Bolted Steel tank to API 12 series or API 650 for Process tank ( Demin Boiler feed water) 1

[Brunel777]

Hi All,

I am working on a project looking at either using a bolted steel tank ( similar to API 12D/F, etc) vs a API650 Welded tank. I am looking for advice from people on the maintenance differences, if it is worth doing the Bolted steel tank over the Welded tank? In my own opinion I want to have a welded Tank to API 650 since the current tanks have been in operation for >25 yrs and have not had any issues, but the price difference is making it difficult to justify.

Tank Details:
Size: 950m3
Contained Liquid: Demineralised water with a high oxygen content
purpose: Demineralised water for a 12,000 kPag HP boiler circuit ( through deaerator to remove oxygen)
Pressure: head pressure only from the water
Design Temperature : 70 deg C

Options
Bolted Steel tank :
- Mild steel shell
- internal liner ( lifetime 10 yrs)
installed cost $250,000

Welded API 650 tank:
- 304 SS
installed cost of $650,000

Any help is appreciated.

 

[LittleInch]

Why SS?? If a liner or internal coating is good for the bolted tank why not the same for the welded tank?

You're not comparing the same thing.

What are the dimensions of the two alternatives?

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

 

[MJCronin]

Brunel,

Every demin Storage tank that I have ever seen in power plant service has been 304SS..... and much smaller

I suggest the following:

- Investigate industry tank experience with Demin Water and the proposed liner material. Who else has done this ? ... Can you talk with others who have used this liner ? What is the ACTUAL service life in this agressive oxygenated environ ?.... Do not believe the salesman ! ...... These people had problems

https://kta.com/kta-university/installation-lining-water-tank/

- How much does a relining cost for the proposed bolted steel tank ? ..Will your system configuration permit a "tank outage" and still operate ?

- Investigate if your quoted lining system is compatable with any proposed nitrogen sparging and blanketing system. This is commonly done in power plants

https://www.powermag.com/prevent-co...tanks-using-nitrogen-sparging-and-blanketing/

- How sensitive is your tank cost comparison over 30 or 40 years when repeated relining is considered ? What if the relining must be done every 8 years ? ..... every 6 years ?

- Do not forget to include peroidic maintenance/repair fo the EXTERIOR of your mild steel tank .... The SS has an advantage here

Lastly, my experience with quotes of ANYTHING made from stainless steel was that the numbers were all over the place. Stainless plate costs changed rapidly with international marketplace conditions, ,,, I hope that your $650,00 figure is the average of at least 5 quotes

My opinion only

MJCronin
Sr. Process Engineer

 

[MJCronin]

This may be of help:

https://www.ast-forum.com/ast_forum...have been,an acceptable low cost alternative.

MJCronin
Sr. Process Engineer

 

[Brunel777]

Hi All,

thanks very much for the response.

@LittleInch A coated carbon steel tank is a potential option as well however site experience with the Polyviyl ester resins has been bad and management didn't want the risk of the tank and coating failure also since the costs of a welded and coated steel tank is still 70% the cost of a SS tank management doesn't see a benefit in saving 30% for a coated tank if there is a risk of coating failure. Bolted tanks are much lower capital cost hence getting looked at...

MJ Cronin:
I have spoken to another plant which has a similar size tank and lined tank and they have issues with constant leaks from the bolted connections and in their Demin buffer tank a liner failure causing the tank level to be limited. However the lower cost is still winning for the lined tank when I do an NPV over a 30 year life and changing liners every 8 yrs.

- relining cost of the bolted tank is approx $60k and 6 days down time, our plant can't run without the tanks in question being in service.

- in the cost comparison I have also included the cost of a repsray in 15 years time which is starting to change the balance of the economic equation.

Quotes were recieved from 4 bidders but there was a very large disparity in those prices so I have taken the average. looking at a spread of over 120% between the lowest and the highest bid.

 

[LittleInch]

I don't really see much of a difference between coating failure ( the application is everything to be sure ) versus failure of the internal liner also leading to a hole / replacement of the liner.

But if mgt are happy for their plant to look dreadful with all sorts of leaks and streak marks where the bolted tank leaks a little bit plus the time and disruption to replace the liner on a maintence basis before it leaks and starts corroding your bolted tank.

So cost of fit and forget versus a lifetime of grief.

NPV calculations are the work of the devil and miss out soooo much and have wild assumptions on interest rates and inflation they are not worth the paper or spreadsheet they are calculated on IMHO. Factor in an unexpected 10 week shutdown while you repair your bolted tank and see what the NPV does for you....

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