Sulfuric Acid Tank Overflow Liquid Trap 6

[Gaply]

We have a 98% sulfuric acid tank of mid 1980's construction on site that was recently inspected. The tank is a vertical tank with a cone roof of about 40m3 capacity. The roof has a 1" vent pipe that goes through a dessicant trap. The tank also has a 3" overflow pipe off the tank wall. The overflow connection has a goose-neck trap at the bottom that was filled with an inert liquid (I think this was a silicon oil) to ensure normal venting goes through the vent line rather than the overfill line. When the gooseneck in the overfill line was inspected it was found liquid in had reacted with the acid vapour and set solid. Obviously this would have resulted in a very dangerous situation if the tank had ever been overfilled in error, possibly causing catastrophic failue. As a result no liquid has been put into the gooseneck yet. Apart form raising this issue my question is does anyone know what is a safe inert liquid that can be used in the gooseneck in future?

 

[hacksaw]

water?

most traps like this simply corrode away that is safer than a plugged one

 

[psafety]

If water is used expect to regularly refill due to evaporation. Glycerine is used quite often.

 

[ash9144]

Use a seal box. Small fiberglass box with PVC dip pipe into a water level 6-12" deep. Water inlet continuous flow with small overflow tubing to drain. Box vents to atmosphere.

 

[cheute79]

I'd be wary of water, and have to be very careful to avoid even evaporative losses to get into the acid, leading to stratification of the acid and higher corrosion rates. You obviously want a relatively high density, low volatile liquid. Glycerine can work, but be wary of acid/organic reactions.

You don't way what the tank is constructed of. I imagine it must be SS or CS with plenty of corrosion allowance. A one inch vent seems undersized.... corrosion products can build up and begin to restrict flow, plus there is the issue of overflow vent plugging.

Although I don't agree with everything in this source, you may benefit from perusing this website dedicated to sulfuric acid:

http://www.sulphuric-acid.com/TechManual/Storage/storage.htm.

Good luck!

 

[Gaply]

Thanks Cheute79,
The tank and pipwork is CS. I'll check out the website and also get the lab to check if glycerine is compatable with 98% sulfuric acid.

 

[psafety]

Be wary of hydrogen liberation from corrosion with carbon steel. Keep that vent at the uppermost part of the top and open.

 

[Nicksun]

In my application, we put pressure retaining valve with minor back pressure on the bottom of overflow pipe.
This is constructed but not running yet.

 

[JimCasey]

98% sulfuric can be used with CS. Atmospheric moisture will dilute the surface layer, and it does not take much dilution of Sulfuric before it becomes corrosive to carbon steel.

Suggest you use a conservation vent from Groth, Protectoseal/Varec to limit environmental interchange with the tank atmosphere. These can be fitted with a dessicant cartridge on the inlet port so that any atmospheric air going into the tank is dried to approx -40 dewpoint, thus preventing corrosion. All conservation vents are available in various polymers to resist chemical effects.

API has procedures for sizing conservation vents on tanks. Thes require the fill rate, the drain rate, and the exposed area above ground. The highest inflow rate occurs on a hot day when the tank gets drenched by a sudden cold rain shower.

 

[cheute79]

Rather than using an air drier, it is preferable (and probably ultimatley less risky) to use nitrogen as a blanket. I even suggest that at least some of the N2 be introduced below the minimum liquid level to facilitate some blending of the contents to avoid the stratification of acid that causes the higher corrosion rate. All CS are not the same in sulfuric service, so use caution. I've used this method several times.

RE hydrogen control, the vent MUST be the highest point and not restricted - makes for an interesting vent cover challenge. I used to use a hydrogen recombiner catalyst, but it is no longer manufactured, and the "replacement" is many thousands of US$ per kg, and probably need less than 200g.

 

[MJCronin]

gap....

Also be aware that by setting the depth of the seal loop on a vent, you are, in effect, setting the operating pressure and vacuum limits that this old tank will see.

The 1" vent to the dessicant trap doesn't sound right to me unless you have a very small tank .....what are your dimensions and capacity ? How fast can the tank be filled ?

I agree with the commnents above regarding the prefernce for a nitrogen blanket..... especially if you already have a system in place for other tanks.

A suggestion: I believe you should carefully look at the top-centerline of acid tank nozzles where there has been much flow over the years ( inlets, outlets) at the 12 o'clock position there may be hydrogen grooving damage. we have found this on tanks in the past.

Good luck and send us any pictures available of the project...

regards..

-MJC

 

[Gaply]

Thanks for the replies. My issue isn't with the vent line which does come off the highest point and goes through a silica gel desscicant trap (and is 3" not 1" as per the orginal post). The issue is that with no liquid in goose trap of the overfill line this line then acts as the vent as there is no backpressure on it. So moist air is drawn into the tank as it empties, not normally an issue due to high turnover rate of tank but as plant is now idled I am concerned with top level of acid becoming corrosive over time. Glycerine is not suitable for use with conc. sulfuric acid so if some-one knows of a suitable inert liquid let me know. If not modification to use a pressure retaining valve seems the best option.

 

[JimCasey]

THe P-trap should contain a liquid that does not react with the vessel contents and also does not evaporate, or react with atmospheric components. I know of no suitable fill fluid.

That puts you back to the conservation vent. There are various resins available so picking one for sulfuric acid vapors compatibility is not too tough. The vacuum vent acts as a failsafe and will keep you from damaging the tank if the N2 system is down and someone pulls fluid from the tank.

 

[Latexman]

How long had the silicon oil been in the trap before it was found to have solidified?

Good luck,
Latexman

 

[Gaply]

Unsure if had ever been replaced as early maintenance records are not available but the silicon oil (or whatever is was) is likely to have been place for at least ten years.

 

[djack77494]

Using air passing through a dessiciant vent dryer makes much more sense to me than piping over expensive nitrogen for vacuum relief. Carbon steel is just fine for this service, but ensure moisture is kept out. Your biggest challenge will be to find the compatable (with concentrated H2SO4 and moist air)fluid. I don't have that answer. Good luck.

 

[WhiskeySierra81]

Disclaimer: I work in an R&D environment on small-scale equipment that doesn't need to last for decades, so this might not be the most relevant answer.

Could you replace the CS gooseneck material with something that is widely corrosion resistant (some kind of lined steel or maybe something like tantalum) and then use 98% H2SO4 to fill the trap? Obviously that removes chem compatibility issues.. and if the overflow pipe material is corrosion resistant, then it wouldn't matter if the acid slowly becomes more dilute. Just a thought

 

[cheute79]

WhiskeySierra;

although this might work out for small and temporary installs, by mixing metals, you now introduce galvanic issues (that can be worked around, but takes it's own maintenance), and part of the problem is avoiding both volatility of sulfuric acid, and avoiding stratification by absorption of atmospheric water vapor (leading to dilution and stratification, which aggravate corrosion). The economics of tantalum is a challenge on larger scale installs. The best piping system would be PVDF (Kynar), which is becoming cost competitive on larger scales.