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
- ???? = 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.)
There are currently various software packages that can be used for analysis:
- AFT Impulse
- PASS/Hydro system
- Flownex Simulation Environment
Methodology (Fig. 5):
- 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.
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Frequently Asked Questions
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.
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
Pressure Surge, Water Hammer, Fluid Hammer or Hydraulic Surge, all these refer to the same event. There is no difference.
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
Pressure Surge can be avoided by the following methods:
1. Rapid Changes in fluid velocity occurs when valves are opened or closed suddenly. So by reducing the fluid velocity or by increasing the time taken for closing/opening the valve it can be avoided.
2. Surge can be avoided by installing Surge Relief Valve, Surge Tank, Viscous Damper, etc in the system.
3. The impact of surge can be reduced by reducing the number of elbows.
4. Eliminate the Presence of Air
Surge Analysis is the analysis of pressure changes in the piping system, normally performed by Process Engineers for proper pipe sizing or finding the peak surge pressure.