SURGES IN PIPELINES Raghuvee Raghuveerr Rao Pallepati Pallepati Department of Civil Engineering Indian Institute of Science Bangalore Bangalore – 560012 560012 India India
Water supply pumping mains Lift irrigation schemes Cooling water systems for thermal and nuclear power plants
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Water supply schemes • Diam Diamte terr of pip pipe e – 0.4 m to to 2.5 2.5 m • Leng Length th 5 km km to to 150 150 km • Disc Discha harg rge e – 20 to 270 270 MLD MLD • Head – 30 m to 250 m
Lift irrigation schemes • Diam Diamte terr of pip pipe e – 1.5 m to 3 m • Leng Length th 0.2 0.2 km km to to 50 50 km km • Disc Disch harg arge – 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 Reservoir – water level - min & max Delivery Reservoir – ground level Ground level profile Quantity of water required MLD - Million liters / day Number of hours of pumping Velocity – 1 m/sec – fix diameter
Delivery
Source
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Design parameters Discharge required - Diameter of pipe Pipe material – Steel/PSC/DI/CI/BWSC/GRP Thickness / pressure rating Calculation of pump head Static head + Frictional loss + Minor losses + Pump house loss
Delivery Static Lift
Source
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Friction loss in pipe Darcy-Weisbach
Hazen-Williams
H f = f
H f =
L v2 D 2g
10.67 L Q1.852 C1.852 D 4.87
Minor losses bends, valves etc. Pump house loss Valves, bends etc
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 velocity V = flow velocity ∆H = Surge pressure g = gravitational acceleration Δ H =
aV g
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 m
This 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 velocity K = bulk modulus of elasticity of water ρ = density of water E = Young’s modulus of elasticity of pipe material D = diameter of pipe t = wall thickness of pipe
a
=
K ρ KD
1+
Et
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Surge – The Phenomenon Rapid Change in Discharge (Velocity) & Associated Change in Pressure
Surge – The Causes Operation of Valves (Closure & Opening) Starting of Pumps Stopping of Pumps Power Failure Single 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 Problems Pressure
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 pressure Column separation due to occurrence of vapour pressure Rejoining of separated columns causing pressure rise (indirect upsurge)
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Column Separation Local Peak Pipe alignment at a local peak
Inflow
cavity
Cavity development Cavity volume with time
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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N/Nr
Q/Qr
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Parameters Influencing Surge Picture • Pipeline constant, B = (aVo)/(gHo) • Friction loss parameter, H /H f o • Pump inertia parameter which is inversely proportional to combined GD 2 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 Results The 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 design Valves • Pressure rating of valves in the pump house • Type of NRVs • Pressure rating of valves along rising main HP & 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 strength to 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 • Controls upsurge and down surge • Air vessel • Controls down surge directly, upsurge indirectly • One way surge tank • ZVV and Surge relief • Controls upsurge only • Control down surge directly, valve upsurge indirectly • Air valves/ACVs • Stand pipe
• 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 Vessel Except for Air valves/ACVs, this general cost trend may be changed in specific cases.
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