Urban Storm Drain Design: Pump Station Design

Purposes To lift stormwater to higher elevation when discharge of local collection system lies below regional conveyance. To lift stormwater to higher elevation when terrain or man-made obstacles do not permit gravity flow to discharge point.

Pond and pump station

Types Submersible Lower initial cost Lower capacity Smaller footprint Wet-well / dry-well Higher initial cost Easier inspection/ maintenance

Submersible lift station

Wet-well / dry-well lift station

Design criteria Size the pumps and the wet-well (sump) storage capacity to accommodate inflow variability and detention time limits. Match the pumps to the flow and head requirements. Provide ‘near-absolute’ reliability Automated controls Redundant systems Alarms Regularly scheduled, preventive maintenance Assess and mitigate environmental factors Flood risk, noise pollution, visibility

Site plan and facilities Protected and accessible during a major flood Redundant power supplies Intruder-resistant with controlled access

Height Inflow pipe elevation Ground elevation Excavation costs Cross section Constant Variable Wet-well (sump) dimension design

Pump capacity Greater than average flow Less than the maximum flow Depends mainly upon Inflow variability Marginal costs of increasing sump volume

Introduction to FHWA-HDS Detailed “how-to” Reviews hydrology Reviews hydraulics

Pump System Hydraulic Operation 1)Runoff enters the collection system and is conveyed to the storage unit and wet well of the pump station. For a period of time there is no outflow as the runoff is stored within the storage unit and wet well. 2)The water level in the wet well rises and a hydraulic gradient develops based on the rate of inflow, the water level in the wet well, and the conveyance capacity of the storage unit and collection system. 3)The first pump starts when the water level in the wet well reaches a specific elevation. The pump evacuates the flow at a rate that varies with the pump characteristics and total dynamic head.

Pump System Hydraulic Operation 4)The inflow rate will vary as defined by the inflow hydrograph. If the pumping rate is lower than the inflow rate, the water level in the storage unit and wet well continues to rise and the volume stored in the system increases. 5)Additional pumps start at predetermined elevations as the water level rises. 6)At some point, the inflow rate will be lower than the total pumping rate. The water level in the wet well drops as water is evacuated from storage.

Pump System Hydraulic Operation 7)At preset elevations, as the water level drops, individual pumps are stopped and the discharge rate drops accordingly. 8)When the water level drops to the minimum level required for submergence, the last pump is stopped. The last pump off is usually the first to have been switched on, although different switching schemes are feasible.

Lift Station Design Process Design criteria include: Design frequency Peak outflow Station type Design philosophy Minimum storage volume Maximum allowable highwater Discharge velocity Criteria must be established before design of storage and pump system (FHWA-HDS pg 4-9)

Lift Station Design Process Hydrologic Analysis: Drainage boundary and area Runoff characteristics Design runoff hydrographs Cumulative inflow

Lift Station Design Process Trial Pumping Configuration: The design is typically iterative Minimum of two pumps recommended

Lift Station Design Process Hydrologic/Hydraulic Analysis/Estimates Inflow hydrograph Inflow mass curve Stage-storage relationship Trial pump/switching scheme Mass curve routing System head curves Pump selection/operation

Hydrographs – review A hydrograph is a graphical representation of discharge (volume/time) with time The area under the hydrograph curve represents the total volume of runoff during the time represented The highest point on the graph is the peak discharge

Hydrographs – review

Storage Storage is collection system, plus any sump (wet well) on suction side of lift station.

Time distributed discharge Route inflow hydrograph through lift station (and collection system) to generate the time distributed discharge.

Inflow Hydrographs Suggested design AEP is 2% (50-yr) NRCS DUH is the typical method (Chapter 4, HDM) Modified rational is an option.

Basic assumptions Pump station hydrologic design is conceptually identical to reservoir routing design Inflow-outflow=change in storage Inflow is the hydrograph produced by the storm sewer system Outflow is derived from the sump geometry (stage/storage) and Pump Performance Curves for the pump or pumps in the station