Jul. 08, 2024
Modern bolted steel tanks are quite different than those from 40 years ago, and the distinctions begin during the design process. Older tanks were drawn in two dimensions, which was very time consuming, and some drawings were not properly prepared, making it difficult to fabricate the tank. New bolted steel tanks, however, are designed using 3D modeling software, making the process considerably faster and more accurate.
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Some premium tanks are designed using the simplified building information modeling (BIM) software, which encompasses the entire project from engineering to manufacturing and construction. Additional information can be contained within the BIM system, including weights and dimensions of materials and components used to construct the tank. Current design standards such as AWWA D103 have progressed over time, resulting in tanks that last longer and perform better. In addition, advances in seismic and wind design methods have led to safer and more structurally sound tanks.
Customer approval of drawings is also considerably faster, as engineering drawings can be sent electronically instead of mailed as hard copies. If a change to a tank accessory is needed, the 3D modeling system makes the revision very simple and accurate.
A second major difference between older tanks and modern bolted steel tanks is how they are manufactured. Older tanks were constructed of material that was individually fabricated by hand or with basic equipment. The slight variations in the process or differences in how parts were measured resulted in parts not fitting properly, which led to leaks. Modern tanks are precision-crafted in state-of-the-art factories with specialized machinery. Tank drawings and details are exported from the 3D modeling software into sophisticated computer-aided manufacturing programs that directly control the CNC machinery. Further, the automated process reduces the fabrication time and ensures accuracy. To ensure that steel panels are fabricated to exact measurements, the entire panel is locked into place within the machinery to prevent the panel from shifting during forming. Tank components are also welded together using robotic welding equipment for extreme accuracy and 100-percent perfect joints.
In addition, numerous procedures have been implemented to ensure that all aspects of manufacturing are extremely accurate. Select tank manufacturing facilities are ISO : certified, guaranteeing product consistency and quality. These companies have invested time and effort into solidifying the way operations perform to eliminate errors and improve quality. All aspects of engineering, manufacturing and shipping processes are analyzed to ensure the customer receives exactly the product they ordered. The ISO process is so comprehensive that even measuring tools such as tape measures and calipers are officially certified to be accurate.
Another significant advancement with modern bolted steel tanks is the availability of new tank finishes. Innovative new options such as powder coating have significantly increased the longevity of bolted steel tanks. The thermoset powder coating process requires careful preparation and application for greatest durability. Steel panels are first cleaned and prepared using standards from the Society for Protective Coatings (SSPC) and the National Association of Corrosion Engineers (NACE) to ensure maximum coating adhesion. To further improve the durability and longevity of the tank powder coating, a special silica-zirconium (SiZr) liquid seal is sprayed onto the prepared steel panels before the powder coating is applied. The SiZr seal is designed to prevent the spread of corrosion if the coating is damaged by impact.
The zirconium component increases the adhesion of the powder coating to the steel panel and has been used for decades in similar applications. The silica in the SiZr seal blocks the steel oxidation process, reducing the amount of corrosion; without the seal, undercutting can occur, increasing the amount of coating damage. Laboratory tests have shown that the SiZr seal isolates the corrosion to a small area and can make the difference between a minor coating patch and a significant coating renovation. After the seal dries on the steel, the powder coating is electrostatically applied to steel panels and other components in an environmentally controlled area. During the application process, a small static charge is used to hold the particles of powder to the steel panels until they are heated and cured. The charge ensures that the powder coating forms a smooth, even surface without voids or thin spots.
For the interior of the tank, an epoxy powder coating specifically designed for continuous emersion is used. For potable water storage, the interior epoxy coating must be NSF 61 certified. For maximum resistance to ultraviolet exposure and harsh weather conditions, a triglycidylisocyanurate (TGIC) polyester powder is then applied to the exterior of the steel panels. Directly after application, the powder coating is heat-cured in special ovens, and the panel surface is completely sealed. This thermoset process heats the powder coating to the point that it begins to melt then chemically reacts to form a higher molecular weight polymer that permanently bonds to the steel surface.
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Bolted Steel tank to API 12 series or API 650 for Process tank ( Demin Boiler feed water)
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(Mechanical)
(OP)
21 Jun 22 04:46Hi 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.
(Petroleum)
21 Jun 22 06:56Why 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?
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(Mechanical)
21 Jun 22 13:10Brunel,Every demin Storage tank that I have ever seen in power plant service has been 304SS..... and much smallerI 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 theservice life in this agressive oxygenated environ ?.... Do not believe the salesman ! ...... These people had problems- 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- 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 hereLastly, 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 quotesMy opinion onlyMJCroninSr. Process Engineer
(Mechanical)
21 Jun 22 14:51This may be of help:
MJCronin
Sr. Process Engineer
(Mechanical)
(OP)
23 Jun 22 03:57Hi 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.
(Petroleum)
25 Jun 22 10:05I 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....
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