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Design of Pipelines and Pump Stations
May 2013 Slide 1
Water Hammer
Design of Pipelines and Pump Stations
May 2013 Slide 2
What is Steady State? • When flow and pressure in the system is kept
unchanged with time, this is called a steady state condition.
• Steady state conditions are very rare in most water distribution works as the demands keep changing all the time (especially in networks).
• One steady state condition is when the fluid is at rest.
Design of Pipelines and Pump Stations
May 2013 Slide 3
What is Transient Flow? • Transient flow are transitions of long or short
duration, from one steady flow state to another. • There are Two types of transients:
– Quasi-Steady Flow • Variation of discharges and pressures with time is
gradual. • Over short time intervals the flow appears to be steady • Draining of large tank, demand variation in a network,
etc… are all examples of Quasi-Steady Flow • Characterized by the absence of Inertial or Elastic
effects on the flow behavior • Quasi-Steady Flow can be modeled in WaterGEMS as
Extended Period Simulation (EPS)
Design of Pipelines and Pump Stations
May 2013 Slide 4
What is Transient Flow? • There are Two types of transients:
– Water Hammer Condition • Variation of discharge and pressure with time is Abrupt. • The effects of Fluid Inertia, Fluid Compressibility, and
Pipe Elasticity are Essential and must be considered. • Sudden Valve Closure is an example of a water hammer
condition • It is a more complete and general characterization of the
flow but is more complicated. • Water Hammer condition can be modeled in HAMMER • A less severe form of water hammer is called Surge
Design of Pipelines and Pump Stations
May 2013 Slide 5
Causes of Water Hammer • Includes any scenario that causes rapid change
in velocity • Uncontrolled Pump trip (Power failure) • Rapid Closure of Valves • Pump Startup • Filling of water mains (Venting of pipelines) • Check Valve Slam • Water Hammer is accompanied by excessive
pressures (positive and negative) and forces in the system
Design of Pipelines and Pump Stations
May 2013 Slide 6
Sudden Valve Closure
Design of Pipelines and Pump Stations
May 2013 Slide 7
Sudden Valve Closure • Ideal Flow:
Assuming Ideal Flow (No Friction) and the valve was initially opened
When the Valve is Suddenly Closed, Large force is needed to decelerate the fluid and this force appears in the system as pressure
Design of Pipelines and Pump Stations
May 2013 Slide 8
Sudden Valve Closure • Ideal Flow:
This pressure wave propagates upstream at the speed of sound in the medium (a)
Sudden increase in pressure occurs at the valve location
This is accompanied by pipe stretching (pipe bulge) and increase in fluid density inside the pipe
Extra volume of fluid is stored in the pipeline !
Design of Pipelines and Pump Stations
May 2013 Slide 9
Sudden Valve Closure • The ΔH Equation
ΔH: increase in pressure
head
a: Wave speed
ΔV: Change in velocity
To Calculate ΔH
Design of Pipelines and Pump Stations
May 2013 Slide 10
Sudden Valve Closure • Wave Speed Calculations
– Ranges from 300m/sec for PVC pipes up to 1400m/s for steel pipes.
– Depends on Fluid properties, Pipe material, and longitudinal restraint allowance
K: Bulk Modulus
ρ: Fluid Density
D: Pipe Diameter
e: Pipe Wall Thickness
E: Young’s Modulus
C: Pipe Restraint Case
μ: Poisson’s Ratio
Case (a): Fixed from one end
Case (b): Totally Fixed
Case (c): Flexible joints
Design of Pipelines and Pump Stations
May 2013 Slide 11
Sudden Valve Closure • Wave Speed Calculations
K: Bulk Modulus
ρ: Fluid Density
D: Pipe Diameter
e: Pipe Wall Thickness
E: Young’s Modulus
C: Pipe Restraint Case
μ: Poisson’s Ratio
For Thick Walled pipes D/e > 40
Design of Pipelines and Pump Stations
May 2013 Slide 12
Sudden Valve Closure
Design of Pipelines and Pump Stations
May 2013 Slide 13
Sudden Valve Closure • Ideal Flow
Fluid accumulates inside the pipeline Relief Backflow to the tank starts
Design of Pipelines and Pump Stations
May 2013 Slide 14
Sudden Valve Closure • Ideal Flow
The valve stops the Relief Backflow and Negative wave is produced Relief flow from the tank starts again
Design of Pipelines and Pump Stations
May 2013 Slide 15
Sudden Valve Closure • Considering Friction
When considering Friction, effect of Line Packing and Wave attenuation appears especially in long pipelines
The produced pressure wave Dampens with time
Design of Pipelines and Pump Stations
May 2013 Slide 16
• Downstream of Valves – Pressure will drop suddenly by the decelerating water
column forming a huge vacuum cavity – Often the fluid will return to collapse the cavity causing
severe water hammer pressure
Liquid flowing steadily
Liquid stationary
High pressure Low pressure
Cavity growing (vacuum)
Closed valve (or tripped pump)
Sudden Valve Closure
Design of Pipelines and Pump Stations
May 2013 Slide 17
• Downstream of the pumps – Similar to valve closure, Pressure will drop by the
decelerating water column and can form a huge vacuum cavity
– Often the fluid will return to collapse the cavity causing severe water hammer pressure
• Upstream of the pumps – Pressure will rise
Tripped Pumps
E-43
Cavity growing (vacuum)Water hammer is not as
severe as sudden valve closure because pumps don’t stop rotating suddenly
Pressure Rise Upstream
Design of Pipelines and Pump Stations
May 2013 Slide 18
• After Power Failure – Downstream column decelerates till it completely stops
and then reverse flow starts
Check Valve Slam
E-48
Reverse flow after pump trip
Slams shut
– Check valves form an uncontrolled positive feedback closure rising pressures in the system
Design of Pipelines and Pump Stations
May 2013 Slide 19
• Unstable operation while gas is being flushed out • Sudden pressure rise due to collapse between
moving liquid column and static liquid column
Filling of Water Mains
P
Moving Liquid Column Air
Static Liquid Column
Design of Pipelines and Pump Stations
May 2013 Slide 20
• Sudden pressure rise when obstacles to the flow are present (Restrictor Orifice, Partially closed Valves)
Filling of Water Mains
) Just be o e t e st e
E-39
Restrictor - orifice
AirLiquid flowing
quickly
Liquid flowing quickly
AirLiquid flowing less quickly
st e
Shock wave
Design of Pipelines and Pump Stations
May 2013 Slide 21
How Severe a Water Hammer Condition Can be
Design of Pipelines and Pump Stations
May 2013 Slide 22
Water Hammer • Pressure surge during a repressurizing of the
pipeline serving the Point McIntyre field in Alaska led to BP shutting down the 20,000 b/d field.
• The surge caused the line to shift from some of its supports. No oil was spilled as a result of the surge. (Reuters, 10:51 August 16, 2006)
Blown Expansion Joints
Design of Pipelines and Pump Stations
May 2013 Slide 23
Water Hammer
Design of Pipelines and Pump Stations
May 2013 Slide 24
Water Hammer
Pipe burst during commissioning tests of 150 km, 1.6 m Diam. Pipe. Control valve closed too early
Design of Pipelines and Pump Stations
May 2013 Slide 25
Water Hammer
Check valve closure caused severe pipe motion
Design of Pipelines and Pump Stations
May 2013 Slide 26
Conclusions
Design of Pipelines and Pump Stations
May 2013 Slide 27
Conclusions • Water Hammer results from any rapid change in
flow conditions • This is accompanied by high pressures and
forces in the system that may cause damage of pipeline and equipment
• Do we need Protection from water hammer?
YES ! Then we must predict it first
Design of Pipelines and Pump Stations
May 2013 Slide 28
Conclusions • Where does it occur? • In Any pipeline system vulnerable to rapid changes in flow
conditions – Any length – Any Diameter – Any Liquid – Any Pipe Material
• Warning Signs: – High Flow Rates – Significant Elevation Differences – Intermediate Peaks along pipelines – Long Pipelines
Design of Pipelines and Pump Stations
May 2013 Slide 29
Conclusions • It is necessary to know enough to spot the Danger
Signs • Solutions to the problems can be quite simple • Ignoring the problem can lead to a disaster • Complex high risk situations need expert
modeling and design solutions • Keep in mind that we all keep learning all the
time!!