Tank fabrication is the work of turning steel plate, heads and fittings into a finished storage or process tank, through design, cutting, rolling, welding, inspection, testing and finishing. Paul Industries fabricates and installs custom stainless and carbon steel tanks for pharmaceutical, biotech, food and beverage, dairy, data center, oil and gas and general industrial plants nationwide, from small jacketed mix tanks to field-built storage tanks. This page covers materials, shop-built versus field erected construction, the governing standards, design inputs, the fabrication process, testing and cost.
What is tank fabrication?
Tank fabrication builds a tank to a specific duty instead of picking one from a catalog. The fabricator turns a process requirement, such as volume, fluid, temperature, pressure, cleaning method and footprint, into drawings, then builds, inspects and tests the tank to the standard that governs it. Our tank fabrication services cover that whole chain, plus delivery, setting and piping tie-in. Tank fabricators differ from catalog tank manufacturers in one main way: a manufacturer builds a standard product range in fixed sizes, while a fabricator builds to your drawing. Stainless steel tank manufacturers with standard lines are a good fit for common sizes and simple duty; custom tank fabrication earns its place when the nozzle layout, jacket, agitator, finish, height limit or documentation has to fit a particular plant and process.
What steel are tanks made of?
Industrial tanks are made of carbon steel or austenitic stainless steel, with duplex and higher alloys for aggressive service. The fluid, the cleanliness required and the cost decide which. Carbon steel plate such as ASTM A36, A283 Grade C or A516 Grade 70 is the usual steel plate for tank fabrication in water, oil, fuel and many chemical services, with internal linings or a corrosion allowance where the fluid demands it. Mild steel tank fabrication is the lowest-cost route for large volumes, but the tank needs a coating system inside and out. Stainless plate is bought to ASTM A240, and 304L or 316L is standard for welded construction, because the low-carbon L grades, at 0.03 percent carbon maximum, resist intergranular corrosion beside the welds.
| Material | Strengths | Watch for | Typical tanks |
|---|---|---|---|
| Carbon steel | Lowest cost, easy to weld, high strength | Needs coating or lining; rust contaminates clean fluids | Fire and process water, fuel and oil, condensate, many chemicals |
| 304L stainless | Good general corrosion resistance, no coating needed | Pitting and stress corrosion cracking in warm chlorides | Water, many food products, mild chemicals |
| 316L stainless | Two to three percent molybdenum improves chloride resistance | Higher cost; still limited in hot, concentrated chlorides | Purified water, pharmaceutical and biotech, CIP, many process chemicals |
| Duplex and higher alloys | Much higher chloride and stress corrosion resistance | Stricter welding heat input control, longer lead times | Brines, seawater, aggressive chemicals |
Steel tank fabrication in carbon steel and stainless steel tank fabrication run on separate tools and work areas, because iron picked up from carbon steel grinding wheels, brushes or clamps rusts on stainless surfaces. Custom stainless steel tanks are specified by grade, interior and exterior finish, weld finish and documentation, and welded stainless steel tanks are passivated after fabrication to restore the chromium oxide layer. Chemical tank fabrication starts from a compatibility check of the fluid, concentration and temperature against the alloy, gaskets and any lining, since the cheapest material that survives is rarely obvious.
Shop-built vs field erected construction
Tanks small enough to ship are built complete in a shop; larger tanks are built where they will stand. The limit is transport: as a rule of thumb, a tank about 12 feet in diameter, typically 50,000 gallons or less, is near the practical limit for road shipping, and loads wider than 8.5 feet already need permits. Field erected tanks, built from rolled plates delivered to site, cover everything larger, and on site tank fabrication is also used when a tank must be built inside a building or a congested plant where a finished tank cannot be brought in.
| Factor | Shop-built | Field erected |
|---|---|---|
| Size | Limited by road transport and site access | Limited by the design standard and foundation |
| Quality control | Controlled environment, positioners, easier inspection | Weather protection and access planning needed for welding |
| Schedule | Built in parallel with site work, short install | Longer site time, but no shipping limit |
| Typical uses | Process, mix, hold, day and chemical tanks | Large water, fire water, fuel and product storage |
Most shop-built process tanks are vertical tank fabrication on legs, skirts or a flat bottom; horizontal tanks on saddles suit long, low rooms and buried or vaulted service.
Tank fabrication standards
The governing standard depends on the tank’s pressure, temperature, contents and size, and each standard sets the design rules, materials, welding, inspection and testing. The main references are:
| Standard | Covers |
|---|---|
| API 650 | Welded atmospheric storage tanks; base scope to 200 degrees F, internal pressure up to 2.5 psig with Annex F, higher temperature with Annex M |
| API 620 | Large welded low-pressure storage tanks, up to 15 psig |
| AWWA D100 and D103 | Welded and bolted carbon steel tanks for water storage |
| UL 142 | Shop-fabricated steel aboveground tanks for flammable and combustible liquids |
| ASME BPVC Section VIII, Division 1 | Pressure vessels; operating pressures of about 15 psi and below fall outside its scope |
| ASME BPE and 3-A | Hygienic design, finish and welding for bioprocessing, dairy and food contact |
API 650 tank fabrication applies to welded steel tanks for oil storage, which is the standard’s own title subject, and owners routinely specify it for other atmospheric liquids. AWWA D100 covers welded steel tanks for water storage. The tank design pressure, internal and vacuum, decides which column a tank falls in: a vented atmospheric tank, a low-pressure tank, or a pressure vessel. Code stamps and listings, such as the ASME U stamp and the UL label, are issued to the shop that builds the tank under that program; the purchase specification should state which standard applies and whether a stamp or label is required, and we confirm that with the owner before design starts. Welders and welding procedures are qualified under ASME Section IX.
Tank design inputs: volume, heads, nozzles, vents and foundations
Storage tank design starts from the working volume, the fluid, and the site; process tank design adds what happens inside the tank, such as mixing, heating, cooling and cleaning. The inputs every tank drawing needs:
- Volume and shape. Working volume plus freeboard, the height-to-diameter ratio that fits the room, and the bottom: flat, sloped, dished or cone for full drainage. Our tank volume calculator converts dimensions to gallons.
- Heads and roof. Tank roof design ranges from flat and cone roofs on atmospheric tanks to dished and elliptical heads on process and pressure-rated tanks, with manways, lifting lugs and access platforms placed for maintenance.
- Nozzles. Inlet, outlet, drain, overflow, vent, instrument, sample, CIP and manway connections, each with size, rating, orientation and projection on the drawing.
- Venting. Tank vent design sizes the normal vent for fill, empty and thermal breathing, and the emergency vent for fire exposure, under API Standard 2000 for atmospheric and low-pressure tanks. A blocked or undersized vent can collapse a tank under vacuum when it is pumped out.
- Supports and foundation. Legs, skirts, saddles, lugs or a flat bottom on a slab or ringwall. Tank foundation design checks the full hydrostatic load, settlement, anchorage for wind and seismic load, and drainage under the floor; API 650 Annex B gives recommendations for storage tank foundations.
- Insulation and finish. Insulation, cladding, interior surface finish and exterior coating, matched to the service and the room.
Custom water storage tanks for fire protection, process water and RO permeate break tanks follow the same inputs, and industrial steel tanks for fuel, oil or chemicals add containment, overfill protection and the fire code. Fabricated steel tanks in any of these services are only as good as the drawing they are built to.
Mixing tank design, baffles and jackets
Good design here matches the agitator, the vessel shape and the internals to what the batch must do: blend liquids, suspend solids, dissolve powders, or move heat. Tank baffle design keeps the liquid from swirling as one body around the shaft; four baffles, each about one-twelfth of the tank diameter wide and set slightly off the wall, is the standard starting point for turbulent mixing, and sanitary tanks often use removable or off-wall baffles so they can be cleaned. Tank jacket design chooses among dimple jackets, half-pipe coils and conventional jackets: dimple jackets are light and economical for moderate pressures, half-pipe coils take higher jacket pressures and temperatures, and conventional jackets suit low-pressure heating and cooling media. Stainless steel mixing tank fabrication then adds the agitator mount, which must be stiff enough to hold the shaft in alignment, and a nozzle and spray device layout that does not leave shadowed surfaces.
Sanitary tank fabrication adds polished product-contact surfaces, ground and polished welds, fully drainable bottoms and hygienic nozzle design under ASME BPE or 3-A. For the details, see sanitary process tanks, pharmaceutical process tanks and vessels and food and beverage process tanks, and ASME BPE surface finish for finish designations.
The tank fabrication process
Fabrication runs in the same order on every job, with a hold point before each step that cannot be undone.
- Drawings and approval. The tank fabrication drawing shows dimensions, plate thicknesses, nozzle schedule and orientation, welds, finishes, supports and the design basis; the owner approves it before material is cut.
- Material receipt. Plate, heads, pipe and fittings arrive with mill test reports, and heat numbers are tracked to each part.
- Cutting and forming. Shell plates are cut, beveled and rolled to the diameter; heads are bought formed or formed to the drawing.
- Fit-up and welding. Shell courses, heads and bottoms are fitted and welded, then nozzles, jackets, supports and internals. Each stainless steel tank welding procedure controls heat input and back-purges the root so that the inside of the weld is not oxidized; welds are identified on a weld map.
- Inspection. Visual inspection of every weld, dimensional checks, and the radiography, dye penetrant or other examination the standard and specification call for.
- Testing. Hydrostatic or pneumatic testing of the shell and jacket as required. See hydrostatic testing.
- Finishing. Grinding and polishing, cleaning to ASTM A380, passivation to ASTM A967, or blasting and coating for carbon steel. See tank passivation.
- Documentation and shipping. Turnover package, protection of nozzles and finishes, and shipping to site.
Bottom and roof welds are leak tested too: on flat-bottom storage tanks, bottom welds are commonly checked with a vacuum box before the hydrostatic test, and jackets are pressure tested separately from the shell. The turnover package for a finished tank normally contains the approved drawing, mill test reports traced to each part, the weld map, welding procedure and welder qualification records, examination reports, the test report, the cleaning and passivation certificate for stainless, the coating record for carbon steel, and the nameplate data. For regulated plants, that package is what lets the tank enter installation qualification without a document hunt.
An ITP for tank fabrication, the inspection and test plan, lists every inspection and test, its acceptance criteria, the record produced, and the hold and witness points where the owner or a third-party inspector signs off. The same plan structure covers process tank fabrication for a single mix tank and industrial tank fabrication for a set of utility tanks; storage tank fabrication in the field adds settlement checks during the hydrostatic test and inspection of the foundation before the bottom is laid.
Welded tanks vs bolted tanks
The choice is between a permanent, gasket-free shell and a faster, factory-coated kit. Welded steel tanks have continuous welded seams with no gaskets to age or leak, can be repaired and modified by welding, and suit process, chemical, product and hot service. Bolted tanks, built to standards such as AWWA D103 for water, use factory-coated panels bolted together with gaskets on site, which is quick to erect and suits remote water, fire water and some dry storage, but the gaskets and coating are the maintenance points. For process plants and anything that must be cleaned, held hot or kept free of contamination, welded construction is the norm.
Process tank vs storage tank
The difference is what happens inside the tank. A storage tank holds a fluid until it is needed and is designed mainly for volume, containment, venting and corrosion. A process tank changes the fluid, so it adds agitators, jackets, spray devices, instruments, load cells and tighter finish and drainability requirements. Pressure-rated reactors and vessels sit at the end of that range, and vessel fabrication for pressure service is governed by the boiler and pressure vessel code. For installation of finished vessels, see process and storage tank installation.
Testing, inspection and service life
Every new tank is tested before service: atmospheric storage tanks under API 650 are filled with water to the design liquid level before permanent external piping is connected, and pressure-rated vessels are hydrostatically tested at the pressure their code sets. As for how often should tanks be inspected in service, owners run routine visual checks, and the formal intervals come from the inspection standard: for API 650 tanks, API 653 requires an external inspection by an authorized inspector at least every five years, or sooner where the corrosion rate demands, with internal inspection intervals set from measured corrosion; shop-fabricated tanks commonly follow STI SP001. Stainless tanks in clean service mainly need checks of welds, nozzles, supports and passivation condition.
Tank fabrication cost
Tank fabrication cost is driven by material and alloy, plate thickness, the number of nozzles and their finish, jackets, agitator mounts and internals, interior finish and weld polishing, inspection and testing level, documentation, and shipping and setting. A plain carbon steel storage tank costs far less per gallon than a polished, jacketed 316L process tank of the same volume, because the labor goes into nozzles, welds and finish, not plate. The fastest way to an accurate price is a clear data sheet: volume, fluid, design pressure and temperature, material, finish, nozzle list, supports and the governing standard.
Tank fabrication and installation
Paul Industries handles fabrication, delivery and setting as one scope, so the tank that leaves the shop is the one that fits the room. Our crews ship, rig and set the tank, then make the piping, jacket and instrument connections; see millwright and rigging and stainless steel piping. We also repair and modify existing tanks: new or relocated nozzles, jacket repairs, manway changes, support repairs, re-passivation and pressure retesting. A modification starts with the original drawings and data report where they exist, or a field survey with thickness readings where they do not, so the change is designed against the tank as it actually stands. Stainless work inside an operating plant is done under hot work and confined space permits, with the tank isolated, cleaned and ventilated first, and with carbon steel contamination kept away from the product surfaces. Tanks are inspected and maintained under our industrial maintenance services.
What Paul Industries does
Paul Industries designs, fabricates, tests and installs stainless and carbon steel storage, process, mix and jacketed tanks, in the shop and in the field, with qualified welding procedures, material traceability, inspection and test plans, passivation and full documentation. We do not sell catalog tanks or plastic tanks. Larger projects often combine tanks with skids and piping; see process skid fabrication and engineering and design. Request a quote with your tank data sheet or drawing.
Standards referenced: API 650 · API 620 · API 653 · API Standard 2000 · ASME BPVC Section VIII Division 1 · ASME BPVC Section IX · ASME BPE · 3-A Sanitary Standards · AWWA D100 · AWWA D103 · UL 142 · STI SP001 · ASTM A240 · ASTM A380 · ASTM A967
Frequently asked questions
How are stainless steel tanks made?
Stainless plate is cut and rolled into shell courses, welded to formed heads, then fitted with nozzles, supports, jackets and internals. Welds are made with TIG welding under an argon purge, inspected, and ground or polished where the finish requires it. The tank is then pressure or leak tested, cleaned and passivated.
How long do stainless steel tanks last?
Correctly specified, fabricated and passivated stainless tanks in clean service commonly last for decades. Service life is shortened mainly by chloride pitting or stress corrosion cracking, surface iron contamination, fatigue at nozzles and supports, and cleaning chemicals stronger than the alloy was chosen for.
What are stainless steel tanks used for?
Holding and processing fluids that would corrode carbon steel or must not pick up iron: purified water, pharmaceutical and biotech products and buffers, food, dairy and beverages, cleaning solutions, many process chemicals, and RO permeate.
What is a field erected tank?
A tank built on its foundation at the site from plates and components delivered there, rather than built complete in a shop and shipped. Field construction is used when the tank is too large to ship by road or cannot be brought into its final location in one piece.
Buffer tank vs storage tank: what is the difference?
A storage tank holds inventory until it is used. A buffer tank adds volume to a running system to smooth it out: in a chilled water or glycol loop it stops a chiller from short-cycling, and in a process it absorbs differences between supply and demand flow.
What is tank freeboard?
The vertical space between the maximum operating liquid level and the top of the shell or overflow. It is reserved for surge, foam, thermal expansion and, in seismic regions, the sloshing wave, so the tank does not overflow or load its roof.
What information do you need to quote a tank?
Working volume, fluid and concentration, design pressure and vacuum, design temperature, material and finish, nozzle list, jacket and agitator requirements, support type, height or footprint limits, the governing standard and the documentation package required.
Is a stainless tank worth the extra cost over carbon steel?
Where the fluid would corrode carbon steel, must stay free of iron, or is cleaned repeatedly, yes: stainless avoids linings, recoating and contamination. For water, oil and fuel storage where a coating system works, carbon steel usually gives the lower life-cycle cost.
Can new nozzles be added to an existing tank?
Usually. The new nozzle is checked for reinforcement and loads, cut in and welded under a qualified procedure, examined and pressure tested as the governing code requires, and stainless surfaces are cleaned and passivated afterward. For a code-stamped vessel the alteration must follow the National Board Inspection Code.
Should a tank be hydrotested in the shop or in the field?
Shop-built tanks are normally tested in the shop, where water, supports and inspection are easier. Field-built tanks are tested on their foundation, which also checks settlement. A tank with field welds or field modifications needs those joints tested after the work is done.
Which head type should a tank have?
Flat or cone roofs suit atmospheric storage, dished or elliptical heads suit process tanks and pressure or vacuum service, and cone bottoms suit tanks that must drain completely or handle solids. The choice also sets height, cost and how well the tank cleans.
What causes rust spots on a new stainless tank?
Usually free iron embedded during fabrication or handling, from carbon steel tools, clamps, grinding dust or contact with carbon steel structures. Cleaning and passivation remove it; contamination control in the shop prevents it.
Can carbon steel tanks hold potable or process water?
Yes, with an internal lining or coating suited to the water and, for drinking water, certified for that use. The coating then becomes the main maintenance item, so it is inspected on a schedule and repaired before corrosion starts underneath.
Why do tanks collapse when they are emptied?
Pumping a tank out faster than air can enter through the vent pulls a vacuum, and a thin-walled atmospheric tank is not designed for it. Blocked vents, undersized vents, frozen vent valves and steaming or rinsing followed by rapid cooling are the usual causes.
Do you fabricate plastic or fiberglass tanks?
No. Paul Industries fabricates stainless steel and carbon steel tanks. Plastic and fiberglass tanks are made by specialist manufacturers, and we can connect them to the plant's piping where they are the right choice.
Related: Process & Storage Tank Installation · Pharmaceutical Process Tanks And Vessels · Food And Beverage Process Tanks · Sanitary Process Tanks: Types & Design · Stainless Steel Piping · Tank and Vessel Passivation · Request a quote