PWHT Requirements for Pressure Equipment and Pipes

Post weld heat treatment (PWHT) is often required of pressure equipment and pipes in power plants, oil refineries, and petrochemical industries. PWHT helps mitigate residual stresses resulting from welding while increasing mechanical properties of welded steels.

Fabrication standards differ in their PWHT requirements; this article compares and contrasts them using fracture mechanics approaches, to compare and contrast limiting thickness requirements at which PWHT should be performed for welds in different codes.

Pressure Vessels

Pressure vessels can be highly hazardous devices that must be carefully constructed in order to withstand internal pressures and stresses without compromise to their structural integrity. Materials chosen must meet both strength and fabrication/weldability/machinability specifications for specific applications, among other factors. Temperature conditions must also influence wall thickness and reinforcement requirements, vessel dimensions, including dimensions for openings such as nozzles, container head shapes such as hemispherical, elliptical or flanged and dished to meet vessel uses, while selecting carefully in order to minimize strain on head or neck muscles.

Most countries mandate all vessels over a certain size and pressure be constructed according to an official code, such as the ASME Boiler and Pressure Vessel Code in the US or European Pressure Equipment Directive in Europe.

General requirements of most codes dictate PWHT for materials thicker than a specified value; however, this varies between codes; for instance, low alloy steel exemption from PWHT differs between U.S. and British codes as does its thickness limit for exemption from PWHT testing; these differences also can include different inherent Charpy test requirements or allowable defect sizes.

Pressure Piping

Pressure piping is used to transport liquids, gases and slurries at high pressure. While its primary use lies within the oil and gas industries, pressure piping also plays a vital role in power generation industries – sending steam, water and other fluids between power plants in order to ensure their operations run efficiently.

Pressure pipes can endure considerable stress. Their internal pressure fluctuates with changing operating conditions, forcing the pipe designer to account for various levels of pressure and require higher design safety factors, thicker components, and specialized materials in its design.

Flange and valve ratings must be increased to accommodate higher pressure levels, leading to an increased cost. Furthermore, pipes must have adequate support and tie-in points as this increases load on support structures – it is crucial that these structures can manage this weight without jeopardizing integrity of pipework.

Pressure pipe weld joints can generate high levels of residual stresses that approach yield strength, creating unsafe weld joint conditions and potentially endangering safety and environment. PWHT reduces these stresses to safe levels while improving weld integrity – this is especially useful when transporting hazardous chemicals or harmful vapors – helping prevent injuries while mitigating environmental damage.

General Structural

Dependent upon equipment and fabrication standards involved, steel and metal fabrication projects often necessitate mandatory PWHT requirements as part of their development, such as pressure vessels, piping, storage tanks, buildings, bridges or offshore structures. PWHT treatment usually is necessary in order to reduce residual stresses caused by welding as well as prevent hardening in the heat affected zone (HAZ).

PWHT (Post Weld Heat Treatment) is a controlled process where material that has been welded is heated back up to and held at its desired temperature for the specified amount of time. This critical operation must adhere to precise specifications; otherwise, distortion, temper embrittlement or oversoftening could occur from incorrect heating/holding temperatures.

As one example of PWHT technology, high-frequency coil currents generate enough heat to raise metal temperatures; this can be accomplished through careful control over number and length of coils as well as automatic recorder equipment that monitors temperature in real time.

ASME codes have detailed requirements for pipe wet heat loss temperature (PWHT), depending on its materials, in Clause 331.1.1 and Table 331.1.1 respectively. However, certain pipe thicknesses have specific exemptions within these tables that provide further insight.

Exemptions

Petrochemical industry and power generation users show an enormous interest in finding an exemption from PWHT where possible, but due to differences between requirements used by these steel users (through EEMUA and EGWP – Electricity Generators Welding Panel) and those of other industrial groups, no easy reconciliation exists between their requirements.

Assumptions suggest that the criteria used by various codes for exempting PWHT of chromium-molybdenum (P-4 and P-5A) weldments used in nuclear service from PWHT may have been developed using good historical engineering practice rather than specific design calculations or experimentation. Current requirements vary among Codes in terms of both minimum limiting weld thickness and pipe diameter limit requirements – although differences could also arise due to differences in toughness requirements between Codes or interpretation of similar technical data sources.

This article compares and contrasts exemptions from PWHT in several current codes, examining any similarities or discrepancies in terms of maximum weld thicknesses for as-welded conditions as well as toughness requirements (based on Charpy tests) as well as attempts at rationalisation. Part two will explore fracture mechanics assessments of steel toughness requirements that demonstrate that many thickness restrictions could be increased without significantly diminishing impact toughness, providing room for further liberalisation of PWHT requirements.