Help with Storage Tank Bolt Failure 5

[bottle77]

My company installed two 10,000 gal 304 SS tanks Just outside our plant back in February November 2018 (time flies). They are roughly 8.25' in diameter. One is holding a chemical that is 7.9 lbs/gal the other holds a chemical that is 8.69lbs/gal. The material comes in at 140F, and we try to keep it that way, though it may dip down to ambient temperatures (60-80F).

The tanks were installed about a foot apart on a level high strength concrete pad. 6 holes were drilled in the 2 inch angle iron flange around the tank bases and drilled down roughly 18" into the concrete. Then 24"x0.5" anchor bolts were epoxied into the holes and nuts were used to tighten the tank down to the concrete. After installation the tanks were equipped with band heaters at the bottom to help keep the material warm.

Last week the operators came in to find that the tank holding the 7.9 lb/gal liquid had snapped 3 of its bolts and bent a fourth considerably. Half of the base is sitting up 1.75". They emptied the 4,500 gallons of stored material out of it, and now we're trying to determine the cause of the failure, and how to remediate it. The other tank was just under half full at the time, probably also around 4,500gal.
Both tanks have vents to keep them at atmospheric pressure, so I don't think a pressure buildup could have caused this. The welds to the outer flange have been broken in a couple spots. The other tank doesn't really show any signs of stress, though there are some hairline cracks in the concrete around the flange. Could this be a failure from thermal expansion? The broken bolts appear to be sheared off. It was windy the evening it failed, but there was at least 35,000 Lbs of material sitting in the tank. I would think that would be enough to "hold it down" as it were. I would also think other parts of the tank would be damaged from wind before the base.

Any advice would be appreciated. Not sure where to start. If pictures of a different vantage point could help let me know. I am a recent grad with a ChemE degree. I don't know what analysis, if any, was done before they decided on how they wanted to install them outside of what was practical given space constraints. Tank schematic should be attached to the post

 

[Geoff13]

I've only designed API 650 tanks, and thus my venting is designed per API 2000.

The required venting depends on pumping rate (in or out), atmospheric pressure changes, off-gassing from a hot product (solar or heater), vapour condensing during cooling (usually rain for sudden cooling), etc.

A vent that 1.5 times the size of the product nozzle is at best a rule of thumb, but is no substitute for proper calculations.

I'm guessing that the white plastic pipe contraction above is your vent. OMG. A Yugo vent. I can't even begin to list all my concerns but I'll throw out a few:
[ul]
[li]Proper vents are weight loaded so that once the pressure reaches the set point it takes no additional pressure to push the vent fully open. If your vent has a spring then the tank internal pressure will have to keep climbing to push the vent further and futher open or nothing will escape.[/li]
[li]Purchased vents have flow vs back pressure curves published for them so they can be properly sized. The back pressure at the required venting rate (established above) must be less than the tank design pressure to avoid tank damage. How was the flow vs back pressure curve for the "vent" established?[/li]
[li]API 650 allows some very small pressures to be treated as "atmospheric", but your tank is so thin and light that this will be very, very close to zero. Will your "vent" keep the pressure that low?[/li]
[li]The white pipe going to the roof nozzle will further restrict the air flow through your "vent system". Unless it's accounted for in the sizing of the "vent system" a long pipe is a poor idea.[/li]
[li]Is there any chance that product vapors will get into the vent pipe, condense and then run down the pipe? If so, could this product build-up stop the "vent" from working properly or even glue it shut?[/li]
[li]Etc, etc, etc.[/li]
[/ul]

I see others are commenting on the other visible issues on these tanks so I'll stick to venting replies.

Note : The problems with these tanks have nothing to do with a light vs heavy product, so switching products won't fix anything.

 

[bottle77]

Yeah, we're thinking that the vent got clogged and that's why the pressure built up. I will pass on that info, but in the past I have not been asked to help design or install any equipment and I feel like this is going down that path. I think that since I'm young and have about zero hands on experience with this stuff, when I try to bring up concerns it comes across as an arrogant kid who thinks he knows better without having done anything himself. We'll see what happens...

 

[bottle77]

Also @3DDave, I checked the broken bolts again and it looks like a tension break, here's a picture:

It doesn't look too sheared off. I mean that it looks like a tension not shear break because the break isn't flat, although I've never seen a sheared bolt so I can't really say. Also it looks like the rain and elevated temperatures might be causing some corrosion issues.

 

[MJCronin]

bottle ..

Many of us on this forum would be happy to help you out with replacement tanks .. (despite the interjection of a little sarcasm by our more senior members.)

Now that we all have learned more, here are a couple of my points to ponder:

- My opinion is that anchor chairs will not work to repair these damaged, thin-walled dairy tanks.

- You may want to get budgetary prices of a new SS API-650 designed tank and evaluate the many, used SS tanks on the equipment market. This is an easy, cheap activity and will help you make a good decision going forward

- You may want to reconsider the volume of product that you really need to store ? Also consider if horizontal tanks would be suitable.

- Your Tank access seems suspect .... is your single dairy-style manway acceptable to your maintenance people ? Most process chemical tanks of this size have two points of access, with a roof manway/hatch necessary for ventilation during cleaning operations

- Consider the fire protection requirements of the state, municipality and your insurance company. Consider the requirements of NFPA-30.

- Some US states require registration and periodic internal inspection of all storage tanks in their boundaries.

- A clogged vent line will collapse any flat bottomed tank. You may want to consider a better metallic piping system with heat tapes.

etc ...

etc...

This is an excellent starting paper .... was widely available on the internet until it was stolen by SCRIBD....

https://www.scribd.com/doc/38636515/Tank-Design

Please respect us and keep us involved in your final decisions

MJCronin
Sr. Process Engineer

 

[LittleInch]

Bottle,

I echo most of what MJC says, although I don't share his obsession with MBA's....

That venting arrangement though is the work of the devil. How you haven't collapsed or blown up this tank so far is a minor miracle. Atmospheric tanks, means exactly that. The definition in API 650 is something like 2 inches of water column...

it also seems very large compared to the size of pipes in and out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bottle77]

Hey all,

I'll do my best to follow this advice. In the past, my input on equipment has been either ignored or I've been told not to worry about it, but hopefully this time will be different. I can't guarantee he'll listen to me, but he might listen to your responses since you have worked in the industry for awhile. To that end, would you mind commenting how long you've been working in the industry and in what capacity? I see that MJCronin is a Sr. Process engineer.

 

[LittleInch]

Hmmm,

Saying to your boss that you've been discussing company business with a bunch of unknown old farts on a distinguished web forum like Eng tips might not be the best move. IMHO.

you can say that you've been investigating the root causes of the failure and you believe this or that which has been subject to some peer review and this seems to be the way everyone else does it....

I've been in the industry over 30 years and had many titles, but mostly in the technical field.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bottle77]

I asked him about posting the issue here, and he told me to go ahead and seek some feedback. I'm just hoping he'll listen to it.

 

[miningman]

I cant and wont comment on venting nor API standards. If it was me , the first thing I would do is bring in the surveyors and have them advise me how far off of vertical these tanks are today. And while they're at it, how well was the initial concrete slab levelled?? Maybe some salesman did tell someone ( probably verbally ) that they could be sited on raw gravel , but those tanks look EXTREMELY high to me. Altho all the comments here so far are probably relevant they are 98% speculative and judgemental. Get some firm numbers to put in front of the boss

 

[bottle77]

miningman, boss likely won't pay for surveyors. This is a pretty small company (<20 people). I don't have a budget or billing/buying power, I have to ask someone else to buy my lab supplies, let alone be able to pay for an inspection. I know it was a professional company that made the concrete slab, but I guess I could go check it out with a level....

 

[Geoff13]

Click on my name, and my profile page notes my experience. This is true for many other members as well.

 

[miningman]

Well, making the assumption that the concrete pad is truly level, how was the tank levelled during installation?? the only way I know of is the use of shimming washers. and surveyors / a level. If there was zero attempt to level the tank, what discussions took place to allow for the 2 inches of non level tank floor? How was this non level floor tied into the geometry of the six hold down studs?? ( And I would have used a minimum of 12).

Who prepped and compacted the sub grade for the tank base??

Have you considered the question " Why did only one of the two tanks fail??"

 

[MJCronin]

LittleInch ....

I am highly insulted !!!!

You state:

Saying to your boss that you've been discussing company business with a bunch of unknown old farts on a distinguished web forum like Eng tips might not be the best move. IMHO.

I will have you know that I AM NOT AN UNKNOWN OLD FART !

I am a well respected member of my six-AM morning McDonalds coffee group !!!...Until we are asked to leave every morning, we discuss important world topics !!....They all know me !!

All of the pigeons know me when I feed them bread on the park bench !!!

All of the CVS clerks know me as the sarcastic, bumbling senior citizen when I go there twice a week for my medicine !

All of the neighborhood kids know me when I lean out the window and yell " Hey You, Get OFF My Lawn !!!"

So, LittleInch ..... be careful what you say ...

MJCronin
Sr. Process Engineer

 

[bottle77]

Update:

Told the boss I was concerned about the process of getting the tanks replaced, and quoted some concerns brought up in this thread, and he said he'd pass on my concerns to the engineer he used to design the concrete base. They also were the ones who suggested the anchoring scheme. So there was an engineer involved! Sounds like they are a civil/structural, but they didn't take the over-pressurized situation into account. We'll see how the boss decides to move forward after he hears back.