Resistance to Internal Pressure
Long Term Hydrostatic Strength - LTHS
A major element in the design of PVC-U pressure pipe systems, is the long-term resistance against mechanical and thermal loading under internal pressure. This property is represented by the regression curves in a log stress vs. log failure time graph from internal pressure testing on a large number of pipe test pieces. The internal pressure applied to the pipe sample, will create a hoop or radial stress in the pipe wall that is calculated from the Ring Formula :
in which (MPa) is the hoop stress, P (MPa) the applied pressure,
(mm) the measured outside diameter and (mm) the measured minimum wall thickness of the pipe sample.
It is of extreme importance, that in this testing each individual pipe sample is measured to its exact dimensions, in order to calculate the real stress for the thinnest spot in the pipe wall. As pipes are produced within certain tolerances for diameter and wall thickness, the use of the nominal standardised dimensions may lead to substantial errors.
It is practice that at each stress level at least three pipe samples are pressure tested, which unavoidably leads to a certain scatter of the recorded failure times. The testing has to proceed for a minimum time of one year, and a performance level at 50 years has to be determined, extrapolation from the recorded data is requiered.
ISO 9080 describes the multiple linear regression calculation method.
In this method, the data from different temperature levels and the mode of failure are processed and two curves for each temperature are calculated :
- one curve representing the average line of the failure times. The stress recorded at the intersection of the curve at 20° C with the 50 year line is the Long Term Hydrostatic Strenght, LTHS.
- one curve representing the 97,5 % lower limit. The stress at the intersection of this curve at 20° C with the 50 year line is the 97,5 % lower predicted limit (LPL).
The represented curves are the LPL-curves by 20° and 60° C.
The curves in black colour represented the minimum requirement of the norm EN1452.
The requirement are 20° C - 50 years - = 25 MPa.
The blue line represented the real curve (LPL) and predicted by
20° C - 50 years - = 29 MPa.
The stress value of = 29 MPa is supplementary security value of 16 %.
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