Introduction to WaterHammer

Design of Pipelines and Pump Stations

May 2013 Slide 1

Water Hammer


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.


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)


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


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


May 2013 Slide 6

Sudden Valve Closure


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


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 !


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


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


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


May 2013 Slide 12



May 2013 Slide 13

Sudden Valve Closure • Ideal Flow

Fluid accumulates inside the pipeline Relief Backflow to the tank starts


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


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


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)



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


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


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


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


May 2013 Slide 21

How Severe a Water Hammer Condition Can be


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


May 2013 Slide 23

Water Hammer


May 2013 Slide 24

Water Hammer

Pipe burst during commissioning tests of 150 km, 1.6 m Diam. Pipe. Control valve closed too early


May 2013 Slide 25

Water Hammer

Check valve closure caused severe pipe motion


May 2013 Slide 26

Conclusions


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


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


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!!

Introduction to WaterHammer

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