BSI and ASME PWHT Requirements

Current design rules for the piping and pressure vessel industries, including BSI and ASME codes, mandate PWHT when component thickness exceeds certain values; such values typically depend on Charpy test parameters and minimum service temperature requirements. Unfortunately, however, such an approach is sometimes seen as overly conservative and there have been instances where discrepancies among various codes have arisen regarding their limiting thickness limits.

Welding Procedures

Welding, brazing and fusing are integral processes in the fabrication of critical components for industrial applications such as power generation and oil and gas exploration. Standard and regulations such as API 650 for storage tanks and ASME B31.3 for process piping require rigorous qualification and examination procedures in order to adhere to quality and safety regulations.

Post-weld heat treatment (PWHT), commonly referred to as stress relief, serves to alleviate and redistribute residual stresses introduced by welding. It typically involves heating parts of a fabrication to high temperatures for an agreed-upon time – usually one hour for every inch of thickness of material used in its construction.

PWHT is an essential element of welding and fusing procedures needed for the construction of pressure vessels, helping maintain integrity of weldments while assuring their long-term performance under operating conditions. Failure to abide by its requirements could result in poor mechanical properties in weld and base materials as well as stress corrosion cracking within heat-affected zones (HAZs). PQR preheating could reduce or even eliminate PWHT for certain pipes by properly controlling its temperatures during preheating and providing better control during testing procedures.

Requirements

PWHT rules in various piping and pressure vessel codes vary considerably; although sometimes differences are minor. For instance, low alloy steel PWHT temperature requirements differ among codes like BS EN 13445 [21], PD 5500 [25], and ASME VIII depending on limiting thickness and service conditions involved.

As noted earlier, the requirements for preheat and soak times may differ between codes; however, all provide minimum requirements that users should abide by in order to prevent trans-granular cracking in pressure vessels and piping components welded by users; this is an essential aspect of safety and reliability for which users must understand these specifications in full and ensure that they adhere strictly.

Exemptions

As previously discussed, nuclear utilities have found that reducing the number of postweld heat treated piping weldments can significantly lower both expenditures and outage times, so subscribers to EPRI Repair and Replacement Applications Program have requested that current requirements are reviewed to see if relaxation might be technically viable.

Limiting PWHT welding to weldments with wall thicknesses of 0.625 inch or less should be feasible, although exemption requirements can differ based on code or section, due to material grouping using Charpy energy absorption not being uniform across carbon steels. CEN/TS 15608 provides one exception that addresses this.

Add to that, the steel’s chemical composition matters greatly, while some codes require weld preheat to be used as part of their welding procedures – all these factors together make a consistent PWHT requirement impossible to attain for carbon steel welding processes.

Thermocouples

Thermocouples are temperature sensors made up of two metals connected at one end that produce a voltage proportional to their exposure temperature, making them essential tools in fire testing and assuring structural components meet safety standards. They must be securely attached by capacitor discharge (CD) welding or mechanically fixed with heat resistant putty and should also be insulated from direct heating elements to avoid overheating of their junctions.

Post weld heat treatment may be required on some components, and is generally performed using thermocouples located along the center line of welds, at edges of soak bands, heated bands and major changes in wall thickness, known as control zones. They are powered by an external panel containing digital recorder and power contactor with potentiometer controls for easy control.