PWHT Procedure ASME B31.3

Asme B31.3’s PWHT procedure covers fabrication/welding of process piping used in fluid service systems such as oil refineries, chemicals manufacturing plants, pharmaceutical laboratories, hydrogen plants, paper pulp mills and power generation plants.

EPRI has recommended that the 2014 revisions to B31.1 and B31.3 lower the PWHT temperature range for carbon steel in fluid service applications rated Category A or M fluid service; this report supports this change.

Temperature

PWHT may be necessary in some welded steel assemblies depending on their operating envelop and exposure to environmental assisted cracking (EAC). Low carbon or lower alloy steels tend to be particularly susceptible to EAC and therefore require PWHT treatments in order to keep cracks at bay.

Welding processes generate residual stresses in welding regions that, under certain conditions, may approach yield strength of material, leaving assemblies vulnerable to brittle failure. PWHT significantly decreases these residual stresses and minimizes environmental assisted cracking risk.

B31.1 and B31.3 both impose PWHT requirements on various steel components welded together, such as C-Mn and Cr-Mo alloy steels, with 19mm as their maximum weld thickness limit for these welds in B31.1/B31.3 respectively; some general structural codes, such as BS 2633 allow significantly thicker weld thicknesses without needing PWHT. These requirements take into account fracture resistance requirements of different structures.

Testing on P No 4 materials has demonstrated that most of the impact properties changed after PWHT can occur before reaching 1300-1375 degF, meaning an increase in margin could be gained if temperatures for B31.1 and B31.3 PWHT could be decreased further. This article seeks to support such an effort.

Equipment

Post-weld heat treatment can have a dramatic impact on brittle fracture failure risk of welded components, making post-weld heat treatments essential for process piping systems where welding operations often take place on site. ASME B31.3 now permits exemptions from its mandatory PWHT requirement for carbon steels that belong to P-No 1 material groups if a preheating temperature of 95degC is applied prior to welding. An examination was undertaken of the effectiveness of exemption curves by reviewing their technical and historical foundation, and comparing these figures with more contemporary fracture toughness requirements established through elastic-plastic fracture mechanics and master curve methods.

Preheating is typically used to reduce residual stress in the weld zone and thus lower environmental assisted cracking risks, but its application cannot always be found appropriate due to potential thermal distortion that compromises strength and toughness of welds. Therefore, it is vital that proper planning be conducted when using weld preheat.

PetroSync’s ASME B31.3 training course provides an in-depth view of the code requirements for designing piping systems, branch connections, selecting flanges and fittings, considering flexibility requirements, fabrication, welding, non-destructive examination and pressure testing as well as strengthening pressure equipment with composite materials.

Personnel

PWHT requires skilled PWHT personnel. Equipment must be adequately protected from rain and wind; welds under PWHT must be adequately cordoned off with danger signs warning against unknown persons entering the area and becoming exposed to high voltage electrical connections; rubber gloves and safety shoes should also be worn by people working on electrical connections to prevent contact with hot parts and burns from happening accidentally. PWHT may be required on certain carbon steels in order to reduce welding residual stress before entering service, making PWHT an obligatory procedure

Sicherheit

Post Weld Heat Treatment (PWHT) of carbon steel weldments serves to minimise or alleviate any harmful temperature gradients and residual stresses which may develop within weldments, and in particular power and process piping systems. PWHT requirements in these fields vary between codes; toughness requirements also differ according to type. In this article, we investigate current requirements for PWHT in various code sections (B31.1 & B313) while considering whether any rationalisation could be possible.

This paper considers the implications of reducing the maximum PWHT temperature for P No 4 materials susceptible to overheating, specifically according to EPRI Report’s recommended maximum temperature of 1200-1300 degF which is lower than existing temperatures in Power and Process Piping Codes. Furthermore, authors propose an innovative weldment toughness master curve which could support changes to exemption curves across code books.

An ongoing concern in piping and pressure vessel design is the risk of brittle fracture failures, so making the PWHT process as efficient as possible is of utmost importance to minimize this risk. One approach would be using low preheat temperatures with multi-pass welding processes – particularly important if thick walled pipes will be subjected to harsh service conditions.