6 replies to this topic

[rikakose]

We have an old tank (from 80's) whose original design temperature is 25C. The tank is equipped with steam coil in the bottom section to keep the storage temperature at 50C. Design pressure is 0 barg. But it is has very long vent line (DN 50) to a gas washer. Pressure drop in the vent line+ gas washing is around 0.5 barg.

 

The tank has actually been working like this for around 20 years.   

 

What should I do with the tank? Replacement is not an option. Re-do a stress calculation? or any other ideas?

 

Thanks.  

[fallah]

We have an old tank (from 80's) whose original design temperature is 25C. The tank is equipped with steam coil in the bottom section to keep the storage temperature at 50C. Design pressure is 0 barg. But it is has very long vent line (DN 50) to a gas washer. Pressure drop in the vent line+ gas washing is around 0.5 barg.

 

The tank has actually been working like this for around 20 years.   

 

What should I do with the tank? Replacement is not an option. Re-do a stress calculation? or any other ideas?

 

 

rikakose,

 

The tank design conditions were 25 C/atm and now it's intended to be subject to 50 C/0.5 barg. Am i well understood your query?

[rikakose]

Yes. Operating conditions 50C/0.5 barg.

And the new design condition should be 130C /1bag

[Art Montemayor]

Rikakose:

 

From what little or un-important data you’ve given us, I can only conjecture that you have a potential serious problem.  And based on that scarce information, the only way that I can see for you to continue using that storage tank the way you propose is to do a complete mechanical design investigation - including detailed design calculations (which I have to assume you don’t have).

 

I have written a lot of responses to threads such as yours through the years and I continue to be amazed as to how mixed up some storage tank owners and user are when the facts are there in front of you.

50 degrees Celsius should not be a concern for you.  1.0 barg should be.  That is fairly obvious.  Carbon steels are more than capable of 100 or even 200 ^oC without a problem.  The problem is containing PRESSURE in a storage tank - not temperature.

 

Why don’t you tell us if you have the fabrication calculations for your tank?  Or at least tell us the wall and other thicknesses as well as the tank’s configuration, anchors, and type.  The fabrication name plate would even be or some help.  The fact that you have been using the tank for years doesn’t surprise me.  As I’ve stated many times before, I’ve seen much abuse of storage tanks in my 57 years of practice - in many countries.  It is sad how little importance is given to storage tank safe design and operation.

 

I seriously recommend you obtain a thorough and detailed mechanical design rating on the tank before continuing to use it without having any hard and accurate information on its realistic and dependable MAWP and MAWV.

[rikakose]

Thanks a lot Art Montemayor. Indeed I don't have much detailed information. I have only the original mechanical design drawing. (A0 format, no digital version ) It states that design temperature 25C. design pressure 0 barg. It is really surprised me that the steam coil is actually present on the drawing. Steam condition: saturated  at 6 barg.

 

 

We can't do re-rating internally, so I was wondering if there are some other solutions. 

[Art Montemayor]

Rikakose:

 

If I have failed in communicating my concerns to you over the proposed storage tank application in my previous response, then I apologize.  We still have no specific and detailed information on the subject tank and I have to guess that it is a conventional API-style, cylindrical tank with a cone roof.  The working temperature is incidental and of a second nature for the moment.  The important process points to watch out and design for are:

• The tank is obviously being pressure controlled by a back-pressure valve.  How you are generating the internal pressure is not revealed by you;
• You must furnish appropriate pressure relief devices (according to the ASME code, for example, since you are now at the 15 psig design level).  I would also consider pressure alarms also, for safety precautions.
• You don’t tell us the fluid stored, so I can only guess that if the internal steam coil leaks, any steam accumulated in the vapor space would vent and be subject to condensation, creating the nightmare for a storage tank - a partial vacuum.  You must design the tank for vacuum relief as well.
• An API storage tank designed for this pressure normally requires a detailed, calculated anchoring system on its foundation.  Otherwise, the tank will “lift” and probably cause a hazardous tank failure.  I presume you already know this and the tank is presently anchored.
• We don’t know if your tank was built with an API 650 type frangible roof.  If that is the case, I personally do not recommend tank frangible roofs but I would strongly recommend that you study this further.  For example, go to: https://www.osti.gov...igned-api-rules
• Once you have the tank modified or re-built to safely withstand the proposed process conditions, you should hold a detailed Hazop on the application and apply Management of Change principles. 

In my opinion, your objective should be, very simply, the safe and ensured future operation of the tank under the proposed conditions regardless of how many years the tank may have operated without incident or failure.  You should have the tank subjected to a detailed and formal engineering mechanical inspection that will reveal the present physical condition of the tank, its welded seams, its corrosion state, and its ability to withstand the process conditions proposed.  This should be followed by a rigorous mechanical design for the correct tank specifications that conform to the process conditions.  The design should be undertaken by an experienced mechanical engineer.  If you lack the local resources to undertake such a study, then you should contract an outside engineering firm.  I am forced to repeat what I wrote previously:

 

“I seriously recommend you obtain a thorough and detailed mechanical design rating on the tank before continuing to use it without having any hard and accurate information on its realistic and dependable MAWP and MAWV.”

Good luck with safe tank operations.

[rikakose]

Rikakose:

 

If I have failed in communicating my concerns to you over the proposed storage tank application in my previous response, then I apologize.  We 

 

Many thanks Art. I just read your reply. 

This a vertical tank with torispheric head and bottom. It has a open vent line connecting to a gas washer. The gas washer has a open connection which can be considered as open. There is no pressure control on the tank. 

The content is chemical water solution. Because of solubility, we have to keep the temperature high enough.

 

In my option: the main risk is steam coil rupture. The steam valve has big Cv. Too much steam will cause a lot pressure drop in the vent line. And the tank will see the steam temperature (130C) as well which is 100C higher than the design temperature.  Material wise it is a big issue.