Transient Analysis and Design Considerations
for Hydraulic Pipelines
Jonathan Funk, EIT
Transient Analysis
2
Objectives
• 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)
May 29, 2015
Transient Analysis
3
Definitions
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.
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
Transient Response
4
Everyone’s Favorite Analogy
Flow >
Energy Absorption
May 29, 2015
Transient Analysis
5
Overview So Far
• 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
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Case Study – Skookum Creek Power Project6
Transient Analysis
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May 29, 2015
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March 11, 2011
Skookum 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
9
March 11, 2011
Skookum Creek Power Project
10
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
Transient Analysis
11
Max HGL Envelope
Pressure Rise
∆P
∆P
Reservoir
Hydraulic Grade Line (Steady State)
Flow >
∆
∆
Min HGL Envelope
∆P
Gate valve
(negative pressure)
where
∆P = change in head (m) (pressure rise)
a = wave speed (m/s) (“communication” speed)
g = acceleration of gravity (m/s2)
(m/s)
∆V = change in velocity
May 29, 2015
Transient Analysis
12
Negative Pressure & Cavitation
Siphon
-14 psi
At standard temperature and
pressures, cavitation starts
at -10m HGL (-14 psi)
May 29, 2015
Skookum Creek Power Project
13
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
Skookum Creek Power Project
14
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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Normal Operations (12 minute shut-down)
15
Outcomes
•
Nothing to worry about
•
Ongoing, successful operation
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Emergency Shut-down
16
Max Transient HGL
Min Transient HGL
Negative Pressure
Surge Tower
Initial Outcomes
•
Negative Pressures (enough for cavitation)
•
Initial ‘Emergency Period’ too short
•
Additional protection required
•
Recommendation: Add a Surge Tower
May 29, 2015
Emergency Shut-down (90 seconds)
17
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
Transient Analysis
18
Overview
• ∆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
May 29, 2015
Transient Analysis
19
Common Sources of Water Hammer
• Valve Operation (Fast AND Slow)
• Pump Start-up / Pump Shut-down
• Power Failure
Mitigation
• Perform a transient analysis
• Slow down your flow changes (prevention!)
• Pressure relief / vacuum break
• Combination air release / vacuum valves
• Surge tanks
May 29, 2015
Special thanks to:
Adrian Gygax (Gygax Engineering Associates)
Peter Zell (Run of River Power)
Ted Steele (KWL)
Steve Mills (KWL)
Transient Analysis and Design Considerations
for Hydraulic Pipelines
Jonathan Funk, EIT