An EXTRAN pump station is conceptually represented as either an in-line lift station or an off-line node representing a wet-well, from which the contents are pumped to another node in the system according to a programmed rule curve. Alternatively, either in-line or off-line pumps may use a three-point pump curve (head versus pumped outflow). For an in-line lift station, the pump rate is based on the water depth, Y, at the pump junction.
The step-function rule is as follows:
Pump Rate = R1 for 0 < Y < Y1
R2 Y1Y < Y2
R3 Y2Y < Y3 (47)
When Y = 0, the pump rate is the inflow rate to the pump junction.
Inflows to the off-line pump must be diverted from the main sewer system through an orifice, a weir, or a pipe. The influent to the wet-well node must be a free discharge regardless of the diversion structure. The pumping rule curve is based on the volume of water in the storage junction. A schematic presentation of the pump rule is shown in the Pump Diversion dialog. The step-function rule operates as follows:
- Up to three wet-well volumes are prespecified as input data for each pump station: V1 < V2 < V3, where V3 is the maximum capacity of the wet well.
- Three pumping rates are prespecified as input data for each station. The pump rate is selected automatically by EXTRAN depending on the volume, V, in the wet-well, as follows:
Pump Rate = R1 for 0 < V < V1
R2 V1 V < V2
R3 V2 V < V3
- A mass balance of pumped outflow and inflow is performed in the wetwell during the model simulation period.
- If the wet-well goes dry, the pump rate is reduced below rate Rl until it just equals the inflow rate. When the inflow rate again equals or exceeds R1, the pumping rate goes back to operating on the rule curve.
- If V3 is exceeded in the wet-well, the inflow to the storage node is reduced until it does not exceed the maximum pumped flow. When the inflow falls below the maximum pumped flow, the inflow "gates" are opened. The program automatically steps down the pumping rate by the operating rule of (2) as the inflows and wet-well volumes decrease over time.
A conceptual head-discharge curve for a pump is shown in the Pump Diversion dialog. When this method is used for either type of pump, an iteration is performed until the dynamic head difference between the upstream and downstream nodes on either side of the pump corresponds to the flow given on the pump curve. In other words, the pump curve replaces equation 12.