For the analysis, nonlinear static structural was selected. The penalty contact with higher stiffness was used for all the contacts. Steel was selected as a default material for the full assembly. In order to preset a pressure limit for the valve, the top spring step was moved downward by 0.15 m (1.5 cm) restricting the closed disk of the valve to open only when the pressure from below increases to ~0.18 MPa (~180 kPa). Two pressure boundary conditions were applied; one on the disk from below and other on the internal surfaces of the body after the disk is opened. First two figures below show the displacement plot of the valve in open state. Vectors in second figure show moving directions of the spring and spindle.

The further three figures below show the stress plots; the first two figures show the stresses in spring and body in opened valve state whereas the third figure shows the stresses in body after the valve was closed. It is worth noting that high stress areas in the body are due to thinner wall in the middle.

The graph below shows the Cauchy stresses in spring plotted over time.

Finally, the figures below show the animation of the final result.

HOW DOES SPRING RATE EFFECT THE LOAD VARIATION OF THE SPRING IN STRESS ANALYSIS?

⬇️ Because the deflection of the pipe system is predicated on its operating conditions, the difference in load between our starting condition and our operating condition is dependent upon the spring rate. A higher spring rate would produce a higher difference in load and a lower spring rate would produce a smaller difference in load. The stress analysis would be utilized to confirm which, if any, of those load differences are acceptable.

How to model a trunnion in Caesar II?

Step 1 :Model a rigid element from center of the pipe to the Surface of the pipe with Line Propeties.
Step 2: Model a trunnion with ambient temperature and without Fluid density and pressure.
Step 3: Apply SIF as per the attachment.

⬇️ ASME B31.3 Weld Joint Strength Reduction Factors

Weld joint strength reduction factors were added to ASME B31.3 in the 2004 edition. These apply at temperatures above 510°C (950°F), and are based on consideration of the effects of creep. They apply to longitudinal and spiral weld joints in pressure design, and to circumferential weld joints in evaluation of stresses due to sustained loads, So They were added because weldment creep rupture strength has been determined to be lower than base metal creep rupture strength in some circumstances. The designer may determine the weld joint strength reduction factor for the specified weldments based on creep rupture test data. This is encouraged to develop factors specific to the base material/weld material combinations used in the design. However, a simplified factor was provided for use by the designer, in the absence of more applicable data. Because it is impractical at this time to establish factors for specific materials, a general factor was used. 

The factor varies linearly from 1.0 at 510°C (950°F) to 0.5 at 815°C (1500°F).

The Designer can use other factors, based on creep tests. The tests should be full thickness cross-weld specimens with test durations of at least 1000 hours. Full thickness tests are required unless the Designer otherwise considers effects such as stress redistribution across the weld.

The factor is applied to the allowable stress used when calculating the required thickness for internal pressure and when evaluating longitudinal stresses due to sustained loads.

The factor is not included when evaluating occasional loads because of their short durations. A reduction of short term allowable stress based on long term creep strength is not appropriate or required.

The weld joint strength reduction factor is not applied to the allowable stress range for displacement stresses, Sa, because these stresses are not sustained. The displacement stresses relax over time. The allowable stress criteria for displacement stress range is designed so that the piping system will self-spring so that the highest level of displacement stresses only occurs at the hot condition once over the lifetime of the piping system.

Weld joint strength reduction factors, W, for weldments at elevated temperatures were introduced as para. 302.3.4 (e) in the 2004 edition of ASME B31.3. They were added because weldment creep rupture strength had been determined to be lower than the base metal creep rupture strength in some circumstances. Background on how those factors were developed is provided in Insert 3.2. A general factor was used for all materials, as its developer, this author, did not have sufficient data to propose material specific factors covering the wide range of materials permitted in elevated temperature service in ASME B31.3. However, the factor was introduced to address a known issue and safety concern, with the expectation that further work would lead to improvements.

Following ASME B31.3, the Power Piping Section Committee (B31.1) and Subcommittee on Power Boilers (Section I) began work to include weld joint strength reduction factors into their respective codes. In order to develop consistent requirements in the ASME Codes, including ASME B31.3, a project team was formed to develop consistent rules for all three codes. The starting point was the rules in ASME B31.3; they were then significantly enhanced to provide requirements that were more material specific.

The factor is used when calculating the required thickness of longitudinal and spiral welded pipe and fittings in pressure design. In the 2004 and 2006 edition, it was also used in evaluating longitudinal stress due to sustained loads at girth weld locations. In the 2008 edition, this was changed from a requirement, to stating that application of weld joint strength reduction factors to girth welds is the responsibility of the designer. This change was made as a compromise to avoid inconsistency between ASME B3I.3 and ASME B31.1; there was not general agreement that it should be required for girth welds.

The factor does not apply to the following conditions.

It is not used to reduce the allowable displacement stress range, SA, because these stresses are not sustained. The displacement stresses relax over time.

It is not used for evaluating stresses due to occasional loads, as such loads have short durations.

It is not used when considering the allowable stress for permissible variations, as provided in para. 302.2.4, as such loads have short durations.