Some systems use a VFD to control a pump. When a second pump is required, controls will become more complicated. Add a PLC, run wires between the two pumps, some programming and we could make it work. With Cycle Stop Valves there is a better way. The two or more pumps could be installed close to each other or, they could be in different locations with considerable distance between them. The question is how do we get one pump to know what the other pump is doing?
Leave the one pump running on the VFD system as is. Let it run at whatever pressure the drive is trying to hold steady. Install a Cycle Stop Valve with pressure switch and a small pressure tank on the other pump or pumps. There are two settings possible. This depends on which pump you want to run as the primary.
If the pump controlled by the Cycle Stop Valve is to be primary, simply set the Cycle Stop Valve 5 PSI higher than the pressure held by the drive on the VFD controlled pump. The pressure switch should start the pump at the same pressure as the setting of the Cycle Stop Valve. The pressure switch should be set to shut this pump off 10 PSI higher than the setting of the Cycle Stop Valve. Anytime the pump with the Cycle Stop Valve is unable to supply the demand, pressure will drop 5 PSI and the VFD pump will ramp up to meet the additional need. When flow is reduced low enough that the pump with the Cycle Stop Valve can supply the need, the pressure will increase by 5 PSI. This makes sure that the VFD pump ramps down.
There is a small envelope where this can become a problem. If for instance both pumps were 100 GPM pumps and the demand required a steady 102 GPM, cycling of the pump controlled by the VFD could occur.
If the pump with the VFD is to be the primary, simply set the Cycle Stop Valve on the other pump 5 PSI lower than the pressure maintained by the VFD pump. The pressure switch should start the pump at the same pressure as the setting of the Cycle Stop Valve and shut it off one PSI lower than the pressure maintained by the VFD on the primary pump. Anytime the pump with the VFD is unable to supply the demand, pressure will drop 5 PSI and a pressure switch will start the other pump. The Cycle Stop Valve on this pump will supply only the amount of water needed to meet the additional demand. When flow is reduced low enough that the VFD pump can supply the need, the pressure will increase until the pressure switch shuts off the pump with the Cycle Stop Valve.
There is a small envelope where this can become a problem. As with any variable speed pump low flow or leaks can cause the pump to continuously ramp on and off. (See also "VFD Maximum Number of Starts per Day")
When you get tired of the problems with the VFD, or the when the VFD goes out, it will be very easy to install Cycle Stop Valves on all of the pumps. The pumps can still have staggered pressure settings for primary or secondary in the same way describe above. (See also "Multiple Pumps In Parallel")