Since water turbulence can be a challenge when using a level transducer in a manhole, a perforated tube was inserted into the manhole to create a stilling well. This not only protected the transducers from debris, it rendered the turbulence undetectable, resulting in a cleaner signal, thus avoiding rapid cycling. The transducers also featured a level signal selector switch that took the average of both transducers, or switched from one to another if a signal went bad.
Pumping on a force main requires a variable head condition proportional to flow. When only one pump is running, the discharge head will be much lower than with all three pumps in operation. Excessive use of a single pump running at full speed with no head could potentially lead to cavitation and damage. The VFDs each had a PLC programmed to only allow a single pump operation to run up to a lower maximum speed, unless more than one pump was required to handle the flow.
The two diesel pumps were equipped with PrimeGuard Controllers and level transducers, and were controlled independent of the electric pumps. In an effort to exercise the pumps on a routine basis, the first diesel backup would start up once a week. The system backup pump would also start once a week, sending an alarm signal via its Chatter Box Auto Dialer to verify proper backup system function.
Since the Puncheon Run facility is in a residential area, the pumps had to operate with minimal noise levels. The combination of electric pumps and the Critically Silenced pump unit resulted in noise levels no greater than 68 dBA at 30 ft.
Technology proves key to success
The month-long bypass project was successfully completed, largely due to advanced pump technology. The ability to meet a wider range of flow requirements, including initiating backup pumping; routinely exercise the pumps and/or allow time for scheduled maintenance; and send alarm signals ensured reliability and responsiveness.
In addition, the electric pumps coupled with VFDs, PLCs and transducers were able to effectively manage the power requirements associated with drastically varying flow levels; decrease electricity costs associated with power surges; and minimize long-term pump wear.