Transient Analysis and Design Considerations for Hydraulic Pipelines Jonathan Funk, EIT

May 29, 2015

2Transient Analysis

• Develop an intuitive understanding of water hammer and transient response

• Present a case study where transient analysis mattered

• NOT Teach the science of wave formation and propagation

(too many formulas)

Objectives

May 29, 2015

3Transient Analysis

Water Hammer (noun)

The concussion and accompanying

noise that result when a volume of

water moving in a pipe suddenly

stops or loses momentum.

Transient Response (noun)

The response of a system to a

change from equilibrium.

Definitions

Source: water hammer. (n.d.). Dictionary.com Unabridged. Retrieved May 19, 2015, from Dictionary.com website: http://dictionary.reference.com/browse/water hammer

May 29, 2015

4Transient Response

Everyone’s Favorite Analogy

Energy Absorption

Flow >

May 29, 2015

5Transient Analysis

• Decelerating flow increases pressure

• Pressure spikes can travel and oscillate throughout a pipeline

• Design for transient pressures! (not just “Steady State”)

• Prevent or absorb pressure spikes

Overview So Far

May 29, 2015

6Transient AnalysisCase Study – Skookum Creek Power Project

May 29, 2015

7

March 11, 2011

8

March 11, 2011

9Skookum Creek Power Project

• Located near Squamish, BC

• 6.4 km FRP & Steel pipeline

• 1.8 – 2.2 m diameter

• 340m elevation change

• 9,900 L/s design flow

• Rated Capacity: ~25 MW

May 29, 2015

10Skookum Creek Power Project

Hydraulic Scenarios

• Power Generation = Flow x Pressure = $ (Steady State)

• Normal shut-down – No long-term damage (Transient)

• Emergency shut-down – No short-term damage (Transient)

• Needle Valve Failure – Survivability (Transient)

May 29, 2015

11Transient Analysis

∆� � ��

�∆�

where

∆P = change in head (m) (pressure rise)

a = wave speed (m/s) (“communication” speed)

g = acceleration of gravity (m/s2)

∆V = change in velocity (m/s)

Pressure Rise

Gate valve

Reservoir

Flow >

Hydraulic Grade Line (Steady State)∆P

∆P

∆P

(negative pressure)

Max HGL Envelope

Min HGL Envelope

May 29, 2015

12Transient Analysis

Negative Pressure & Cavitation

At standard temperature and

pressures, cavitation starts

at -10m HGL (-14 psi)

-14 psi

Siphon

May 29, 2015

13Skookum Creek Power Project

Design Limitations

• Topography

• Hydrology & Wetlands

• Old Growth Management Areas (protected)

• Site Access (de-activated forest service roads)

• Geotechnical Conditions

• Max/Min Elevations

• Hydraulics!

May 29, 2015

14Skookum Creek Power Project

Steady State Analysis

• More headloss = less pressure = less power generation

• Design flow and pipeline deterioration

• Siphon! Hard to release air. Closer to Cavitation.

• Remote site considerations

Recommendation: Eliminate Siphon

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15Normal Operations (12 minute shut-down)

Outcomes

• Nothing to worry about

• Ongoing, successful operation

May 29, 2015

16Emergency Shut-down

Surge Tower

Initial Outcomes

• Negative Pressures (enough for cavitation)

• Initial ‘Emergency Period’ too short

• Additional protection required

• Recommendation: Add a Surge Tower

Negative Pressure

Max Transient HGL

Min Transient HGL

May 29, 2015

17Emergency Shut-down (90 seconds)

Surge Tower

Final Outcomes

• Determined 90 second threshold for emergency shut-down

• Addition of surge tower (doubles as air release)

• Successfully completed project, potential problems prevented

May 29, 2015

18Transient Analysis

• ∆P ∝∝∝∝ ∆V (“is proportional to”)

• Design for pressure spikes (not just steady state)

• -14 psi = Cavitation (please avoid)

• Change flows as slow as you can manage

• Transient Analysis is understanding and mitigating these phenomenon

Overview

May 29, 2015

19Transient Analysis

• Valve Operation (Fast AND Slow)

• Pump Start-up / Pump Shut-down

• Power Failure

Common Sources of Water Hammer

Mitigation

• Perform a transient analysis

• Slow down your flow changes (prevention!)

• Pressure relief / vacuum break

• Combination air release / vacuum valves

• Surge tanks

Transient Analysis and Design Considerations for Hydraulic Pipelines Jonathan Funk, EIT

Special thanks to:Adrian Gygax (Gygax Engineering Associates) Peter Zell (Run of River Power)Ted Steele (KWL)Steve Mills (KWL)