9 replies to this topic

[S J]

Dear Sirs,

I'm process engineer, I have a question regarding sulfuric acid line.

Currently, in the plant, 98% sulfuric acid is used for water packages backwashing and neutralization pit.

Because sulfuric acid velocity in the pipe is lower than 1 m/s, all material for sulfuric acid tank and pipe is carbon steel. Recently, some leakages happened at ball valves on sulfuric acid distribution line and ball valves at sulfuric acid distribution line were stuck.

I think because 98 % sulfuric acid is very dense , valve stuck happened. (Ambient temperature never fell down below zero.)

If anybody has experience or knowledge in handling ulfuric acid line, please share with me and advise regarding why valves at sulfuric acid line were stuck.

[Bobby Strain]

If you use Alloy 20 valve bodies with stainless trim, you can avoid such problems.

 

Bobby

[S J]

Bobby

 

Could you advise why ball valves are stuck ?

[breizh]

Hi ,

On top of what Bobby is sugesting , consider reading the document attached .

 

Breizh

 

[shvet]

 

I think because 98 % sulfuric acid is very dense , valve stuck happened. (Ambient temperature never fell down below zero.)

 

 No. You answer is - water.

 

I had been working for 9 years with H2SO4 and we had a lot of problems this kind. You consider H2SO4 as simple solution of H2SO4 and H2O. Actually it isn't so. There are H2SO4 hydrates in water solutions - H2SO4*H2O, H2SO4*2H2O and so one, each of them should be considered as separate chemical. Some of this hydrate-water solution has crystallisation temperature ~+10 °C!

 

I had been working with H2SO4 from 98% to ~15% and majority of problems we had with 85% and 98% H2SO4. Storage vessels, pipes and pumps were made of carbon steel. From time to time we "caught" water (as a result of operator negligence or with feedstock). Main plant headers (we had two headers), filters, globe valves, vortex breackers etc. were plugged or/and corroded. It was a real problem and we haven't found effective way to solve it.

As a result we were reluctant to replace CS pipes with SS pipes but it didn't remove plugging. My country is rather cold and most of months we had to "fight" against plugging. There are two ways unplug filter/valve etc.:

- or to warm it up and then to pump deluted H2SO4 to storage vessel

- or to scrub it manually by operator. Be aware! This operation is dangerous and we had some accidents.

[demank]

Refer to NACE RP0391, for small diameter less than 3 in, SS is preferred than carbon steel.

In case of ball valve is stuck? is there any probability in the system for mixing water-acid on that valve? back-flow case for example. because mixing water - acid is prohibited to occurred in the carbon steel.

[S J]

Gentlemen , thank you so much for replying to my question.

Sulfuric acid lines have been operated for several months and ball valves are stuck at several points of block valve and drain valve at  98% sulfuric acid line and some valves located near acid-water mixing point. Currently, we are also checking NACE 0391, but, CS is already applied to line and valve material and socket weldings are already done on several points. And concentration of H2SO4 is decreasing slowly though silica gel is installed at vent of tank. Current concentration of sulfuric acid measured is 95~96 % at tank bottom. Anyway it looks we should consider replacement of CS lines with SS line and elimination of plugging someday.

Additionally, I'm wondering whether we should choose HCl instead of H2SO4 for bachwashing and neutralization when we chose chemicals for the plant. Could you advise which one is preferred and used between H2SO4 and HCl ?

[breizh]

Hi ,

HCL ....corrosion issue , Fume .....difficult to manage !

 

Breizh

Edited by breizh, 23 May 2016 - 07:36 AM.

[shvet]

 Anyway it looks we should consider replacement of CS lines with SS line and elimination of plugging someday.

 

 

You should replace concentrated H2SO4 pipes CS with SS only if you have problems with corrosion. If water-caused contingencies are very rare this is redundant. SS pipes are subjected to plugging in the same way as CS pipes.

Anyway in my case even SS pipes couldn't solve problems with corrosion. From time to time we had to replace some SS pipe sections (only 85% conc.). Acttualy I don't know the reason - it was caused by bad pipes quality or high H2SO4 activity.

 

 

 

Additionally, I'm wondering whether we should choose HCl instead of H2SO4 for bachwashing and neutralization when we chose chemicals for the plant. Could you advise which one is preferred and used between H2SO4 and HCl ?

 

H2SO4 is preferred. It is cheap and it is produced in every refinery. If you don't need high quality of H2SO4, for example you use H2SO4 for neutralization, then H2SO4 from refinery is the best choice.

Edited by shvet, 25 May 2016 - 01:56 AM.

[demank]

Halo shvet,

I believe your case is caused by sulfuric acid high activity.

Again, as per NACE RP0391, which is applicable only 90% - 100% sulfuric acid conc, SS (or CS for size 4 in above) pipe is sufficient but have some limitation such as low velocity, weld type (socket weld or threaded is prohibited), etc...