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SURGES IN PIPELINES

Raghuveer Rao PallepatiDepartment of Civil Engineering

Indian Institute of ScienceBangalore – 560012 India

Water supply pumping mains

Lift irrigation schemes

Cooling water systems for thermal and nuclear power plants

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Water supply schemes

• Diamter of pipe – 0.4 m to 2.5 m • Length 5 km to 150 km• Discharge – 20 to 270 MLD • Head – 30 m to 250 m

Lift irrigation schemes

• Diamter of pipe – 1.5 m to 3 m • Length 0.2 km to 50 km• Discharge – 2 to 14 m3/sec• Head – 10 m to 140 m

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Cooling water systems

• Diamter of pipe – 2 m to 3.2 m • Length 1-2 km• Discharge – 6 to 16 m3/sec• Head – 10 m to 20 m

Source

Delivery

Source Reservoir – water level - min & maxDelivery Reservoir – ground levelGround level profileQuantity of water required

MLD - Million liters / dayNumber of hours of pumpingVelocity – 1 m/sec – fix diameter

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Design parameters

Discharge required - Diameter of pipePipe material – Steel/PSC/DI/CI/BWSC/GRP

Thickness / pressure ratingCalculation of pump head

Static head + Frictional loss + Minor losses + Pump house loss

Source

Delivery Static Lift

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Friction loss in pipe

Darcy-Weisbach

Hazen-Williams

Minor losses bends, valves etc.

Pump house loss

Valves, bends etc

g2v

DLfH

2

f =

87.4852.1

852.1

f DCQL67.10H =

Hydraulic gradient line

At pump = Minimum water level in sump + Pump Head

At delivery end = Delivery level

Uniformly varies with length

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Non-return valve

Butterfly valve

Isolation valve

Uniform clousure / dual speed closure

Single door / multi door

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Dual Plate Check Valve

The “Simple & Simplistic” Formula for Surge

a = pressure wave velocityV = flow velocity∆H = Surge pressure

g = gravitational acceleration

gaVH =Δ

Focus on upsurge only, no attention to down surge

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Anomaly of the simple formula –An example

Pressure wave velocity, a (approx.) = 981 m/sec

Flow velocity, V = 1 m/sec∆H = (981 x 1)/9.81 = 100 mThis pressure rise is independent of static lift, pipe

length & pump head !Same pressure rise for two schemes with 100 m

pump head, one 200 m long with 96 m static lift & another 60 km long with 10 m static lift !!!

Pressure Wave Velocity

a = pressure wave velocityK = bulk modulus of elasticity of water

ρ = density of waterE = Young’s modulus of elasticity of pipe material

D = diameter of pipet = wall thickness of pipe

EtKD

Ka+

=1

ρ

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Surge – The PhenomenonRapid Change in Discharge (Velocity) & Associated Change in Pressure

Surge – The CausesOperation of Valves (Closure & Opening)Starting of PumpsStopping of PumpsPower FailureSingle Pump Failure

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Phenomenon of Power Failure• Power supply cut; Pump speed starts dropping

from rated speed• Discharge & pressure (head) starts reducing• Pressure wave (down surge) transmits through

the rising main• At delivery reservoir, down surge wave gets

reflected as upsurge wave and moves towards pump end

• At some reduced pump speed, flow starts reversing at pump

• NRV at pump closes due to flow reversal causing a pressure rise or upsurge

Power Failure (contd.)

• These waves (upsurge & down surge) move along the rising main, reflected at the delivery reservoir & at the closed NRV at pump end

• Speed of wave movement approx. 1 km/sec• Reflection at delivery reservoir (+) wave becomes

(–) wave• Reflection at closed NRV (+/-) wave doubles up,

that is reflected wave same sign as direct wave• Final result: Low & high pressures all along the

rising main

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Surge – The ProblemsPressure rise due to NRV closure too high (depends on type of NRV & closure pattern)Pressure drop due to down surge immediately following power failure causes negative pressure, which may go down to vapour pressureColumn separation due to occurrence of vapour pressureRejoining of separated columns causing pressure rise (indirect upsurge)

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Column Separation

Local Peak Cavity development

Cavity volume with timePipe alignment at a local peak cavity

Inflow Outflow

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Effect of Column Separation• At a peak location, pressure goes down to vapour pressure• This location becomes a pressure control, that is

temporarily like a pseudo-reservoir• Upstream & downstream water columns separate with

different flow velocities• Initially outflow velocity more increasing vapour pocket

or cavity size• Later inflow velocity becomes more (outflow velocity

changes direction – reverse flow) shrinking the cavity• When cavity volume becomes zero, sudden pressure rise

due to column rejoining occurs • Pressure rise travels on both sides of rising main

increasing pressure all through

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Surge Analysis Program Version 2 Software Developed at department has been acquired by 44 organizations through Technology Transfer and also used for analysis

and design of surge protection systems for over 400 Consultancy Projects

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Phenomenon of Single Pump Failure• Two or more pumps working in parallel;• One pump suddenly trips due to a fault;• The pressure in the manifold drops slightly, but the running

pumps control the drop (more the running pumps, less the drop in pressure at manifold);

• Running pumps get slightly over-loaded;• Water from running pumps flows through the failing pump;• NRV on failing pump closes with associated pressure rise;• Pressure rise depends on type of NRV, delivery pipe size, and

extent of pressure drop at manifold;• Pressure rise local to pump house, endangering NRV and BFV

or sluice valve.

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Q/Qr

N/Nr

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Parameters Influencing Surge Picture

• Pipeline constant, B = (aVo)/(gHo)• Friction loss parameter, Hf/Ho• Pump inertia parameter which is inversely

proportional to combined GD2 of motor and pump • Longitudinal alignment of the pipeline• Type of NRV in the pump house• Number of working pumps (for effect of single

pump failure)• Delivery pipe size from individual pumps (for

effect of single pump failure)

General Trends

• Larger the value of B, surge relatively more;• Larger the friction loss, upsurge less critical &

down surge more critical;• Larger GD2 value, surge less critical;• Alignment effects are very important and quite

varied;• Choice of NRVs in pump house and their closure

characteristics may be adjusted to suit requirements of surge control;

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Non-return Valves - Types

• Swing check valve (small size valves)• Swing check valve (large size valves)• NRVs with dash pot arrangement• Multi-door NRVs• Dual plate check valves• Pump discharge valves (NRV cum isolation valve)• Zero velocity valves (special type upsurge control

valve to be used at intermediate locations along the rising main)

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Interpretation of Surge ResultsThe Pipe Material Aspect

• Pipe material (MS,DI,CI,PSC,BWSC,AC,GRP,PVC)

• Pressure class or wall thickness• Vulnerability to upsurge or pressure rise

(PSC,AC,PVC,CI)• Vulnerability to down surge or pressure drop

(large size MS pipe,GRP,PVC)• D/t issue for MS pipes (6 mm for 1000 mm dia

thumb rule)

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Interpretation of Surge Results(contd.)

Pump House & Rising Main• Use more conservative design criteria for pump house pipes • Manifold design to be atleast as conservative as rising main

designValves• Pressure rating of valves in the pump house• Type of NRVs• Pressure rating of valves along rising mainHP & LP Reaches• Pros & cons

Design Criteria for Surge Protection

Upsurge• Max. pressure not to exceed 1.5 times (or 1.25 times) working

pressure or pump head • Low head schemes, particularly with MS pipe, max. pressure

upto twice pump head may be quite safe (criterion: check against hoop stress)

Down surge• No sub-atmospheric pressure• Sub-atmospheric pressure upto (-) 5 m• Vapour pressure allowed, but upsurge due to column

separation to meet upsurge limit (criterion: check pipe strengthto withstand full vacuum)

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Control of Surge - Principles

• Primary surge immediately following power failure is down surge or pressure drop, which occurs due to reduction of flow velocity;

• If some stored water can be supplied into the rising main immediately after power failure, the down surge intensity will reduce;

• This is the concept used in air vessel & one way surge tank (OST) protection devices.

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Control of Surge (contd.)

• Upsurge or pressure rise is essentially associated with the development of return flow after power failure;

• Hence, if return flow is controlled, upsurge reduces;

• This is the concept used in air vessel (for control of upsurge) and Zero velocity valve;

• Alternately, if safe passage is allowed for return flow, upsurge is again controlled;

• This is the concept used in Surge anticipating valves (Surge relief valves)

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Zero Velocity Valve

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Surge Protection Devices

• Air vessel

• One way surge tank

• ZVV and Surge relief valve

• Air valves/ACVs

• Stand pipe

• Controls upsurge and down surge

• Controls down surge directly, upsurge indirectly

• Controls upsurge only• Control down surge directly,

upsurge indirectly

• Controls down surge

Cost of Surge Protection Devices – In ascending order (general trend)

• Air Valves/ ACVs• Stand Pipe• Surge Relief (Anticipating) Valves• Zero Velocity Valves• One Way Surge Tanks• Air VesselExcept for Air valves/ACVs, this general cost trend

may be changed in specific cases.