Inspectioneering Journal

Tank Inspections per STI SP001 5th Edition

By Dana Schmidt, Standards Engineer at STI/SPFA. This article appears in the July/August 2013 issue of Inspectioneering Journal

Tank inspectors and owners, did you know?

  • Each year about 14,000 oil spills are reported, according to the EPA website.
  • It is estimated that there are 618,000 facilities that are regulated under EPA SPCC in the United States.
  • There are more regulated aboveground storage tanks than underground storage tanks in the United States.
  • Tanks under 300,000 gallons represent 90% of the actual tank units in operation.
  • STI estimates that as many as 4 million shop-fabricated oil storage tanks have been produced since the mid-1960’s. It is difficult to assess how many of these tanks are still in use today, but anecdotal evidence suggests that tanks are often used for 40 years or more.

It is no wonder that the Steel Tank Institute/Steel Plate Fabricators Association (STI/SPFA) continually reviews and updates standards, researches new technologies, and provides education and information to the public and industry professionals. STI/SPFA is a trade association representing fabricators of steel construction products and their suppliers. Member companies produce steel storage tanks, field erected water tanks, pressure vessels and heat exchangers, and pipe and pipelines. Their customers are from the petrochemical, power generation, food, pharmaceutical, fuels, wastewater and water transmission industries.


The U. S. Environmental Protection Agency (EPA) originally promulgated the Spill Prevention, Control, and Countermeasure (SPCC) Rule in 1973, requiring certain non-transportation related facilities to develop and implement an SPCC Plan. The purpose of the SPCC regulation is to prevent the discharge of oil into U.S. navigable waters. A facility’s SPCC Plan describes the preventive measures established so that a spill at the facility is contained. It also describes countermeasures in the event of a spill to prevent the spill from reaching navigable waters. A facility must have a Spill Plan, certified by a licensed Professional Engineer, if it has an aboveground oil storage capacity greater than 1,320 gallons or a buried underground oil storage capacity greater than 42,000 gallons.

After several interim modifications, the EPA formally amended the SPCC Rule to make it largely performance-based in 2002. The EPA then issued the Final Revised SPCC Rule regarding the storage and handling of oils, both petroleum and non-petroleum (40 CFR Part 112).

  • Facilities affected: This Rule has bearing on a multitude of facilities that store oil in containers of many constructions and sizes. Facilities affected are those with total aboveground (i.e. not completely buried) oil storage capacity greater than 1,320 gallons. Containers of oil, petroleum and non-petroleum, of 55 or more gallons are to be included when determining the total oil storage capacity at a facility. Prior to 2002, the smallest container considered for inclusion in the total oil storage at a facility was 660 gallons (Refer to further requirements on the type of tank installations, such as partially buried tanks, found in §112.1 of 40 CFR Part 112).
  • Integrity testing: In §112.8(c)(6) [petroleum oils] and §112.12(c)(6) [non-petroleum oils], the Rule states,

“Test or inspect each aboveground container for integrity on a regular schedule and whenever you make material repairs. You must determine, in accordance with industry standards, the appropriate qualifications for personnel performing tests and inspections; and the frequency and type of testing and inspections which take into account container size, configuration, and design (such as containers that are shop-built, field-erected, skid-mounted, elevated, equipped with a liner, double-walled, or partially buried). Examples of these integrity tests include, but are not limited to: visual inspection, hydrostatic testing, radiographic testing, ultrasonic testing, acoustic emissions testing, or other systems of non-destructive testing. You must keep comparison records and you must also inspect the container's supports and foundations. In addition, you must frequently inspect the outside of the container for signs of deterioration, discharges, or accumulation of oil inside diked areas. Records of inspections and tests kept under usual and customary business practices satisfy the recordkeeping requirements of this paragraph.”

Note that the original SPCC Rule (dating from 1973) did not mandate integrity testing, but rather stated that tanks should undergo periodic integrity testing. This means that, historically, a large number of facilities may not have had a tank inspection program, but as of 2002, they are now required to have one.

Implementation of the Rule’s 2002 amendment was delayed and altered several times. The Final SPCC Rule was published in November 2009, with compliance dates for most facilities to prepare (or amend, if applicable) and implement a Spill Plan no later than November 10, 2010.

Importance of Industry Standards

Prior to 2000, only the American Petroleum Institute (API) Standard 653, “Tank Inspection, Repair, Alteration, and Reconstruction,” included requirements for the inspection of aboveground storage tanks. The focus of API 653 is large, field-erected tanks built to API Standard 650, “Welded Steel Tanks for Oil Storage." API 650 includes equations for calculating the steel thickness needed for a particular tank. These equations take into account pressure encountered, the type of steel and weld joints used, and the weld inspection testing employed. Extensive inspection requirements are included in API 653 because of the large volumes and hydrostatic pressures encountered in these larger tanks.

STI/SPFA developed its first edition of Standard SP001 in 2000. With the support of the EPA, STI/SPFA used a consensus process to further modify SP001. The committee that developed the 3rd Edition and subsequent editions of SP001 included representatives from major oil companies, petroleum marketers, federal and state regulating agencies, tank fabricators, inspectors, and tank equipment suppliers.

SP001 includes inspection requirements for welded metal, shop-fabricated and small field-erected tanks. Also included is the inspection of smaller, portable containers such as 55-gallon drums, intermediate bulk containers (IBCs), and other such containers that may be of metal or plastic construction.

The construction and installation of smaller, shop-fabricated tanks differs greatly from that needed for large, field-fabricated tanks. The shop-fabricated tanks covered by STI SP001 are commonly manufactured to third-party standards, such as UL 142 or UL 2085. These Underwriters Laboratories standards use tables, rather than equations, to specify steel thickness based on tank diameter and capacity. Shop-fabricated tank capacity is rarely more than 50,000 gallons, and this smaller capacity means a lower hydrostatic pressure encountered at the tank bottom. Further, whereas API 650 tanks are always vertical, shop-fabricated tanks can also be horizontal cylindrical or rectangular. The bottom of the horizontal cylindrical tank is visible, and the tank supports used to elevate the tank must be inspected.

Small, portable containers are also included in the scope of SP001, as the inspection of these is not covered by another inspection standard.

Small, field-erected tanks with a maximum shell height of 50 feet and a maximum diameter of 30 feet are also included in the scope of SP001. Many facilities include a mix of tank types (see Figure 1) and inclusion of these tanks allows inspection of sites with a variety of tank sizes by a single inspector.

Figure 1. Facility with a mix of tank types
Figure 1. Facility with a mix of tank types

SP001 Terminology

Key to understanding the inspection schedule in SP001 are definitions of the terminology used in the standard. Here are some key concepts:

CONTINUOUS RELEASE DETECTION METHOD (CRDM) is a means of detecting a release of liquid through inherent design. It is passive because it does not require sensors or power to operate. Liquid releases are visually detected by facility operators. The system shall be designed in accordance with good engineering practices. Several acceptable and commonly used CRDM systems are as follows:

  • Release prevention barrier (RPB) (described in definition of “release prevention barrier”).
  • Secondary containment AST, including double-wall ASTs, double-bottom ASTs, or other ASTs described in definition of “secondary containment.”
  • Elevated AST with release prevention barrier.

Field-Erected AST is a welded metal AST erected on the site where it will be used. For the purpose of this standard, ASTs meeting either of the following descriptions are to be inspected as field erected ASTs:

  1. An AST where the nameplate (or other identifying means such as accurate drawings) indicates that it is a field-erected AST. These are limited to a maximum shell height of 50 feet (15.24 meters) and a maximum diameter of 30 feet (9.14 meters).
  2. An AST without a nameplate (or other identifying means such as accurate drawings) that is more than 50,000 U.S. gallons (189,271 liters) and a maximum shell height of 50 feet (15.24 meters) and a maximum diameter of 30 feet (9.14 meters).

Formal External Inspection (FEI) is a documented external inspection conducted by a certified inspector to assess the condition of the AST and determine its suitability for continued service, without entry into the AST.

Formal Internal Inspection (FII) is a documented internal inspection conducted by a certified inspector to assess the internal and external condition of the AST and determine its suitability for continued service. This includes the inspection requirements of a formal external inspection. A formal internal inspection satisfies the requirements of a formal external inspection and shall be considered equivalent to or better than a formal external inspection for the purposes of scheduling.

Leak Testing Method (LTM) is a point-in-time test method to determine if an AST is liquid tight. Leak testing is not preventive in the sense that it provides an indication only if the AST integrity has already been breached. Therefore, it may be used as a tank integrity measure or as a supplement to other inspection procedures, as part of a tank integrity assessment. LTMs may include the following technologies:

  • Gas pressure decay (includes vacuum decay)
  • Gas pressure soap bubble testing
  • Gas tracers (e.g. helium tracer)
  • Soil tracers (chemical marker)
  • Mass measurement
  • Level measurement
  • Hydrostatic test

Overfill Prevention is defined as systems, procedures, or devices used to prevent liquid in ASTs from running over or spilling out of the AST during the filling process. A person who is physically present and in control of a shutoff device during the entire tank filling process is an acceptable procedure to achieve overfill protection.

Periodic AST inspection is a visual, documented inspection conducted by an owner’s inspector, to assess the general AST condition, as best as possible, without suspending AST operations or removing the AST from service.

Release Prevention Barrier (RPB) is a liquid containment barrier installed under the AST. Its purpose is to divert leaks toward the perimeter of the AST where they can be easily detected, as well as to prevent liquid from contaminating the environment. RPBs are composed of materials compatible with the liquid stored in the AST and meet proper engineering standards. Examples are steel (such as in steel double-bottom tanks), concrete, elastomeric liners, or other suitable materials, provided the above criteria are met. (See Figure 2)

Figure 2. Release Prevention Barrier
Figure 2. Release Prevention Barrier

Secondary containment AST is an AST which is either double-walled or has an integral secondary containment dike. These integral secondary containment dikes may be pans, boxes or containers, and are designed to contain the contents of the primary tank if the primary tank fails. A secondary containment AST may be open or closed to the atmosphere. If precipitation cannot readily enter the integral secondary containment, then the containment need only be sized for the primary tank volume. If precipitation can enter the secondary containment, then the secondary containment is sized to contain the primary tank volume and with sufficient freeboard to contain precipitation.

Shop-fabricated is a welded metal AST fabricated in a manufacturing facility, or an AST not otherwise identified as field-erected, with a volume less than or equal to 50,000 U.S. gallons (189,271 liters).

Spill control is a means of preventing a release of liquid to the environment, including adjoining property and waterways. Methods include the following:

  • Remote impounding
  • Secondary containment dike/berm
  • Secondary containment AST
  • Secondary containment system

Risk Based Inspections in SP001

Inspection of storage tanks based on the risk to the environment is important to both the owners of tanks and regulators, although for different reasons. To the tank owner, reducing inspections by the addition of safeguards sufficiently extensive to reduce or simplify expensive tank inspections makes good business sense. Regulators wish to minimize and/or eliminate releases. In developing relevant standards, STI considered whether tanks that have these safeguards, and are therefore less likely to create a problem, can be inspected less frequently and intensively than tanks without them.

Beginning with the July 2005 3rd Edition, Steel Tank Institute’s Standard SP001, “Inspection of Aboveground Storage Tanks,” the inspection schedules are risk-based.

As shown in the Table 1 below, equipment, such as a tank, is ranked based on the likelihood of its failure in the columns of the table, and the consequence of failure in the rows of the table.

Table 1. Risk matrix example
Table 1. Risk matrix example

The optimum scenario is that the likelihood of failure is deemed “not likely” and the consequence is “small.” Conversely, the scenario to avoid is where the likelihood of failure is “very likely” and the consequence is “major.”

However, the SP001 inspection schedule is prescriptive and does not rely on assigning values to various likelihoods and consequences. Rather, a table of types of tank installations and tank sizes is given, with varying periods between inspections. For example, single-wall tanks sitting directly on soil without secondary containment or spill control were deemed to pose the greatest risk for an incident and therefore require more frequent inspections. The basis for this assessment was empirical and determined by experts in the field based on good engineering practices.

SP001 divides ASTs into three categories. Tank installations are classified based on whether they have safeguards to prevent spills from entering the environment. The determination of which category is applicable for a specific tank is the responsibility of the tank owner, but the owner should consult the SPCC Plan for the facility, if applicable.

Examples of the tank categories in SP001 include:

Category 1: ASTs with spill control and with CRDM

Double-wall tank with overfill prevention
Double-wall tank with overfill prevention

Tanks in concrete dike
Tanks in concrete dike

Category 2: ASTs with spill control, without CRDM

Single-wall tank in earthen berm
Single-wall tank in earthen berm

Category 3:- ASTs without spill control and without CRDM

Single-wall tank directly on soil and without spill control
Single-wall tank directly on soil and without spill control

Below is the corresponding “Table of Inspection Schedules” from SP001:

*Note the following:

  • P – Periodic AST inspection (PI)
  • E – Formal External Inspection by certified inspector (FEI)
  • I – Formal Internal Inspection by certified inspector (FII)
  • L – Leak test by owner or owner’s designee (LT)
  • ( ) indicates maximum inspection interval in years. For example, E (5) indicates formal external inspection every five years.

Table of Inspection Schedules
Table of Inspection Schedules

This table from SP001 shows that Category 3 tanks must have the most complex and most frequent inspections (with their associated costs), whereas Category 1 tanks have the least inspection complexity.

Tank Examples

These few examples will show how to assign a tank to the various categories, which then determines the inspection requirements:

  1. 1,100 gallon double-wall tank, horizontal cylindrical on saddles: In the chart above, this is a Category 1 tank and requires only Periodic AST inspections by the owner.
  2. 3,500 gallon single-wall elevated horizontal tank in an earthen dike: According to the chart above, this is a Category 2 tank (does not have CRDM, but has secondary containment). The tank requires Periodic AST inspections by the owner, as well as both formal external inspections by a certified inspector and leak tests by owner or owner’s designee at 10-year intervals.
  3. 5,000 gallon single-wall tank in an earthen dike; tank is vertical cylindrical and rests directly on the ground. In the chart above, this is a Category 2 tank and requires Periodic AST inspections by the owner, as well as both formal external inspections by a certified inspector and leak tests by owner or owner’s designee at 10-year intervals.
  4. 10,000 gallon single-wall tank without secondary containment of any type; tank is vertical cylindrical and rests directly on the ground. This is a Category 3 tank and in the chart above two choices are given for inspection schedules:
    • Choice 1 - Periodic AST inspections by the owner are necessary. Also, formal external inspections by a certified inspector and leak tests by owner or owner’s designee are needed at five-year intervals. Lastly, formal internal inspections by a certified inspector are necessary at ten-year intervals.
    • Choice 2 - Periodic AST inspections by the owner are necessary. Also, leak tests by owner or owner’s designee are necessary at one-year intervals. Lastly, formal external inspections by a certified inspector are necessary at five-year intervals.

Notice that the two inspection alternatives for the 10,000 gallon tank offer the owner a choice of inspection type. STI SP001 provides alternatives where warranted in an effort to equalize the risks associated with each alternative. In this case, the two choices were offered because entering an AST poses significant risk to the inspector. Also, many shop-fabricated ASTs do not have manways and therefore entry is difficult. Noted, though, is the fact that formal internal inspections provide more information about the condition of an AST than formal external inspections combined with leak tests; therefore, Choice 1 has longer intervals between inspections than Choice 2. Hence, if we can reduce the frequency of internal inspection while maintaining equivalent protection, we reduce the risk of harm to both the inspector and the environment.

Benefits of a Risk-Based Approach

As mentioned earlier, for tank owners, reduced inspections by the addition of safeguards helps their bottom-line. For regulators, targeting inspections at those tanks that are installed without safeguards prevents leaks. Implied in the Table of Inspection Schedules is encouragement for a tank owner to improve a tank installation to reduce its risk to the environment. By applying generally accepted good industry and engineering practices, such as those given for Category 1 tanks, the tank owner is rewarded with fewer inspections. While short-term costs may be higher, the long-term total cost of ownership may be much lower due to fewer incidents, environmental cleanups, regulatory citations, extended tank life, and many other factors. Further, when comparing inspection requirements for underground tanks to aboveground tanks, most tank owner/operators realize that the visual inspection of elevated ASTs is considerably simpler than inspecting an underground tank.

Clearly, tank owners should consider upgrading tank installations. This should be accomplished by incorporating CRDM and secondary containment to ASTs at the first available opportunity. The negative impact of not upgrading is evident when the next scheduled formal external or informal inspection reveals that missing safeguards require an increased inspection frequency or are needed to comply with regulations. Numerous tank types are found in the United States. Although Table 5.5 in SP001 includes many tank types that are installed with various features, including all types is impossible. For example, inspection methods for plastic tanks are not included in SP001. Therefore, a Professional Engineer assessing tank inspection programs must use professional judgment to adapt this standard to specific industries or facilities. Per paragraph 1.3 of SP001,

“The Professional Engineer must use other standards, recommended practices, and other equivalent engineering and best practices that exist and provide alternative inspection requirements for tanks defined within the scope of this standard and for tanks outside the scope of this standard.”

Tank Inspector Certification

Periodic Inspections are performed by a tank owner’s designated inspector. The personnel performing such inspections must be knowledgeable about storage facility operations, the type of AST and its associated components, and characteristics of the liquid stored. Owner’s inspectors must also be familiar with pumping, piping, and valve operations of the AST system. Checklists for periodic inspections are found in the SP001 standard, intended as a guide for recording inspection data.

Formal external (FEI) and formal internal inspections (FII) are to be performed by a Certified Inspector. These inspections include the following tank components, as applicable: primary and secondary tank; tank supports, anchors, foundations, release prevention barriers, and spill control systems; gauges, alarms, and overfill valves; normal and emergency vents; and other tank appurtenances. The tank system is compared to the requirements of applicable industry standards.

A “Certified Inspector” is certified by one or more of the following programs:

  • Steel Tank Institute (STI) Certified SP001 AST Tank System Inspector: A week-long training seminar is provided by STI at several locations each year. Certification requires attendance at all seminar sessions, passing the exam given at the end of the training, and proof of visual acuity exam within the past 12 months. In addition, there is a requirement for a minimum amount of related work experience based on the level of education of the individual prior to enrolling. Information about course scheduling and enrollment is found at under “Education.”
  • American Petroleum Institute (API) Standard 653 Authorized Inspector Certification with STI SP001 Adjunct Certification. This program is offered online through the STI/SPFA website and is available to individuals with current API 653 Certification. The program was developed at the urging of API, to make sure that API inspectors are thoroughly familiar with the requirements of STI SP001. Information about this online program is found at under “Education.”

STI/SPFA lists all Certified SP001 Inspectors online at As of this writing, 928 individuals are certified. The certification expires after five years, at which time the inspector must get recertified. Currently, recertification is provided through an online program.

Suitability for Continued Service

The purpose of conducting inspections is to identify the condition of and changes to the AST. The SP001 Standard provides inspection and evaluation criteria required to determine the suitability for continued service of aboveground storage tanks until the next scheduled inspection. The determination of this suitability is based on the tank category as well as any flaws found. Reference is made to Steel Tank Institute SP031 Standard for Repair of Shop-Fabricated Aboveground Tanks for Storage of Combustible and Flammable Liquids for alterations or repairs to an AST.

If the suitability criteria indicate that the particular tank must be repaired, the certified inspector documents in his report that the next formal external or formal internal inspection is to occur within five years and each subsequent five year interval thereafter, until the condition that caused the tank degradation has been fully corrected. When the tank degradation has been arrested or is in a steady-state condition, then the inspection intervals shown in Table 5.5 for subsequent inspections resume.

Latest Edition Revisions

In September 2011, after nearly two years of study and revision by a broad-based committee of experts, the Steel Tank Institute published the 5th Edition: SP001 Standard for the Inspection of Aboveground Storage Tanks.

Changes were developed through the STI AST Inspection Standards Committee review process. The revised standard was unanimously approved by a 21-member committee balanced among STI tank fabricators, federal and state regulators, bulk storage tank owners, tank repair companies, and several other interested parties. Significant changes from the previous edition include:

  • Elevated tanks previously required a release prevention barrier (in addition to secondary containment) to qualify as a Category 1 tank. This requirement was deleted, as leaks from elevated tanks are visible.
  • Double-wall tanks must have overfill prevention in order to be considered to have secondary containment. In the revised standard, however, overfill prevention may simply be a person watching the fill.
  • The requirement for a release prevention barrier to be sufficiently impervious was deleted, to avoid confusion with EPA's definition of secondary containment.
  • Language was added to the schedule of inspections concerning the initial inspection date, which is now based on the initial service date of the tank.
  • Additional examples of tank configurations were added as examples of tank categories.
  • Inspection requirements for several types of valves, leak detection equipment, spill containers, etc. were added to the equipment checklists in the Appendix.

For more information on SP001 Standard 5th Edition and SP031 Standard for Repair of Shop-Fabricated Aboveground Tanks for Storage of Flammable and Combustible Liquids, go to and click on “Publications”. SP001 covers the requirements for tank inspections and SP031 covers repairs that may be required as a result of findings from these inspections.

Comments and Discussion

Posted by Khan Amir Younus Kamal on October 27, 2015
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