6.3 Pressure Vessel General Arrangement, Detail Sheets, Coordinates & Elevations
Key Takeaways
- General Arrangement (GA) drawings establish the master orthographic layout of boilers and pressure vessels, integrating plan views, elevation views, section cuts, and detail callouts.
- The vessel Tangent Line (T.L.) defines the precise geometric junction between the cylindrical shell and formed heads, serving as the universal reference datum for axial dimensions, nozzle elevations, and tray spacing.
- ASME BPVC Section VIII rules require staggering longitudinal shell seams by at least 5 times plate thickness (or 12 inches) and orienting seam welds away from nozzle cutouts to minimize stress concentrations and avoid mandatory seam radiography.
- Nozzle schedules define nozzle identity, size, schedule, flange class, service, radial projection, and angular azimuth orientation (0 to 360 degrees measured clockwise from Plant North in plan view).
- Internal detail sheets define tolerances for tray support rings (levelness within +/- 1/16 to 1/8 inch), downcomer bars, vortex breakers, impingement baffles, and demister pads, while structural gridlines and TOS (Top of Steel) datums coordinate vessel support framing.
Last updated: August 2026
Pressure Vessel General Arrangement, Detail Sheets, Coordinates & Elevations
Core Concept: Industrial boilermakers construct, rig, align, and overhaul massive steam drums, fractionating columns, reactors, and shell-and-tube heat exchangers using General Arrangement (GA) Drawings and Detail Sheets. Precision blueprint reading requires mastering vessel coordinate datums (Centerline and Tangent Lines), 360-degree nozzle orientation azimuths, shell weld seam staggering rules under ASME Boiler and Pressure Vessel Code (BPVC) Section VIII, internal tray support leveling tolerances, and structural steel framing gridlines.
1. General Arrangement (GA) Drawing Structure
A General Arrangement (GA) Drawing is the primary architectural blueprint of a pressure vessel or boiler. It provides a complete overview of the assembled vessel, defining overall envelope dimensions, nozzle locations, support structures (skirts, saddles, legs, or lugs), design operating parameters, and bill of materials.
+---------------------------------+
| GA DRAWING ARCHITECTURE |
+---------------------------------+
|
+---------------------------------------+---------------------------------------+
| | |
v v v
+--------------------+ +--------------------+ +--------------------+
| PLAN VIEW | | ELEVATION VIEW | | DETAIL CALLOUTS |
+--------------------+ +--------------------+ +--------------------+
| * 360 deg Azimuth | | * Tangent Lines | | * Section Cuts A-A |
| * Nozzle Clocking | | * Elevation Datums | | * Tray Support Bar |
| * Ladder/Clips | | * Shell Seam Stagg | | * Vortex Breaker |
+--------------------+ +--------------------+ +--------------------+
Primary Orthographic Views & Callouts
- Plan View (Looking Down Vessel Axis): Depicts the 360-degree circular cross-section. Establishes the angular orientation (azimuth) of all nozzles, manways, ladder clips, and platform brackets relative to Plant North ($0^\circ$).
- Elevation View (Side Profile): Illustrates the vessel standing vertically or resting horizontally on saddles. Displays vertical centerline datums, base ring elevations, nozzle centerline elevations, skirt heights, and shell course seams.
- Section Cuts (e.g., Section A-A, Section B-B): Cutting planes designated by thick dashed lines with directional arrows indicating viewing direction. Reveals internal components such as fractionator tray support rings, downcomers, demister pads, and steam drum cyclone separators.
- Detail Callouts (e.g., Detail "D", Sheet 3): Enlarged, high-magnification sub-drawings displaying intricate weld preps, bolting assemblies, reinforcing pad dimensions, and clip weld profiles.
2. Coordinate Referencing, Vessel Datums & Weld Seam Layout
Pressure vessel fabrication relies on precise reference datums to locate components in three-dimensional space.
VERTICAL PRESSURE VESSEL DATUMS
TOP T.L. ========================================
| Top Head (2:1 Ellipsoidal) |
+--------------------------------------+
| | ^
| Shell Course 3 | |
| (Long Seam @ 90 deg) | |
|--------------------------------------| | T.L.-to-T.L.
| | | Cylindrical
| Shell Course 2 | | Shell Length
| (Long Seam @ 270 deg) | |
|--------------------------------------| |
| | |
| Shell Course 1 | |
| (Long Seam @ 90 deg) | v
BOTTOM T.L. ========================================
| Bottom Head |
+--------------------------------------+
| Skirt & Base Ring Assembly |
BASE FLANGE +======================================+ EL (+) 100'-0"
Primary Vessel Coordinate Datums
- Vessel Centerline ($CL$): The primary longitudinal and radial symmetry axis from which all radial offsets, nozzle projections, and angular degrees originate.
- Tangent Line ($T.L.$): The precise line of tangency where the curvature of a formed head (2:1 semi-ellipsoidal, ASME torispherical, or hemispherical) transitions into the straight cylindrical shell cylinder.
- Tangent-to-Tangent Length ($T.L.\text{ to }T.L.$): The true cylindrical length of the vessel shell, excluding formed heads. This is the master baseline for all nozzle elevation dimensions.
- Overall Length / Height ($OAL / OAH$): Total distance from the crown apex of the top head to the bottom flange base ring or bottom head apex.
ASME BPVC Weld Seam Staggering & Nozzle Interference Rules
Under ASME BPVC Section VIII, Division 1 (Paragraphs UW-9, UW-14, and UW-15), shell plate layouts must satisfy strict structural seam spacing criteria:
- Longitudinal Seam (Long Seam) Staggering: Longitudinal weld seams in adjacent cylindrical shell courses must be staggered (offset) by a minimum of $5 \times t$ (5 times the plate thickness) or at least $12\text{ inches} (300\text{ mm}) / 30^\circ$ around the circumference. Long seams carry twice the hoop stress of circumferential seams ($S_{hoop} = 2 \times S_{longitudinal}$); aligning long seams across adjacent courses creates a continuous plane of weakness.
- Circumferential Seams (Circ / Girth Seams): Welds joining adjacent cylindrical shell courses or heads.
- Nozzle Cutout Clearance Rules: Shell openings for nozzles and manways should avoid intersecting weld seams whenever feasible. If an opening or reinforcing pad must intersect a shell weld seam:
- ASME Section VIII mandates 100% Radiographic Examination (RT) or Ultrasonic Examination (UT) of the intersected shell seam for a distance of at least $1.5 \times \text{Hole Diameter}$ (or a minimum of $6\text{ inches}$) on each side of the opening.
- Nozzle reinforcing pad welds must maintain minimum clearance from adjacent girth welds (typically $\ge 2\text{ inches}$ or $\ge 3 \times t$ of shell) to prevent overlapping Heat-Affected Zones (HAZ).
3. Nozzle Schedule & Orientation Specifications
Every vessel GA drawing features a comprehensive Nozzle Schedule table that provides exact technical parameters for every shell penetration.
+-------------------------------------------------------------------------------------------------------+
| VESSEL NOZZLE SCHEDULE TABLE |
+-----+------+---------+------------+----------+-----------+------------+------------+------------------+
| MARK| SIZE | SCH/THK | FLANGE CL. | FACING | ELEVATION | AZIMUTH | PROJECTION | SERVICE / REMARKS|
+-----+------+---------+------------+----------+-----------+------------+------------+------------------+
| N1 | 24" | Sch 80 | ASME 300# | RF | EL 112'-0"| 0 deg (N) | 48.00" | Top Vapor Outlet |
| N2 | 12" | Sch 120 | ASME 600# | RTJ | EL 105'-6"| 90 deg (E) | 36.00" | Hydrocarbon Feed |
| N3 | 8" | Sch 80 | ASME 300# | RF | EL 98'-0" | 180 deg (S)| 30.00" | Reflux Return |
| N4 | 2" | Sch 160 | ASME 900# | RTJ | EL 92'-4" | 270 deg (W)| 24.00" | Level Transmitter|
| M1 | 24" | 1.5" thk| ASME 300# | RF | EL 86'-0" | 45 deg (NE)| 42.00" | Vessel Manway |
+-----+------+---------+------------+----------+-----------+------------+------------+------------------+
NOZZLE AZIMUTH ORIENTATION (PLAN VIEW)
0 deg / NORTH
[ N1 ]
|
|
|
270 deg / WEST [ N4 ] --------+-------- [ N2 ] 90 deg / EAST
|
|
|
[ N3 ]
180 deg / SOUTH
Nozzle Geometric Parameters Defined
- Azimuth (Clock Angle): The angular orientation of the nozzle centerline in plan view, measured in degrees ($0^\circ\text{ to }360^\circ$) clockwise from Plant North ($0^\circ$).
- Elevation ($EL$): The vertical height of the nozzle centerline relative to project grade or bottom tangent line ($T.L.$).
- Radial Distance / Offset: The distance from the vessel centerline axis to the nozzle centerline (for radial nozzles, offset is zero; for tangential nozzles, offset is specified).
- Projection: The physical distance from the vessel vertical centerline (or outer shell plate surface) to the finished mating gasket face of the nozzle flange. Critical for verifying piping fit-up clearances.
- Flange Rating & Facing: Specifies the pressure class (ASME Class 150, 300, 600, 900, 1500, 2500) and gasket sealing geometry (RF = Raised Face with serrated finish; RTJ = Ring Type Joint with machined octagonal metallic ring groove; FF = Flat Face).
4. Internal and External Detail Sheets
Detail sheets provide high-resolution mechanical drawings for internal process hardware and external structural attachments.
+-----------------------------------------------------------------------------------------+
| VESSEL INTERNALS & EXTERNALS TAXONOMY |
+-----------------------------------------------------------------------------------------+
| 1. TRAY SUPPORT RINGS : Rolled flat bars welded to vessel ID supporting valve/sieve |
| distillation trays. Strict leveling tolerance required. |
| 2. DOWNCOMER BARS : Vertical/horizontal flat bars bolting tray downcomer plates.|
| 3. VORTEX BREAKERS : Cruciform (cross-shaped) or slotted baffles over bottom |
| liquid outlets preventing swirl cavitation in pump suction. |
| 4. IMPINGEMENT BAFFLES : Sacrificial wear plates positioned directly opposite high- |
| velocity fluid inlet nozzles to prevent shell wall erosion. |
| 5. DEMISTER PADS (MIST) : Stainless steel wire mesh pads supporting gas-liquid droplet|
| separation, held by internal support grids and hold-downs. |
| 6. LADDER & PLATFORM CLIPS: External structural steel brackets welded to shell course |
| PRIOR TO Post-Weld Heat Treatment (PWHT) to support catwalks.|
| 7. INSULATION RINGS : Rolled angle rings welded horizontally at intervals on shell|
| to support thermal insulation and weatherproofing jacket. |
+-----------------------------------------------------------------------------------------+
Internal Tray Support Ring Leveling Tolerances
Distillation columns (fractionators) rely on uniform liquid levels across every tray deck. If a tray support ring is out of level, liquid pools on the low side and vapor bypasses through the high side, destroying fractionation efficiency:
- ASME / Licensor Leveling Tolerance: Tray support rings must be level within $\pm 1/16\text{ inch} (1.5\text{ mm})$ to $\pm 1/8\text{ inch} (3.0\text{ mm})$ across the entire diameter of the vessel, measured using optical levels or precision water manometers during turnaround installation.
TRAY SUPPORT RING DETAIL VORTEX BREAKER DETAIL
| | | |
| Vessel Shell ID | | Bottom Head |
| +----------+ | | +---------+ |
| | Tray Ring| | | | Cruciform |
|===\+----------+ | |===| Baffle |===|
| / Continuous | | +----+----+ |
| Seal Weld | | | |
| | +-----[Nozzle]----+ Liquid Drain
5. Structural Framing Gridlines & Elevation Datums
Boilers, HRSGs, and heavy pressure vessels are supported by massive structural steel superstructures. Boilermakers must coordinate vessel elevations with structural steel blueprints.
Alphanumeric Column Gridlines
Structural drawings utilize an alphanumeric Cartesian grid coordinate system to pinpoint column locations:
- North-South Grid Lines: Designated by letters (e.g., Grid Lines
A,B,C,D). - East-West Grid Lines: Designated by numbers (e.g., Grid Lines
1,2,3,4). - Example: Column
C-3identifies the physical column at the intersection of Grid LineCand Grid Line3.
Standard Elevation Datums
| Elevation Abbreviation | Full Engineering Term | Definition & Trade Purpose |
|---|---|---|
| TOS | Top of Steel | Elevation of the top flange surface of a structural steel beam where vessel support lugs or grating rest. |
| BOS | Bottom of Steel | Elevation of the bottom flange surface of a structural beam (critical for overhead clearance). |
| TOC | Top of Concrete | Elevation of finished concrete foundation piers, equipment pads, or pedestals. |
| BOP | Bottom of Pipe | Elevation of the bottom exterior surface of a process pipe resting on a pipe rack beam. |
| WP | Working Point | A precise spatial reference point where structural framing centerlines intersect. |
| EL (+) 0'-0" | Project Reference Datum | Master site elevation datum (e.g., mean sea level or finished plant grade). |
6. Realistic Trade Scenario: Setting a 120-Foot Fractionator Column
+-----------------------------------------------------------------------------------------+
| FIELD TRADE SCENARIO |
+-----------------------------------------------------------------------------------------+
| Task: Erect, plumb, orient, and secure a 120-foot vertical fractionator column onto a |
| reinforced concrete foundation pedestal with 24 anchor bolts. |
| |
| Verification Checklist: |
| 1. Foundation Verification: Verify Top of Concrete (TOC EL (+) 102'-6") and check anchor|
| bolt circle diameter against GA Sheet 1 foundation detail. |
| 2. Base Ring Azimuth Alignment: Lower column skirt onto foundation; rotate vessel until |
| Skirt Base Centerline aligns with Plant North (0 deg Azimuth). |
| 3. Nozzle Orientation Check: Check Nozzle N1 (Top Vapor Outlet @ 0 deg Azimuth) and |
| Nozzle N2 (Feed Inlet @ 90 deg Azimuth) using optical transit from structural grid. |
| 4. Vertical Plumbness Verification: Setup dual theodolites / optical transits 90 deg |
| apart (North-South and East-West axes). Verify vessel plumbness is within tolerance |
| (typically <= 1/8" per 10 feet of height, or max 1/2" over total 120-foot height). |
| 5. Shimming & Grouting: Place precision steel shims under skirt base ring adjacent to |
| anchor bolts; torque nuts to engineered tension; apply non-shrink epoxy grout. |
+-----------------------------------------------------------------------------------------+
Test Your Knowledge
On a pressure vessel General Arrangement (GA) drawing, what is the precise engineering definition of the 'Tangent Line' (T.L.)?
A
B
C
D
Test Your Knowledge
A vessel nozzle schedule indicates that Nozzle N1 has an orientation of 270 degrees and a projection of 42 inches from the vessel centerline. In plan view where 0 degrees represents Plant North, in what direction and to what physical point does the nozzle extend?
A
B
C
D
Test Your Knowledge
Why do ASME Boiler and Pressure Vessel Code (BPVC) Section VIII rules require that longitudinal weld seams in adjacent shell courses be staggered and that shell nozzle cutouts avoid intersecting existing weld seams?
A
B
C
D
Test Your Knowledge
On an industrial structural framing blueprint supporting a top-hung utility boiler steam drum, what does the callout 'TOS EL (+) 144'-6"' specify to the erection crew?
A
B
C
D