Urban Storm Drain Design: Pump Performance Curves

System Sketch Collection Lift Station System

System Sketch

Inflow/outflow relations A Pump Performance Curve is a graph or equation that represents the “performance envelope” of a pump of a particular design One axis usually represents energy, or “head” that the pump is working against The other axis usually represents discharge; the rate of water moved (volume/time) against a given head

Inflow/outflow relations For each value of head, there will be only one discharge, but the converse may not be true- there may be two different heads that produce the same discharge (due to differences in efficiency over the operating range of the pump)

Inflow/outflow relations There are three basic types of rotary pumps we are concerned with. They are classified by how they move the water; Axial flow pumps- push water without changing it’s direction of flow. A boat or airplane propeller is an open axial flow pump Radial flow pumps- take in water parallel to the axis of rotation of the impeller, then “spin it off”, changing it’s direction of movement at a right angle Mixed flow pumps- elements of both types of motion; changes direction of flow from parallel to the axis of rotation, but less than a right angle

Inflow/outflow relations Axial flow pumps are good for moving large amounts of water against low head values At higher head values, the impeller will “cavitate”. Radial flow pumps are good for moving much smaller quantities of water against much higher head values. Mixed flow pumps are a flexible compromise- they can be designed to optimize performance against known head values through a broad range.

Pump Curve(s) Wire-to-water efficiency Discharge Total dynamic head One (of many possible) Operating conditions Q= 9000 gpm, TDH=67ft

System Sketch

System curve: head loss vs flow

Inflow/outflow relations For design purposes, the performance curves of pumps selected serve the same purpose as the stage/discharge curve for detention design; Given: Initially dry sump, inflow hydrograph, sump storage curve, pump performance curves. Inflow rate*time step= inflow volume Volume-> depth in sump -> head on pump Head on pump ->point on performance curve -> outflow rate Outflow rate*time step= outflow volume Inflow volume-outflow volume=volume in sump at next time step->

Pump selection important points Operating point is where pump h vs. Q curve crosses the system h vs. Q curve. Net pump suction head (NPSH) available > NPSH required. Parallel pumps add flows, but operate at same head. Check pump operating conditions for allowable cycle times. For submerged pumps, check allowable operating time when unsubmerged. Check suction pipe velocity – should be below 5 fps.

Pump notes: Pumps should be self-priming for suction lift service. Suction piping arrangements for multiple pumps must be designed to prevent hydraulic interference. Submerged pumps are often allowed up to 20 or 25 BHP. Variable speed pumps can simplify design process, but may dramatically increase installation costs Using smallest impeller allows later increase in pump capacity Motor can be changed (higher speed) to increase pump capacity, but this is a more expensive option.

Example pump selection problem Proposed configuration: 10 pumps in parallel Each pump required to lift ft static lift

Choosing the pump First pick out the points above the design flow rate (450,37) (450,25) (350,27) (350,19)

(450,37) (350,27) (450,25) (350,19)

(450,37) (350,27) (450,25) (350,19)

(450,37) (350,27) (450,25) (350,19)

(450,37) (350,27) (450,25) (350,19)

C140 system curve C100 system curve