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AWEA CollectionSystem Committee

Pump StationDesign

Facilitator: Mike Stickley, P.E.

Date: September 14, 2017

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Safety Moment

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Topics

• Wet Well Design

• Surge Analysis

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Wet Wells

• Sources of Information

• Potential Issues

• Wet Well and Pump Suction Design

• Modeling

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Sources of Design Information

• Hydraulic Institute (HI) standards andpublications

• Sanks “Pumping Station Design”

• General industry standards,regulations and guidelines (AWWA,OSHA, NACE, NFPA, NEC, etc.)

• Journal articles, webinars, andconference presentations

• Manufacturer’s literature, includingwhite papers

• Folks in this room

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PotentialProblemsAssociated withImproperlyDesigned orConstructedIntake or WetWell

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Pumps

• Reduced flow andhead capacity

• Effects on powerrequirements

• Cavitation damage

• Vibration and noise

• Clogging of Pumps

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Causes

• Submerged vortices

• Free-surface vortices

• Excessive pre-swirl (rotationalvelocity) of flow entering thepump

• Unbalanced flow enteringpumps

• Entrained air or gas bubbles

• Solids and grit accumulation

Just a Minor Reduction inPumping Capacity

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Wastewater Wet Wells

• Deposition of Solids

• Corrosion

• Odors

The “InsertDesigner’sName” GritIsland

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Causes• Wet Well Layout results in

areas of low velocity

• Creates islands of solids thatrequire ongoing maintenance

• Deposited solids are re-suspended during higher flow

• Pump wear

• Plugging

• Influent drops from significantheight or is introduced in amanner that causesturbulence

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Intake and Wet WellDesign – HI

• Flow Chart Addresses• Free surface or not

• Clear Liquid or not

• For non-clear liquids 3 typesof wet wells configurationsare addressed• Rectangular

• Circular

• Trench

• When model test is required• Flow – 100,000 gpm

• > 40,000 gpm/pump

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Approach Flow Conditions(wet well or sump configuration)

From Hydraulic Institute, ANSI/HI-2012, Pump Intake Design

All pumpsOperatingResults on

Velocity

ApproachPatterns

for VariousCombinati

on ofPumps

Operating

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HI Recommended Intake Structure Layout

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Formed Suction Intake

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Multiple Pump Wet Pit

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HI Thoughts on a Rectangular Wet Well

“The geometry of a rectangular wet well is not particularly suitedfor use with solids bearing liquids but with special provisions for

frequent cleaning such wet wells may be acceptable”

• Use of Mixer

• Return flow frompump back to wetwell

• Just plan oncleaning it out allthe time

Pump MFG to the Rescue- Recommendations

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Trench-type Wet Well

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Open Bottom Can Intakes

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Closed Bottom Can Intake

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RemedialMeasures(Example shown forsub-surface vortices)

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RemedialMeasures(Example shown forsub-surface vortices)

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Sanks Pump Station Design Manual

• Includes a “Survey of TwoThousand Wastewater PumpingStations”– Failure to follow HI standards

– Compares dry and wet pitarrangements

– Maintenance – ability to access wetwells and pull pumps

– Base elbow tearing away from slab –loose bolts, improper embedment,etc.

– Deep wet wells and submersiblepump cables

– Flow entrance conditions – waterconstantly falling on the pumps

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Modeling

• Computational Fluid Dynamics (CFD)

• Physical Model

Vortex

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To Line or Not to Line

• Only get one opportunity to easilyline a wet well

• Depending on the system the linershould provide a service life of 15to 50 years

• Can reduce maintenance byreducing cleaning effort

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When can you use Computational FluidDynamics (CFD) ?

• Essentially any fluid flow conditions

• Closed conduit or open channel

• Subcritical or supercritical flow

• Steady-state or transient

• Newtonian or non-Newtonian fluids

• Laminar or turbulent flow

• Single or multi-phase (air-water, sediment-water)

• Complex combinations of above conditions

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CFD results providepowerful visualization offlow characteristics

• 3D solutions (x, y, & zcomponents) at each finitevolume element

• Velocity and acceleration

• Pressure (or water surfaceelevation)

• Volume fraction of air

• Concentration of tracer

• Particle flow path

• Several other parametersincluding user definedparameters

Free SurfaceVortices

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Physical Modeling or CFD?

• Physical modeling is acceptedby Hydraulic Institute where asCFD modeling is not

• CFD modeling is an order ofmagnitude less expensive

• CFD modeling can beperformed with significantlyless time

• CFD modeling allows forefficient evaluation of severalalternatives

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Summary

• CFD and physical modelingprovide valuable and costeffective design guidance

• Building computer files ischeaper than plywood, nails,Plexiglas, and glue (i.e. CFD ischeaper than physical models)

• Plywood, nails, Plexiglas, andglue are cheaper thanconcrete, rebar, pipes, andpumps (i.e. a physical model ischeaper than the actual pumpstation)

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Surge Analysis

Water hammer(or HydraulicTransients)

in Pump andPiping Systems

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Surge Analysis

• Causes

• When to Perform an analysis

• Analysis Methods

• Mitigation Methods

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Common Causes of Transients

• Uncontrolled Pump startup/shutdown (managed by design andoperation procedures)

• Uncontrolled valve opening/closure (managed by design andoperating procedures)

• Uncontrolled pump shutdown (power failure)

• Valve Malfunction or operator error

• Specialty valve operation (check, air release, pressure reducing,pressure relief)

• Pipe rupture

• Turbine load rejection

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When to Perform a Surge Analysis

• You feel the hair stand up on the back of your neck?

• No Standards or Guidelines

• Flygt “Transient Analysis” Document

• Pipeline Profile

• Length – Greater Than 1000 lf

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Analysis Methods

• Bentley Hammer

• AFT Impulse

• Numerous Vendor Packages

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Mitigation Methods

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Combination Air Valve

• Have a largeorifice for airinflow but asmall orifice forair outflow.

• This feature isextremelyimportant toavoidtransientscaused by valveslam.

• But they are often in remote locations that do notallow easy/consistent maintenance

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Surge Chamber

Surge chambersare closed tankswhich containpressurized air andwater. Dualconnections can beused to minimizetheir size.

Air compressor is used tocontrol the water level.(Note: a bladder systemcan be used to eliminatethe need for thecompressor)

• Provide very good control but are much more expensive than air valves

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• Pump motor flywheelsprovide a source of inertialenergy to keep pumpsspinning longer to reducedown surge pressures.

• May be less expensive than asurge chamber

• May be difficult to get reliabledesign from pumpmanufacturers which canincrease pump operatingrisks.

Pump Motor Flywheels

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Surge Anticipator Valves

• Valve opens whenpressure fallsbelow specifiedpoint

• Valve remains openfor pre-set time

• Valve closes slowlyto prevent highpressures resultingfrom rapid valveclosure.

• If not set properly they can actually make the waterhammer problem worse

• Require a discharge location

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Surge Relief Valves

• Valve opens when pressure exceeds specified point

• Can be used to control positive pressures in thesystem

• Do little to control negative pressures and columnseparation

• Require a discharge location

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Check Valves• Limit reverse flow

• Standard swing check valves cangenerate transients due toslamming operation

• If system requires a surgechamber, must use quick closing,non-slam type check valve atpump discharge

• Slanting disk buffered check valve

• Nozzle check valve

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Surge analysis cost is often very smallcompared to the risk of not evaluating

and protecting the system

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Questions?