11.3 Essential, Non-Essential & Supplementary Essential Variables

API 653 Aboveground Storage Tank Inspector Exam

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1 Chapter 1: API 653 Scope, Inspector Certification & Roles

1.1 Scope of API 653, Tank Types & Construction Standards 1.2 Inspector Roles, Responsibilities & Certification Requirements 1.3 Inspection Planning, Safety, Confined Space Entry & Preparation

2 Chapter 2: Tank Shell & Roof Components Inspection

2.1 Fixed Roof Types, Structural Supports & Inspection Practices 2.2 Floating Roofs, Seals, Drainage & Operational Issues 2.3 Shell Courses, Welds, Attachments & Wind Girders 2.4 Tank Appurtenances, Nozzles, Manways & Piping Connections

3 Chapter 3: Tank Bottom & Foundation Inspection

3.1 Tank Bottom Types, Annular Plates & Internal Bottom Inspection 3.2 Foundation Types (Concrete Ringwall, Earth, Asphalt) & Inspection 3.3 Leak Detection Systems, Secondary Containment & Under-Tank Monitoring

4 Chapter 4: Corrosion & Damage Mechanisms in Storage Tanks

4.1 Atmospheric Corrosion, Condensation & External Deterioration 4.2 Corrosion Under Insulation (CUI) & Piping/Nozzle Interfaces 4.3 Soil-Side Corrosion, Under-Bottom Pitting & Microbiologically Induced Corrosion (MIC) 4.4 Product-Side Deterioration: Chemical Attack, Ethanol SCC, Concentration Cell & Sulfuric Acid Corrosion

5 Chapter 5: Materials, Toughness & Brittle Fracture Prevention

5.1 Tank Steels, Material Specifications & Historic Standards 5.2 Brittle Fracture Assessment: Figure 5.1 Decision Tree & Exemption Curves (API 653 Section 5) 5.3 Charpy V-Notch Impact Testing & Minimum Design Metal Temperature (MDMT)

6 Chapter 6: Cathodic Protection & Interior Linings

6.1 Cathodic Protection Principles & Systems for Tank Bottoms (API RP 651) 6.2 CP Criteria, Surveys, Potential Measurements & Interference (API RP 651) 6.3 Tank Bottom Interior Linings, Surface Prep & Inspection (API RP 652)

7 Chapter 7: Corrosion Rates, Remaining Life & Inspection Intervals

7.1 Determining Corrosion Rates (Short-Term vs. Long-Term) 7.2 Remaining Life Calculations & Limiting Components 7.3 Establishing In-Service & Out-of-Service Inspection Intervals (API 653 Section 6)

8 Chapter 8: Tank Shell Minimum Thickness Evaluation

8.1 Shell Minimum Thickness Calculations (tmin for Hydrostatic & Operating Levels) 8.2 One-Foot Method, Variable Design Point & Reconstructed-Tank Shell Thickness 8.3 Joint Efficiency (E) Selection, Missing Data & Radiography Factors 8.4 Evaluation of Pitting, Localized Thin Areas (LTA) & Critical Length (L)

9 Chapter 9: Bottom Plate & Annular Ring Thickness Evaluation

9.1 Minimum Bottom Thickness Formulations (MRT, MRTbottom, MRTlinear) 9.2 Annular Plate Evaluation, Stress Criteria & Critical Zones 9.3 Bottom Pitting Evaluation & Remaining Thickness Before Next Inspection

10 Chapter 10: Tank Repairs & Alterations Standards

10.1 Classification: Repair vs. Alteration vs. Reconstruction (API 653 Section 9) 10.2 Shell Plate Replacement, Lapped Patch Plates & Insert Plates 10.3 Shell Penetrations, Door Sheets & Nozzle Additions 10.4 Hot Tapping Requirements, Limitations & Pre-Engineering (API 653 Section 9.15 & API Publ 2201)

11 Chapter 11: Welding Procedures, Qualifications & Quality Control

11.1 Welding Procedure Specification (WPS) & Procedure Qualification Record (PQR) per ASME IX 11.2 Welder Performance Qualification (WPQ) & Continuity per ASME IX 11.3 Essential, Non-Essential & Supplementary Essential Variables 11.4 Preheating, Post-Weld Heat Treatment (PWHT) & Weld Flaw Remediation

12 Chapter 12: Nondestructive Examination (NDE) Methods

12.1 Visual Examination (VT) & Magnetic Particle Testing (MT) per ASME V 12.2 Liquid Penetrant (PT) & Ultrasonic Thickness Measurement (ASME V Article 6 & SE-797) 12.3 Radiographic Testing (RT): Acceptance Criteria, Film Interpretation & ASME V/API 650

13 Chapter 13: Hydrostatic Testing & Post-Repair Examination

13.1 Hydrostatic Test Triggers, Exemption Criteria & Major Alterations (API 653 Section 12.3) 13.2 Leak Testing: Vacuum Box, Tracer Gas, Diesel & Air Pressure Tests 13.3 NDE Examination Requirements for Shell, Bottom, Roof & Nozzle Welds (API 653 Section 12.1)

14 Chapter 14: Relief Devices, Tank Settlement & Nameplate Records

14.1 Pressure & Vacuum Relief Devices, Venting Principles (API RP 576 & API 2000) 14.2 Tank Settlement Types: Uniform, Tilt, Out-of-Plane & Edge Settlement (API 653 Annex B) 14.3 Evaluating Settlement Limits, Cosine Curve Analysis & Remedial Action 14.4 Tank Marking, Nameplates, Inspection Reports & Recordkeeping (API 653 Section 13)

11.3 Essential, Non-Essential & Supplementary Essential Variables

Key Takeaways Explain This Topic AI

ASME Section IX QW-250 classifies procedure variables into essential, non-essential, and supplementary essential categories based on their mechanical and metallurgical impact on the finished weldment.

Essential variables directly influence tensile strength, yield strength, or ductility; changing an essential variable beyond qualified limits invalidates the WPS and mandates welding a new test coupon for PQR requalification.

Non-essential variables affect operational deposition, bead geometry, or productivity without altering mechanical properties, allowing editorial modification on the WPS without requalifying the PQR.

Supplementary essential variables are activated exclusively when notch toughness (Charpy V-notch) testing is mandated by API 650 (e.g., Section 9.2.2) or API 653 (e.g., Section 11.1.2) for low-temperature or impact-tested materials.

Welding heat input (calculated as HI = [V * I * 60] / [S * 1000]) is a critical supplementary essential variable because excessive heat input coarsens grain microstructures and severely degrades low-temperature impact toughness.

Last updated: September 2026

11.3 Essential, Non-Essential & Supplementary Essential Variables

The Engineering Hierarchy of Variables: The core mechanism by which ASME Section IX controls welding quality is the systematic classification of welding parameters into Essential Variables , Non-Essential Variables , and Supplementary Essential Variables . In aboveground storage tank construction and repair, an Authorized Inspector must instantly discern which field alterations require a completely new Procedure Qualification Record (PQR) with destructive mechanical testing, and which may be resolved through an administrative revision to the Welding Procedure Specification (WPS).

1. Classification Architecture of Welding Variables (QW-250)

Under ASME Section IX Article II, each welding process—Shielded Metal Arc Welding (SMAW, QW-253), Submerged Arc Welding (SAW, QW-254), Gas Metal Arc Welding (GMAW/FCAW, QW-255), and Gas Tungsten Arc Welding (GTAW, QW-256)—has a dedicated variable table. Variables are categorized into three distinct classes: +-------------------------------------------------------------------------+

| ASME SECTION IX VARIABLE TAXONOMY |
+-----------------------+-----------------------+-------------------------+
| Variable Category | Metallurgical Effect | Action Required on |
| | | Parameter Change |
+-----------------------+-----------------------+-------------------------+
| **Essential** | Alters mechanical | **Mandates New PQR** |
| **Variables** | tensile, yield, or | with coupon welding and |
| (QW-200.1(b)) | bend properties | destructive testing |
+-----------------------+-----------------------+-------------------------+
| **Non-Essential** | Affects bead shape, | **WPS Revision Only**; |
| **Variables** | deposition rate, or | no new coupon or PQR |
| (QW-200.1(c)) | operational technique | testing required |
+-----------------------+-----------------------+-------------------------+
| **Supplementary** | Alters Charpy V-notch | **Mandates New PQR** |
| **Essential** | low-temperature | **ONLY when toughness** |
| (QW-200.1(d)) | impact toughness | is invoked by code |
+-----------------------+-----------------------+-------------------------+
2. Essential Variables: Mechanics and Requalification Triggers

An Essential Variable is defined under QW-200.1(b) as a condition where a change in the variable is considered to affect the mechanical properties (other than notch toughness) of the weldment. If an essential variable is changed beyond the permitted range, the existing WPS is invalidated, and a new test coupon must be welded and mechanically tested to generate a supporting PQR.

Critical Essential Variables Across Storage Tank Welding

Base Metal P-Number (QW-403.11 & QW-420):

ASME Section IX assigns base metals to P-Numbers based on chemical composition, weldability, and mechanical properties. Carbon steels commonly used in storage tanks (ASTM A36, A283 Grade C, A516 Grade 70, A573 Grade 70) are assigned to P-No. 1 .

A change from one P-Number to another (e.g., from P-No. 1 carbon steel to P-No. 8 austenitic stainless steel) is an essential variable requiring full requalification. Welding dissimilar P-numbers (e.g., P-No. 1 to P-No. 8) requires a dedicated PQR qualifying that specific bimetallic combination.

Base Metal Thickness Range Qualified ( T T T ) (QW-403.8 & Table QW-451.1):

The thickness of the test coupon welded during procedure qualification dictates the minimum and maximum base metal thickness envelope qualified for production:

For test coupon thickness T < 3 / 8 in. (10 mm) T < 3/8\text{ in. (10 mm)} T < 3/8 in. (10 mm) : Qualified range is 1 / 16 in. (1.5 mm) 1/16\text{ in. (1.5 mm)} 1/16 in. (1.5 mm) to 2 T 2T 2 T .

For 3 / 8 in. ≤ T < 3 / 4 in. (19 mm) 3/8\text{ in.} \le T < 3/4\text{ in. (19 mm)} 3/8 in. ≤ T < 3/4 in. (19 mm) : Qualified range is 3 / 16 in. (5 mm) 3/16\text{ in. (5 mm)} 3/16 in. (5 mm) to 2 T 2T 2 T .

For T ≥ 3 / 4 in. (19 mm) T \ge 3/4\text{ in. (19 mm)} T ≥ 3/4 in. (19 mm) : Qualified range is 3 / 16 in. (5 mm) 3/16\text{ in. (5 mm)} 3/16 in. (5 mm) to 2 T 2T 2 T , except that when T ≥ 1.5 in. (38 mm) T \ge 1.5\text{ in. (38 mm)} T ≥ 1.5 in. (38 mm) , maximum qualified thickness is 8 in. (200 mm) 8\text{ in. (200 mm)} 8 in. (200 mm) .

If a contractor qualifies on a 0.500-inch plate, the maximum shell plate thickness they may weld in production is 2 × 0.500 = 1.000 inch 2 \times 0.500 = 1.000\text{ inch} 2 × 0.500 = 1.000 inch . Welding a 1.250-inch shell insert plate requires a new PQR welded on a coupon of sufficient thickness.

Welding Process (QW-401.1):

A change from SMAW (stick) to GMAW (MIG), FCAW (flux-cored), or SAW (submerged arc) is a fundamental essential variable. In multi-process procedures (e.g., GTAW root followed by SMAW fill), each individual process must be qualified for its deposited thickness ( t t t ) per QW-404.30.

Filler Metal F-Number (QW-404.4):

A change in filler metal from one F-Number (Table QW-432) to another (e.g., switching from F-No. 3 cellulosic E6010 to F-No. 4 low-hydrogen E7018) is an essential variable requiring procedure requalification.

Filler Metal A-Number (QW-404.5):

A-Numbers (Table QW-442) classify the chemical composition of deposited carbon and alloy weld metal. A change from A-No. 1 (plain carbon steel weld metal) to A-No. 2 (carbon-molybdenum) or A-No. 8 (austenitic stainless steel) is an essential variable.

Post-Weld Heat Treatment (QW-407.1):

A change in PWHT condition—such as qualifying without PWHT and subsequently applying PWHT in production, or vice versa—is an essential variable requiring a new PQR.

3. Non-Essential Variables: Engineering Flexibility & Administrative Revisions

A Non-Essential Variable is defined under QW-200.1(c) as a condition where a change may be made in the WPS without requalifying the procedure. These variables influence the appearance, bead shape, operating ease, or deposition rate of the weld puddle, but do not alter the tensile strength, ductility, or bend properties of the completed joint.

Common Non-Essential Variables in Storage Tank Procedures

Joint Design and Geometry (QW-402.1): Modifying groove preparation, such as changing a single-V butt joint to a double-V butt joint, changing the included bevel angle from 60° to 75°, or adjusting root face land and root opening dimensions.

Backing (QW-402.4): Adding or deleting a permanent steel backing strip, ceramic backing, or changing backing material.

Electrode Diameter (QW-404.14): Changing the core wire size of an electrode (e.g., from 1/8-in. E7018 to 5/32-in. E7018) within the same AWS classification and F-Number.

Method of Cleaning and Preparation (QW-410.1): Changing from mechanical wire brushing to disc grinding, chipping, or carbon-arc backgouging.

Bead Technique (QW-410.5): Switching between a stringer bead technique and a weave bead technique (provided supplementary essential toughness rules do not restrict heat input).

Peening (QW-410.6): The addition or deletion of mechanical peening on intermediate weld passes.

Administrative Rule: When a non-essential variable is changed, the contractor updates the WPS document, increments the revision number, logs the date, and issues the document for production. No test coupons are welded, and no mechanical testing is conducted.

4. Supplementary Essential Variables: The Notch Toughness Regime

A Supplementary Essential Variable (QW-200.1(d)) is a specialized variable that is classified as non-essential under standard conditions, but becomes an Essential Variable whenever the referencing code mandates notch toughness (Charpy V-notch / CVN) testing .

When is Toughness Mandated in Storage Tanks?

In storage tank engineering, notch toughness is not universally required for every atmospheric vessel. However, under API Standard 650 Section 4.2.9, Table 4.4, and Section 9.2.2 , as well as API 653 Section 11.1.2 , toughness testing becomes mandatory under specific operational and material envelopes:

Tanks constructed from Group IV, Group IVA, Group V, or Group VI steels (e.g., ASTM A537 Class 1 or 2, normalized A516 Gr. 70, A662, A737).

Tanks operating at low Design Metal Temperatures (DMT) falling below the exemption curves in API 650 Figure 4.1.

Shell plates exceeding specific thickness thresholds (e.g., plates thicker than 1.5 inches) operating under high hydrostatic tensile membrane stress.

Low-temperature or refrigerated product storage.

Critical Supplementary Essential Variables

When notch toughness is specified, the following parameters immediately become essential variables, requiring a new PQR if altered:

Increase in Heat Input (QW-409.1): An increase in calculated heat input or volume of weld metal deposited per unit length over that qualified on the PQR.

Change in Welding Position (QW-405.2): A change in position (e.g., from flat 1G to vertical 3G) or a change in vertical progression from uphill to downhill.

Decrease in Preheat Temperature (QW-406.1): A decrease of more than 100°F (55°C) in the minimum preheat temperature qualified.

Increase in Interpass Temperature (QW-406.3): An increase of more than 100°F (55°C) in the maximum interpass temperature recorded on the PQR.

Change in Filler Metal Classification (QW-404.12): A change in the specific AWS classification (e.g., from E7018 to E7018-1) or manufacturer trade name where impact properties are guaranteed by classification.

Change in PWHT Time and Temperature (QW-407.2): A change in PWHT soaking temperature or a change in soaking time exceeding 25%.

5. Heat Input Calculation and Microstructural Toughness Mechanics

Heat input ( H I HI H I ) is the primary thermodynamic metric governing the cooling rate and final grain morphology of the weld metal and heat-affected zone (HAZ).

The Heat Input Formula (ASME IX QW-409.1)

Heat input is quantified as electrical energy delivered per unit length of weld bead: H I = V × I × 60 S × 1000 HI = \frac{V \times I \times 60}{S \times 1000} H I = S × 1000 V × I × 60

Where:

H I HI H I = Heat Input in kilojoules per inch ( kJ/in. \text{kJ/in.} kJ/in. ) or kilojoules per millimeter ( kJ/mm \text{kJ/mm} kJ/mm )

V V V = Arc Voltage (Volts)
I I I = Welding Current (Amperes)

S S S = Travel Speed of the arc in inches per minute ( in/min \text{in/min} in/min ) or millimeters per minute ( mm/min \text{mm/min} mm/min )

60 60 60 = Conversion factor (seconds per minute)

1000 1000 1000 = Conversion factor (Joules to kiloJoules) LOW HEAT INPUT (Fast Travel Speed) HIGH HEAT INPUT (Slow Weave / High Amps)

---------------------------------- ----------------------------------------
Rapid Cooling Rate (T8/5) - Slow Cooling Rate (Extended T8/5)
Fine-Grained Acicular Ferrite - Coarse Grain HAZ (Grain Growth)
High Tensile Strength - Upper Bainite / Coarse Ferrite Networks
EXCELLENT CHARPY IMPACT TOUGHNESS - SEVERE LOSS OF CHARPY IMPACT TOUGHNESS
Why High Heat Input Destroys Impact Toughness

When heat input is excessively high (e.g., dragging a heavy weave pass at slow travel speed or operating at excessive amperage), the cooling rate through the critical transformation range (800°C down to 500°C, known as Δ t 8 / 5 \Delta t_{8/5} Δ t 8/5 ) is severely retarded.

Slow cooling allows austenite grains in the coarse-grained HAZ (CGHAZ) immediately adjacent to the fusion line to grow excessively large. Coarse grains transform into coarse pearlite, upper bainite, and pro-eutectoid ferrite networks along prior austenite grain boundaries.

Under dynamic shock loading at low operating temperatures, these coarse microstructures provide low-energy cleavage planes, causing catastrophic brittle fracture at dramatically reduced impact energy levels.

Consequently, when toughness is specified, ASME Section IX strictly prohibits increasing the production heat input above the maximum heat input validated during PQR coupon fabrication.

6. Comparative Welding Variables Matrix for Storage Tank Processes

The following comparative table contrasts the classification of critical welding variables across the three primary processes deployed in aboveground storage tank fabrication and repair:

Variable Description ASME Section IX Code Variable SMAW (QW-253) Stick Welding GMAW / FCAW (QW-255) MIG / Flux-Cored SAW (QW-254) Submerged Arc Impact on Mechanical / Toughness Properties

Change in P-Number QW-403.11 Essential Essential Essential Directly changes base metal chemistry and weldability

Change in Base Metal Thickness ( T T T ) QW-403.8 Essential Essential Essential Changes cooling rate, restraint, and tensile load capacity

Change in Welding Process QW-401.1 Essential Essential Essential Changes fluxing, shielding, and arc physics completely

Change in Filler F-Number QW-404.4 Essential Essential Essential Changes slag chemistry, arc characteristics, and deposit strength

Change in Filler A-Number QW-404.5 Essential Essential Essential Alters chemical composition of deposited weld metal

Change in PWHT Condition QW-407.1 Essential Essential Essential Relieves stress; alters tensile and yield strength

Increase in Heat Input QW-409.1 Supplementary Essential Supplementary Essential Supplementary Essential Coarsens HAZ grain size; degrades low-temp CVN energy

Decrease in Preheat > 100 ∘ F > 100^\circ\text{F} > 10 0 ∘ F QW-406.1 Supplementary Essential Supplementary Essential Supplementary Essential Accelerates quench rate; increases local hardness and lowers toughness

Increase in Interpass > 100 ∘ F > 100^\circ\text{F} > 10 0 ∘ F QW-406.3 Supplementary Essential Supplementary Essential Supplementary Essential Retards cooling; causes extreme grain coarsening in HAZ

Change in Position (e.g., 1G to 3G) QW-405.2 Supplementary Essential Supplementary Essential Non-Applicable (SAW is 1G/2G) Out-of-position alters bead thickness and thermal cycle

Change in Groove Joint Design QW-402.1 Non-Essential Non-Essential Non-Essential Affects access and economy; does not alter metallurgy

Change in Electrode Size (Diameter) QW-404.14 Non-Essential Non-Essential Non-Essential Alters deposition rate; does not alter chemical properties

Change in Shielding Gas Flow Rate QW-408.2 Non-Applicable Non-Essential Non-Applicable Affects atmospheric shielding; minor impact if within range

Method of Cleaning / Backgouging QW-410.1 Non-Essential Non-Essential Non-Essential Workmanship variable; does not dictate metallurgy