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1
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)
21
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)
23
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.