Two existing steel cooling water headers were to be replaced with Glass Reinforced Epoxy (GRE) piping under two Technical Change Requests. This case study covers the first request, for the main cooling water header “boiler”. The second, covers the “lube” header. A flexibility analysis was required, together with recommendations, to enable NOBO approval.
The replacement is not GRE throughout. The GRE line runs to a heat exchanger, while the branches and the line close to that exchanger are steel, and the isometrics show the system connecting to two heat exchangers. Where the line passes below road crossings it runs inside mantel pipes, which protect it from traffic loads.
Across the considered isometrics the design pressure is 10.6 barg and the operating pressure 3.5 barg, with a hydrostatic pressure of 15.2 barg. The design temperature range is -20°C to 50°C, the operating temperature 23°C and the installation temperature 4°C. The fluid is water, at a density of 1000 kg/m³.
One constraint was set by the equipment. The allowable nozzle loads provided for equipment 5015-C Nozzle T1 are low: 1149 N for P, 173.4 Nm for MC and 224.4 Nm for ML.
Flexibility analysis in CAESAR II to ISO 14692 and ASME B31.3
The system was modelled in CAESAR II and analysed for sustained, operational, occasional and hydrostatic loads, with GRE assessed to ISO 14692 and steel to ASME B31.3. Certain isometrics are included for continuation reference only; these are steel pipes of which the stresses are not assessed.
Material properties for the GRE pipe and fittings come from the manufacturer’s system specification for Wavistrong EST 32. Allowable stresses at 65°C are SHS(2:1) 250 MPa, SAS(0:1) 65 MPa, σqs 125 MPa and SAL(0:1) 32.5 MPa, and the same values apply across pipe pressure classes and sizes. From these an allowable stress envelope was built, using part factors of 0.67 for sustained, 0.83 for operational and 0.89 for occasional cases.
Sixteen load cases were run. They combine the weight of the pipe and its contents, water-filled weight, design and operating pressure, hydrotest pressure, design and operating temperature, and wind. One case takes the design temperature together with the difference between the cold and hot installation temperatures of -16°C and 4°C, and is used for loads on sensitive equipment. Another repeats the maximum design case without friction.
The occasional cases are split by purpose. ISO 14692 does not require secondary loads from temperature and displacement to be included in the occasional cases, so the four used for the stress assessment carry weight, pressure and wind only. Four further cases add the design temperature and are used for the restraint assessment, because they produce the maximum restraint loads. Wind is applied to all piping as an occasional load in combination with both sustained and operational loads. Two wind loads are defined, one from the West (-X) and one from the North (-Z), each applied with both a positive and a negative sign. The wind profile follows General Specification 74-14-00-004, taken from NEN6702 (Gebied II, Onbebouwd) with a shape factor Ct of 0.7.
Flange loads were checked with the flange check feature in CAESAR II. Axial force and bending moment are converted into an equivalent pressure and added to the line pressure, then compared with an allowable pressure rating equal to twice the pipe pressure rating times the part factor for the load case type.
Where the arrangement on the isometrics as provided was seen to be overstressed, modifications were made to bring the stresses within the allowable limits. These are changes of restraint type and location, recorded on the marked-up isometrics. At 5015-C Nozzle T1 the supporting of the connecting pipe was modified in order to bring the loads down to acceptable values.
Below the road crossings the pipe is carried inside a mantel pipe. The manufacturer’s example shows the pipe resting on spacers inside a casing below the road surface, with a shrinking sleeve at one end and a link-seal at the other. The metal spacer ring is not to be in direct contact with the GRE pipe: a sheet of neoprene or similar material between the two protects the pipe against excessive wear. The isometrics give the number of spacer rings required.
What the analysis found
Stress results are reported for nine of the sixteen load cases, as a percentage of the ISO 14692 allowable for that load case type, where the limit is 100%.
The highest value is 83.4%, at node 5610, in an occasional case with wind in the +X direction. Next is 83.2% at node 515 under maximum design conditions, and the same case without friction gives 79.2% at node 2965. The six remaining reported cases range from 37.7% under weight alone to 73.0% in the sustained case with design pressure, with the hydrotest case at 65.1%. Where necessary the isometrics were modified to ensure that all the stresses conform with the allowables set by the standard.
All GRE flanges are loaded within their capability as specified by the pipe manufacturer.
Nozzle loads are given as three components, P, MC and ML, in the orientations shown below. At 5003-C Nozzle N4 the calculated values are 3505 N (L14), 2527 Nm (L4) and -621 Nm (L3), against allowable loads of 9000 N, 3600 Nm and 4680 Nm. At 5015-C Nozzle T1 they are 149 N (L13), 106 Nm (L3) and -47 Nm (L13), against 1149 N, 173.4 Nm and 224.4 Nm. Every component in every load case is below the allowable. Each is taken from the load case producing its worst value, which makes the assessment conservative.
Axial and hoop allowables, restraint modifications and equipment nozzle limits in GRE systems
Acceptance is determined by a combined axial and hoop check against an envelope constructed from the allowable stresses, and the same allowable stresses apply across pipe pressure classes and sizes.
- GRE behaves differently along the pipe and around it. The axial modulus of 10500 MPa is below the hoop modulus of 20500 MPa, and axial and hoop allowables are quoted separately, SAS(0:1) 65 MPa and SAL(0:1) 32.5 MPa axial against SHS(2:1) 250 MPa hoop. The envelope is constructed with the part factor for the load case type, and the figure below shows a separate bound for the sustained, operational and hydrotest cases.
- The changes were made in the restraints. Where stresses had to be brought within the allowable limits, the modifications were to restraint type and location, and they are recorded on the marked-up isometrics.
- Splitting the occasional cases by purpose keeps each check consistent with the code. The stress cases carry weight, pressure and wind only, as ISO 14692 permits, while the restraint cases add the design temperature and produce the maximum restraint loads. The highest stresses occurred in an occasional wind case and in the maximum design case.
- Equipment nozzle limits can decide the supporting. The allowable loads at 5015-C Nozzle T1 are low, and they were met by modifying the supporting of the connecting pipe, although all the reported pipe stresses are within the allowables.
