A flue-gas overhead line runs from a third stage separator to a waste heat boiler. It carries gas at a maximum operating temperature of 740 °C upstream of the orifice chamber and is designed for 755 °C in the hot-walled sections. At those temperatures the hot-wall material, stainless steel A240 type 304H, is in its creep range, and the failure mechanism of concern is excessive pipe strain due to creep.
Long term strain results from long term sustained loads, so the code requires the sustained stress, the stress due to weight and internal pressure, to stay within the creep value for the material. That value is relatively low. For A240 type 304H the allowable stress is 137.9 MPa at 21 °C, and at elevated temperature it is 34.6 MPa at 670 °C, 31.5 MPa at 680 °C, 21.7 MPa at 730 °C, 18.8 MPa at 740 °C and 16.6 MPa at 755 °C, creep governing at each of them. The values correspond to those indicated in ASME B31.3 Appendix A.
The low material allowable stress imposes relatively strong requirements with respect to supporting. Simultaneously, the large thermal line expansion in combination with the thermal growth of the third stage separator requires pipe system flexibility.
The analysis covers a modified layout. In the revised arrangement the flow through the expander is blocked by the introduction of blindflanges and endcaps, and the duct is no longer connected to the expander. The analysis is based on the layout and support arrangement indicated on the issued isometrics for bypass operation.
Pipe stress model, load cases and support arrangement
The overhead line between the third stage separator and the waste heat boiler was split at the orifice chamber, which was anchored, and both parts were integrated in one model. A model of the third stage separator was included in the piping model for completeness. At the waste heat boiler side the line was anchored immediately upstream of the waste heat boiler.
Line flexibility is generated by hinged and gimbal type expansion joints. Nine are included in the section downstream of the third stage separator, six of gimbal type and the remaining three of hinged type. Their stiffnesses and weights were taken from the applicable manufacturer’s drawings, and the joint deflections were determined as an output of the analysis.
Eight load cases were considered. Case 1 combined the design temperature, 755 °C for the hot-walled sections and 343 °C for the cold-walled section at the separator, with design pressure of 4.5 Barg upstream of the orifice chamber and 0.5 Barg downstream. Case 2 applied the maximum operating temperature, 740 °C and 320 °C upstream of the orifice chamber and 730 °C downstream, and case 3 the minimum operating temperature, 680 °C and 240 °C upstream and 670 °C downstream, both at operating pressure of 2.5 Barg upstream and 0.3 Barg downstream. Cases 4 and 5 applied ambient temperature of 21 °C at design pressure and at operating pressure respectively. Cases 1 to 5 include pipe and insulation weight. Cases 6, 7 and 8 were the expansion cases, formed by differencing the displacement results: case 1 less case 4, case 2 less case 5, and case 3 less case 5.
For a hot-walled system operating at elevated temperature the main objective of the support arrangement is to keep both the sustained and the expansion stresses within allowable limits, and because the creep stress value above 700 °C for type 304H materials is relatively low, the arrangement must be designed particularly to accommodate the weight loads of the piping system. The results reported are the outcome of a number of iterative analyses of adjusted spring support arrangements for the section downstream of the separator, and the final arrangement is the one indicated on the isometrics. Due to the new layout, only one spring is required at position 04.
Stress results and the nozzle load boundary
The sustained stresses for both normal operating and design conditions are within the allowable limit. The maximum sustained stress during design conditions is 15.1 MPa against the allowable of 16.6 MPa at the design temperature of 755 °C, and the highest stress occurs at node 100 in the duct immediately adjacent to the third stage separator.
For thermal expansion the mechanism is a different one. Those stresses are self limiting, the failure mechanism of interest is low cycle fatigue, and the allowable stress range is much larger. The expansion joints provide sufficient flexibility to keep the secondary stresses within allowable limits. The secondary expansion stress range is 8.0 MPa against an allowable range of 191.6 MPa, far below the allowable limit.
On close examination of the material stress levels the analysis concluded that all material stresses are acceptable for at least 20 years of steady operation, and that the indicated support arrangement is fit for purpose and guarantees the structural integrity of the duct system for at least 20 years of sustained operation at working conditions. Restraint and hanger loads, the hanger table, stresses, nodal displacements for all load cases, deflected model shapes and summaries of the spring hangers and bellows deflections are presented in the appendices.
One boundary of the study should be noted. The waste heat boiler nozzle loads were not assessed. The arrangement with the anchor immediately upstream of the waste heat boiler nozzle always results in artificially high nozzle loads. It is assumed that this arrangement is not changed and has proven its fitness for purpose during operation in the past.
Design points for supporting hot-walled lines
Above 700 °C the creep stress value for type 304H is relatively low, and it is the weight of the pipe and its insulation that sets the requirement on the support arrangement.
- The support arrangement carries the design effort. The allowable stress range for thermal expansion is much larger than the sustained allowable, and the support arrangement must be designed particularly to accommodate the weight loads of the piping system.
- Hinged and gimbal type joints covered the movement. The flexibility required by the large thermal line expansion and the thermal growth of the third stage separator was provided by the nine joints in the section downstream of the separator, and the secondary stress range stayed far below its allowable.
- Stiffness came from the drawings and the spring arrangement was iterated. The expansion joint stiffnesses and weights were taken from the applicable manufacturer’s drawings, and the support arrangement was reached through a number of iterative analyses of adjusted spring arrangements rather than a single pass.
- A terminal anchor changes what can be concluded about nozzle loads. An anchor immediately upstream of a nozzle always results in artificially high nozzle loads, which is why the waste heat boiler nozzle loads sit outside this assessment and rest on the unchanged arrangement having proven its fitness for purpose in past operation.
