What Is a Field-Erected Welded Steel Tank?
A field-erected welded steel tank is a large-capacity storage vessel built on-site by welding steel plates together — one course at a time — directly on the prepared foundation. Unlike shop-fabricated or bolted tanks, a field-erected tank has no practical upper limit on diameter or height. If the steel can get to the site, the tank gets built.
Field-erected construction is the standard for large-diameter atmospheric storage tanks governed by API 650 (petroleum and chemical service) and AWWA D100 (potable water and water storage). It’s also the only construction method accepted for ASME-coded pressure vessels that cannot be shop-fabricated in a single piece due to size or transport constraints.
MMI Tank has been designing, fabricating, and field-erecting welded steel tanks from our 8-acre Phoenix, Arizona facility for decades. Our ASME S and U stamps, National Board R stamp, and OSHA/MSHA/HAZWOPER certified field crews mean we handle every phase — engineering, plate fabrication, field welding, NDE inspection, coating, and startup — under one roof.
How MMI Builds a Field-Erected Tank
Engineering and Design
Every tank starts in our engineering department. We size the shell courses, select steel grades and plate thicknesses, calculate wind and seismic loads per the governing code (API 650 Appendix E, AWWA D100, ASCE 7), and produce stamped fabrication and erection drawings. Foundation reaction loads, anchor bolt patterns, and nozzle schedules are delivered before the first plate is cut.
For ASME pressure vessels, our U-stamp program includes full Manufacturer’s Data Reports (MDRs) filed with the National Board.
Shop Fabrication
Steel plates are cut, beveled, and rolled in our Phoenix fabrication plant. Computer-controlled plasma tables handle precision cuts; plate rollers form shell courses to the required diameter; and overhead cranes (up to 7.5 tons) stage materials for shipment. Every plate is marked with a heat number, course number, and orientation so field crews can erect in sequence without rework.
Field Erection
MMI field crews erect tanks using conventional (bottom-up) or jacking (top-down) methods depending on tank diameter, site access, and schedule. Our fleet includes mobile cranes to 60,000 lbs, welding rigs, and self-contained crew support for remote sites.
All field welds are performed by AWS-certified welders and inspected per the applicable code — radiographic (RT), ultrasonic (UT), magnetic particle (MT), or liquid penetrant (PT) examination as required. We maintain full weld maps and NDE documentation for turnover.
Surface Preparation and Coatings
After erection and hydrostatic testing, the tank is abrasive-blasted and coated. MMI’s industrial coatings division handles surface prep and application in-house — epoxy, polyurea, zinc-rich primers, high-temperature coatings, and NSF/ANSI 61 potable water linings. Keeping coatings in-house eliminates subcontractor scheduling gaps between erection and coating.
Tank Codes and Standards We Build To
| Standard | Application | Key Requirements |
|---|---|---|
| API 650 | Petroleum, chemical, water (non-potable), industrial liquids | Atmospheric storage, welded steel, seismic per Appendix E |
| AWWA D100 | Potable water, treated water, fire protection | Welded steel standpipes and reservoirs, seismic per ASCE 7 |
| API 620 | Low-pressure storage (refrigerated, cryogenic) | Design pressures up to 15 psig |
| ASME BPVC Section VIII | Pressure vessels exceeding atmospheric | U-stamped construction, National Board registration |
| API 653 | Tank inspection, repair, alteration, and reconstruction | Existing tank assessments and repair welding |
| NFPA 22 | Water tanks for private fire protection | Capacity, seismic, and freeze protection requirements |
| UL 142 | Aboveground flammable/combustible liquid tanks | Steel construction, secondary containment options |
Capacities and Configurations
| Configuration | Typical Range | Common Applications |
|---|---|---|
| Vertical flat-bottom (API 650) | 10,000 – 10,000,000+ gal | Crude, refined products, process water, chemicals |
| Standpipe (AWWA D100) | 50,000 – 2,000,000 gal | Potable water, fire protection |
| Horizontal vessel (ASME) | 500 – 50,000 gal | Pressure service, process vessels |
| Rectangular / specialty | Per project | Secondary containment, custom process |
Roof options include cone roofs, dome roofs, internal and external floating roofs, geodesic aluminum domes, and open-top configurations.
Industries We Serve
MMI field-erected welded tanks are in service across:
- Water and Wastewater — potable storage, reservoirs, clarifiers, equalization basins, digesters
- Oil and Gas — crude storage, produced water, frac water, terminal tankage
- Food and Beverage — process water, ingredient storage, CIP systems
- Agriculture — fertilizer, irrigation, anaerobic digesters
- Mining — process water, leach solutions, tailings
- Municipal — water distribution, fire protection, wastewater treatment
- Power and Energy — cooling water, condensate, thermal energy storage
- Industrial and Manufacturing — chemical storage, wash water, cooling towers
Field-Erected vs. Bolted: When Does Welded Win?
| Factor | Field-Erected Welded | Bolted |
|---|---|---|
| ASME pressure vessel service | ✅ Required | Not available |
| High-temperature storage | ✅ No gasket limits | Gasket degradation risk |
| Custom or unusual geometry | ✅ Unlimited | Round vertical only |
| Very large diameters (100+ ft) | ✅ Standard | Available but less common |
| API 650 / API 620 code compliance | ✅ Standard | Typically AWWA D103 |
| Speed of installation | Longer | ✅ Faster |
| Factory coating quality | Field-applied | ✅ Factory-baked |
| Future relocation | Permanent | ✅ Disassembles |
MMI builds both. Read our full comparison: Bolted vs Welded Steel Tanks: Choosing the Right Construction
Tank Repair, Inspection, and Reconstruction (API 653)
MMI’s National Board R stamp and API 653 expertise mean we don’t just build new tanks — we assess, repair, reline, and reconstruct existing field-erected tanks. Services include:
- API 653 fitness-for-service assessments
- Floor and shell plate replacement
- Nozzle additions and modifications
- Internal and external relining
- Seismic retrofits and anchor bolt upgrades
- Settlement correction
Learn more about our API 653 repair services →
Why MMI for Your Next Welded Tank
- ASME S, U, and National Board R stamps — code-stamped tanks and pressure vessels fabricated and erected in-house
- 8-acre Phoenix fabrication plant — plasma tables, plate rollers, welding bays, overhead cranes to 7.5 tons
- AWS-certified welders in all processes and positions, all alloys
- In-house NDE — RT, UT, MT, PT examination by certified Level II technicians
- In-house coatings — surface prep and coating application with no subcontractor gap
- Licensed across 11 western states — AZ, CA, CO, NV, NM, OR, WA, WY, HI, AK, SD
- Proven clientele — Chevron, Shell, Honeywell, SRP, and repeat industrial customers
Field-Erected Tank FAQ
What is the largest welded tank MMI can build?
There is no practical upper limit on diameter or capacity. We have engineered and erected tanks exceeding several million gallons. Tank size is constrained by site footprint and foundation capacity, not our manufacturing.
How long does it take to erect a field-welded tank?
Timeline depends on diameter, height, code requirements, and NDE scope. A mid-size API 650 tank (50–100 ft diameter) typically takes 6–12 weeks for erection, exclusive of foundation and coating. Bolted tanks of similar capacity erect faster — that’s part of the bolted vs welded decision.
What welding processes does MMI use?
SMAW, FCAW, GMAW, GTAW, and SAW — all positions, all common structural and pressure vessel alloys including carbon steel, stainless, and chrome-moly. All welders are AWS-certified and procedures are qualified per ASME Section IX or AWS D1.1 as applicable.
Can MMI repair an existing field-erected tank?
Yes. Our National Board R stamp authorizes repairs and alterations to coded tanks and vessels. We perform API 653 assessments, floor and shell replacements, nozzle modifications, and relining on existing tanks across the western U.S.
Does MMI handle the foundation?
Yes. Our foundations team designs and pours concrete ringwalls, full-slab foundations, and pier foundations. Keeping foundation work in-house eliminates coordination delays between the civil and tank erection contractors.
What coating systems are available for field-erected tanks?
Epoxy (interior and exterior), zinc-rich primers, polyurea, high-temperature coatings, and NSF/ANSI 61 potable water linings. All surface preparation and application is performed by MMI’s industrial coatings division.
Request a Quote
Contact our engineering team to discuss your next field-erected tank project:
