- Chemical Resistance
- Safety Data Sheets (SDS)
- Material Properties
- PRO Systems
- PE Pressure Pipe
- PE Pipe Selection
- MAOP for PE Pipes
- Temperature Influences
- Selection of Wall Thickness for Special Applications
- Hydraulic Design for PE Pipes
- Surge and Fatigue
- Slurry Flow
- Pneumatic Flow
- Expansion and Contraction
- External Pressure Resistance
- Allowable Bending Radius
- Thrust Block Support
- Conductivity, Vibration and Heat Sources
- Polyethylene Jointing
- Handling and Storage
- Trench Preparation for Buried Pipes
- Relining and Sliplining
- Pipeline Detection
- Above Ground Installation
- Accommodation of Thermal Movement by Deflection Legs
- Service Connections for PE Pipes
- Concrete Encasement
- Fire Rating
- Testing and Commissioning
- PVC Pressure Pipe
- PVC Pressure Pipe Standards
- Pressure Considerations
- PVC Temperature Considerations
- Mine Subsidence
- Water Hammer
- Thrust Support
- Air and Scour Valves
- Soil and Traffic Loads
- Bending Loads
- PVC Pipe Jointing
- Jointing Components with Ductile Iron Flanged Joints
- Service Connections for PVC Pipe
- PVC Pipe Handling and Storage
- Below Ground Installation
- Above Ground Installation for PVC Pipe
- Testing and Commissioning for PVC Pressure Pipe
- Detecting Buried Pipes
- FLUFF – Friction Loss in Uniform Fluid Flow
- Technical Notes
Water hammer is a temporary change in pressure in a pipeline due to a change in the velocity of flow in a pipe with respect to time, e.g. a valve opens or closes or a pump starts or stops. Accidental events such as a pipe blockage can also be a cause. The effects are exacerbated by:
- fast closing/stopping valves/pumps
- high water velocities
- air in the line
- poor layout of the pipe network, positioning of pumps, etc.
Note that water hammer pressure may be positive or negative. Both can be detrimental to pipe systems; not only pipes, but pumps, valves and thrust supports can be damaged. Negative pressures can cause ‘separation’ (vacuum formation), with very high positive pressures on ‘rejoinder’ (collapse of the vacuum). For these reasons, water hammer should be eliminated as far as possible.
Water hammer pressures can be reduced by:
- Controlling and slowing valve and pump operations
- Reducing velocities by using larger diameter pipes
- Using pipe materials with lower elastic modulus
- Astute layout of network, valves, pumps and air valves
- Fast-acting pressure relief valves, e.g. Neyrtec (Trade mark of Alsthom International Pty Ltd. For further information contact Sofraco International Pty Ltd, Sydney, Australia)
It is beyond the scope of this manual to give a complete description of water hammer analysis and mitigation. However, it is appropriate to highlight some important aspects related to PVC pipes.
Celerity is the speed (expressed in metres per second) that the pressure waves travel in a closed circuit. This should not be confused with the velocity of the water.
This is a function of the pipe geometry (dimension ratio) and material and may be estimated from:
where: W = density of fluid (water = 1,000) (kg/m3)
A = cross-sectional area of the wall of the pipe per unit length (mm2/mm)
= wall thickness for plain wall pipes
D = mean diameter of the pipe (mm)
k = the bulk modulus of the fluid (2150 for water) (MPa)
E = the elastic modulus for the pipe (MPa)
The wave celerity induced in PVC pipes are shown in Table 3-12. As PVC has a celerity about one third that of metallic pipes, analyses for metallic pipes should not be used to check PVC classes.
|≤ DN 150||> DN 175|
|DR||a (m/s)||DR||a (m/s)|
|PVC-O All sizes|
|PN||PVC-M All sizes|
For buried pipes, increase the wave celerity (a) by 7%.
The advantage of a low celerity can be demonstrated by Joukowsky’s Law, which gives an estimate for the water hammer pressure rise due to instantaneous valve closure.
P = W a • ΔV (Pa)
where: ΔV = change in flow velocity (m/s)
This equation should NOT be used for design purposes. Water hammer analysis is fairly complex and computer analysis by a competent consultant is recommended wherever it is suspected that water hammer may be significant.
Selection of class should be based on peak operating pressures including water hammer. Control devices may be useful in reducing peak pressures and enable a more economic pipe class to be used.
The response of the pipe to occasional abnormal pressures, for example due to the failure of protective devices, is important.
PVC has a high factor of safety on short term stress effects, and is able to withstand occasional events at higher than normal pressures. This advantage should be considered when determining the validity of basing a design purely on the pressures induced by events that may be rare in the design lifetime, e.g. power failure on a pump.