The hydrostatic pressure capacity of HDPE pipe is related to a number of variables:
- The ration between the outside diameter and the wall thickness (Standard Dimensional Ratio – SDR)
- The hydrostatic design stress of the HDPE material being used (PE 63, PE 80, PE 100)
- The operating temperature.
- The duration and variability of the stress applied by the internal hydrostatic pressure.
- The chemical resistance of the pipe to the chemical being carried (the standard pressure rating is based on a pipe carrying water).
Although HDPE pipe can withstand short-term hydrostatic water pressures at levels substantially higher than the pressures rating, or class, (see item 4.3 “the stress regression line” and item 4.4 “design stress and safety factor”) the duty of HDPE pipe should always be based on the pipe’s long-term strength at 20°C to ensure a design life of at least 50 years. As stated earlier, the relationship between the internal pressure, the diameter and wall thickness and the circumferential hoop stress in the pipe wall, is given by the Barlow Formula, which can also be expressed as follows.
T = p x d / (2σ + p)
- t = minimum wall thickness (mm)
- p = internal pressure (MPa)
- d = mean external diameter (mm)
- σ = design stress (MPa)
These formulae have been standardized for use in design, testing and research and are applicable at all levels of pressure and stress. For design purposes, p is taken as the maximum allowable pressure and σ, the maximum allowable hoop stress at 20°C.
The design hoop stress used in SANS ISO 4427 are as follows:
The pressure classes of SANS ISO 4427 HDPE Pipes are based on constant internal water pressures. HDPE pipes are however capable of handling dynamic pressure events which exceed the values given by the classes but such occurrences can have a negative effect on the standard 50 year life expectancy and in extreme cases can result in product failure.
Pipelines may be subjected to short-term increase in pressure above the normal working pressure due to water hammer. Water hammer will occur in a pipeline when its equilibrium is disturbed by rapid changes in flow conditions.
Examples of such conditions are:
- Starting and stopping of pumps
- Rapid opening and closing of valves
- Pipe failures etc.
A rapid change in the velocity Δv of water in the pipeline gives rise to a pressure increase Δp according to the formula:
Δp = cΔv/g
- c = the wave celerity (meters per second)
- g = the acceleration due to gravity
The approximate wave celerities for HDPE pipes are as follows:
Note: Since part of the formula for calculating wave celerity incorporates the ratio between diameter and wall thickness (SDR), which is roughly constant for all sizes within a pressure class, the wave celerity’s are also constant for all sizes within a pressure class.
Note: By way of comparison the wave celerity for steel pipes is about 3-5 times higher than for HDPE – 1000 to 1400 m/s.
It is important to note that the pressure increase due to water hammer in a particular class of pipe is a function of the change in velocity and it is therefore important (for this and other reasons) to keep pumping velocities in a pipeline within the conventional norm of 1 to 2 m/s.
In general steps should be taken during design and operation to minimize the frequency and intensity of water hammer. However, the total pressure may be permitted to reach a value 50% higher than the nominal pressure if the frequency can be described as “occasional”.