The control of the pumps shall be through ultrasonic level instrumentation.

The level in the wetwell shall be sensed by an ultrasonic level-sensing transducer connected to a level transmitter (Rosemount 1151GP Smart Option) capable of producing a four to 20 mA DC signal proportional to the level in the wetwell. The level transmitter output shall be wired in a current loop to three AGM modules (one for high wetwell level alarm and low wetwell level alarm, one for start/stop of the lead pump and one for the start/stop of the lag pump), a digital level indicator and the remote terminal unit for transmission of level to the Control Center.

Float switches (teardrop, tilting mercury type) shall be furnished as a back up for the high wetwell level alarm and the low wetwell level alarm and pump cut-off, as the AGM modules are susceptible to voltage surges.

The control system shall function as follows:

In the automatic mode, the lead pump shall start when its “start” level is reached, and shall be stopped when the wetwell has been pumped down to its “stop” level. If the inflow is more than what the lead pump can handle or if the lead pump fails, the lag pump shall start when the wetwell level reaches its “start” point; and when the two pumps running together bring down the level to the “stop” point of the lag pump, the lag pump shall be stopped. The lead pump shall continue to run until the wetwell level has reached its “stop” level. A high wetwell level alarm shall be initiated after the lag pump is called to start.

The pumps are not to be automatically alternated at every start.

A wetwell low level/pump cut-off bypass circuit with a timer shall be provided to enable operations staff to periodically “pump down” the wetwell so that floating solids could be sucked up by the pump. (Design Guideline DG-08 § 16.2, dated August 2006)