A line in the creep range, where the weight loads set the support design
The line studied here is the revised fluegas overhead line between the third stage separator and the superheater. It operates at 725 °C in the hot walled sections and 200 °C in the cold walled section, with design temperatures of 750 °C and 343 °C respectively.
At 725 °C the hot wall material is in the creep range, so the allowable stresses for primary sustained loads are low. The failure mechanism of concern is excessive pipe strain due to creep, and long term strain is the result of long term sustained loads. The code therefore requires the sustained stress, being the stress due to weight and internal pressure, to stay within the creep value for the applicable material, so the support arrangement must be designed particularly to accommodate the weight loads of the piping system. At the same time the large thermal line expansion, in combination with the thermal growth of the third stage separator, requires pipe system flexibility.
The allowable stresses used correspond to the values indicated in ASME B31.3 Appendix A. For the hot wall material, Stainless Steel type 304H, the allowable stress is 137.9 MPa at 21 °C, 21.4 MPa at 725 °C and 17.3 MPa at 750 °C, creep setting the allowable at both elevated temperatures. Above 700 °C the creep stress value for type 304H materials is relatively low. The second material recorded is Carbon Steel A672 C65, with allowable stresses of 149.6 MPa at 21 °C and at 200 °C, and 138.3 MPa at 343 °C.
Two models, six load cases and the final support arrangement
The overhead line between the third stage separator and the CO boiler was split at the superheater and represented by two separate models. The upstream model covers the section between the third stage separator and the superheater, which is the section reported here; the downstream model covers the section between the superheater and the CO boiler. A model of the third stage separator was included in the piping model for completeness, as were the 30% bypass line and the fourth stage overflow line. The layout and support arrangement analysed are those indicated on the three isometrics listed in the report.
Six load cases were considered. Case 1 combines working temperature (725 °C for the hot walled sections and 200 °C for the cold walled section), working pressure (1.57 Barg upstream of the orifice chamber and 0.15 Barg downstream of the orifice chamber) and pipe plus insulation weight. Case 2 applies design temperature (750 °C / 343 °C) and design pressure (4.5 Barg) with pipe plus insulation weight. Cases 3 and 4 are both at ambient temperature (21 °C) with pipe plus insulation weight, at working pressure and at design pressure respectively. Cases 5 and 6 are the expansion cases, formed by subtracting the case 3 displacements from the case 1 displacements, and the case 4 displacements from the case 2 displacements.
The load case definitions quote the values for the hot walled and cold walled sections and either side of the orifice chamber, but temperature and pressure are not applied uniformly along the line. The operating temperature plot carries levels of 0, 21, 200, 343, 400, 600 and 725 °C, and the operating pressure plot levels of 0, 15, 30, 157 and 483 kPa.
For hot walled systems at elevated temperature the main objective of the support arrangement is to keep both the sustained and the expansion stresses within allowable limits. Line flexibility is generated by means of hinged and gimbal type expansion joints, identified on the model plot as XJ-1 and NXJ-101 to NXJ-106. The allowable stress range for thermal expansion is much larger than the sustained allowable, because the stresses resulting from thermal expansion are self limiting, and the failure mechanism of interest for them is low cycle fatigue. The expansion joints provide sufficient flexibility to keep the secondary stresses within allowable limits.
The support arrangement of the section downstream of the third stage separator was reviewed and adjusted in iterations, and the results reported are for the final arrangement. The spring hanger arrangement includes a new hanger at an expansion joint, recorded on the model plot as a measure to reduce the superheater nozzle load.
Sustained stress at the bypass bend, nozzle loads above the basic allowable levels, and an arrangement fit for purpose
The highest sustained stress in the section upstream of the superheater occurs at node 9190, at the bend in the 30% bypass line. For normal operating conditions, load case 1 at 725 °C, it is 18.6 MPa, which is lower than the allowable stress at 725 °C of 21.4 MPa. Strictly in accordance with ASME B31.3, however, the allowable is imposed by the highest temperature, being the design temperature of 750 °C, at which the allowable stress is 17.3 MPa, and against that value there is a stress excess of 8%. For the design conditions, load case 2, the highest stress is 23.4 MPa and again occurs at node 9190 in the 30% bypass line, the increase being the result of the higher design pressure.
Nozzle loads. The superheater nozzle loads are in excess of the basic set of allowable load levels, and the loads are to be submitted to the superheater supplier. The moment loads are substantial and are the result of the lateral and the bending stiffness of the untied universal bellow in combination with the nozzle displacements resulting from the thermal expansion of the superheater. The nozzle load summary reported in the appendices includes the pressure thrust, which in the present case acts at the superheater body rather than at the superheater nozzle.
Other results. Wall thickness, temperature and pressure distributions, nozzle displacements, expansion joint and spring hanger arrangements, deformed shapes, restraint loads, hanger results and the nozzle load summaries are reported in the appendices.
The analysis results demonstrate the fitness for purpose of the support arrangement for the overhead line between the third stage separator and the superheater. The indicated support arrangements guarantee the structural integrity of the duct system for at least ten years of sustained operation at working conditions.
What the work shows for a line assessed at its design temperature
The design conditions are the envelope of the maximum occurring operating conditions, not the situation for ten years of sustained operation. Applying the creep allowable for the design temperature to a ten year service assessment puts an artificial constraint on the result.
- The realistic allowable is the one for the temperature the line runs at. Against the long term allowable for operating conditions at 725 °C, and upon close examination of the material stress levels, the material stress in the bend in the 30% bypass line is relatively high but still acceptable for at least ten years of steady operation.
- In the creep range the weight loads set the support design. The sustained allowable falls from 137.9 MPa at 21 °C to 21.4 MPa at 725 °C, so the support arrangement has to be designed particularly to carry the weight of the pipe and its insulation, while the expansion joints provide the flexibility.
- A stress excess and a fitness judgement are separate statements. The 8% excess against the design temperature allowable and the acceptance of the same stress for sustained operation at 725 °C are both true, and the reasoning connecting them should be documented rather than left implicit.
