+3 votes
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Postweld Heat Treatment of Equipment Handling Alkaline Solutions

asked Aug 13, 2014 in Material by sabdullayev (5,290 points)
The need for Postweld Heat Treatment (PWHT) and the prevention of alkaline stress corrosion cracking of all welded carbon steel equipment.

3 Answers

+3 votes
answered Aug 13, 2014 by sabdullayev (5,290 points) | selected Aug 15, 2014 by Normandy
 
Best answer

The following equipment for handling of Sodium Hydroxide (caustic solutions) should be postweld heat treated:

  • All equipment provided with heat tracing, with the exception of piping that is heat traced for winterization purposes using temperature controlled electrical tracing that will not cause the pipe metal temperature to increase beyond the limits shown in Figure 1, "Area A" of NACE RP0403 in the event of controller or tracing malfunction.
  • All equipment that contains heating coils with no high temperature alarm.
  • All equipment for handling solutions that have coincident metal temperature and concentration above the curve per Figure 1 of NACE RP0403.
  • All equipment in caustic service below the curve in Figure 1 of NACE RP0403 in the presence of sulfides.
+3 votes
answered Aug 13, 2014 by cchang219 (2,310 points)

The equipment that handle potassium carbonate (except Catacarb solution or FLEXSORB HP solvent, each of which contains vanadium inhibitor) having a concentration greater than 2% (by weight) should also be postweld heat treated as well if any of the below is true:

  • Metal temperature exceeds 200°F (93°C), or
  • Equipment that contains a heating coil not provided with high temperature alarms.

If the potassium carbonate solution is generated in situ from potassium hydroxide, then the equipment should be postweld heat treated as well.

+1 vote
answered Aug 14, 2014 by Tadd Pham (5,620 points)

Additional information on the PWHT of equipment in Alkaline solutions.

All equipment that handle amine solutions, such as MEA, MDEA, DEA and FLEXSORB amines, with concentrations greater than 2% should be postweld heat treated except the following:

  • Atmospheric storage tanks if metal temperature does not exceed 100°F (38°C), and
  • The tank does not contain a heating coil, or
  • The tank contains a heating coil and is provided with a high temperature alarm set at 100°F (38°C) or less.

Coker and cracker fractionation and light ends equipment handling ammonium carbonate/bicarbonate mixtures (excluding piping) should be postweld heat treated regardless of metal temperature.

All equipment that may be exposed to alkaline solutions as a result of operational upsets should be postweld heat treated as well.

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