acid under tank? 1

[chameleone]

I have a 93% sulfuric acid tank. It was installed in '91. There was a leak in '95 where the base of the tank was submerged in approximately 2 feet of sulfuric acid. That was neutralized and cleaned up. Recently, within the past year, there has been another leak from the tank (from a vent pipe with entrained amounts of acid). This leak is much smaller in that it was drips down the side of the tank. The tank rests on a cement pad with a 4" cement combing. The combing has recently begun to peal away from the tank. A fellow engineer is worried that the acid leaking down the side and the acid previously spilled may have collected under the tank. It is not practical to raise the tank to check or repair. What other methods are there of finding if there is acid? Is it worth attempting to neutralize the acid with bicarbonate? Short of fixing the leak, which has been done, what steps should next be taken?

 

[PBroad]

Two problems I can see first of all most concrete is corroded by sulphates. Suggest you core drill into the base and examine the drillings. This will tell you two things, the state of the concrete and the pH under the tank. If the concrete is so damaged that it will no longer support the weight of a full tank of acid then the only solution is to remove the tank ASAP before it tips over.

If the foundation is still solid then a glass resin spray on the inside of the tank after washing it out may be adequate but check local regulations, and follow confined space rules when entering the tank, and ensure that the coating is both heat and acid resistant. If you are a consultant then your judgement is what you are being paid for. If not a conbsultant then hire one. It will at least get the monkey off your back.

 

[bimr]

You have neglected to state what the tank is fabricated from nor the capacity of the tank. These are important considerations when making a recommendation.

The acid tank that you have should not have had several leaks in the limited lifetime it has seen.

 

[chameleone]

The tank is a 10,000 gallon carbon steel tank. Unfortunately, it appears that the steel and the concrete are touching at the base of the tank as the concrete was not sealed before the tank was emplaced. The interior of the tank is lined with a Halar lining.
The earlier leak was from too much stress through an outlet nozzle. They set up a recirculating system so that the nozzle was under contstant flow. Eventually the steel pipe corroded and leaked near the outlet nozzle of the tank.
The other leak was from an improperly positioned vent pipe. Small dripping from the joining between the vent pipe and the acid tank flange during blowdown periods when the acid tank was filled.
Both problems have been corrected.

 

[bimr]

Most applications for concentrated sulfuric acid use steel tanks that are unlined. Steel has a good service life in concentrated acid. Also, there usually is some type of liner placed under the tank, between the tank and the concrete.

You should have some type of tank inspection program in your facility where you inspect tanks every 1-2 years. With a tank inspection program, any problems that develop over time will be identified during the inspection.

Steel tanks are ideal for holding sulfuric acid. Unlike plastic tanks that are subject to catastrophic failure, any leakages in a properly fabricated steel tank will occur gradually over time and allow plenty of time for repair to be scheduled.

One would think that flushing the area with water will remove all of the spilled acid. It is probably not practical to lift the tank and inspect under it.

 

[pmureiko]

I am worried that concentrated acid has collected under the tank bottom and is eating away at the bottom. Carbon steel is compatible with highly concentrated acid. Unfortunately the acid will be diluted with water from rain and humidity over time and aggresively attack the steel. You have already experienced this attack with damage to the concrete foundation. Depending on the location of attack, there is potential for a catastrophic tank failure at the bottom tank seam since the ring wall may be damaged and is not supporting the tank.

I would rinse the bottom of the tank with a bicarbonate solution to neutralize and acids. From there you should consider repairing and putting a protective coating on the concrete. Conrete is basic in nature and will be attacked by acids. Given the frequency of acid exposure based on past events it will be money well spent.

 

[bimr]

I beg to differ with pmureiko. You will not get a catastrophic failure with a properly fabricated steel tank at atmospheric pressure. Steel tanks will start to weep, drip, and leak long before any extensive failure occurs which will allow plenty of time for tank repairs. Every catastrophic failure of a steel tank that I have seen has been associated with poor, improper, and noncode fabrication techniques.

 

[chameleone]

The tank is indoors under a steel reinforced concrete parking deck. This causes a large portion of the impracticallity of raising the tank to check the underside and cement support structure. We are currently considering core drilling as a method of determining the status of the underside of the tank.

We are also considering flooding the area with a bicarbonate solution to neutralize any acid remaining under the tank that may be diluted with groundwater.

We do have plans to coat the cement support sturcture and bottom of the tank in an acid rated paint or Derakene coating to prevent any further material from getting under the tank.

Our major concern is with the concrete support structure and whether or not it will continue to support the acid tank. Our tank inspections over the past years of the tank itself have been good. An inspection program for the flanges attached to the tank is being initiated to prevent any further leaks.

 

[radwaste]

I would suggest pumping the acid to stainless tankers. Then pull the storage tank and inspect the tank and containment for damage. I would guess the damage to the tank was caused from an introduced contaminant. With pure sulfuric, especially 93%, even small amounts of water could cause problems.

The vent pipe you mentioned in your description....is the vent pipe vertical from the top of the tank? Or does the vent pipe contain a bend to prevent precipitation from entering the acid tank?

Do not neutralize 93% sulfuric with bicarbonate or any other base. You could experience a very dangerous reaction.