Double Wall Storage Tanks

A double wall tank is a tank-within-a-tank with a primary storage container enclosed by an integrated outer shell. The outer shell functions as a built-in secondary containment unit that catches leaks should the inner tank fail instead of reaching the workfloor, soil, or a drain. Choosing a double wall tank depends on the storage contents, which affects product resin type and specific gravity rating, and applicable codes, standards or legal requirements: SPCC, RCRA, fire code, insurer, or company policy, as each can set a different containment target. If these two are understood, the rest is tank-to-application configuration such as fittings, monitoring, and accessory components.

Tell us your chemical, concentration, temperature, and volume, and we can help match the resin, SG, and containment unit with a freight quote to your site.

How to Choose a Double Walled Plastic Tank

1. Resin & SG Selection

Double wall tanks are available in either linear HDPE (HDLPE) or crosslinked polyethylene (XLPE), in specific gravity ratings of 1.5 and 1.9, depending on the tank line and material. The rating must be matched to the density of the storage fluid, at minimum, (a 1.9 SG rated tank can handle roughly 15.8 lb/gal liquids). Resin choice is application-specific: XLPE offers higher environmental stress-crack and impact resistance, in general; HDLPE is weldable and can be modified to accommodate custom fittings.

For aggressive oxidizers, there are chemical-specific resin packages, including proprietary sodium hypochlorite (resin #880059), often UV-opaque to slow degradation, and sulfuric acid (resin #880046) formulations.

2. Containment Requirements by Rule

Secondary containment applies to more than one rule. Oil is governed by SPCC (40 CFR 112); hazardous waste tank systems by RCRA (40 CFR 264/265.193); hazardous materials generally by the IFC/IBC and your state fire code; and insurers or corporate EHS policy may establish its own rules. Each rule may set a different required containment volume and freeboard.

A satisfactory tank product depends on the liquid type and properties, total volume, site parameters, and often the approval of a PE or local governing body. A dual wall tank provides engineered secondary containment in one unit, supporting compliance, and an engineer can confirm it meets your company-specific obligation. We won’t tell you it makes you compliant; no honest vendor can.

3. Primary Volume & Footprint

Double wall storage tank capacities run 35 to 12,500 gallons. The benefit of a double walled tank over a single wall tank placed in a separate concrete dike is footprint: the containment is built-in, so one system can potentially be specified over engineering and pouring a basin around a tank.

4. Draw Configuration, Fittings, Accessories, and Freight

Double wall tanks can be configured with drains, fill lines, draw pipes, U-vent assemblies, OSHA rated ladders, tie-down systems / seismic restraints, level indicators, manways, and interstitial leak detectors.

For discharge, the double wall design requires a bottom outlet to pass through both walls using a through-containment fitting, or a top draw line. Sidewall bulkheads cannot be added the same way they would be on a single wall tank. Decide draw configuration before you order.

Always use flexible fittings as inner and outer walls expand and flex independently under hydrostatic load. Rigid piping tied to a through-containment fitting can shear pipes or crack nozzles, and typically voids tank warranty.

Double wall tanks also ship heavier and bulkier per gallon than single walled equivalents and may require a different freight class and offloading.

5. DWC vs. Captor Style Tanks

DWC series dual wall tanks come in 35–1,500 gallons, ideal for mini-bulk and day tanks; ≤250 gal options clear standard doorways. Forklift channels on 275, 360, and 500 gal. Captor tanks range from 550–12,500 gallons, ideal for bulk and tanker-fed uses; feature a U.F.O. fitting, 18” standard manway up to 24”, and add-on options for OSHA ladders, tie-down lugs, sensors, level indicators, fill/draw lines, and vent assemblies.

6. Used Oil Containment Tanks

Used oil tanks answer to 40 CFR 279.22, not the chemical-containment rules, and they’re built for it: safety-yellow secondary (black primary), 2” top-draw quick-connect for the hauler’s pump-out, hinged lockable manway, and a debris strainer/sink. Containment runs 110–115% (Snyder WOS) or 110% (Chem-Tainer Oil-Tainer). Browse used oil tanks for our full selection.

Double Wall Tank Spec & Logistics

Dual Wall Polyethylene Resins & Compliance Standards

Resin

HDLPE for weldability, customization capacity, and value on chemically compatible, non-critical, and water related applications. XLPE for roughly an order of magnitude more stress-crack resistance and higher impact strength on demanding chemical duty. Neither is universally better for all services. The right choice is a fluid-by-fluid determination, and for oxidizers, selection is specific per compatibility data. We evaluate your quote resin selection against verified data to ensure no validated concerns.

Specific gravity

Ratings of 1.5 and 1.9. Match tank SG rating to fluid density, minimum. Confirm chemical, concentration, activity, and temperature compatibility together as full application suitability and tank structural derating shifts with temperature. For example, a heated liquid with a 1.45 specific gravity may be better matched to a 1.9 SG tank over a 1.5 SG tank, especially when factoring in long term service.

ASTM D1998

Primary, vertical cylindrical tanks of 500 gallons or more are designed to meet or exceed ASTM D1998, the specification for one piece rotomolded polyethylene upright storage tanks in atmospheric service (Type I XLPE continuous service below 150°F, Type II HDLPE below 140°F). ASTM D1998 does not apply to the smaller day tank sizes or to non-cylindrical containment units. ASTM D1998 does not cover external, imposed loads like seismic or wind that are covered by local building codes and an engineer.

Containment

The outer shell is engineered to hold a defined percentage of the primary tank’s contents, commonly to bring the full containment capacity to 110% of the volume rating. This is a build specification, not a compliance certification. Whether a double wall tank satisfies SPCC, RCRA, or fire code obligations depends on the fluid, volume, jurisdiction, and professional sign-off. One practical advantage: because double wall tanks are fully enclosed, precipitation never enters the containment. This eliminates the concerns with open dikes that have to be sized with added freeboard for storm events and then pumped out after rainfall.

Features → applications

Logistics & freight

Double wall tanks ship fully assembled from our warehouse network either LTL or flatbed depending on size. They are heavier and larger per gallon of storage than a single wall tank of the same capacity, which affects freight class and handling. Plan for liftgate or forklift offload on larger units and confirm site access. We quote freight to your delivery point and flag offload requirements up front to eliminate surprises at receipt. Honest lead times by size on request.

Frequently Asked Questions about Double Wall Tanks

Related Products