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What is Pressure Surge or Water Hammer?

Pressure Surge or Water Hammer or Fluid Hammer or Hydraulic Surge is a pressure wave that is caused by the kinetic energy of the moving fluid when there is a sudden change in flow velocity.

In Piping/Pipeline system networks this phenomenon is a major concern for Piping/Pipeline/Process engineers. This sudden change is velocity can arise due to various reasons.

What Can Cause Pressure Surge?

Pressure Surge can cause these following effects:

Hydraulic transients that occur at changes in flow in piping/pipelines and this could be due to:
Pump start & stop, specifically due to load shedding or sudden power failure
Quick operation of Valve (Sudden closure/opening)
Sudden closure of the check valve
Presence of Air pockets inside piping/ pipeline systems, especially during pump start
A sudden release of Air
Quick Pipeline filling

Pressure Surges can occur in open channels and partly liquid-filled pipes, as well

Is Water Hammer Dangerous?

Refer Fig. 1 to understand what a pressure surge can cause to a Piping System. Pressure Surge of Significant nature creates high pressure and velocity rise that can lead to:

High pressure or vacuum in the piping/pipeline system which in turn may cause:
Failure of pipe/pipeline fittings
Bursting of pipes
Damage of Pump/pumping system
Deformation of valves and piping supports

Basic Definitions concerning Pressure Surge:

Pressure Surge:- It is basically a pressure wave caused due to a sudden change in flow velocity.
Wave speed or acoustic velocity:- The velocity at which pressure waves travel through the liquid/fluid.
Joukowsky equation:- Relationship relating head change to velocity change and acoustic velocity.
Pipeline Period:- Time required for a pressure wave to traverse the pipe/pipeline length and come back.
Pressure Head:- Pressure measured as the height of fluid (10 m head of water is roughly 1 atmosphere)

Analysis of Water Hammer/Pressure Surge:

The most important parameters to estimate the magnitude of transient pressures is:

Acoustic wave speed, a
Pipe/Pipeline period, T
Joukowsky head, Δh

The acoustic wave speed formula depends on the fluid and the pipe characteristics expressed as:

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

The time that a pressure wave takes to travel from its origin through the system and back to its source is defined as the pipe period. For a single pipeline with pipeline Length, L this is provided as given below:

T = Critical period
L = Length of the pipe
a = Velocity of the pressure wave

Events that take place in less than T are called ‘fast’ events and these are likely to cause pressure surge issues.

As per the Joukowsky formula, the pressure head change (Δh) due to an instantaneous velocity change (ΔV) is expressed as shown above. Here,

Δh = head rise
ΔV = change in velocity
a = wave speed
g = acceleration due to gravity

This is a very useful guide that explains the likely severity of a pressure surge event but is not a replacement for a proper surge analysis!

How to Avoid Pressure Surge

To avoid pressure surge system must be protected. Protection of systems against water hammer can be parted into three groups:

1.0: System Design Solutions:

Use of pipework with a higher pressure rating
Rerouting of the pipeline avoiding high/low points
Changing of piping material, thus altering the wave speed
Increase of the pipe diameter, thereby reducing the velocity
Increase of pump inertia by incorporating a flywheel
Adding bypass lines

2.0 Active Protection:

Piping/Pipeline systems can be protected against Surge impact by using devices during pipeline normal operation like:

Variable speed pumping
Soft starters
Slow closing and opening valves

Be informed that these devices require power and during load shedding or power-failure cannot be of use.

3.0: Passive Protection (Fig. 3):

There are several passive protection equipments available in the market that operates without the need for additional power. Few examples of these are:

Surge Vessels
Surge Shafts
Air Valves
Vacuum Breakers
Pressure Relief Valves
Surge Anticipation Valves
Intermediate Check valves

Selection of System for Surge Protection (Fig. 4.)

Modeling Softwares:

There are currently various software packages that can be used for analysis:

HyTran
Flowmaster
WANDA
Hammer
AFT Impulse
PIPENET
PTRAN
PASS/Hydro system
Flownex Simulation Environment

Methodology (Fig. 5):

Conclusions:

The Pressure Surge phenomena during transient events are very important as they can put the system’s integrity at high risk.
During risk and HAZOP analysis, Pressure Surge Events and the corresponding mitigation devices should be always taken into account.
System operations staff must be trained in order to prevent operations likely to damage the system integrity.
Surge protection equipment must be maintained periodically.
It is highly possible to increase the reliability and life expectancy of systems by taking preventive measures for reducing the risk of failure due to pressure surge events,
Pipe/Pipeline system should be properly supported with the hold downs, guides and line stop and the supports along with supporting structures must be designed considering dynamic forces during a Surge event.

Some more Resources for you..

Understanding Centrifugal Compressor Surge and Control
Water Hammer Basics in Pumps
Pipe Stress Analysis from Water Hammer Loads

References:

Frequently Asked Questions

What is Pressure Surge?

Pressure Surge is a pressure wave that is caused by the kinetic energy of the moving fluid when there is a sudden change in flow velocity.

What is the pressure surge in piping?

If the high-velocity flow in a pipe is forced to stop or change direction suddenly, a pressure wave generates and moves back at the speed of sound in the liquid. This can produce huge forces in the piping or pipeline system. This is called Pressure Surge in Piping

What is the difference between Pressure Surge and Water Hammer?

Pressure Surge, Water Hammer, Fluid Hammer or Hydraulic Surge, all these refer to the same event. There is no difference.

What Can Cause Pressure Surge?

The Pressure Surge in a Piping system can be caused by any of the following Events:
1. Pump start & stop, specifically due to load shedding or sudden power failure
2. Quick operation of Valve (Sudden closure/opening)
3. Sudden closure of the check valve
4. Presence of Air pockets inside piping/ pipeline systems, especially during pump start
5. A sudden release of Air
6. Quick Pipeline filling