PUMP EFFICIENCY
Getting smarter all the time
Variable speed drives are getting smarter every day – but are operators taking advantage of this, asks Craig Rapson of Danfoss Drives
n 1968 Danfoss released the world’s first massed-produced variable speed drive (VSD) – the VLT5. This was an oil-filled unit with no display and looked very much like a transformer. Back then a three-phase supply would be connected, the unit given a speed reference and told to start. Now, over 40 years later, the VLT Aqua drive has been designed specifically for water and wastewater applications with great intelligence and many dedicated software features – and guess what? The majority of the time the product is still utilised as a ‘dumb’ speed controller as it was in 1968.
I
In the past 10 years or so, although we have seen the cost of VSDs drop by around 3%, year on year, the intelligence and efficiency of this technology has exceeded all expectations. VSDs now have the ability to do more than just run a motor at different speeds. Onboard microprocessors have the power and memory to run more application specific tasks. This power can be used for various features that would have previously been carried out by additional hardware connected to the control systems. The result is a lower installed capital cost to control the process, while achieving the lowest possible running costs.
Three features that have been designed for
the water industry to improve reliability, simplify installation, while reducing the initial and lifecycle costs associated with the investment: Low Flow Detection, Cascade Control – Master Follower and Impeller Deragging.
Low Flow Detection
Low Flow Detection is primarily used to protect the pump in a situation where we have a loss of flow, for whatever reason. In a traditional system this can mean investing in a low flow transmitter and the associated controller.
The pipe would be drilled, the transmitter fitted then the pipe welded. Installation also includes running the cable back to the MCC in the switch room to the detection controller and the commissioning of the PLC. Typically in a medium to large water treatment plant there could be around 40-50 of these types of installations.
If this is compared to what is available in the new VLT AQUA Drive, every piece of hardware used in the traditional installation can be eliminated. No external transmitter is needed, no drilling of the pipe, no transmitter cabling at all, no additional PLC programming and it gives a very high accuracy level of protection. This integrated feature in the standard drive is totally non-intrusive.
During commissioning of the drive, operators are asked to close the outlet valve to dead head the pump. The low flow detection feature in the drive is then selected. The drive will then request permission to proceed. After selecting yes, the drive will run to pump up to the default value of 85% speed at which point it records the power consumption. It then slows the pump down to 50% speed, all the time recording the power consumption to give us a power curve. This whole sequence takes around 10-20 seconds to complete. After the commissioning is complete and the pump is started, the drive will continuously monitor the pump power and if the power for a certain speed drops below the curve, the pump can be tripped or a warning given. A slight delay can occur if required to stop nuisance tripping. For a site with 40-50 low flow systems, by utilising this technology, the savings could be in excess of £20,000 inclusive of labour and hardware.
Cascade Control – Master Follower The Master Follower function means pumps can be automatically staged and destaged to meet the demand requirements of the system. For example – Pump 1 will start and run up to a speed where the drive system decides that is now more efficient to run with two pumps in parallel. At the point a second drive/pump is staged in and both pumps run at the same speed following a common speed reference between all drives. Again the demand may rise and once again a third pump is staged in for best efficiency.
Danfoss VSDs to pump systems have saved 40% of energy costs in a wastewater treatment plant retrofit for Aguas de Cartagena in Colombia. By applying Danfoss VLT Aqua drives to control the levels in the holding tanks, operation of the four 370kW pumps realised a return on investment within 6 months
16 Water & Wastewater Treatment June 2011
The same also applies when the demand drops and we automatically destage a pump. Control of up to 13 parallel pumps is possible. If required the system can be set to runtime balancing and this means the system looks at the running hours of all pumps and alternates the duty & assist and standby pumps for even- wear across the pumps. In some circumstances it may be a preference to have the running hours unbalanced as there can be an argument it is better to have a different service schedule for each pump rather than all at once. The balancing feature is simply disabled by the user if this is a specified.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52