SURGE ANALYSIS

WATER HAMMER

HYDRAULIC TRANSIENT

Himadri Enviro-Protection Consultants Pvt. Ltd.

Ahmedabad – 380006.

By Devang P. Shah, Director

Steady flow – no change in conditions at a point with time.

Unsteady flow – conditions at a point may change with the time.

Uniform flow – average velocity at a given cross section is same at any given instant

Non-uniform flow – velocity varies at any given instant

BASIS FOR HYDRAULICS AND FLOW CONDITIONS

CAUSE OF SURGE

1.Sudden change in discharge of system due to

Valve closure or opening

2.Change in velocity of system due to Starting of

pump/ Valve

3.Stopping of pump/ Valve

4.Power failure

Water hammer or surge or hydraulic transient phenomena could develop in both - Gravity and Pumping systems.

Since for gravity system, flow can be controlled manually and thus surge effect in gravity system could be controlled easily as compared to the pumping system due to pump failure and power failure phenomena beyond control.

Due to rapid change in discharge in pipelineChange in velocity and change in pressurePressure wave (down surge) transmits through the rising mainAt delivery reservoir, down surge wave gets reflected as upsurge wave

and moves towards pump end At reduced pump speed, flow starts reversing at pump NRV at pump closes due to flow reversal causing a pressure rise or

upsurge

CAUSE OF SURGE

When power fails motor speed decreases and thus discharge and head get reduces and hence development of down surge pressure wave travel along the pipe alignment towards delivery end at the speed of 1 km/s

After reaching wave at delivery end again it gets reflected as upsurge wave and thus pressure rise in the pipeline above working or test pressure.

CAUSE OF SURGE

• Increase of pressure in pipe beyond it’s sustained pressure• Reduction in pressure near vapor pressure squeezing the pipe• These waves move along the rising main, at the delivery reservoir

& at the closed NRV at pump end • Speed of wave movement approx. 0.8-1.2 km/sec• Reflection at delivery reservoir positive wave becomes negative

wave • Reflection at closed NRV wave doubles up, i.e. reflected wave

same sign as direct wave• Final result: Low & high pressures all along the rising main

Extent of damage to pipe depends upon• Occurrence of wave velocity depends upon, type of pipe material

and wall thickness of pipe

SURGE PHENOMENA

The Problems associated with the Surge

POSITIVE SURGE – RUPTURE OF PIPE

Pressure rise due to NRV closure too high (depends on type of NRV & closure pattern)

NEGATIVE SURGE – DENTING OF PIPE

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 and down surge)

SCHEMATIC DIAGRAM OF A TYPICAL PUMPING MAIN

Air Valve

Air Valve

Air Valve

Scour Valve

Scour Valve

Delivery Chamber

Pump House

Sump

During down surge wave pressure gets below atmospheric pressure and at peak vapour pressure occurs.

At certain location in pipeline flow travel upstream and downstream simultaneously, this phenomena known as column separation.

Due to negative pressure and development of vapour pressure is the main cause of pipe collapse.

In single pump failure pressure wave travel from the pump to delivery point of manifold. And hence local air valves on the pipeline does not control such case. Hence in delivery pipe proper NR valve is required to install.

SURGES IN A PUMPING MAIN FOLLOWING

POWER FAILURE

Closing valve at delivery end induce upsurge or pressure rise behind the valve and pressure rises towards the source reservoir.

SURGE IN GRAVITY MAIN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400180

200

220

240

260

280

300

320

340

360

380

400

420

Fig.:Minimum and Maximum Piezometric Heads

Alignment Minimum Head Maximum Head HGL

Path Chainage, m

Ele

vati

on, R

L, m

Schematic Diagram of Gravity Main

∆H = a X Vo / g

Where , Vo = flow velocity

a = water Hammer velocity

g = gravitational acceleration

∆H = rise in pressure head

Calculation of Maximum Head as per CPHEEO manual

PRESSURE WAVE VELOCITY 1425

a = -----------------------------

(1 + (KD / Ee)) ^ 0.5

where a = pressure wave velocity (m /sec)

K = bulk modulus of elasticity of water (kgf / m2)

D = diameter of pipe (m)

E = modulus of elasticity of pipe material (kgf / m2)

e = pipe wall thickness (m)

This equation gives idea about maximum pressure in system

Actual field problem throughout the alignment at different point and time is not tracked from this formula.

basic limitation due to lack of pipe length and geo-spatial hydraulic transient are not reflecting effectively.

Hence systematic transient analysis with help of analytical tool plays significant role

Material E (kg / m2)Polyethelene – soft 1.2 X 10^7Polyethelene – Hard 9 X 10^7PVC 3 X 10^8Concrete 2.8 X 10^9Asbestos cement 3 X 10^9RCC 3.1 X 10^9PSC 3.5 X 10^9Cast Iron 7.5 X 10^9Ductile iron 1.7 X 10^10Steel 2.1 X 10^10GRP 2.55 X 10^9

Water hammer velocity may be as high as 1370 m/sec for a rigid pipe or as low as 850 m/s for a steel pipe and for plastic pipe may be as low as 200 m/s.

INPUT DATA REQUIRED FOR SURGE ANALYSIS Design discharge

Internal diameter of pipeLength of pipelinePipe material and other related dataPump headWater level in the sump and delivery levelThickness of pipe wallPump data – efficiency, shutoff head, pump characteristic

etc.L – section of pipeline alignment

For the surge analysis of pipe line different softwares are available in the market

COLUMN SEPARATIONGenerally at peak when vacuum occurs, velocity at upstream and down stream of the location differ. This phenomena known as column separation.At a peak location, pressure goes down to vapour pressure (-10) This location becomes a AIR LOCK (pressure control), temporary pseudo-reservoirUpstream & downstream water columns separate with different flow velocitiesInitially outflow velocity is more resulting into increasing vapour pocket or cavity sizeLater inflow velocity becomes more (outflow velocity changes direction — reverse flow) shrinking the cavitycavity volume becomes zero - sudden pressure rise due to column reunion occursPressure rise transmits on both sides of rising main increasing pressure throughout the system

Parameters Influencing Surge Picture

Pipeline constant,

Friction loss parameter,

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)

Involve down-surge or pressure drop and control of upsurge or pressure rise.

Critical condition is power failure. Down surge occurs as the primary surge and upsurge follows as secondary stage.

Certain devises are to control down surge and thus controlling upsurge.

SURGE CONTROL

Control of Surge

Supplement the water to control vacuum

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.

Restrict the velocity and bypass the upsurge

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)

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)

Devices

Air vessel Controls upsurge and down surge

One way surge tank Controls down surge directly, upsurge indirectly

ZVV and Surge relief valve Controls upsurge only

Air valves/ACVs Control down surge directly, upsurge indirectly

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.

DEVICES

1. Air vessel

2. One way surge tank

3. Surge control valve

1) To control both up surge and down surge

2) Lower part water and upper part air3) When power failure water from vessel travel to pipe line

and air expand and when velocity reduce flow gets reversal and air gets compressed-

4) Location near pump houseArrangement - Type: 1 Rounded Orifice

Type: 2 NRV

AIR VESSEL

Air Vessel : Function of Air Vessel - When pump fails water is supplied from air vessel to pipe due to compressed air, again during reverse flow air is compressed and water travel from pipe to air vessel, thus control both downsurge and upsurge.

Orifice Type : 1 Velocity 12-20 m/s.

Design part – Water level in AV to be maintained.

Compressor is required as an accessories.

Diameter normally 3 to 4 mm of AV.

AIR VESSEL

Schematic Diagram of Air Vessel Installation

Schematic Diagram of Alternate Types of Design for Air

Vessel

ONE WAY SURGE TANK

1) To control down surge directly and the upsurge gets controlled indirectly

2) Location at the middle of alignment and where location is available at peak

3) Height of tank may be around 3 to 5 m.

4) When pressure gets reduce in the main pipeline water flow from tank to fill main pipeline and thus reducing down surge.

ONE WAY SURGE TANK

5) When pressure rise in the main pipeline in between valve of connecting pipeline gets automatically close.

6) One has to check water level in the tank periodically and filling of water is required in the tank for better operation of the system

7) Filling arrangement can be different inlet pipe to tank from main pipeline

as a regular usually RCC tank, filling of tank is required regularly, NRV on connecting pipe between surge tank and main height below HGL ,circular RCC tank, Design of tank size and connecting pipe.

Schematic Diagram of One Way Surge Tank Bypass Filling Arrangement

SURGE CONTROL VALVES

1) Non return valve Dual plate check valve

2) Pressure relief valve

Location near the pump house. Spring loaded valve with separate low pressure and high pressure pilot setting enable opening and closing of valve. When line pressure reaches to normal range , the valve again closes.

SURGE CONTROL VALVES

3) Air valves

Act as vaccume breaker to control the sub atmospheric pressure. i.e. To control down surge under vaccume

4) Air cushion valve

To allow large quantities of air in the pumping main

The valve has spring loaded air inlet port , an outlet normally closed by float.

With diferential pressure the spring loaded port opens and admits outside air into the main.

With differential pressure the spring loaded port opens and admits outside air into the main.

When the pressure in the main becomes near atmospheric, the inlet valve closes under spring pressure. The entrapped air is than compressed by returning water column till the poppet valve opens with float is dropped position, the air is expelled through pappet valve and controlled orifice under predetermined pressure thus dissipating the energy of the returning water column.

Air Valve & Air Cushion Valve for Surge Control

Air valves are always very useful as supplementary surge control devices along with some other devices

low cost and useful of filling & draining purpose

Air valves for surge control must be distinguished from air valves required for normal pipe filling & draining purposes

Air cushion valves are special type of air valves with separate inflow port & controlled outflow rate

Limitation associated while analysis for air valve

Time lag in the vacuum breaker function of air valve;

Inlet capacity of air valve;

Air exhaust characteristics of air valve;

Movement of air introduced along the pipe;

Two phase flow effects.

Surge protection based on only air valves may be suitable only if the pipe strength is good for both upsurge & down surge;

Myth - If all air valves in a rising main are considered as ideal vacuum breaker, there will be no severe down surge,

Reliability of air valve function as vacuum breaker may be dependent on the rate of pressure drop following power failure, in the absence of air valve

If the rate of pressure drop is too rapid, less reliability

Schematic Diagram of Stand Pipe

4) Zero velocity valve

To arrest the forward moving water column at zero movement. Spring loaded non return valve intended for upsurge control. Closing occurs gradually with reduction in velocity zero. Size of valve normally same size as main . Valve is provided with bye pass arrangement. The springs are so designed that the disc remains in fully open position for velocity of water equal to 25% of the designed maximum velocity in the pipeline.

5) Duel plate check valve

Spring loaded non return valve. Installed at intermediate location of main pipe line. Size of valve as main pipe line size. When velocity in the pipe line reduce to zero, valve gets close and prevent flow reversal. Suit to control up surge , not down surge

6) Stand pipe

Location of stand pipe is near to delivery end where HGL line very near to Ground elevation

Schematic Diagram of Valve With Bypass Outlet

Zero Velocity Valve

Special - spring loaded NRV integral bypass

Regulates reverse flow

The springs initiate closure as the flow velocity reduces followed by full closure occurring at zero velocity

Bypass has an important role and must be kept open under normal operation

Not suited for down surge control

Suitability of ZVV

High head schemes with significant reverse flow development potential;

Pressure rise due to column separation pronounced, particularly due to local vulnerable peaks;

Pipe strength good to withstand vacuum or down surge not severe;

Suitability of ZVV

High head schemes

Pressure rise due to column separation, due to local peaks;

Pipe strength good to withstand vacuum or down surge not severe;

Under down surge, pressure drop rate is such that air valve or Air cushion valve can control vacuum reliably.

Dual Plate Check Valve

Spring loaded NRV, but unlike ZVV, torsion spring is used;

Two semi-circular discs;

Can also be looked upon as a spring loaded multi-door (two door) NRV;

Commonly used NRV in pump houses;

Application as upsurge control device along the alignment in cross country pipelines (like ZVV) requires provision of an external bypass.

Surge Relief Valve or Surge Anticipating Valve

Special - piloted pressure relief valve with a low/ high pressure pilot;

Located near the pump house on a tee like a pressure relief valve;

When the pressure crosses stipulated limit either valve opens;

When the pressure comes back between these range, the valve closes;

When pressure drops below the low pressure pilot setting following power failure. the SRV opens discharging water;

It is this aspect of SRV anticipating upsurge that it is also referred as surge anticipating valve;

As pressure reaches static pressure, the SRV closes gradually.

Example of case study of surge analysis and graph with different alternatives.

Minimum and maximum piezometer head with no protection

With column separation

With Air valves

With surge relief valve

With zero velocity valve

SEVERITY OF SURGE – UPSURGE AND DOWNSURGE EFFECTS

Direct effect of down-surge due to occurrence of vacuum, causing buckling of pipe or dislocation of joints in case of flexible joints.

Effect of direct upsurge resulting in undue pressure rise causing failure of the pipe.

Effect of secondary upsurge resulting from water column separation (occurrence of vapor pressure) and associated shock pressure rise due to the rejoining of the separated water column.

Minimum and Maximum Piezometric Heads

Rate of Pressure Drop at Specified Locations

Alignments Vulnerable to Down-surge

Alignment with very rapid rise of ground level near delivery chamber

Air valves To control down-surge

Intended to release air at summit during filling of pipeline

Located at an average spacing of 0.8 km to 1 km, but not at uniform interval

If pressure drags from sub-atmospheric pressure to vapour pressure in less than 1 sec (in the absence of AV), the AV function as vacuum breaker will be uncertain. If drop in pressure from sub-atmospheric pressure to vapour pressure take few seconds (3 sec. or more), air valve function as vacuume breaker may be considered reliable.

Surge relief valve :

Rate of Pressure Drop at Specified Locations

Minimum and Maximum Piezometric Head – An Example

Schematic Diagram of the Installation of Surge Relief Valve

CASE STUDY GWIL NC 9 pipe line Maliya to Bhachau

  ANALYSIS OPTION : Option -1 w/o any protection

  TRANSMISSION MAIN DETAILS:

  DESIGN DISCHARGE (cum/sec) = 3.700

INTERNAL DIAMETER OF TRANSMISSION MAIN (mm) = 1800.

LENGTH OF TRANSMISSION MAIN (m) = 54700.

PRESSURE WAVE VELOCITY (m/sec) = 947.

  PUMP DETAILS:

  NUMBER OF WORKING PUMPS = 6

DISCHARGE PER PUMP (cum/sec) = .617

PUMP HEAD (m) = 100.0

CASE STUDY GWIL NC 10A Bhachau to Varshamedi

Flow 85 mld

  ANALYSIS CASE : Option -1 w/o any protection

  TRANSMISSION MAIN DETAILS:

  DESIGN DISCHARGE (cum/sec) = 1.026

INTERNAL DIAMETER OF TRANSMISSION MAIN (mm) = 900.

LENGTH OF TRANSMISSION MAIN (m) = 34680.

PRESSURE WAVE VELOCITY (m/sec) = 1012.

  PUMP DETAILS:

  NUMBER OF WORKING PUMPS = 4

DISCHARGE PER PUMP (cum/sec) = .257

PUMP HEAD (m) = 80.0

Zero Velocity Valve consists of a Spring loaded closing disc for stopping reverse flow in case of failure of pumps. It is enclosed in an outer shell. A well designed dome is located in back of disc to stream line the flow in routine operation. Disc is mounted on a central shaft and is further supported by guide rods. Valve is provided with a bye pass arrangement. Valves are generally supplied with barrel ends but can also be flanged, if so desired. The springs are designed in such a manner that the valve remains full open when 25% of designed velocity is achieved. In case of closure of pump, disc starts closing in relation to decrease of velocity and fully closes when velocity drops near to zero. Thus upstream water column is prevented from creating water hammer wave. Bye pass arrangement keeps pressure balance on both sides of disc. It also prevents creation of vacuum in downstream side.

Surge Relief Valves

Surge relief valves are known for their quick speed of response, excellent flow characteristics, and durability in high pressure applications. Surge relief valves are designed to have an adjustable set point that is directly related to the max pressure of the pipeline/system. When the product on the inlet of the valve exceeds the set point it forces the valve to open and allows the excess surge to be bled out in to a breakout tank or recirculated into a different pipeline. So in the event of the surge, the majority of the pressure is absorbed in the liquid and pipe, and just that quantity of liquid which is necessary to relieve pressures of unsafe proportions is discharged to the surge relief tank. Some valve manufactures use the piston style with a nitrogen control system and external plenums, while others use elastomeric tubes, external pilots, or internal chambers.