Using Carpenter 20 in H2SO4 5

Are you sure you have the correct alloy designation? As I understand it, Carpenter 20 Alloy is nickel wire with 18% Mo added for use in high temperature thermocouples, and therefore not the same as the conventional Alloy 20 (Ni-Fe-Cr-Cu-Mo alloy). Carpenter has a range of alloys called 20Mo-4, 20Mo-6, etc. that are similar to Alloy 20.

Carpenter 20 is fine depending on the temperature. You always have to be cautuious about the 93 acid;it can be tricky the details are important.

Alloy 20 is no longer proprietary (the patent ran out) and now consists of a half dozen varieties, some okay, some not okay. with piping components it is generally okay either because it is a cast version or because of adequate corrosion allowance...

The Carpenter 20 would be used in a 6000 gallon storage tank. 96-98% H2SO4 is received by truck. Because the tank is vented to atmosphere, the moisture pickup drops the concentration to approximately 93% (+/- a few %) before the next load. A nitrogen pad to keep the moisture out is probably a good idea. Another issue is in winter months - cold temperatures increase the viscosity of the acid, which in turn cavitates the pumps. Plate coil heaters (with steam) were installed to keep things warm. Problem was it was too warm (no controls on the temp), which corroded the existing steel tank (don't know what grade of steel was used).

Consequently, a new storage tank is being installed. The material being considered by the plant is Carpenter 20. The new tank will be heated by hot/warm water (rather than steam) to keep the viscosity down and also have better temp controls to keep it below 110F.

Hacksaw - what details are you referring too with regards to 93% acid being "tricky" in Carpenter 20? Sorry, I am by no means an expert in alloys. Simple terms or just enough info to ask the right questions would be beneficial.

Thanks

ghopkins,

I suggest that you consider a cheaper, more "standard configuration" for your H2SO4 storage tank.

This would include:

- Carbon steel tank, (A36 or A515-70) built to ASME VIII but not stamped, at least 1/2 inch thick

- Desicant vent Drier to stop dillution

- No nitrogen pad or heater required

- Apropriate pump for cold weather service

(This assumes that you can tollerate some iron in the acid...this iron will be generated during the first fill only....the 6000 gallons is a good size, because it is about 1.5 truckloads)

I have seen horizontal, carbon steel tanks constructed in this manner last over 20 years....

(Oh, do not forget to tell the structural guy about the weight of the tank.......98% H2SO4 has a pretty high SG)

More info is available from OXYCHEM and from NACE on this stype of tank......

Oh yeah........could you send me the difference in the tank cost between the Carpenter 20 you proposed and the cheaper CS ???!!! ( small bills in a brown paper bag would be good)

mcronin@sdengineers.com

MJC

FYI, Carpenter 20 is a generic name for the alloy Carpenter calls 20Cb-3® Stainless, with the UNS designation N08020. 20Mo-4 and 20Mo-6 are further developments of this alloy. You can obtain quite a bit of information, including corrosion resistance, from their website:

click on Technical Information then Alloy Name and enter the above.

Ghopkins:

Your metallic options in rough order of increasing price / likely life cycle:

Carbon Steel
Carbon Steel w/ internal coating
304L
316L
Alloy 20 / 20cb3

Refer to the isocorrosion charts on pages 59-61 in the following document from the nickel development institute:
While CS would be the cheapest option in the metallics, history has shown that minor operational issues can quickly destroy the tank (faulty valves, loose flanges, idiots responsible for keeping the desicant fresh, etc.) If you do go with CS make sure that the nozzles are extra thick, penetrate into the tank, and are not exposed to excessive heat to minimize hydrogen grooving effects.
In most operational conditions the 316 is the most cost effective metallic solution.


Your non-metallic options if the concentrations are typically <96% in rough order of increasing price / likely life cycle are:

HDPE ( provides a 3 year warranty < 97%)
HDPE Dual laminate - you may know a good fabricator :-D the but difference in CTE / bonding issues must be taken into account.
CPVC Dual laminate
PP Dual laminate
ECTFE Dual laminate or
ECTFE Dual laminate
While the strait HDPE option makes me a little nervous (a little water could make some destructive heat), if the tank is in a low trafic area with a containment dyke, and located inside a building, it would be extremely cost effective. Sunlight and fittings are the primary limiting factors in HDPE tanks. A half step between HDPE and HDPE dual laminate would be an FRP jacket (not full encapsulation). I belive the freezing point of H2S04 to be around -20°F. If you design conditions are conservative, then you could get away from the heating which is a pain for either system, but possibly a deal killer for the dual laminates.

G

A correction in my previous e-mail. PP should have been PVDF, PP would not be acceptable! The last dual laminate should have been ETFE, not ECTFE.

Carpenter 20 tankage would be pricey to say the least, but it depends on what your design objectives are.

93 acid starts to be agressive for most alloys (temperature dependent).

freeze protection (acid) is the usual reason for heat, not viscosity control, and the heating requirements are far less stringent.

One other point to mention:
Velocity
Sulfuric acid will not attack carbon/stainless steel if the passivated layer on the surface of the metal is in tact. Velocity of the fluid will wash off that layer. The limit to the fluid velocity is dependant on the metal. Rules of thumb that I have seen are
CS - 1-3 ft/s (I have not used CS, so I can't explain the rational of the minimum 1 ft/s)

304l - 0-6ft/s
316l - 0-8ft/s
Alloy 20 - 0-20ft/s
Teflon lined - 0-50+ ft/s

The 93% sulfuric system I have delt with is mostly 316l, including the storage tanks and piping. We use alloy 20 in areas with localized high velocity like on the discharge of pumps, tanks, and control valves. Teflon lined pipe is also used frequently in these areas.

On a tank this small I would not try to save a nickle or two by buying carbon steel. This material works, but leaves you little room for error. A little water or velocity and you are buying another tank. Go with 316L and be done with it.