Pump Control Signals
Continuous Operation using On-Off Switch
The simplest method for controlling a pump is to turn it on and off manually, such as with an On-Off switch in the control panel. Using this configuration, the pump runs continuously to supply the process. In a fuel oil system, the pump supplies a loop to a burner, with excess oil returning to the storage tank. In a boiler feed system, the pump supplies a header to a boiler. Excess water is not returned from the boiler. Instead, a recirculation line from the pump discharge is piped back to the tank.
On/Off Operation using Hand-Off-Auto Switch
For cases where a pump is cycled on and off in response to process demand, a Hand-Off-Auto switch is used in the control panel. The Hand (manual) setting runs the pump continuously, usually for startup and testing. The Automatic setting runs the pump based on one or more Pump Control Switches, located outside the control panel.
The description of these two types of operation can be confusing. For continuous pump operation, an On-Off Switch is used in the control panel. For On/Off pump operation, a Hand-Off-Auto Switch is used in the control panel.
One Pump Control Switch per Active Pump
When a single Pump Control Switch is used, the active pump runs whenever the switch closes. The switch could be a boiler level switch, an operator switch in a control room, or a relay output from some other control system. When a single level switch is used, the distance between on and off is usually small. This may cause operational problems if the pump is not sized properly. To increase the on/off differential, use two Pump Control Switches.
Two Pump Control Switches per Active Pump
When using two Pump Control Switches, the effective on/off differential is determined by the mounting locations. The switches are configured as a traditional start/stop circuit, with a holding relay in the control panel. For pump-up applications, such as an oil day tank, the switches are each normally closed (tank empty). As oil falls below the bottom switch, the pump starts and adds oil until the top switch opens. For pump-down applications, such as a condensate system, the switches are normally open (tank empty). As water rises above the top switch, the pump starts and removes water until the bottom switch opens.
Duplex Alternating Float Switch
In small condensate systems, a Duplex Alternating Float Switch (DAFS) may be used to control two pumps. Such a switch actually contains two Pump Control Switches, one for each pump. On successive cycles, the DFAS mechanically alternates the pumps. (No Electric Pump Alternator is needed in the control panel.) If the water level continues to rise even after the first pump is turned on, the second pump is turned on as well. This lead/lag feature is implemented mechanically in the DFAS.