Despite being on holiday this week, I attended a seminar by ABB on energy efficiency and smart sensors.
There were two presentations, the second presentation was about permanent condition monitoring devices that remain in place on each piece of equipment and gather data every hour. Then the data is collected via bluetooth and aggregated in ABB’s cloud system for analysis. I don’t intend to talk more about these devices in this post.
The first presentation was on the energy savings of using variable speed drives on electrical motors around the site. They tried to frame the normal operation as driving with the brake on. This description annoyed me.
How pumps really work
The traditional way to operate a pump is to have the pump spinning at a fixed speed and to control the flow by partially closing a valve downstream of the pump. When you need more flow, you open the valve up, when there is too much, you close it in. The pump continues to spin at the same speed regardless.
It may sound similar to running a car with the throttle fully open and controlling the process by operating the break. But it is not. In the car analogy, the amount of fuel used is based on the throttle position. So applying the brake would not decrease the energy consumption. But that doesn't happen with a pump. If you close a valve on a pump discharge, the power usage goes down not up. This is somewhat counterintuitive and I have had many a debate with someone on nightshift about this. If a pump is running with high amps and is close to tripping out, don’t open the valve to ‘release the brake’. This means more liquid is pumped and requires more energy. Instead close the valve in, pumping less liquid and using less energy.
Of course in the grand scheme of things the ABB guys are right. Installing a variable speed drive to adjust the speed of the motor as required and keeping all the valves open is more efficient. Just not as dramatic as the initial metaphor would suggest.
Best places to find savings
For ideal centrifugal systems, the efficiency is a product of a cube law, as such small decreases in speed can have significant savings on energy consumption. In cases like positive displacement pumps, or systems with very high static heads, the relationship is more linear. As such the biggest savings can be found on centrifugal pumps and fans with low differential pressures across them. Things like FD fans or transfer pumps.
An idea I had never considered
One idea that was put forward was to use the variable speed drives to power two pumps at less than half the required duty. In many cases, we have a duty/standby pair of pumps. One runs and the second sits ready for use. We regularly swap them over to ensure that the standby still works and hasn’t been damaged by sitting still for prolonged periods of time.
Of course, the startup and shutdown times are the ones when most damage is done. If you could run both at reduced loads continuously you could save on the energy consumption and potentially also reduce maintenance loads. You also guarantee that every time condition monitoring people do their rounds, they can test both pumps at the same time.
I also like that if one fails, the other is already spinning so there is a higher chance that the control system can respond and increase the speed enough to stop the full plant from tripping.
We were warned that this would not work in all situations due to the hydraulics of the system but it was worth investigating.Go Top