Integrity of Horizontal Atmospheric Tank 1

[Maz07]

Hi Buddies,

We are having a CS Horizontal Atmospheric Tank (SA 510 Gr. 60) built in 1966 resting on two saddle supports. Material thickness is 6.35mm having no corrosion allowance. The ID is 2425.7mm and the service is NaOH. t(required) is calculated as 3mm. Based on the UT inspection report end of 2021, 46% readings revealed material thickness below 4mm and only 4% readings were under 3mm. The corrosion rate for the critical area is established -0.11mm/ yr and overall other parts of tank is -0.6mm/ yr. Some theoretical stresses are reported around saddle horns calculated by Finglow software PD:5500

Based on the above scenario, is it possible to repair it with some kind of inner coating or lining to make it more out of it rather replacement?

 

[LittleInch]

Personally I think it's time to retire that tank.

The act of blast cleaning it could lead to thickesses <3mm.

You need to really get a good map of where the corrosion is. "46%" is not clear if this is evenly spread over the tank or did they take more readings in the areas of corrosion?

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

 

[shvet]

Residual thickness 3mm does not mean that effective thickness is 3mm too. Good metal appearance does not mean metal is in a good condition. What metallurgy tests of these spots have showed? Is metal degraded or it has its original strength? Are there thermal/cycling fatigue zones?

Risk based inspection is recommended for such cases.

 

[MJCronin]

I agree with LittleInch .... It is time to replace this 57 year-old geezer ... Nothing lasts forever ...

Many highway bridges built in the 1960s have met their design life and have been replaced ... You expect more from this tank in corrosive NaOH service ??... Was this tank "repurposed" or was it designed for Caustic Soda service with NO CORROSION ALLOWANCE ?..... I don't understand

Your question about adding a "coating" will require surface prep (blasting) and you have nothing left to blast

When your MBA boss[i][/i] gives you a hard time and demands that you find some way to "make it work" .... tell HIM to take responsibility for the future tank integrity and safety.... be prepared to change jobs..

I strongly suggest that you have at hand a detailed estimated price for tank removal and repacement ... it costs very little to have this information available for future decisions

How long is this tank ? ... Where is it located and what is the operating temperature ??

Can you provide pictures?... design drawings ???

MJCronin
Sr. Process Engineer

 

[Maz07]

@LittleInch: Totally agreed with the point of have proper mapping of the corroded areas. I'm trying to look in to previous reports but I think the inspector didn't submit the detailed map. Waiting for the shut down to have detailed internal inspection. Thanks buddy!

@shvet: Just UT performed. can you suggest some metallurgy test particular to the scenario? caustic solution is 47% and steam tracing is there to prevent it from freezing. Thanks!

 

[GD2]

Maz07,
This is a classic example to do a FFS. FFS also allows you to repair/rebuilt. Do some economics and see the cost impacts of repair Vs new. You don't really require FFS if you decide to repair.

GDD
Canada

 

[r6155]

Alternatives
1) Reduce the actual fill height.
2) Contact a paint company
3) Contact a company for the Acoustic Emissions Examination (AEE see ASME V) to define a corrosion map and detect any other defect. This AEE is during the operation of the storage tank.

Remember: this is not a pressure vessel.

Regards

 

[shvet]

Just UT performed. can you suggest some metallurgy test particular to the scenario?

You should repost this topic at corrosion forum. I am a process engineer, not a mechanical one. So answer is negative. Looks like you should find&consult a proven/reputable/experienced local contractor to create a proper inspection plan for such equipment. If there is no such available then preventive/proactive replacement of deteriorated equipment/piping is advised. Note that there is an option for money saving if such equipment will not be sold as a scrap, but as used equipment at relevant international market.
bit.ly/3OySJH2

Agree with r6155 - a proper coating is able to save this tank.

Note that your issue is not related to a particular engineer/person (e.g. you). This is an issue of an operating company of assets maintaining system. How expenses are forecasted, planned, spent, and reported.

For info - recent week I have been at a phosgenation process unit where some process vessels have been manufactured before 1945 (marked with nazi swastika). Those looked pretty well, no problems were reported. So as per my experience 50 years is a common situation if proper inspection, maintenance, and repair had been applied.

caustic solution is 47% and steam tracing is there.

Sounds bad. High temperature and caustic is a dangerous combination. See NACE SP0403 for details.

 

[Maz07]

@MJCornin: You got it right regarding the task given by the boss "make it work "

It's very old tank so I cant say either it was designed particularly for caustic or some other service as the GA drawing is not having any service mentioned on it. Working is in progress for a trade off between repair or purchasing a new tank.

Regarding dimensions, length is 5.4m having dia of 2.44m. Capacity is 5000 Gallons. The product inside is at 50 degree C.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1690818976/tips/098-T8900_GA_Drawing_ywzopx.tiff[/url]

 

[MJCronin]

Maz07 ...... How is it that I already knew about the bosses demands and we have never met ???? .... Hmmmm

I have been involved in several 50% Caustic Soda storage and transfer systems in the past.

Due to problems in level control and localized corrosion, horizontal 50% Caustic Soda tanks are not optimum

These are my suggestions:

1) Get an up-to-date price quote for a replacement... Tell your MBA that more information is always better.

2) Make sure that price quote is for a coded tank/vessel designed for caustic soda with at least 0.25" CA

3) Suggest a 30-50 psig design pressure

4) Putting an expensive coating on this ancient tissue-thin Methuselah is an MBA excercise in waste and vanity

5) Pay attention to tank venting, grounding and interior access in your new design

There are many, many, many discussions on eng-tips about caustic soda (NaOH) systems design .... start learning !!

Please keep us posted about your final decisions and repair/replacement choices .... Everyone can benefit !!

MJCronin
Sr. Process Engineer

 

[Maz07]

I really appreciate your response and thanks for the guidance .

Surely, I'll update the final decision once made.

 

[Maz07]

@SHVET: Thanks for providing reference of NACE SP0403

 

[Maz07]

@shvet, @r6155 & @GD2: Thanks for further suggestions buddies.

 

[LittleInch]

In the shutdown and internal inspection make sure you do a small amount of serious blast cleaning and then measure UT again. Sometimes you find your thickness reduces significantly... I still think your repar / coating cost benefit is going to be pretty marginal especially if you want > 5 years out of this tank.

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

 

[shvet]

caustic solution is 47% and steam tracing

The product inside is at 50 degree C.

Working is in progress for a trade off between repair or purchasing a new tank.

I'll update the final decision once made.

50°C is a bulk temperature of liquid, correct? This means that local wall temperature is much higher, especially at hot weather.
47% is an commercial grade of caustic, correct? This means that actual concentration can be say 50%, especially localized caused by local vaporization e.g. at draining.

What is a safety margin in your case? Are you sure that in your case stress relieving was enough to ensure cracking did not occur? Are you sure that new vessel should be made of CS, not Ni-alloys? How local overheating is controlled and ensured? Are you sure that heat tracing is a safe design for such environment? Are you sure that the vessel has not been damaged by local cracking already? Are you sure that in future during detailed design of new vessel one will not find out that CS is not suitable for such environment and cost will increase significantly making this trade off meaningless?

Based on the UT inspection report

As for me API 571 states that spot UT is not applicable for cracking control. A corrosion engineer might correct me.

4.5.3.7 Inspection and Monitoring
a) Although cracks may be seen visually, crack detection is best performed with WFMT, EC, RT or ACFM techniques [not UT!]. Surface preparation by grit blasting, high pressure water blasting or other methods is usually required.
b) PT may not be effective for finding tight, scale-filled cracks and should not be used as the only means of detection.
c) Fine cracks may be difficult to find with RT.
d) Crack depths can be measured [measured, not found!] with a suitable UT technique including external SWUT.
e) AET can be used for monitoring crack growth and locating growing cracks.

Sounds like this trade off is going to be done by unverified arguments. This is an engineer's work to provide sound engineering and assessment to a decision maker, right? See para. 6 API 581 for a risk-based inspection guide.

 

[MJCronin]

Have you gotten any cost estimates for internal blasting and coating ? .... Will you also add-on an external coating ?

How does this cost estimate compare to the replacement cost of a new, thicker tank ... designed for 50% caustic ?

Most importantly .... What cost advantage have you assigned to the option of a new, thicker tank that will last for 25-40 tears versus a new internal coating which may last ~5 years ?

MJCronin
Sr. Process Engineer

 

[shvet]

an example of caustic SCC and improper inspection

https://www.youtube.com/watch?v=uo7H_ILs1qc

 

[Maz07]

@shvet: Yep 50 c is the bulk temperature of the liquid and its a commercial grade of caustic. May be the electrical tracing could be beneficial.

Thanks for the CSB link. Tank in our case is atmospheric. I think better to have AE testing in combination with UT during upcoming inspection.

 

[Maz07]

@MJCronin: not yet as will do in particular after the third party inspection report. Regarding coating what do you suggest as per your experience epoxy will be good or we can go for glass coating as well?

 

[MJCronin]

I believe that you are looking for a replacement caustic tank that would look similar to this:

https://hansontank.com/horizontal-air-receivers/84hm5000w/

The 2023 USA cost for a NEW, ASME-VIII, properly coated, carbon steel tank suitable for your caustic service should be somewhere between $35,000 and $70,000 ... in my opinion ... Importantly, this NEW tank will be good for 20-30 years

Your coated tank work will be somewhere between $25,000 and $45,000 (my guess) .... The coating will last 3-6 years

Who disagees ? ....... Who has better cost estimates than me ?

MJCronin
Sr. Process Engineer