A refinery operator is replacing two existing steel main cooling water headers with Glass Reinforced Epoxy (GRE) piping. The two changes were raised as Technical Change Requests, and this case study covers the second of them, the “lube” cooling water header. The GRE consists of a header and a branch towards heat exchangers, and carries many small bore steel branch connections. The analysis supports recommendations made to enable NOBO approval, and was performed in full consideration of ISO 14692 for the GRE, ASME B31.3 for the connected steel and the project objectives set out in the documents provided.
The GRE laminate is not isotropic. Its moduli differ along and around the pipe, 10500 MPa axial against 20500 MPa hoop, its thermal expansion coefficient is 20×10⁻⁶ mm/mm/°C, and the same properties apply to plain pipe and fittings alike.
How the system was analysed
The system was modelled in CAESAR II, with ISO 14692 applied to the GRE and ASME B31.3 to the connected steel. The GRE is represented as Wavistrong EST 32 with the manufacturer’s laminate properties and stress intensity factors on fittings from ISO 14692, and its elbows as moulded elbows.
ISO 14692 does not check a single stress value against one allowable. Axial and circumferential stress are judged together against an allowable envelope, and the envelope that applies depends on the type of loading, through part factors of 0.67 for sustained, 0.83 for operational and 0.89 for occasional and hydrotest conditions.
Twenty load cases are used in the assessment. They cover the weight of the piping and its contents, the design, operating and hydrotest pressures of 10.6, 2.5 and 15.9 Barg, the design, operating and minimum design temperatures of 50, 23 and -20 °C against an installation temperature of 4 °C, and wind to NEN6702 from two directions. Some cases analyse material stresses against the applicable code, others evaluate loads on restraints and the compliance of nozzles and flanges, and one uses the design temperature with a virtual installation temperature of -16 °C for the analysis of loads on strain sensitive equipment.
What was changed
Where the original system was seen to be overstressed, modifications were made to bring the stresses within the allowable limits. They take the form of restraint type and location changes and additional laminate. At one location the routing of an expansion loop was modified in concert with the engineering contractor, a steel section there was replaced with GRE and the position of a valve was changed, the gain in flexibility being needed to ensure proper flange loadings. The minimum length required before a lateral restraint may be placed on small diameter branches is also indicated, and all modifications are marked up on the isometrics.
What the analysis found
The stresses conform to the allowables set by the standard in all eleven load cases for which stress results are reported. The highest is 92.9 per cent of allowable under maximum design conditions, with the sustained case at design pressure reaching 88.5 per cent and the hydrotest at 87.6 per cent, and the expansion stress range at 53.5 per cent.
Two nozzles on one heat exchanger carry loads above their allowables. The circumferential moment exceeds on both nozzles, the longitudinal moment on one of them, and the axial force on one when the GRE is left out of the calculation. Every load component is lower in magnitude with the GRE branch included than without it. The steel line has been present since the plant was installed, the GRE branch that connects the header to it is very flexible and so has little influence on it, and the expansion of the GRE header counteracts the expansion of the steel line. The system exists and operates satisfactorily and, on that basis together with these effects, the supporting and routing do not need to be altered according to ASME B31.3 section 319.4.1.
All GRE flanges are loaded within the capability specified by the pipe manufacturer. Four steel flanges are non-compliant according to the equivalent pressure method. That method is regarded as over-conservative for metal flanges, so the four were reassessed by an ASME VIII Division 1 Appendix 2 calculation. All four pass. The flanges are assessed for operation at design loads only, on the assumption that they will not fail during seating if installed according to ASME.
Because the continuation of the small bore steel branches is uncertain, the GRE part of these connections was covered by this analysis and the steel sections by the engineering contractor. The minimum needed unrestrained steel pipe length was provided, and the maximum header displacement to be accommodated is carried on the marked up isometrics. Stresses in these steel branches are within the allowable values, but certain small bore flanges show high loadings and may therefore be subject to leakage. High flange loadings there are mainly caused by high bending moments, and a bending moment can be transformed into a torsional moment by adding a loop in the plane perpendicular to the header displacement, loading the flange more favourably.
What transfers to other projects
Every nozzle load component is lower in magnitude with the GRE branch included than without it.
- A conservative screen is a filter, not a verdict. The equivalent pressure method is regarded as over-conservative for metal flanges, so a flange it flags is not necessarily non-compliant. A detailed ASME VIII Division 1 Appendix 2 calculation cleared all four cases here.
- Acceptance under ISO 14692 is a combined check. Axial and circumferential stress are judged together against an envelope whose part factor depends on the load type, so the load category matters alongside the magnitude.
- A flexible new line can reduce the loads on the existing one. At the interface between the new GRE and the existing steel, the branch has little influence on that line and the header expansion counteracts the steel expansion, so nozzle loads are better judged with and without the GRE than component by component. Where a scope boundary falls at such an interface, issuing the maximum displacement and the minimum unrestrained length lets each party design its portion against known conditions.
