Steel and fiberglass underground storage tanks are both used to store fuels, chemicals, and other regulated liquids below grade. The best choice depends on the stored product, site conditions, regulatory requirements, installation method, corrosion protection needs, monitoring requirements, and long-term ownership goals.
The main difference is that steel underground storage tanks provide structural strength and fabrication flexibility, while fiberglass tanks rely on fiberglass reinforced plastic construction for corrosion resistance. Modern steel underground storage tanks are not limited to bare steel construction. Protected steel tank technologies may include cathodic protection, external coatings, FRP jackets, double-wall containment, and interstitial leak detection.
For many applications, steel may be preferred when structural integrity, proven fabrication, product compatibility, monitoring options, and long-term service experience are important decision factors.
Quick Comparison Table
| Topic | Steel Underground Storage Tanks | Fiberglass Underground Storage Tanks |
|---|---|---|
| Primary material | Steel tank structure, often with corrosion protection, coating, jacket, or cathodic protection system | Fiberglass reinforced plastic construction |
| Main advantage | Strength, structural integrity, fabrication flexibility, and proven protected tank technologies | Nonmetallic corrosion resistance |
| Corrosion protection | May use cathodic protection, FRP jacketing, urethane coating, or other corrosion protection systems | Fiberglass material is inherently nonmetallic |
| Double-wall options | Available through protected steel tank technologies with interstitial monitoring | Commonly available in double-wall configurations |
| Monitoring | Double-wall steel tanks can allow continuous interstitial leak detection and monitoring | Double-wall fiberglass tanks can also include interstitial monitoring |
| Installation considerations | Requires proper handling, bedding, anchoring when needed, backfill, and compliance with installation instructions | Requires proper handling, bedding, anchoring when needed, backfill, and compliance with installation instructions |
| Repair considerations | Steel fabrication and repair knowledge is widely established | Repairs depend on fiberglass-specific procedures and qualified personnel |
| Typical applications | Fuel storage, petroleum, commercial, industrial, and regulated storage applications | Fuel storage, petroleum, commercial, industrial, and regulated storage applications |
| Key specification question | Which protected steel tank technology best fits the site and service? | Does fiberglass construction fit the stored product, site, and owner requirements? |
What Is a Steel Underground Storage Tank?
A steel underground storage tank is a shop-fabricated tank designed for below-grade storage. Modern steel USTs may include corrosion protection systems, double-wall containment, coatings, FRP jackets, cathodic protection, and monitoring features depending on the tank technology and project requirements.
STI/SPFA licenses underground steel storage tank technologies including ACT-100®, ACT-100-U®, PERMATANK®, and sti-P3®. STI/SPFA describes ACT-100® as a steel underground storage tank with corrosion-resistant secondary containment, structural integrity from the steel tank, long-lasting corrosion protection from the outer coating, and double-wall design that allows continuous interstitial leak detection and monitoring.
STI/SPFA also describes sti-P3® as a cathodically protected double-wall underground storage tank with a pre-engineered corrosion protection system and ongoing monitoring of corrosion protection.
What Is a Fiberglass Underground Storage Tank?
A fiberglass underground storage tank is made from fiberglass reinforced plastic, often referred to as FRP. Fiberglass tanks are nonmetallic and are commonly selected for their corrosion-resistant material properties.
Fiberglass tanks may be used for many of the same broad underground storage applications as steel tanks, including fuel and petroleum storage. As with any underground storage system, proper specification, installation, compatibility review, monitoring, maintenance, and regulatory compliance are essential.
Key Differences Between Steel and Fiberglass Underground Storage Tanks
Material and Structure
Steel tanks use steel as the primary structural material. This gives the tank strength, rigidity, and resistance to many external loads when properly designed, installed, and protected.
Fiberglass tanks use fiberglass reinforced plastic as the primary structural material. This provides nonmetallic corrosion resistance, but the tank’s performance still depends on proper design, handling, installation, backfill, anchoring, and service compatibility.
Corrosion Protection
Corrosion protection is one of the most important differences in the steel vs. fiberglass discussion.
Modern steel underground storage tanks use engineered corrosion protection systems. Depending on the technology, this may include coatings, cathodic protection, FRP jackets, urethane coatings, secondary containment, and monitoring. STI/SPFA notes that ACT-100® tanks use fiberglass-reinforced plastic on the exterior, while PERMATANK® jacketed tanks are double-wall tanks with the entire outer tank being FRP.
Fiberglass tanks are nonmetallic, so they are often discussed in terms of inherent external corrosion resistance. However, corrosion resistance is only one part of tank selection. Product compatibility, installation, structural requirements, monitoring, repairability, and owner preference also matter.
Double-Wall Containment and Monitoring
Both steel and fiberglass underground storage tanks can be specified in double-wall configurations.
For steel tanks, double-wall technologies such as ACT-100® allow continuous interstitial leak detection and monitoring. Double-wall designs are especially important in regulated storage applications where leak detection and environmental protection are major concerns.
Product Compatibility
Product compatibility should be reviewed for any tank material. Stored products may include gasoline, diesel, biofuels, heating oil, aviation fuel, chemicals, or other regulated liquids.
STI/SPFA’s underground storage tank maintenance guidance notes that storage tank systems of any material and storing many types of products can be affected by water-related maintenance issues, including fuel quality degradation, microbial contamination, and system damage.
Installation and Site Conditions
Both steel and fiberglass tanks require proper installation. Installation quality can affect long-term performance regardless of tank material.
Important installation factors may include excavation, bedding, backfill, anchoring, groundwater conditions, traffic loads, site access, handling, tank placement, and compliance with applicable federal, state, and local requirements.
STI/SPFA’s ACT-100-U® installation instructions state that installation instructions are intended as an aid to knowledgeable and experienced underground tank installers and that compliance with those instructions does not necessarily satisfy all applicable federal, state, and local laws, regulations, and ordinances.
Repairability and Long-Term Service
Steel tanks benefit from a long-established fabrication and repair knowledge base. Steel is weldable, strong, and familiar to tank fabricators, installers, inspectors, and maintenance professionals.
Fiberglass repairs require fiberglass-specific procedures, materials, and qualified personnel. The right approach depends on the type of tank, location of the issue, service conditions, manufacturer requirements, and applicable regulations.
When Steel Underground Storage Tanks
Are Typically Used
Steel underground storage tanks are commonly used in fuel, petroleum, commercial, industrial, and regulated storage applications. They may be especially appropriate when the project requires:
| Application Need | Why Steel May Fit |
|---|---|
| Structural strength | Steel provides a strong primary tank structure |
| Protected tank technology | Steel USTs can be specified with coatings, jackets, cathodic protection, or secondary containment |
| Double-wall monitoring | Protected steel tank technologies can include interstitial leak detection |
| Fabrication flexibility | Steel is well suited for shop fabrication and project-specific requirements |
| Proven industry use | STI/SPFA notes long-term performance for more than a quarter million sti-P3® steel underground storage tanks |
When Fiberglass Underground Storage Tanks
Are Typically Used
Fiberglass underground storage tanks are commonly used when a nonmetallic tank material is preferred. They may be considered when the owner, engineer, or project specification prioritizes:
| Application Need | Why Fiberglass May Fit |
|---|---|
| Nonmetallic construction | Fiberglass reinforced plastic is not steel |
| External corrosion resistance | FRP is commonly selected for corrosion-resistant material properties |
| Double-wall containment | Fiberglass tanks are commonly available in double-wall configurations |
| Certain owner preferences | Some owners or specifications may standardize around fiberglass tanks |
| Specific site or product conditions | Suitability depends on compatibility, installation, and project requirements |
Why Steel May Be Preferred for
Underground Storage Tanks
Steel may be preferred for underground storage tanks because it combines structural strength with modern corrosion protection technologies. The comparison should not be framed as bare steel versus fiberglass. Today’s protected steel USTs may use engineered systems that include coatings, FRP jackets, cathodic protection, secondary containment, and monitoring.
Steel underground storage tanks may be a strong choice when the project requires:
| Decision Factor | Steel Advantage |
|---|---|
| Strength | Steel provides structural integrity and load resistance when properly designed and installed |
| Corrosion protection options | Steel tanks can be protected through coatings, cathodic protection, FRP jackets, or composite systems |
| Monitoring | Double-wall steel tank technologies can support interstitial monitoring |
| Fabrication knowledge | Steel tank fabrication, inspection, and repair practices are well established |
| Product and application range | Steel tanks are used across fuel, petroleum, commercial, industrial, and regulated storage applications |
| STI/SPFA technology support | STI/SPFA licenses multiple underground steel storage tank technologies, including ACT-100®, ACT-100-U®, PERMATANK®, and sti-P3® |
Common Confusion Points
Are modern steel underground tanks just bare steel?
No. Modern steel underground storage tanks are often protected by engineered corrosion protection systems. Depending on the tank technology, that protection may include coatings, cathodic protection, FRP jackets, urethane coatings, double-wall construction, and interstitial monitoring.
Are fiberglass tanks always
maintenance-free?
No. Corrosion protection is important, but tank selection should also consider structural loads, installation conditions, groundwater, anchoring, product compatibility, monitoring, repairability, regulatory requirements, and long-term ownership needs.
Is corrosion the only factor in tank selection?
When a heat exchanger contains pressurized fluids, its pressure-retaining parts may need to comply with pressure vessel design and fabrication requirements. The equipment may be selected for heat transfer, but it still must safely contain pressure.
Can steel tanks
include fiberglass?
Yes. Some protected steel underground storage tank technologies use fiberglass reinforced plastic as part of the corrosion protection or jacket system. STI/SPFA notes that ACT-100® tanks are externally coated with fiberglass-reinforced plastic, while PERMATANK® jacketed tanks have an FRP outer tank.
FAQs
Related Resources
STI Technologies
Underground Shop-Fabricated Storage Tanks Member Directory
ACT-100®
sti-P3®
Underground Storage Tank Maintenance
Codes & Regulations
ACT-100-U®
ACT-100-U® Installation Instructions
Steel Tanks & Pipe Fabrication
Choosing an underground storage tank material?
STI/SPFA members fabricate protected steel underground storage tanks for fuel, petroleum, commercial, industrial, and regulated storage applications. Learn more about STI/SPFA underground storage tank technologies or find a member fabricator for your next project.
