In many pumping applications, it’s common practice to use several smaller pumps, which can be switched in and out of service as required, rather than one large pump. Providing efficient and effective control for these multipump systems has in the past been something of a headache, but convenient, cost-effective and easy-to-implement solutions are now available.
It’s an excellent idea to use, say, three modestly sized pumps which between them can meet the peak demand on a pumping system, rather than a single large pump. For a start, with the multipump arrangement, it’s easy to save energy by running only one or perhaps two out of the three pumps when demand is low. By contrast, with a single large pump, this has to operate – and its fixed losses have to be supplied – to satisfy even the smallest demand.
And there’s another big benefit. With the multipump arrangement, if one of the pumps fails or gets jammed, most of the pumping capacity is still available whereas if a single large pump is used and it fails, no capacity at all is available until the problem is rectified. Multipump arrangements also allow pumps to be taken out of use for servicing or replacement without the need to shut down the whole system.
Efficient and effective?
With these important benefits in mind, it’s easy to see why multipump arrangements are so popular. But, as always, there’s a snag! How can the pumps in such an arrangement be controlled efficiently and effectively? In fact, the general idea is simple enough.
One of the pumps (the lead pump) starts and, if it is able to satisfy the immediate demand, usually indicated by a pressure or flow switch, nothing else happens. If it can’t meet the demand, however, the second pump is started and, if these two together can’t meet the demand, the third pump is started – and so on, if there are more than three pumps. When the pumps running have excess capacity compared with the immediate demand, they are shut down in reverse sequence.
A complication is that it is usual for each of the pumps in turn to take on the role of lead pump so that the wear on the pumps is equalised. In the old days, when pumps mostly ran at full speed or not at all, devising a control system based on a few contactors and relays to provide the required control sequence wasn’t too difficult.
There are, however, very few pump users today who would accept the inefficiencies of fixed speed control, even if the current energy efficiency regulations allowed this. And when variable speed control is introduced things can get a lot more complicated especially if an attempt is made to use just one variable speed drive (VSD) to control the lead pump, with the others controlled only by contractors.
Each pump has its own VSD
The complex nest of contactors and wiring needed to ensure that the VSD is connected to the pump which is currently acting as the lead, and that the other pumps are ready to operate when needed, is neither a pretty sight nor particularly cost-effective, especially when fault finding and maintenance challenges are factored in.
Fortunately, there’s now a much better way. Modern VSDs are very cost-effective, which means that it makes good financial and technical sense to use one VSD for each pump. In fact, a recent study has confirmed that the most economical arrangement for multipump systems is for each pump to be controlled by its own VSD.
The attractions of this arrangement are further enhanced by the ability of modern VSDs to talk to each other via data links that typically comprise just one plug-in connection and because the best of them incorporate, as standard, software that’s been specifically developed to support multipump operation. No additional hardware or software is therefore needed, which means that cost and complexity are kept to a minimum.
Multimaster and multifollower
The built-in multipump software can be expected to provide two control modes: multimaster and multi follower. Multimaster is the most widely applicable of these modes and, to explain how it operates, we’ll consider a three-pump system.
When demand is low, Pump 1 runs as the master and its speed is controlled to match the immediate demand. When the maximum flow rate for a single pump is reached and more capacity is needed, Pump 1 locks itself at its optimum operating speed and the master functions are transferred to Pump 2. The process repeats for Pump 3 if even more capacity is needed.
As demand falls, the speed of Pump 3 is reduced until it reaches zero flow, when it stops and transfers the master function back to Pump 2. The same procedure is adopted when Pump 2 reaches zero flow, leaving just Pump 1 running as the master. If Pump 1 also ramps down to zero flow, as the overall demand on the system is minimal, it goes into energy-saving sleep mode but continues to monitor demand so that it can restart automatically when needed.
Note that this functionality is pre-programmed, as is the ability to transfer duties between pumps so that they receive approximately equal use. No contactors or relays are needed! All that the user has to do is install the communications links between the drives and select the appropriate operating parameters.
Multifollower operation is similar in many respects, except that when more than one pump is running, the speed of all of the pumps is controlled. That is, they all follow the speed reference passed on by the leader, hence the name multi follower operation.
In some applications, multi follower control can give smoother control, quieter operation and increased overall system efficiency. As with multimaster operation, multi follower operation is a pre-programmed function that needs no special wiring and no contactors or other extra components.
Simple design and implementation
It’s clear that VSDs with built-in multipump control options greatly simplify the design and implementation of control systems, but that’s not all they have to offer. Setting up and commissioning are much easier and dependable duty rotation means that wear and tear on the pumps is optimally controlled. If each pump is provided with its own flow or pressure sensor, redundant operation is easy to arrange, so that if one pump fails the others remain operational and properly controlled.
Further, if the VSDs have a high IP rating, no control enclosure at all is needed, as the multipump functionality has eliminated the need for contactors and other external components, and the drives themselves require no additional protection.
With the best VSDs other smaller but still very useful benefits are available including, for example, soft fill for pipes to prevent water hammer and a boost facility for the sleep function, which boosts pressure just before the drive goes into sleep mode so that this energy-saving mode can be maintained for longer.
Simultaneous control of multiple pumps is a very common requirement and, as we’ve seen, it no longer needs to be difficult or expensive to achieve. All that’s necessary is to choose VSDs like those that support versatile and easy-to-use multipump functionality without the need for expensive add-ons or external components.
