist lifting equipment. Each inspection or extraction also results in downtime.

Some drives can deal with this problem through an anti-jam or anti-ragging function. This commands the pump to go through a series of forward and reverse rotations to shake off any build-up rags or other debris on the impeller – the pump can then continue with normal operation. As well as preventing potential blockages before they can happen, this improves pumping efficiency, and decreases the need for manual inspections and cleaning. This is also better for on-site health and safety.

Intelligent Pump Control (IPC)

Many water treatment facilities use multi-pump configurations, to balance flows, manage duty time across the pumps, and to provide redundancy in the event of planned or unplanned downtime. There can also be energy saving benefits - since centrifugal pumps are variable torque applications, running two pumps at 60 percent speed can cost much less to move the same volume of water compared to running one pump at 100 percent speed.

IPC makes it easier to integrate and operate multi-pump systems, which typically consist of several pumps, each connected to a separate drive. Using this arrangement allows high flexibility, allowing operators to share the load between the pumps while keeping each pump in the running optimally. This makes it easier to maintain a stable water flow, while also taking account of varying water pressures and variances in pump speeds. With the control function provided by the drive, there is also no need for an external logic controller.

Multi-pump functions can help achieve a high level of redundancy, whereby if one pump fails or requires maintenance, then others can use their spare capacity to pick up the slack. This provides stability and ensures continuous operations.

The method can also be used in combination with another feature known as pump auto change. This shares out the operating time of pumps in a parallel pump system, reducing strain on both individual pumps and collectively.

Other functions include sleep boost. Just before shut down, this feature runs the pumps to boost the pressure or the water level, extending the pump’s sleep time and saving energy. It avoids unnecessary starting and stopping of the pumps, which can lead to damaged pipes. Sleep boost can also help to flush the pipelines.

More complex operations can also be programmed. These can include favouring the most efficient pump in a series. A similar feature is pump priority control. With this feature, drives can be programmed to operate higher capacity pumps during daytime and smaller units at night.? As well as saving energy, pump priority reduces the risk of leakage, as the higher the water pressure, the more chance there is of a leak occurring. Water utilities can benefit from lower maintenance costs throughout the life cycle of the pump system.

Mitigating harmonics

Drives can in some circumstances produce a high amount of harmonic content. In areas with poor power quality this can affect system efficiency and cause electrical equipment to run erratically. Usually, harmonics are mitigated by adding additional equipment such as filters, multi-pulse transformers, or by over-sizing equipment. ABB’s Ultra-Low Harmonic (ULH) drives avoid this by having built-in harmonic mitigation. This incorporates an active supply unit and integrated low harmonic line filter, which can reduce harmonic content by up to around 90 percent compared to a conventional VSD.

Detection and prevention of cavitation

Cavitation is a major issue in pumping applications. Left unchecked it can severely damage the components within a pump. Cavitation is caused by a pump’s impeller blades creating a vacuum on the suction side/inlet of a pump, resulting in tiny steam bubbles that then collapse, producing a shockwave. These shockwaves gradually erode the impellers, and will eventually damage or even destroy the pump.

Modern drives such as the ABB ACQ range can detect and prevent cavitation in real-time. The drive monitors a pump’s speed and torque directly from the motor shaft - this allows any potential flow issues to be picked up immediately, without the need for additional sensors or other equipment. Anti-cavitation software in the drive looks for specific patterns in the motor and VSD operating data, automatically reducing pump speed to prevent cavitation and helping to significantly increase the pump’s lifetime.

In summary

VSDs are widely known for their energy saving capabilities, but they also have a lot more to offer. From controlling multiple pumps, to avoiding damage to impellers and clearing pumps of debris, VSDs have a lot to offer to utilities seeking to keep their pumping operations efficient, effective and available.

There are tens of thousands of motors in operation across the UK water industry, and each one represents an opportunity to improve resilience and operational efficiency, as well as saving energy. Even small changes, when combined, can add up to significant savings across a site. ABB is inviting water companies across the UK to challenge us to find the untapped opportunities for energy saving ?across their facilities and operations.

Changing the operating parameters of some water handling processes by as little as 1% can improve the energy efficiency of pumping equipment by a significant amount, with no negative process impact. Cumulatively, improvements in operating efficiency could help the sector to save up to ￡60 million per year in energy costs.

To find out more, visit: https://campaign-mo.abb.com/l/961052/2023-07-06/5569c