Stess Relief Of Carbon Steel Caustic Piping

[NARL]

We are in the process of revising our plant standard for stress relieving carbon steel piping welds in caustic service. From NACE recommendations stress relief should be performed to minimize caustic SCC for concentrations of 5% and higher in operating temperatures of approximately 120°F and higher. However, they also make reference to the fact that localized concentration effects and other contaminants may cause caustic SCC at lower concentrations and temperatures. In addition to this, there is the practical aspect of how close the actual operation is run to design conditions, upsets do occur. Should stess relief be performed on all carbon steel piping in caustic service regardless of temperature and concentration, I would appreciate any comments on the criteria used to determine when stress relief is used (i.e. what concentration and temperature).

 

[aariahi]

Dear NARL:
We have a graph for caustic services which is a material selection guide for caustic services,this graph is drawn Temperature Versus Caustic Concentration:
Usually you have three Areas at this chart:
1)Use Carbon steel without stress relief in low temperature and low content of caustic(<180F, <5% Caustic)
2) you have to stress relieve C.S by standard which is told in ASTM, as example for A179( material of heat exchanger tubes) you have to heat 200C/hr until 660 C then after 2 hours at this T you should cool down with the same speed.( <200F, 10% Caustic )
3) more temp and conc. of caustic you should use Ni base alloys
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you can see this chart at:
1) Corrosion Engineering by:Fontana,Third ed. page:120
or
2)corrosion in the petrochemical industry, ASM,ed. by linda gaverick page:437
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if you need more details let me know.

sincerely yours
ariahi

 

[MJCronin]

NARL,

You may consider contacting one or more of the major caustic suppliers (PPG-Natrium for example) and try to find someone with production experience with carbon steel usage in caustic service.

I have seen carbon-steel piping systems and carbon-steel tanks in service (ambient temperature)that were conveying 40-50% caustic that were over 30 years old...... The piping still had almost it's original wall thickness... Schedule 80 piping is often used.......

Isn't thin-walled stainless piping cheaper than stress relieved carbon steel ?

Lots of good info at this link....

http://corporate.ppg.com/PPG/SBU/ChlorAlkali/default.htm

Good luck

MJC

 

[ThomasEun]

As mentioned above, the most popular guide is “Caustic Soda Service Graph” in Corrosion Data Survey, NACE.

Additionally some trouble shootings are very useful to understand caustic soda corrosion as following;

1. Storage tanks

(1) Concentration Point of View: when the injection pipe (with rich caustic) at tank inside is installed, the inside distributing pipe should be located at the center of stored fluid in order to minimize local concentration.
(2) Temperature Point of View: The actual temperature may be increased due to exothermic reaction when the caustic are charging in tanks. To be confirmed the actual exothermic temperature to use the caustic Soda Service Graph.

2. Piping
Normally the caustic lines are required (electric) heating coil or steam tracing to keep the fluid velocity/flux.
(1) When the insulation (especially between pipe and heating coil) is not appropriate, the pipes under the heating lines can be instantly corroded due to over heating.
(2) When the insulation (heating coil covering) is not appropriate, the caustic can be solidified with white scale, and plugged the pipe inside.

Good luck,

NACE Corrosion Specialist

 

[hiliter]

I was involved in the installation of a 2&quot; caustic line which was not stress relieved. It was traced with a pre-insulated &quot;Dekeron&quot; tube to prevent overheating in the line.

Within 1 year, numerous cracks appeared at the welded joints and the line had to be replaced with a stress relieved line.

From personal experience (20 + years) in the industry, every caustic line that I have seen installed has since been stress relieved.