Killed Carbon Steel & Post Weld Heat Treatment

1. Purpose

This procedure describes when killed carbon steel and postweld heat treatments are required for process equipment and piping.

2. Guidelines

The Mechanical and Materials Engineering Skill Center uses the following general information for determining killed carbon steel and postweld heat treatment (PWHT) requirements for process equipment and piping. These guidelines are general; expect exceptions and additions (or deletions). Design Engineers shall follow these guidelines and consult with the Metallurgical Specialists as required.

2.1 Killed Carbon Steel

Specify killed carbon steel for all equipment and piping (where higher metallurgy is not required), except as follows:

a. Heat Exchanger Baffles

Specify carbon steel where higher metallurgy is not required.

b. Tower Trays

Specify carbon steel where higher metallurgy is not required.

2.2 Postweld Heat Treatment

Specify PWHT for killed carbon steel equipment regardless of Code requirements under the following environmental conditions:

a. Wet H2S

For equipment in H2S service where the process conditions meet the definition of wet H2S service in NACE Standard MR0103(3), paragraph 1.3.5.1. Specifically, these environments contain an aqueous water phase and one or more of the following:

(1) >50 ppmw total sulfide in the aqueous phase, or

(2) ≥1 ppmw total sulfide in the aqueous phase and pH<4, or

(3) ≥1 ppmw total sulfide and ≥20 ppmw free cyanide in the aqueous phase, and pH>7.6, or

(4) >0.3 kPa absolute (>0.05 psia) partial pressure H2S in the gas phase associated with the aqueous phase of a process.

b. Hydrofluoric Acid (HF)

For equipment and piping where the process stream contains any amount of HF. (This is not a requirement where seal or strength welding of heat exchanger tubes is required.)

c. Caustic (NaOH)

For equipment and piping, reference the attached curve "Caustic Soda Service Chart" to determine if PWHT is required. Use operating temperature and concentration of NaOH (weight per cent). Follow the same criteria for KOH.

If equipment and/or piping will be steamed out, specify PWHT regardless of operating temperature.

d. Amines

For equipment and piping in MEA, DEA or other amines at a concentration greater than 2.0 weight percent.

e. Carbonate

A risk of carbonate stress corrosion cracking (SCC) can exist in certain areas of FCC units running hydrotreated feeds with a feed sulfur level below 1000 ppm (0.1 wt%). In these cases, specify a “special” PWHT. The specifics of the “special” PWHT shall be given in the 801 specification for the project.