ASME B31 3 PWHT Requirements

According to E2G’s findings from its brittle fracture assessments, reducing or eliminating mandatory PWHT may increase brittle cracking while diminishing its positive impacts on stress relief and increasing weld metal toughness.

ASME B31 3 establishes requirements for fabrication/welding of process piping. Table 331.1 lists exemptions from mandatory post weld heat treatment requirements in terms of material groups, control thickness and weld type.

Preheat

The 2014 edition of ASME B31.3, Process Piping, introduced several significant modifications in terms of fabrication/welding practices for process piping fabrication/welding practices. One key change was the inclusion of PWHT requirements based on pipe material type; Clause 331.1.1, along with Tables 331.1.1 and 331.1.2, provide this information; additionally there may be exclusions if preheating techniques are utilized appropriately.

Preheat temperatures should be carefully monitored with temperature-indicating crayons, thermocouple pyrometers, or other suitable instruments to ensure they reach and remain within the range specified in your Work Procedure Specification (WPS). A 25 mm (1 in.) buffer zone should extend at least beyond each edge of the weld for optimal results.

Once welding has been completed, the area should be heat treated according to Table 330.1.1. This treatment helps minimize or mitigate adverse effects caused by high temperature welding temperatures or temperature gradients and residual stresses caused by forming and bending, and helps increase weld metal toughness.

To qualify for the preheat exemption, P1 carbon steel material must be welded at a controlled thickness with a 95 degrees Celsius preheat. Otherwise, after welding is completed it would require post heat treatment which increases costs and delays commissioning of piping systems.

Post Weld Heat Treatment

Current design codes in the pressure vessel and piping industries stipulate that PWHT should be conducted if weld thickness exceeds a specified value, determined by Charpy test properties of material used. This approach provides an easy and direct means of identifying when PWHT is necessary; generally accepted by industry practices; however it’s important to remember that various codes have different limiting values that must be respected.

EPRI’s report (Ref 1) clearly documents how the power utility industry’s PWHT temperature requirements are far too high; these temperatures approach or even approach critical threshold temperatures of 1-1/4″ Cr Mo materials that could damage weld metal during inspection, repair, or maintenance activities and significantly extend outages during these activities.

PWHT requirements for general structural applications in bridges and buildings can also be overly conservative. While some structures have adequate fracture toughness to avoid the need for PWHT, large structural components or weld connections in thick sections typically need extensive PWHT with long hold times and slow heating rates; it would therefore be preferable if exemption from this process could be achieved wherever possible.

Strength Tests

Engineers constructing and operating pressure piping systems must understand all of the requirements set out by ASME B31 when doing so, such as leak testing procedures for internal lines and jackets; vacuum considerations; flanged joint assembly qualifications, ultrasonic examination acceptance criteria, as well as any additional specifications needed in terms of design, fabrication, erection inspection documentation of such systems.

Leak testing of piping systems is conducted to identify any points that require repairs. While hydrotesting requires testing fluid to reach above its design pressure, pneumatic tests instead take place at lower pressures that won’t result in explosive rupture of the pipe system.

For safe operation of pipework, it is crucial that the test pressure not surpass the material yield strength or its established rating at test temperature. This is particularly essential when using carbon steel, where its thickness may not cover all of its hoop or longitudinal stresses in components.

In a forced-flow steam generator, the piping system must be tested at 1.5 times the system design pressure – this requirement exceeds those set forth by boiler and pressure vessel code which mandate structural tests prior to assembly of all components.

Hydrotest

Hydrotesting, or fluid pressure resistance testing, is an essential element of quality assurance and must be conducted with great care and precision.

Equipment required for such pressure tests includes water pumps, calibrated pressure gauges, pressure chart recorders and shut-off valves. Furthermore, qualified personnel should conduct pressure tests.

Hydrotesting relies on an equation which takes several factors into account, including test temperature and allowable stress limit (calculated from yield strength at specific temperature). Hydrotest pressure should exceed design pressure, but care must be taken not to surpass allowable stress limit.

Hydrotesting is an integral component of ensuring the safety of any piping system, as it exposes defects that might otherwise go undetected through other nondestructive evaluation (NDE) techniques. Unfortunately, hydrotesting should not be seen as an exhaustive fitness-for-service test as there may still be issues that will not become evident through water testing – fine cracks and welds may remain hidden by flowing water; while larger internal defects like structural collapse will likely go undetected through hydrotesting alone.