UPDATE 2014-01-15: The attached .zip file now contains a Mathcad Prime 3.0 worksheet (.mcdx) and—for those of you who are still using earlier verisions of Mathcad—an Adobe Acrobat printout (.pdf) of the worksheet so can see how it is put together.
Many small sewer and storm drainage lift stations are designed with two constant speed pumps—one duty, one standby (alternating)—that pump out of a circular wetwell. The capacity of each pump equals the design peak flow rate into the wetwell. The working volume of the wetwell is sized to limit the frequency of pump starts based on single pump operation (in case one pump is inoperable). The pumps are controlled by the water level in the wetwell, with the duty pump turning ON at HWL (High Water Level) and turning OFF at LWL (Low Water Level). If the actual inflow exceeds the design inflow for long enough, the wetwell level can exceed the HWL; in such cases (assumed to be rare), the standby pump is turned ON to augment the duty pump.
The purpose of this worksheet is twofold. Section 1 below determines the required wetwell working volume and depth based on single pump operation. Section 2 analyzes the final design (or an existing wetwell) for pump cycling (pump ON/OFF cycle duration and pump starts per hour) based on single pump operation.
This worksheet does not consider the use of pipe storage to augment the wetwell storage volume. Pipe storage is covered in a separate worksheet (Lusk_Pipe Storage.mcdx).