For those who are new to or have some knowledge of pump multiplexing, here is a different way of looking at it. When multiplexing pumps are controlled by variable frequency drives (VFDs), understanding how the system operates and the benefits it offers can be as easy as understanding the process of moving from one home to another.
Typical Operation
First, imagine the typical moving process (or in some cases, think back to the last time you moved). At the start of the moving process, you are carrying light boxes and do not need a lot of strength to lift and move them. As you progress, you find that the boxes are getting heavier and heavier, and you will need more and more strength to lift and carry them. Eventually, you are going to get to boxes and items that are too heavy to lift.
At this point, you may start to consider one of two things: (1) start weightlifting to get stronger so you can lift more or (2) call for help. Calling for help is probably going to be your best bet when moving. With another person helping you, heavy items you had a hard time lifting are easier to lift and move. You may even get to a point where you will need to call a third or fourth friend to lift objects that two people cannot lift. Once you have moved all of your heavy items, you can start thanking your friends for their help and sending them home so they can get some rest. Then, the next thing you know, you are all done and ready for some hard-earned sleep.
So, how is that analogous to VFD pump multiplexing? Like the beginning of moving, the demand for water can be low at various times of the day—the middle of the night, for example. The pump will rotate slowly (like moving light boxes) to generate enough flow to maintain the required pressure. As the day continues, the demand for water starts to increase (like needing to move heavier boxes). The increase in demand will result in the pump rotating faster to increase flow, which results in more power being needed to accomplish this. Like maxing out how much weight you can lift while moving, the pump will eventually reach a point where it will not be able to keep up with the demand and will be unable to maintain pressure.
At this point, it is important to consider one of two things: (1) change the pump out so it is a higher horsepower (hp) (strength training) or (2) increase the number of pumps in the system, i.e., pump multiplexing (call friends for help). Going on the multiplexing route, if the water demand reaches a point where one pump cannot handle it anymore (not enough strength to lift), it will call for a second pump to turn on and assist (first helpful friend).
As the second pump ramps up, the flow of the first pump may start to decrease in speed (lifting a lighter item) and require less power to run. If the demand continues to increase (lifting heavier items), then more pumps (more friends) will be needed and turned on to meet this demand and maintain pressure. Eventually, the demand will decrease (fewer heavy items), and the additional pumps that were turned on can start to be turned off (sending friends home). At some point in the day or night, it will be back to a single pump operating. If there is little to no demand, the pump can go from rotating at a minimal speed that creates minimal flow and pump lubrication to a standby mode (sleep).
Benefits
Though perhaps an unconventional way to look at VFD pump multiplexing operation, the two scenarios certainly have similarities. With the help of friends, moving takes less energy. And depending on the number of friends helping, it is possible to move a range of boxes with different weights. If someone gets tired, they can sit out for a bit. Or worse, if they hurt themselves during the move and cannot help anymore (should have stretched first), they can sit out indefinitely.
Just like having good friends who can help with moving, a VFD multiplex system has various benefits as well. VFD multiplexing can help save energy in the same way moving heavy objects requires less energy when more people are participating. When operating a pump with a VFD, it is possible to take advantage of the affinity laws: flow is directly proportional to pump speed, torque is proportional to the square of pump speed and power is proportional to the cube of pump speed.
When demand exceeds what the system can currently do—typically one or more pumps operating at full speed (60 hertz [Hz])—a new lead pump is brought online to supplement the flow. The new lead pump is then responsible for the regulation of pressure, speeding up and down as needed. To help reduce the energy consumption of the system, the lag pumps are reduced in speed. To visualize this a little better, consider a duplex system as an example. A new lead pump is brought in and starts operating at 40 Hz to provide the flow necessary to maintain pressure. After the lead pump gets up to speed, the lag pump reduces its speed from 60 Hz to 54 Hz, resulting in an energy reduction of 27%. But if the speed of the lag pump is reduced, the lead pump will need to run faster to account for the reduction in flow. The lead pump increases its speed from 40 Hz to 46 Hz to make up the flow, but this results in an increase of 52% in energy usage. How is this considered energy savings? That is one of the problems when trying to look at things purely in percentage. Instead, it is better to use some discrete values.
Operating point 1 = 60 Hz + 40 Hz -> 100 kW + 29.6 kW = 129.6 kW
Operating point 2 = 54 Hz + 46 Hz -> 72.9 kW + 45.1 kW = 118.0 kW
Comparing operating points 1 and 2, which offer the same amount of flow, it becomes clear that point 2 requires 11.6 kW less, or 9% less energy.
The second benefit of a VFD multiplex system is the ability to control flow over a wider range (like lifting a range of differently weighted boxes). Pumps typically have a minimum speed they need to operate at or above to generate flow and provide proper lubrication to the pump. By using smaller pumps, it is possible to generate lower flow and maintain pressure when the system demand is not present. A single pump may not be able to do that as well and could result in a large variance in water pressure when the demand for water is low.
A third benefit is pump cycling (like the tired friend). A VFD multiplex system can use different methods to bring pumps offline when the demand drops. The method can be based on first-in-first-out (FIFO) or on the pump’s operating hours. The advantage of cycling through pumps is that they can keep their runtime around the same, resulting in even wear on the pumps.
And last but not least is redundancy (like the hurt friend). With a single pump installation, if maintenance needs to be performed on a pump or it suddenly fails, there is no means to create water pressure until the situation is resolved. With a VFD multiplex system, even if a single unit is taken out of the system, the other units can still offer a means to provide flow and create pressure in the system. Though it may not be able to create the full amount of pressure one expects, it is still enough to satisfy the needs of those using the system.
Who knew that VFD pump multiplexing would be like moving from one home to another? And just like moving, the requirements and needs can vary depending on the system. It is always good to evaluate what the needs are and what the optimal solution is. And yes, this does apply to both pumping and moving.
For more on variable frequency drives, visit pumpsandsystems.com/tags/variable-frequency-drives.
