What is pressure surge analysis

Causes for Pressure Surge
Consequences of Surge
Few Definitions
Analysis of Pressure Surge
Variation of wavespeed with pipeline characteristics
System Protection against Surge
System Design Solutions
Selection of System for Surge Protection

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What is Pressure Surge Analysis?

Causes for Pressure Surge:
·
Hydraulic transients occur at flow changes in pipelines and this is due to:
·
Pump start and stop, especially due to power failure
·
Valve operation (closure/opening)
·
Check valve closure
·
Air pockets in pipelines, especially at pump start
·
Air release
·
Pipeline filling
Surges can also occur in open channels and part full pipes.
Consequences of Surge (Fig. 1):
Pressure Surge of Significant change in pressure and velocity leads to:
·
High pressure or vacuum along the pipelines causing:
·
Failure of fittings
·
Burst pipes
·
Pump damage
·
Deformed valves

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Above Photo, Few Consequences of Pressure Surge
Few Definitions:
·
Pressure Surge:- Is a pressure wave caused by a sudden change in flow velocity.
·
Wave speed or acoustic velocity:- The speed at which pressure waves travel through
the fluid.
·
Joukowsky equation:- Relationship relating head change to velocity change and
acoustic velocity.
·
Pipeline Period:- Time for a pressure wave to travel the length of the pipe and come
back.
·
Head:- Pressure measured as height of fluid (10 m head of water is roughly 1
atmosphere)
Analysis of Pressure Surge:
The most important parameters to estimate the magnitude of transient pressures are:
·
Acoustic wave speed, a
·
Pipe period, T
·
Joukowsky head, Δh
The acoustic wave speed formula depends on the fluid and the pipe characteristics expressed
as:

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·
a = Velocity of pressure wave
·
K = Bulk modulus of fluid
·
ρ = Liquid density
·
D = Internal diameter of the pipe
·
E = Young’s modulus of the pipe material
·
e = Wall thickness
·
( = restraint factor (usually taken as 1)

Variation of wavespeed with pipeline characteristics
The pipe period is defined as the time required for a pressure wave to travel from its source
of origin through the system and back to its source. For a single pipeline with Length L this is
expressed as shown below:
·
T = Critical period
·
L = Length of the pipe
·
a = Velocity of pressure wave
Events which take place in less than T are ‘fast’ events likely to cause surge issues.

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The Joukowsky formula describes the head change (Δh) that results from an instantaneous
change in velocity (ΔV). This is expressed as shown above:
·
Δh = head rise
·
ΔV = change in velocity
·
a = wave speed
·
g = acceleration due to gravity
Note that this is a useful guide to the likely severity of a surge event but is not a replacement
for a proper surge analysis!
System Protection against Surge:
System protection can be divided into three groups:
1.0: System Design Solutions:
·
Use of pipework with a higher pressure rating
·
Reroute pipeline to avoid high/low points
·
Change of pipe material to alter the wavespeed
·
Increase the pipe diameter to reduce velocity
·
Increase pump inertia by adding a flywheel
·
Bypass lines
2.0 Active Protection:
By using devices to actively protect the systems against the effects of pressure surge during
pipeline normal operation like
·
Variable speed pumping
·
Soft starters
·
Slow closing and opening valves
Note that these require power and generally cannot provide protection during a power failure.
3.0: Passive Protection (Fig. 3):

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Passive Equipments for Surge Protection
Passive protection equipment operates without the need for additional power like:
·
Surge Vessels
·
Surge Shafts
·
Air Valves
·
Vacuum Breakers
·
Pressure Relief Valves
·
Surge Anticipation Valves
·
Intermediate Check valves
Selection of System for Surge Protection: Refer Fig. 4.

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Selection of System for Surge Protection
Modelling Softwares:
There are currently various software packages that can be used for analysis
·
HyTran
·
Flowmaster
·
WANDA
·
Hammer
·
AFT Impulse
·
PIPENET
·
PTRAN
Methodology (Fig. 5):

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Surge Analysis Methodology
Conclusions:
·
The phenomena that occur during transient events are very important since they can
put the system’s integrity at risk.
·
Pressure Surge Events and the corresponding mitigation devices should be always
taken into account in risk and HAZOP analysis.
·
System operations staff should have training in order to prevent operations likely to
damage the system.
·
Surge protection equipment must be maintained.
·
Taking preventive measures for reducing the risk of failure due to pressure surge
events, it is possible to increase reliability and life expectancy of systems.

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·
Pipe/Pipeline system should be properly supported with guide and line stops and the
supports and supporting structures should be designed considering dynamic forces
during Surge event.
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