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ASME STP-PT-036-2010 — Bolted Flanged Connections in Elevated Temperature Service
🔥 ASME STP-PT-036-2010 is a technical research report issued under the sponsorship of ASME Standards Technology, LLC (ST-LLC). It provides a comprehensive treatment of the creep and relaxation behavior of bolted flanged connections in elevated-temperature service. The report traces the literature from the pioneering work of Baumann, Bailey, Tapsell, Johnson, and Kerkhof in the 1930s–1940s through modern studies, explains how elevated temperature degrades the mechanical behavior and sealing performance of flanged joints, and quantifies the time-dependent decay of bolt load using uniaxial relaxation tests, finite element models, and full-joint relaxation experiments. It closes with conclusions and recommendations relevant to both code development and engineering practice.
💡 The greatest threat to a flanged joint at high temperature is not strength failure but leakage: creep relaxation of the bolts combined with gasket creep steadily reduces the residual gasket seating stress, and once it falls below the minimum gasket stress required for sealing, the joint begins to leak. This is the central challenge of high-temperature flange design.
1. Overview: The Physical Mechanisms of High-Temperature Joint Behavior 📌
The report first establishes a physical model of high-temperature joint behavior through an extensive literature review:
- Direct effects of temperature on mechanical properties: as temperature rises, the allowable stresses and elastic moduli of flange rings, bolts, and gaskets decline; the strength of carbon steel pipe flanges drops markedly with temperature.
- Bolt stress relaxation: under essentially constant strain (the preload deformation), creep in the bolt material causes stress to decay with time — even without opening the joint, the preload “disappears by itself.”
- Gasket creep: gaskets, especially flexible ones, creep-thin in the thickness direction under the combined action of high temperature and compressive stress.
- Flange ring relaxation: creep deflection of the flange ring itself absorbs and releases part of the preload.
The historical body of work — including “relaxation diagrams” and “joint life diagrams” — established the analytical framework, while modern research relies heavily on finite element (FEA) time-dependent creep analysis of the bolt-flange-gasket system.
2. Creep Behavior and Finite Element Modeling 🔬
The report takes a formal approach to creep: defining a creep law and material properties, then using them in FEA:
- Creep law: a classical strain-hardening creep law whose parameters are calibrated from uniaxial relaxation tests.
- Model hierarchy: from the simplest bolt/cylinder model to realistic joints such as NPS 2 Class 900, NPS 3 Class 150/300, and NPS 6 Class 150, covering a range of sizes and rating classes.
- Time scale: analyses typically extend beyond 217 hours, tracking the evolution of creep strain and bolt stress with time.
| Study Dimension | Method / Object | Key Parameters and Observations |
|---|---|---|
| Uniaxial relaxation tests | Carbon steel, Cr-Mo steel specimens | Carbon steel Omega relaxation curves at 842°F, 932°F; Cr-Mo up to 1000°F |
| Finite element models | Bolt/cylinder; NPS 2 cl.900, NPS 3 cl.150/300, NPS 6 cl.150 | Bolt stress decay with time; creep strain distribution at 217 h |
| Bolt relaxation tests | Actual bolt assemblies | Measured decay of preload stress versus time |
| Joint relaxation tests | Complete flanged joints | Preload retention of the whole joint including the gasket |
⚠️ A common engineering mistake is to torque the joint once at ambient temperature and never retighten. Experiments show that in the 930–1000°F range, carbon-steel bolts can lose a substantial fraction of their preload within a few hundred hours, leading directly to leakage. High-temperature joint maintenance plans must account for relaxation.
3. Experimental Methods and Results 🧪
The report designs two classes of experiments — bolt relaxation tests and joint relaxation tests. In the former, a bolt is loaded to a prescribed preload stress and held at constant deformation while stress decay is measured; in the latter, a complete joint with gasket is preloaded and placed in a high-temperature environment as a whole, and the residual load is measured. The principal findings:
- Relaxation rate is strongly temperature-dependent: the higher the temperature and initial stress, the faster the relaxation.
- Joint relaxation results from the combined action of bolt material relaxation and gasket compressive creep; the gasket type has a strong influence.
- Code allowable stresses at elevated temperature (e.g., the ASME B31 and ASME B16.5 pressure-temperature rating systems) can be used to approximate creep/relaxation behavior, offering a path for simplified engineering calculations.
4. Conclusions, Recommendations, and Engineering Practice 🛠️
The report’s principal conclusion is that the reliability of a high-temperature flanged joint depends on systematic management of bolt load retention. Its recommendations translate into these practical points:
- 🔧 Material selection: prefer creep-resistant alloy bolting (e.g., Cr-Mo and high-temperature low-alloy steels) to lower relaxation rates.
- 📏 Preload control: use controlled tightening (torque, tensioner) and account for modulus differences at operating temperature; perform hot retorque after heat-up where required.
- 🔍 Retightening management: schedule periodic retightening inspections for critical high-temperature joints before gasket stress falls below the sealing threshold.
- 📋 Gasket matching: choose gaskets with stable high-temperature performance and low creep (e.g., spiral-wound and serrated composite gaskets), and size initial seating stress using joint relaxation data.
- 💻 Numerical verification: for critical joints, apply the FEA creep analysis approach of this report to verify the residual preload as design documentation.
✅ Although this is a technical report rather than a mandatory code, its testing methods and data provide a solid technical basis for provisions in ASME B16.5 appendices, ASME PCC-1, and high-temperature piping codes. In practice, it can be used as an “operator’s manual” for high-temperature flange joint design.
5. Frequently Asked Questions ❓
❓ Q: Is the saying “the joint will go on leaking until it settles” correct?
A: No. Leakage is the end-stage symptom of preload decay, not a self-healing process. Once gasket stress falls below the sealing threshold, leakage tends to continue and accelerate gasket damage; only retightening or gasket replacement fixes it.
A: No. Leakage is the end-stage symptom of preload decay, not a self-healing process. Once gasket stress falls below the sealing threshold, leakage tends to continue and accelerate gasket damage; only retightening or gasket replacement fixes it.
❓ Q: Which dominates — bolt relaxation or gasket creep?
A: It depends on temperature and gasket type. With rigid metallic gaskets, bolt creep relaxation dominates; with flexible gaskets (PTFE, elastomer), gasket creep may dominate. Tests or analysis for the actual joint combination are needed.
A: It depends on temperature and gasket type. With rigid metallic gaskets, bolt creep relaxation dominates; with flexible gaskets (PTFE, elastomer), gasket creep may dominate. Tests or analysis for the actual joint combination are needed.
❓ Q: Do B31 or B16.5 directly require retightening of high-temperature flanges?
A: The codes define how flange ratings decay with temperature and the material boundaries, but retightening intervals are normally set by the project based on risk assessment and data like this report, and then written into the operating and maintenance procedures.
A: The codes define how flange ratings decay with temperature and the material boundaries, but retightening intervals are normally set by the project based on risk assessment and data like this report, and then written into the operating and maintenance procedures.
❓ Q: What inputs does an FEA creep analysis need?
A: Creep-law parameters (e.g., strain-hardening constants), elastic and creep properties at the target temperature, gasket compression-recovery behavior, the tightening method and initial load, and the temperature-load history over the design life. Data quality governs result confidence.
A: Creep-law parameters (e.g., strain-hardening constants), elastic and creep properties at the target temperature, gasket compression-recovery behavior, the tightening method and initial load, and the temperature-load history over the design life. Data quality governs result confidence.
