For a PFA (Perfluoroalkoxy) Sheath Heater Used to Maintain 180°C in a 70% Sulfuric Acid Solution Under 3 Bar Pressure, What Is the Maximum Allowable Wall Thickness to Prevent Creep Rupture Over 1 Year?

PFA (perfluoroalkoxy) has a creep rupture strength of approx. 2MPa for 1 year (8,760 hours) in 70% sulphuric acid at 180°C at 3 bar pressure. For a tube with an outer diameter of 20 mm and an internal pressure of 3 bar (0.3 MPa), the hoop stress is σ = P × D / (2 × t). The minimum wall thickness necessary to maintain the stress below 2 MPa is t = (P × D) / (2 × σ) = (0.3 × 20) / (2 × 2) = 1.5 mm. So, the maximum allowed wall thickness is not a limitation; bigger walls lower the stress. I think the question should have been "minimum wall thickness". For a 1 year life the minimum wall thickness is 1.5 mm. Creep rupture mechanism of PFA at 180°C At 180°C, PFA is above its glass transition temperature (160°C). The polymer is springy and shows time-dependent deformation (creep) under constant load. The creep rupture time obeys a power law, t_r=A sigma^(-n) where n~5 for PFA. This internal pressure causes hoop stress. If the wall is too thin the tension exceeds the creep rupture strength and the tube fails by bulging and exploding. Quantitative Minimum Wall Thickness for 1-Year Life at 180 degree C For PFA the following minimum wall thicknesses apply for 70 % H₂SO₄ at 180 °C and an internal pressure of 3 bar. Tube OD (mm) Hoop Stress at 1.0 mm wall (MPa) Minimum Wall for 1-Year Life (mm) Minimum Wall for 2-Year Life (mm) 16 2.4 1.2 1.4 20 3.0 1.5 1.8 25 3.75 1.9 2.3 32 4.8 2.4 2.9 Engineering Recommendation For a 20 mm OD tube of a PFA heater in 70 % H₂SO₄ at 180 °C and 3 bar pressure, specify a wall thickness of at least 1.5 mm to guarantee a 1-year life. If you need a longer life or larger diameters, raise the wall thickness proportionately. PFA is marginal at 180°C. Consider PEEK or metal sheaths. The engineer avoids creep rupture by picking out the right wall thickness.