Temperature derating of PVC pipes for pressure applications

Alan Whittle and Mike Stahmer, January 2005

When PVC pressure pipes are to be operated at temperatures greater than 20°C, the maximum allowable operating pressure should be reduced by applying a thermal derating factor.

When PVC pressure pipes are to be operated at temperatures greater than 20°C, the maximum allowable operating pressure should be reduced by applying a thermal derating factor.

Table 1

The reduced maximum allowable operating pressure under static pressure is determined by multiplying the PN rating of the pipe by the derating factor given in Table 1 or by interpolation from the graph in Figure 1.

Figure 1

The derating factors have been selected from the International Standard ISO 4422.2, Pipes and fittings made of unplasticised polyvinyl chloride (PVC-U) for water supply - Part 2: Pipes (with or without integral sockets). The table has been extended from 45°C up to 50°C and interpolation may be used to select a derating factor for temperatures between those nominated.

These derating factors may be applied to PVC-U, PVC-M and PVC-O pipes.

Design Operating temperature:

The material temperature under consideration is the average temperature of the pipe wall under operational conditions.

In most instances it may be assumed that the pipe temperature is equal to the elevated temperature of the fluid being carried.

Where a temperature differential exists between the fluid in the pipe and the external environment, the operating temperature may be taken as the mean of the internal and external pipe surface temperatures.

For the usual case of turbulent flow of fluid inside the pipe, the inside surface temperature may be taken as the temperature of the fluid. The rate of heat transfer across the wall of a PVC pipe is low, and provided the exterior of the pipe is well ventilated, the external surface will be near ambient. Where heat transfer to or from the surrounding material is very slow, the external surface temperature will be near to that of the internal surface.

It may be necessary in critical cases to establish surface temperature characteristics by experiment. For the situation of a buried pipeline with flowing water, an appropriate ‘rule of thumb’ is:

Tm = 2Tw + Ts
               3
Tm = mean material temperature
Tw = water temperature
Ts = soil temperature

It should be noted that the pressure condition where flow is stopped should also be checked. In this event, water temperature and outside temperature will equalise.

Vinidex Installation Guide

Property Value Conditions and Remarks

Physical Properties

Molecular weight (resin) 140,000 cf: K57 PVC 70,000
Relative density 1.42‐1.48 cf: PE 0.93, GRP 1.4‐2.1, Cl 7.20, Clay 1.8‐2.6
Water absorption 0.12% 23ºC, 24 hours of: AC 18‐20% AS 1711
Hardness 80 Shore D Durometer, Brinell 15, Rockwell R 114, cf: HDPE 60
Impact strength – 20ºC 20 kJ/m² Charpy 250 μm notch tip radius
Impact strength – 0ºC 8 kJ/m² Charpy 250 μm notch tip radius
Coefficient of friction 0.4 PVC to PVC cf: PE 0.25, PA 0.3

Mechanical Properties

Ultimate tensile strength 52 MPa AS 1175 Tensometer at constant strain rate cf: PE 12‐20
Elongation at break 50‐80% AS 1175 Tensometer at constant strain rate cf: PE 500‐900
Short term creep rupture 44 MPa Constant load 1 hour value cf: PE 10‐16
Long term creep rupture 28 MPa Constant load extrapolated 50 year value cf: PE 6‐8
Elastic tensile modulus 3.0‐3.3 GPa 1% strain at 100 seconds cf: PE 0.6‐0.8
Elastic flexural modulus 2.7‐3.0 GPa 1% strain at 100 seconds cf: PE 0.5‐0.7
Long term creep modulus 0.9‐1.2 GPa Constant load extrapolated 50 year secant value cf: PE 0.1 Shear
modulus 1.0 GPa 1% strain at 100 seconds G = E/2/(1+μ) cf: PE 0.2
Bulk modulus 4.7 GPa 1% strain at 100 seconds K = E/3/(1‐2μ) cf: PE 2.0
Poissons ratio 0.4 Increases marginally with time under load cf: PE 0.15

Electrical Properties

Dielectric strength (breakdown) 14‐20 kV/mm Short term, 3mm specimen
Volume resistivity 2 x 10¹⁴ Ωm AS 1255.1
Surface resistivity 10¹³ – 10¹⁴ Ω AS 1255.1
Dielectric constant (permittivity) 3.9 (3.3) 50 Hz (106 Hz) AS 1255.4
Dissipation factor ( power factor) 0.01 (0.02) 50 Hz (106 Hz) AS 1255.4

Thermal Properties

Softening point 80‐84°C Vicat method AS 1462.5 (min. 75°C for pipes)
Max. continuous service temp. 60°C cf: PE 80, PP 110
Coefficient of thermal expansion 7 x 10⁵/K 7mm per 10m per 10°C cf: PE 18‐20 x 10¯⁵, Cl 1.2 x 10¯⁵
Thermal conductivity 0.16 W/(m.K) 0‐50°C
Specific heat 1,000 J/(kg.K) 0‐50°C
Thermal diffusivity 1.1 x 10¯² m²/s 0‐50°C

Abbreviations Conversions of Units

PE – Polyethylene Cl – Cast Iron 1 Mpa – 10 bar = 9.81kg/cm² – 145 lbf/in²
PP – Polypropyiene AC – Asbestos Cement 1 Joule – 4.186 calories=.948x10³ BTU=737ft.lbf
PA – Polyamide (nylon) GRP – Glass Reinforced Pipe 1 Kelvin – 1°C=1.8°F temperature differential