10.3 Shell Penetrations, Door Sheets & Nozzle Additions

Design Principles of Shell Penetrations

Cutting a circular or oval opening through the cylindrical shell of an atmospheric storage tank interrupts the continuous circumferential path of membrane hoop stress. Left uncompensated, this penetration creates severe stress concentrations, with localized peak stresses reaching up to three times ($3.0\times$) the nominal membrane stress at the edge of the opening. To restore structural equilibrium, API Standard 653 Section 9.8 mandates that all newly added or replaced shell penetrations must conform to the strict design and fabrication rules of API Standard 650 Section 5.7.

+-------------------------------------------------------------------------+
|                 SHELL PENETRATION REINFORCEMENT SCHEME                  |
|                                                                         |
|                     Vertical Shell Plate                                |
|                              | |                                        |
|   +==========================+ +==========================+             |
|   | Reinforcing Plate (Repad)   [ 1/4" NPT Telltale Hole ]|             |
|   |                          | |                          |             |
|   |      +-------------------+ +-------------------+      |             |
|   |      | Nozzle Neck (t_n)                       |      |             |
|   |      | <------------- Diameter d ------------> |      |             |
|   |      +-------------------+ +-------------------+      |             |
|   |                          | |                          |             |
|   +==========================+ +==========================+             |
|                              | |                                        |
+-------------------------------------------------------------------------+

The Area Replacement Method (API 650 Section 5.7.2)

The fundamental principle governing penetration reinforcement is the Area Replacement Method. The cross-sectional area of metal removed from the shell plate when boring the hole must be fully replaced by reinforcement metal placed adjacent to the opening:

$A_{required} = d \cdot t$

Where:

• $d$: Diameter of the hole cut in the shell plate (in inches or millimeters).
• $t$: Nominal thickness of the shell plate course, excluding corrosion allowance (in inches or millimeters).

This required area $A_{required}$ is provided through three combined sources:

1. Reinforcing Plate (Repad): The cross-sectional area of the external or internal doughnut-shaped reinforcing plate ($A_{repad} = [D_{outer} - (d + 2t_n)] \cdot t_{repad}$).
2. Nozzle Neck Excess Thickness: The cross-sectional area provided by the nozzle neck wall thickness projecting beyond the minimum required pipe wall thickness.
3. Weld Fillet Metal: The cross-sectional area of the interior and exterior nozzle attachment fillet welds.

Reinforcing Plate Details and the 1/4-Inch NPT Telltale Hole

• Repad Thickness: Typically matches the nominal thickness of the host shell plate ($t_{repad} = t_{shell}$), though thinner repads with larger outer diameters or thicker repads (up to $1.5\times t_{shell}$) may be engineered.
• Telltale Hole: Every reinforcing pad must be equipped with at least one 1/4-inch NPT threaded telltale hole.
  • Pneumatic Proof Testing: Following welding and prior to hydrotesting or service, the repad cavity is pressurized with air or nitrogen to 15 psig (100 kPa). A foaming leak detection solution (soapy water) is applied across the inner nozzle-to-shell weld and the outer repad perimeter fillet weld to detect any through-wall weld leakage.
  • Operational Status: During normal operation, the telltale hole must be left open to the atmosphere (or lightly packed with grease/fitted with a breathable plastic plug) to prevent pressure build-up and provide immediate visual evidence if the internal shell-to-neck weld leaks.

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Door Sheets: Engineering and Field Execution (API 653 Section 9.2.4)

During out-of-service tank overhauls, large industrial equipment—such as bobcat skid steers, mini-excavators, hydro-blasting rigs, plate bundles, and scaffolding systems—must be moved into the tank interior to replace floors or rebuild floating roofs. Because standard shell manways (NPS 20 or NPS 24) are far too small, temporary openings known as Door Sheets are cut into the cylindrical shell.

+-------------------------------------------------------------------------+
|                    DOOR SHEET GEOMETRY & CUT LINES                      |
|                                                                         |
|   Existing Horizontal Shell Seam                                        |
|   ===================================================================   |
|   |                                                                 |   |
|   |      +---------------------------------------------------+      |   |
|   |     / (Min 6" Corner Radius)                              \     |   |
|   |    |                                                       |    |   |
|   |    |                     DOOR SHEET                        |    |   |
|   |    |                 (Temporary Opening)                   |    |   |
|   |    |                                                       |    |   |
|   |     \                                                     /     |   |
|   |      +---------------------------------------------------+      |   |
|   |       >= 6" clearance from existing welds                       |   |
|   ===================================================================   |
|   Existing Horizontal Shell Seam                                        |
+-------------------------------------------------------------------------+

Cut Line Layout and Corner Geometry (API 653 9.2.2 and 9.2.4)

Improper cutting of a door sheet can induce severe shell buckling, permanent out-of-roundness, or notch embrittlement. A door sheet is a replacement shell plate, so 9.2.2.1 applies: the minimum dimension is 300 mm (12 in.) or 12 times the replacement plate thickness, whichever is greater, and the plate may be circular, oblong, square with rounded corners, or rectangular with rounded corners. 9.2.4.2 then adds that in a butt-welded tank the removed section may be reinstalled in its original location or replaced with new plate, but either way the installation shall use joints with complete penetration and complete fusion. Two cutting configurations are used in practice:

1. Cut Entirely Within a Shell Plate: When the opening is bounded within a single shell plate, all four corners must be rounded to a minimum radius of 6 inches (150 mm). Sharp 90-degree corners, torch notches, or square cuts are strictly prohibited because they create extreme stress raisers that can initiate brittle running cracks.
2. Terminating at Existing Weld Seams: The cut lines may extend to and utilize existing vertical or horizontal shell butt welds. When intersecting an existing weld seam, the cut must cross at a perpendicular 90-degree angle. The existing weld must be ground flush, inspected, and back-gouged prior to re-welding.

Choosing the Right Door-Sheet Figure (API 653 Figures 9.1 through 9.5)

API 653 9.2.4 opens by making the figure set mandatory: the requirements of Figure 9.1, Figure 9.2, Figure 9.3, Figure 9.4, and Figure 9.5 shall be used to locate door sheets relative to existing seams, unless an alternative is designed by a storage tank engineer and the owner/operator approves it in writing. The exam expects you to select the right figure from the tank's seam construction.

"Offset" is defined in the note to 9.2.4.6 as the horizontal distance between the vertical welds above and below a horizontal seam, as shown in Figures 9.2, 9.3, and 9.5.

Door Sheet Rules by Seam Type (API 653 9.2.4.1 through 9.2.4.8)

• 9.2.4.1 — door sheet installation shall also meet 9.2.1, 9.2.2, 9.2.3, and 12.2.1.6.
• 9.2.4.3 — large door sheets of the type shown in Figures 9.2, 9.3, 9.4, and 9.5 should have the top and/or sides of the opening stiffened to prevent sagging and deformation at the top of the opening and shell deformation at the sides and upper corners. Stiffening — usually angle, channel, or wide-flange members — should be installed before cutting where it is safe to do so, and should remain until the door sheet is completely and satisfactorily reinstalled. A storage tank engineer determines the arrangement.
• 9.2.4.4 — for lap-welded and riveted tanks, reinstallation of an original plate section that crosses an existing horizontal seam is not permitted.
• 9.2.4.5 — door sheets that cross vertical riveted or lap-welded seams are not permitted in any case.
• 9.2.4.6 — butt-welded tank, cut line crossing an existing seam without an offset, removed section reinstalled: in addition to the examination requirements of 12.1.5.1, the back-gouged surface of the root pass and the final pass (each side) of the new welds shall be examined by magnetic particle or liquid penetrant methods, and the existing horizontal seam intersected by the new vertical weld shall also be examined by MT or PT for 150 mm (6 in.) on both sides.
• 9.2.4.7 — butt-welded tank with an offset: the minimum offset must equal dimension V as shown in Figure 9.5. Prior to welding the new vertical seams, cut the existing horizontal seam weld for a minimum of 300 mm (12 in.) beyond the new vertical joints, and weld the horizontal seam last.
• 9.2.4.8 — cut line crossing a lap-welded or riveted horizontal seam: the replacement assembly shall be constructed of two separate plates. The lower section is butt-welded to the adjacent shell course by full-penetration, full-fusion vertical welds; the upper section laps over or under the lower section and is butt-welded to the existing shell plate; after the butt welds are complete, the horizontal lap is fillet welded along both the inside and outside edges. Per the figure notes, the upper door sheet thickness matches the higher shell course and the lower matches the lower course, and the fillet weld size equals the thickness of the thinner of the two plates.

Sequencing notes carried on Figure 9.1 — and repeated on the door-sheet figures — are frequent exam material:

1. All weld intersections shall be at approximately 90 degrees.
2. Prior to welding new vertical joints, cut the existing horizontal weld for a minimum of 300 mm (12 in.) beyond the new vertical joints. Weld the horizontal joint last.
3. Prior to welding new vertical joints that run to the bottom, cut the existing shell-to-bottom weld for a minimum of 300 mm (12 in.) beyond the new vertical joints. The cut shall extend past or stop short of existing bottom plate welds by at least 75 mm (3 in.) or 5t. Weld the shell-to-bottom weld last.
4. Rivets and existing lap rivet seams within 300 mm (12 in.) of a weld may need to be caulked, coated, or seal welded to prevent product seepage, and rivet holes left in the shell are plug welded to full plate thickness and ground flush.

Door Sheet Clearance Rules

• Vertical cut lines must maintain a minimum clearance of $12\text{ in.}$ ($300\text{ mm}$) or $5t$ from adjacent existing vertical shell seams.
• Horizontal cut lines must maintain a minimum clearance of $6\text{ in.}$ ($150\text{ mm}$) or $5t$ from adjacent horizontal shell seams (unless terminating directly on the seam).
• The bottom cut line of a door sheet in the lowest shell course must maintain at least $6\text{ in.}$ ($150\text{ mm}$) clearance from the shell-to-bottom corner weld, unless the door sheet extends fully to the bottom plate as an engineered section.

Re-Installation and Welding Sequence

Re-welding a door sheet introduces immense thermal shrinkage stresses that can pull the shell inward, creating severe localized peaking or banding. To preserve shell roundness and plumbness within the strict erection tolerances of API 650/653, the welding sequence must be carefully controlled:

1. Edge Preparation: Plate edges must be beveled (e.g., $60^\circ$ included angle V-groove with $1/16$ to $1/8\text{ in.}$ root face) and ground clean of all slag, scale, and carbon deposits.
2. Fit-Up and Strongbacks: Temporary internal strongbacks (bridge dogs and wedges) must be installed across the joints to maintain flush shell curvature while allowing longitudinal shrinkage along the seam.
3. Welding Sequence: Low-hydrogen consumables (e.g., AWS E7018) must be used. Vertical joints are typically welded first using step-back or cascade multi-pass sequences to distribute heat evenly. Horizontal seams are subsequently welded only after vertical joints have cooled, preventing severe biaxial locking stresses.

NDE Requirements for Door Sheets

• 100% Radiographic (RT) or Ultrasonic (UT) Examination: All vertical butt welds joining the re-installed door sheet must be 100% inspected over their entire length.
• Horizontal Butt Welds: Inspected with RT/UT at all weld intersections and spot examined per API 650 Section 8.1.
• Surface NDE: Magnetic particle (MT) or liquid penetrant (PT) examination of the root pass and the final weld cap.

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Weld Spacing Around Nozzles and Repads (API 653 Figure 9.1)

When adding new nozzles or replacing damaged penetrations, maintaining strict spatial separation between nozzle welds, reinforcing pad welds, and existing shell seams is vital. Overlapping weld zones compound residual tensile stresses and create metallurgical hard spots susceptible to environmental cracking.

+-------------------------------------------------------------------------+
|                   NOZZLE WELD CLEARANCES (FIGURE 9.1)                   |
|                                                                         |
|                     Vertical Shell Butt Weld                            |
|                                | |                                      |
|                 <--- >= 3" --->| |                                      |
|                                | |                                      |
|             +==================+ +==================+                   |
|             | Reinforcing Pad Outer Fillet Weld Toe |                   |
|             |          +--------------------+       |                   |
|             |          | Nozzle Neck        |       |                   |
|             |          +--------------------+       |                   |
|             +=======================================+                   |
|                                | |                                      |
|                                | |                                      |
|                                v >= 3"                                  |
|   ===================================================================   |
|                     Horizontal Shell Butt Weld                          |
|                                                                         |
|                                v >= 6"                                  |
|   ===================================================================   |
|                     Shell-to-Bottom Corner Weld                         |
+-------------------------------------------------------------------------+

Prescriptive Clearances from Figure 9.1 and Table 9.1:

1. Repad Weld Toe to Vertical Shell Seam: Minimum $3\text{ in.}$ ($75\text{ mm}$) or $5t$ (whichever is greater).
2. Repad Weld Toe to Horizontal Shell Seam: Minimum $3\text{ in.}$ ($75\text{ mm}$) or $5t$ (whichever is greater).
3. Repad Weld Toe to Shell-to-Bottom Corner Weld: Minimum $6\text{ in.}$ ($150\text{ mm}$) for standard elevated nozzles.
4. Nozzles Crossing Existing Shell Welds: If geometric layout forces a reinforcing pad to cross an existing vertical or horizontal shell seam, the following conditions must be met:
   • The existing weld seam covered by the repad must be ground completely flush with the plate surface.
   • The covered length of the weld seam, plus at least $6\text{ in.}$ ($150\text{ mm}$) beyond each side of the repad, must be 100% radiographically examined (RT) or ultrasonically examined (UT) prior to fitting the pad.
   • The repad outer fillet weld must cross the shell weld seam at an angle of approximately $90^\circ$.

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Weld Sizes for Shell & Roof Openings — the Open-Book Lookup Map

The Body of Knowledge carries a dedicated calculation category for weld sizes for shell and roof openings. It does not ask you to derive anything — it asks you to be fast and accurate with a specific set of figures and tables during the open-book session. Know what each family is for before you open the book.

Three habits decide these questions:

• Every dimensional table comes in an "a" (USC) and a "b" (SI) version. API 653 1.1.3 notes that tanks are built to either US customary or SI units, and the BOK warns that some tables are published in only one system and that you will be expected to use the table as the question requires. Read the units in the stem first and open the matching table.
• Opening weld spacing is measured from weld toe to weld centerline, not plate edge to plate edge. API 650 expresses the shell-opening requirement as 8W — eight times the largest weld size on the reinforcing plate or insert plate periphery weld, measured from the toe of that periphery weld to the centerline of the shell butt weld.
• Classify the opening before you size the weld. Shell openings are sorted into RTR (regular-type reinforced), LTR (low-type reinforced, extending to the bottom), and S-N openings that have neither a reinforcing plate nor a thickened insert plate because they are integrally reinforced or require no reinforcement. The category selects the figure.

The BOK deliberately removes the design side of this topic: nozzle calculations for external loads and flange calculations are both listed among the items candidates are not expected to know. You are tested on reading the details, not on designing the opening.

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Post-Weld Heat Treatment (PWHT) and Preheating Requirements

Thick shell plates subjected to the heavy restraint of large nozzle necks and heavy reinforcing plates develop intense triaxial residual stresses. In materials with high carbon equivalents or thick sections, these localized stresses can initiate brittle failure during cold ambient filling cycles.

Mandatory Furnace PWHT per API 650 Section 5.7.4

API 650 Section 5.7.4.2 and API 653 Section 9.8 dictate that nozzle assemblies must be shop-prefabricated and thermally post-weld heat treated in an enclosed furnace prior to being installed into the tank shell when both of the following conditions are met:

1. Nozzle Size: The penetration is larger than NPS 12 (DN 300) (or larger than NPS 2 for certain specialized quench-and-tempered alloys).
2. Plate Thickness and Material Group:
   • Shell plate thickness exceeds $1.0\text{ in.}$ ($25\text{ mm}$) for Materials Groups I, II, III, or IIIA.
   • Shell plate thickness exceeds $0.5\text{ in.}$ ($12.5\text{ mm}$) for Materials Groups IV, IVA, V, or VI.

Shop Prefabrication Procedure:

In these heavy-wall configurations, the nozzle neck, reinforcing pad, and a section of host shell plate (the insert plate) are completely assembled and welded in a fabrication shop. The entire assembly is placed into a heat-treating furnace and heated to $1100^\circ\text{F} - 1200^\circ\text{F}$ ($595^\circ\text{C} - 650^\circ\text{C}$), soaked at temperature for 1 hour per inch of thickness, and cooled at a controlled rate per ASME Section VIII Division 1. The stress-relieved assembly is then shipped to the field and welded into the tank shell using full-penetration butt welds.

Field Preheating Requirements

When welding insert plate assemblies or thick shell penetrations in the field, preheating the steel to at least $200^\circ\text{F}$ ($93^\circ\text{C}$) is mandatory when:

• Plate thickness exceeds $1.0\text{ in.}$ ($25\text{ mm}$).
• Ambient temperature is below $32^\circ\text{F}$ ($0^\circ\text{C}$).
• Base metal carbon equivalent ($CE$) exceeds $0.43%$.

Preheating slows the weld cooling rate, allowing dissolved hydrogen to diffuse out of the molten pool and preventing the formation of brittle, crack-sensitive martensitic microstructures in the heat-affected zone.