With the advent of AMP6, the onus is on water and wastewater service providers to adopt a TOTEX (total expenditure) approach to driving down capital and operational costs. One of the ways they can do this is to make their existing assets work harder – a practice known as ‘sweating assets’. Here, Martin Richardson, water framework manager – drives & controls, ABB Limited, shares six tips to optimise the reliability and efficiency of variable speed drives (VSDs) installed on pump systems.
1. Check parameter settings to ensure VSDs are set up correctly and optimised for your system
If you’ve ever been shopping for a new television and noticed that the images on some sets are sharper than others, there’s a very simple reason: someone has taken the time – and had the know-how – to set it up properly.
It’s no different with VSDs. Correct installation and regularly updating the parameter settings ensures the VSD achieves optimum performance throughout its life cycle.
If a VSD isn’t set up properly in the first place or the needs of the system changes then this can have a significant impact on the performance of the pump process. What might happen is that site engineers tinker with the set-up, making adjustments without fully appreciating the impact on the overall system. A little knowledge can be a dangerous thing.
To maximise the reliability of the installed VSDs from day one, leave the parameter settings to the vendor and ensure that they are checked at least annually as part of a proactive maintenance schedule.
2. Consider replacement, upgrades and retrofits
Over time almost all pumping stations need to change their capacity, either in response to an increase in the local population, or in preparation for it.
This provides a good opportunity to replace, upgrade or retrofit the installed VSDs and/or electrical motors. The VSDs, for instance, can often be refurbished and reused in the new system, at a fraction of the cost of buying a new unit.
In cases where the pumping station needs to increase its capacity to meet growth in population, existing VSDs may no longer be suitably sized for the new pumps. Yet rather than buy new VSDs, it might be possible to retain the existing cabinet and framework and simply upgrade or retrofit specific parts. This approach not only saves capital cost but can cut the installation and commissioning time too.
Recently, a water company was upgrading its pump system and asked ABB whether the existing VSDs could be reused or replaced. Replacing the 55 kW VSDs would have been more costly than upgrading or retrofitting. However, the environment in which the VSDs operated was clean and they had been well maintained. Despite being some 10 years old, ABB recommended a straightforward cleaning of the VSD heatsink and overall drive cabinet and replacement of a fan. The cost of refurbishing the existing drives was around 10-15 percent of the purchase price of a new drive.
3. Use the smart functions in your VSD
Today’s water industry VSDs come packed with smart functions as standard and can add several percent to the efficiency of pumping equipment and cut maintenance. Yet many VSD users only use a small proportion – if any – of these functions. The reason is that there is a lack of understanding about what these functions do and the benefits they bring.
Smart functions bring both capital and operation benefits. By putting control functions in the drive, it cuts the need for a PLC, at the same time helping to save energy, reduce downtime and prevent pump jamming and pipeline blocking.
Among the intelligent pump functions is sleep boost, which runs the pumps to boost the pressure or water level just before they shut down, extending the pump’s sleep time and so saving energy. It also avoids unnecessary starting and stopping of the pumps.
Another function is pump priority control, which is used to balance the operating time of all the pumps in the system. It can be a great help in planning maintenance and can boost energy efficiency by operating pumps near their best efficiency point - in a system where the consumption rate is greater during the day, the drive can be programmed to operate higher capacity pumps during daytime and smaller units at night.
The pump cleaning function, also referred to as anti-ragging, is the smart function most commonly used by the water industry. The standard way of detecting flow problems is to react if the pump is drawing a higher than normal current or torque as it tries to deal with the extra load. This method of detection is not always reliable and can be too late as the blockage has already reached a critical stage and cannot be undone easily.
However, the pump cleaning function takes action before things get critical. When a blockage is detected, the drive automatically runs the pump back and forward rapidly to clean the impeller, quickly returning flow rates to their normal level and avoiding any reduction in service. This also avoids the needs for more costly intervention, such as lifting the pump to remove the blockage.
Smart functions enable the VSD to sweat the other assets in the pump system – in other words make them work harder and/or better. For example, the pump cleaning function doesn’t benefit the drive itself, but it can extend the life of the pump, reduce maintenance intervals and increase efficiency of the pump system as a whole.
4. Make the most of programmable logic control
Many of today’s VSDs have built-in programmable logic controllers (PLCs) which can bring additional functionality thereby improving reliability of supply and cutting capital and operation costs.
For example, the VSD can be programmed to reduce water pressure during the night when demand for supply is lower. This saves energy, and reduces the risk of leakage, as the higher the water pressure the more chance there is of a leak occurring. This cuts maintenance costs throughout the life cycle of the pump system.
PLCs integrated into VSDs work best in simple applications such as booster sets. Yet even a simple programme can save hundreds of pounds on a single pump. Multiply that by the number of drives in a typical pumping station and the savings can be significant.
5. Check pump system data to ensure the energy control philosophy is adequate
Installing VSDs and motors on pump systems is proven to generate substantial energy savings. However, water companies may not be achieving the level of savings they expect if the control philosophy is inadequate.
We’ve seen cases where water suppliers have invested money in VSDs and high-efficiency motors, but they have failed to achieve the advertised energy efficiency because the control philosophy is wrong. Yet it is possible to improve the energy efficiency of a pump system by between 15 and 20 percent simply by ensuring the control philosophy is adequate.
For example, the traditional method of filling a storage tank is to run the pump at a fixed speed using a sensor to indicate when the tank is full and then switching the pump off. However, greater energy efficiency – and reliability - can be achieved by switching the control philosophy of the VSD to pump slower for longer. Reducing the speed of the pump cuts energy consumption. It also eliminates the constant stopping and starting of the pump, which reduces wear on bearings and seals, cutting maintenance costs and contributing to the reliability of the pump system. And it makes batch processes, such as chemical dosing, easier to predict, as it is easier to deal with a steady flow of water than with rising and falling volumes.
To help water companies optimise the energy efficiency of their existing pump systems ABB has created a CPD (continuous professional development) accredited Lunch ‘n’ Learn training course, entitled System Efficiency.
6. Assess the condition of your plant today – and plan for the future
A reliability assessment from a reputable manufacturer is the recommended first step to help water utilities to sweat their motor-driven assets.
A reliability assessment – also known as a life cycle assessment – provides the foundation for developing a long-term maintenance and improvement plan for VSDs. Customers are provided with a detailed view of the current condition of their installed base of VSDs, helping them to optimise service budgeting by planning plant maintenance actions in advance and prioritising spend on critical applications.
A reliability assessment also predicts how plant conditions might change over time, meaning steps can be taken to tackle potential problems before they arise.
This information was provided by ABB