GFRP Hypochlorite Line: Hoop Stress vs. Environmental Derating
A DN200 filament-wound epoxy FRP pipe carries 12% sodium hypochlorite at 65°C and 8 bar. The inside diameter is 200 mm and the specified wall is 4.5 mm. The manufacturer's 50-year hydrostatic design basis (HDB) in water at 23°C is 70 MPa. The manufacturer lists a temperature service factor of 0.50 at 65°C and a chemical service factor of 0.50 for 12% hypochlorite. Company procedure applies an additional design factor of 0.50 to the HDB after service factors. A short-term burst test on a sample passed at over 25 bar, and the project team used that result to accept the line. After 18 months, the line weeped through a longitudinal crack in a joint region; the crack ran along the glass direction and the exposed fiber showed resin loss.
1. Calculate the hoop stress at 8 bar and the allowable long-term hoop stress. Is the specified wall adequate?
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Use the thin-wall hoop formula with pressure p = 8 bar = 0.8 MPa, D = 0.200 m, t = 0.0045 m:
sigma_h = pD/(2t) = 0.8 x 0.200/(2 x 0.0045) = 17.8 MPa.
Allowable = HDB x temperature factor x chemical factor x design factor = 70 x 0.50 x 0.50 x 0.50 = 8.75 MPa.
The applied hoop stress is about twice the allowable, so the 4.5 mm wall is not adequate. The required wall for the same allowable is:
t_min = pD/(2 sigma_allow) = 0.8 x 0.200/(2 x 8.75) = 0.0091 m = 9.1 mm.
A practical specification would be at least 10 mm, with an allowance for handling and joint details. The calculation uses nominal pressure; any surge or water hammer must be added.
2. Explain why the short-term burst test passed even though the line failed in service. What failure mode is involved?
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The burst test measures short-term, monotonic strength. The short-term hoop strength of the laminate is far above 17.8 MPa, so a sample can pass 25 bar without difficulty. Service failure is a time-dependent, environmental failure, not a burst failure.
The mechanism is environmental stress cracking or static fatigue of the FRP, often called strain corrosion. The resin is attacked by hypochlorite at 65°C, the glass-resin interface degrades, and slow crack growth occurs along the glass direction under sustained hoop stress. The longitudinal crack and exposed, resin-starved fiber are consistent with this. Joint regions are especially susceptible because of stress concentration, incomplete wet-out, and higher local strain.
The design must use the long-term HDB with temperature and chemical factors, not the short-term burst pressure. The burst test is useful only as a quality-control check.
3. Recommend a corrective design and the qualification testing you would require before restart.
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Corrective design: increase the wall to at least 10 mm, or change to a resin with a better hypochlorite rating such as a vinyl ester or a dual-laminate thermoplastic liner. Recalculate the allowable with the actual service factors and include surge pressure. Reduce the temperature if possible; at 65°C the temperature factor is already 0.50, and every 10°C reduction improves the margin. Redesign joints to reduce strain concentration and specify a qualified joint procedure.
Qualification testing: a long-term hydrostatic test in the actual hypochlorite at 65°C, not water at 23°C; at least 1000 h with extrapolation, or a standard HDB regression. Test the joint, not just the pipe body. Check resin cure and glass content on cutouts. After installation, conduct a hydrotest at 1.5 times design pressure and monitor for weeping. Do not accept the line on a short-term burst result alone.