connversion of caustic tank to H2SO4 storage. 1

[Jasmine2000]

I am working on converting an existing caustic tank to 98% sulfuric acid. The tank is made of A-36 and is atmospheric.
I believe moisture ingression into the tank would dilute the concentration, consequently increasing the corrosion rate.for this reason, I am planning to blanket the tank with N2. Does anyone know what steps I need to take to prepare the existing tank for H2SO4 storage? this tank is insulated and vents to ATM. THANK YOU

 

[HEC]

Two issues. 98% H2SO4 is more dense than NaOH, so the hoop stresses in the lower portion of the tank will increase, hence the volume stored will need ot be reduced. Additionally the corrosion allowance would increase for H2SO4. Check the current wall thickness and compare against the design requirements for corosive storage code. This will indicate the limitations.

Regards

Mark Hutton

 

[ash9144]

Your decontamination procedure needs to be very detailed and good. Any caustic left behind is going to react to form a pool of weak sulfuric acid, which could eat through your tank with surprising quickness.
There is no need for nitrogen on a fresh acid tank. Just put a gooseneck on the atmospheric vent to prevent rain ingress. CS is common for sulfuric storage.

As HEC mentions you need to check all of thicknesses and design conditions to ensure it will hold up.

 

[Jasmine2000]

Is heating required?

 

[Jasmine2000]

Dear ash9144

Most of the sufuric acid storage tanks have N2 blanketing why there is no need for 98% H2SO4?

 

[ash9144]

The nitrogen is not necessary just because air ingress didn't seem to be a problem for fresh acid tanks.
That was at a sulfuric acid plant. If your standards call for nitrogen by all means incorporate it, just watch where it vents to so you do not create an oxygen deficient environment where someone would be working.

 

[HEC]

N2 is real nasty, oxygen depletion is a hazard, but extreme levels of N2 cause the victem to stop breathing! This is more of an issue wiht N2 venting.

Mark Hutton

 

[gr2vessels]

jasmine,
The concentrated sulphuric acid is hygroscopic, hence will absorb the moisture from the air the tank breathed in and dilute the top layer of the acid. The top of liquid contact line with the carbon steel tank will show a rapid corrosion, subject of the rate of fresh air intake, external humidity/available moisture and temperature. The N2 blanket will displace the fresh air and limit/exclude the moisture intake, thus preventing or limiting the corrosion. The top vent should be fitted with a pressure relief valve with the outlet piped to a scrubber if possible or other means of emission control from the acid tank. Obviously, an appropriate N2 blanketting system...Apart of that, some pre-cleaning, the rest you seem to have it under control, follow the above advices for tank wall thickness check (API 579?). Perhaps you should check with your local regulator for the bunding requirements and to have a disaster management plan for the tank burst event...

 

[JoeTank]

Knowledgeable and expereinced H2SO4 tank owners have generally found that the typical API-650 storage tank is not suitable for use. Most owner's specs now refer to the NACE standard for sulfuric acid tanks. You will note obvious differences such as butt welded bottom joints and significant corrosion allowance. Not mentioned by NACE (but is well known) is that the shell nozzle welding details are often modified to be more suitable for the acid service. Watch out for flow rates into and out of the tank (normally limited to 2 or 3 fps. All mill scale must be removed from the plate interior surfaces for acid service. If the inlet is thru the roof, do not place it near the shell. As mentioned by others, the weight of the acid is greater, so liquid depth is normally reduced. I would rate the service depth per API-650.

Joe Tank

 

[MJCronin]

Joe..

I am familiar with the NACE standard and have made a little home collection of process engineering department guidelies & suggestions on H2SO4 tanks

You stated:

"Not mentioned by NACE (but is well known) is that the shell nozzle welding details are often modified to be more suitable for the acid service"

Exactly what "nozzle welding details" are you referring to?

There was an article in "Chemical Engineering"by Stanley Grossel some years back on acid tank design. He had some nozzle configutration suggestions.

Can you give us a reference ?

Regards..

-MJC

 

[BenThayer]

you might want to look at max flow rates for your outlet piping as well. the nozzle size may not be large enough.

if you search this site you should find some info but i think 3 feet/sec is considered a max velocity for H2SO4 in carbon steel piping.

 

[JoeTank]

Mr Cronin,
The nozzle details are normally changed to require full penetration of the repad-to-neck. In addition, the shell-to-bottom weld is also changed for full penetration. The idea is to not permnit fillet welds only for any connection due to the agressiveness of acid on slag inclusions. Additional RT is also employed. The only non-NACE reference I know of is a storage tank manual by CIL (formerly Marulex (sp?) and now Noranda) authored by Tivil and McGlynn. By observation, it appears that the NACE document is a watered down version of the CIL manual.

Joe Tank

 

[MJCronin]

Joe....

Are we talking about the same "F. McGlynn"...???

He has written several authoritative specifications/papers about sulfuric acid tanks, piping systems etc..

http://www.xstrata.com/assets/pdf/XPS_Materials_Technology_Publications_2007-08.pdf

-MJC