water resource specialists

SURGE CONTROL STRATEGIES FOR WASTEWATER CONVEYANCE SYSTEMS

Overview

Consequences of surge eventsEvents that create pressure surgesSurge control strategiesSurge analysis objectives and criteriaSurge analysis projects Concluding remarks / Q&A

Consequences of pressure surges

High positive pressure can rupture pipe

Increased thrust force can damage joints

Negative pressure can collapse pipe

Can cause flow reversal

Events that create pressure surges in wastewater systemsPump power failure (creates the worst-

case surge events)Pump shutdown/startupPipeline rupture

Pump operating at 1420 gpm (v = 4 ft/s)

Time = 1 sec

Airvalve

Airvalve

PumpStation

Tank

6800 ft long, 12-inch diameter

PVC pipeline

Drop in pressure proportional to reduction in flow velocity

Time = 2.3 sec

a = acoustic wavespeed for PVC pipe (1300 to 1500 ft/sec)g = gravitational accelerationΔv = change in velocity (v) of flow in pipeline

Δh

Joukowsky Equation:

Drop in pressure head (Δh) = a/g Δv

Pressure drop wave

Pressure drops low enough to openair valve

Time = 3.8 sec

1st air valve opens

HGL drops sufficiently below pipeline to create vapor pressure

Vapor cavity formation and water column separation

Time = 4.6 sec

2nd air valve opens

Water hammer wave reflection re-pressurizes pipeline between air valves

Pipeline re-pressurized bywater hammer wave reflection

Water columns rejoin andVapor cavity collapse

Time = 5.6 sec

Water column rejoinder causesvapor cavities to collapse

Positive pressure spike created-impossible to accurately predict

Time = 6.4 sec

Vapor pressure should be avoided

Controlled venting sewage vacuum relief valves

Regulates air outflow to ensure gentle closure of air valves

(Courtesy of ARI)

(Courtesy of Vent-O-Mat)

(Courtesy of APCO)

Flywheel

Vertical drive shaft to pump on lower level

Motor

Flywheel

Pressurized Surge Tank

Pressure / Surge Relief Valve

Opens quickly when pressure exceeds set point pressure andcloses slowly when it subsides

Variable Frequency Drive

Backup Generator

(Courtesy of MTU Onsite Energy)

Surge Tower/Standpipe

Objectives of Surge Analysis

Identify potential adverse surge pressuresIf needed, recommend additional surge

control measures

Air entrainment in wastewater

Recommend safe startup and shutdown procedures for pumps

Surge Control Design Criteria

Limit positive surge pressures to ~30 percent over the larger of static, working, or rated pressure

Eliminate large negative pressures and vapor pressure

Kihei 10 Wastewater PS Renovation (County of Maui, HI)

Parallel 12” diameter, 2,700’ long, proposed PVC and backup DIP force mains

3 X submersible pumps (1800 gpm @ 150’ ) Swing check valves VFDs Sewage combination air/vacuum valve on

discharge header

Movie - Power failure without additional surge protection (PVC)

Power failure without additional surge protection (PVC)

Strategy 1 Strategy 2 (preferred)

Device involved 7 Controlled venting VVs

100 ft3 surge tank with a controlled venting VV

Negative pressure heads

-16’ -10’

Pros/cons High level of maintenance

force mains subjected to less stress; prolongs effective life of infrastructure;less maintenance

Surge control alternativesProvide satisfactory protection for both PVC and DIP force mains scenarios

Movie - Power failure with surge tank (PVC), negative pressure heads controlled to -10 ft

Power failure with surge tank (PVC)

Movie - Power failure with controlled venting vacuum relief valves (PVC), negative pressure heads controlled to -16 ft

Power failure with vacuum valves(PVC)

Main Wastewater PS (Lexington, MA) Noise from force main Possible causes of noise -

transient events created by Pump shutdown (no VFDs) Accumulated air (manual air

release valves)

24” dia., 5,850’ long, DIP force main

3 X vertical centrifugal pumps (3500 gpm @ 135’)

Movie - Pump shutdown (power failure) without surge protection

Pump shut down (power failure) without surge protection

Strategy 1 Strategy 2 Strategy 3 (preferred)

Device involved 8 Controlled venting VVs and a surge relief valve

2.5’ dia. X13” thick flywheels

396 ft3 surge tank with 2 controlled venting VVs

Negative pressure heads

-17’ -16’ -13’

Effectiveness at controlling pressure/noise

Least effective Moderatelyeffective

Most effective

Pros/cons Requires the highest level of maintenance

Introduces harmonic issues when installed with VFDs

force main subjected to less stress;prolongs effective life of infrastructure;less maintenance

Surge control alternatives

Additional recommendation: automatic air release valves at high points

Movie – Pump shutdown (power failure) with surge tank, negative pressure heads controlled to -13 ft

Pump shutdown (power failure) with surge tank, negative pressure heads controlled to -13 ft

Comparisons of pressure heads at Pump Station

-100

-50

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60 70 80 90 100

Pres

sure

Hea

d (ft

)

Time (sec)

No surge protection Surge Tank Flywheel Sewage vacuum valve with controlled venting feature

Loss of power to pumps

-20 ft

Large diameter, multiple pump stations system near San Francisco, CA Four wastewater pumps stations (Qtotal = 89 MGD) Force main diameters ranging between 30” and 60” HDPE and DIP force main alternatives

Movie - Power failure without surge protection (HDPE)

Power failure without surge protection (HDPE)

Force main Material

HDPE DIP

Device involved 1500 ft3 - 2700 ft3

surge tanks at 3 out of 4 pump stations

3 controlled venting VVs

1500 ft3 - 2700 ft3

surge tanks at all pump stations

2 controlled venting VVs

Negative pressure heads

-11’ -8’

Surge control recommendations

Movie - Power failure with surge protection (HDPE)

Power failure with surge protection (HDPE)

Concluding Remarks

Many surge control options available Proper selection and installation are important Surge control strategy could involve multiple

devices Surge control device can maximize effective life of

infrastructure

water resource specialists

Northwest Hydraulic Consultants80 South Lake Avenue, Suite 800Pasadena, California 91101Tel: (626) 440-0080www.nhcweb.comContact: Nami Tanaka, P.E.ntanaka@nhcweb.com

Valve closure creates upsurge and downsurge

TankTank

FlowControlValve

Valve opening creates downsurge and upsurge

TankTank

FlowControlValve

Dampened Swing Check Valve

Oil controlled bottom buffer recommendedwhen surge tank installed

Pressure Surge Analysis Computer ModelMethod of Characteristics (MOC)

Highest level of accuracy Extensively verified in lab and field Thousands of real-world systems Well suited to multi-core processors

Data Requirements

1. Pump performance curves2. Pump and motor polar moment of inertia3. Pipe length, diameter, friction factor, wavespeed4. Pipeline alignment and elevation drawings5. High and low tank/reservoir/clearwell levels6. Pump station plan and elevation drawings7. Diameter, manufacturer, and model number for existing

valves (e.g., check, air, relief, etc.)8. Whether or not pumps have VFD’s, soft start/stops or

control valves9. Junction demands

Pump Startup

Static HGL