Materials For Sulfuric Acid Piping 2

[NARL]

we are in the process of installing new sulfuric acid piping in our water treatment facility, and I would appreciate any guidance on material selection for the following applications.

1. Pure sulfuric acid up to the dilution point. 66 be sulfuric acid @ ambient temp. We are planning to use carbon steel, A106 pipe, is this suitable, any special fabrication requirements (e.g. PWHT, etc..?

2. After the dilution point we are at a pH of 0.39. ie. 2 wt % sulfuric acid (66 be sulfuric acid) with the balance water at ambient temperature. Here we seem to have a choice between alloy 20 valves and pipe, or 316L stainless steel. 316L is more cost effective and seems to meet our requirements, is this a suitable choice? If so are there any special fabrication requirements (e.g. PWHT, etc...)?

 

[rustbuster1]

Your inclinations follow conventional practice.
CS is acceptable for intermittent service within temperature and flow limits, and generally works well at temperatures up to 40C and with flow rates up to 2 ft per second. The lines cannot allow ambient air in when they drain, so they must be padded with nitrogen or with dessicated (instrument-grade) air.
MTI guidelines recommend 304 or 316 threaded pipe for < 1 in. size and 304L or 316L welded pipe for 1 to 3 in. pipe. (There are similar NACE guidelines.)

The dilution zone is critical design location because of high temperatures genrated by the dilution exotherm.

Diluted acid (2- 4 %) can be conveyed in with Alloy 20. 904L and 317L. 316L is marginal, depending on the temperature. You should also consider non-metal piping for the dilute acid. CPVC and vinyl ester resin piping have very good resistance, but need proper support too.

 

[CdotS]

You could also consider aluminized steel. generally aluminized steel surfaces resist sulphuric acid very well and cost effective as well.

 

[MJCronin]

NARL

I would like to add:

- Consider schedule 80 (XS) Carbon Steel (A53 -GrB or A106 Gr B)piping for the concentrated acid... it is a small additional expense and will give you a long life even if some corrosion occurs. Minimize gaskets in the design

- Ensure that any plastic pipe does not see high temperatures ( over 120F).... it will sag, distort and buckle. Ensure that plastic pipe in dilute acid duty is protected from physical damage in the plant.


MJC

 

[GGH]

You might like to consider PTFE lined carbon steel pipe. It gives you the benefit of the excellent resistance to the dilute acid you get from plastic without the pipe support complications you get from all plastic pipe. The only down side is the number of flanges required as the lining lengths are comparitively short.