The mounting and fixing devices for temperature sensors, such as thermowells, adapters, connecting flanges, and associated valves and fittings, collectively form the pressure boundary between the measurement point and the process system. Their pressure-containing capability is directly related to production safety and measurement reliability. This capability is not an isolated number but a systemic attribute influenced by a combination of material, design, manufacturing process, operating conditions, and installation quality. A deep analysis of these influencing factors is fundamental for correct selection and risk assessment.
Part I: Core Determinants of Pressure-Containing Capability
1. Mechanical Properties of Materials
Material is the cornerstone of pressure capability. Key parameters include:
Yield Strength and Tensile Strength: Determine the limit at which the material undergoes permanent deformation or fracture under pressure load. For temperature sensor accessories, common materials like 304/316 stainless steel, alloy steels, Hastelloy, etc., have strength grades that directly influence wall thickness design.
Creep and Stress Relaxation: Under long-term service at high temperatures, materials undergo slow plastic deformation under constant stress (creep), or bolt pre-load gradually diminishes (relaxation). This is critical for material selection (e.g., high chrome-nickel alloys) and life assessment for high-temperature applications.
Impact Toughness: The material's resistance to brittle fracture at low temperatures or under conditions of pressure shock (water hammer).
2. Geometry and Design
Wall Thickness: For pressure-containing tubular components (like thermowells), wall thickness is the primary factor resisting the hoop stress induced by internal pressure. Calculations typically follow formulas in standards like ASME B31.3 or GB/T 150, incorporating factors like corrosion allowance and thread weakening.
Stress Concentration: Areas of abrupt geometric change, such as thread roots, steps, small holes, and weld zones, experience localized stresses much higher than the average. Good design minimizes the stress concentration factor through smooth transitions and generous radii.
End Closure Form: Thermowells with flat, hemispherical, or conical ends have different stress distributions at the bottom. Hemispherical ends generally offer the best pressure capability.
3. Manufacturing Process and Quality Assurance
Forming Process: Seamlessly forged or extruded components offer more uniform material properties and higher pressure reliability compared to welded constructions.
Heat Treatment: Used to relieve cold work or welding residual stresses, or to achieve specific microstructures to optimize material properties.
Non-Destructive Testing: Performing RT, UT, or PT on critical pressure welds or the component body is essential to ensure the absence of fatal defects like cracks or lack of fusion.
Part II: Pressure-Containing Characteristics of Different Mounting Methods
1. Pressure Capability of Threaded Connections
Threaded connections are often one of the weaker links in the system's pressure boundary due to material removal and complex stress states.
Capability Limit: Generally suitable for low-to-medium and up to high-pressure ranges. Their pressure rating decreases as the thread size increases. A small-diameter (e.g., 1/2") high-quality alloy steel threaded fitting may withstand several hundred bar, while a large-diameter (e.g., 4") fitting will have a significantly lower rating.
Failure Modes: Primary modes include thread shear, stripping (thread pull-out), fatigue cracking initiated at the thread root due to stress concentration, and leakage due to sealing failure. For tapered threads, the sealing capability is part of the pressure limit.
Dependency: Highly dependent on correct installation. Insufficient torque leads to leakage and vibration loosening; excessive torque causes thread deformation, galling, or stress corrosion cracking. Thread sealant is mandatory for lubrication, aiding sealing, and preventing galling.
2. Pressure Capability of Flanged Connections
Flanged connections are the standard method for high-pressure, large-diameter connections. Their capability is defined by the complete "bolt-flange-gasket" system.
Capability Definition: Directly defined by the pressure rating (PN or Class) of the flange at a specific temperature, as found in standard tables (e.g., ASME B16.5 for a Class 300 SS flange at 100°C).
System Behavior: The capability depends not only on the flange body strength but crucially on the force balance between the flanges, gasket, and bolts under bolt pre-load. A flange pair with sufficient stiffness and compatible deflection, coupled with a properly compressed gasket, maintains sealing under operating pressure.
Key Control Point: Bolt Pre-load is paramount. Torque wrenches or hydraulic tensioners must be used to tighten bolts in a cross-pattern, step-by-step sequence to specified values, ensuring even gasket load and preventing flange distortion.
3. Pressure Capability of Welded Connections
A welded connection, ideally, can achieve pressure capability equal to that of the adjoining pipe or vessel parent material, making it the strongest connection form.
Potential: Theoretically, a full-penetration butt weld can have a design strength factor of 1.0, meaning it is as strong as the parent material. Thus, the upper limit is governed by the weaker of the joined materials.
Core Constraint: Its actual capability is entirely governed by weld quality. Any lack of fusion, slag inclusion, porosity, or crack in the weld acts as a stress concentrator and potential failure initiator. Therefore, Welding Procedure Qualification, certified welders, and necessary NDT are mandatory.
Application: Commonly used for directly welding thermowells onto high-pressure pipes or vessels, or for special equipment without flanges.
Part III: Pressure Capability Assessment and Selection Practice
Define Design Conditions: Establish maximum working pressure, maximum/minimum working temperature, media properties (corrosiveness, toxicity), and possible pressure cycling or shock.
Consult Authoritative Standards: Based on the project's applicable codes (e.g., ASME BPVC, GB/T 12224, HG/T 20592), look up the allowable working pressure for the corresponding material, size, and pressure class. Estimation is strictly prohibited.
Apply Safety Margin: Standards already incorporate safety factors. During selection, it is common practice to require the device's rating (e.g., flange Class) to be no lower than the system's maximum working pressure class, with some margin for abnormal conditions
Consider Fatigue and Vibration: For services with cyclic pressure changes or mechanical vibration, select connection types with better fatigue resistance (e.g., Weld Neck over Slip-On) or add supports to reduce vibrational loads.
Rely on Manufacturer Data: For non-standard or complex assemblies (e.g., integrated manifold mounts with valves), pressure test-verified data and technical specifications from the manufacturer are mandatory.
In conclusion, the pressure-containing capability of temperature sensor mounting devices is a subject requiring systematic engineering judgment. It demands that designers understand not only the performance of individual components but also the behavior of the entire pressure boundary system formed by different connection methods. Through rigorous adherence to standards, rational selection, and proper installation, we can ensure that this seemingly minor measurement point performs its duty safely and durably within harsh industrial environments.
