Heat Exchanger Shells, Cones and Nozzles: Rolling, Welding and Reinforcement

Quick Answer

A heat exchanger shell is either pipe — common up to about NPS 24 (600 mm) — or plate rolled into courses and welded. Under ASME Section VIII Division 1 the shell thickness is t = PR/(SE − 0.6P), where the joint efficiency E comes straight from the radiography level: 1.0 for full radiography, 0.85 for spot, 0.70 for none on a double-welded butt joint. ASME limits out-of-roundness to 1% of nominal diameter, but TEMA's shell tolerances are tighter because the bundle must slide through, and internal weld beads must be ground flush along the bundle path. Longitudinal seams in adjacent courses are offset by at least five times the plate thickness. Cones follow the simple ASME rule only up to a 30° half-apex angle, and every cone-to-cylinder junction needs its own reinforcement check. Nozzles must replace the metal removed by the opening — roughly the opening diameter times the required shell thickness — from excess shell and neck thickness, weld metal or a reinforcing pad with a tell-tale hole. Below certain metal temperatures, material and welds need impact testing; low-temperature grades such as SA-333 Grade 6 pipe and SA-350 LF2 Class 1 forgings are tested at −46°C (−50°F).

The shells in this photograph are for a current Kasko Makine heat exchanger order — chalk-marked with their diameter, order number and drawing position. A few metres away, the cone for one of the units is on the submerged-arc station, and a pallet of pipe pieces cut for nozzle necks carries each piece's heat number in paint.

None of these parts transfer any heat. They are the pressure envelope: the shell that contains the shell-side fluid, the cones that change its diameter, the nozzles that connect it to the plant and the body flanges that let it be opened. They are also where most of the code calculations, most of the welding and most of the radiography happen. This guide covers how they are designed and made.

Shells: Pipe or Rolled Plate

Pipe ovality can be a real problem for a bundle with close baffle clearance. Where it is, the shell is specified with a tighter ovality limit or made from rolled plate.

Shell Thickness and Joint Efficiency

The ASME Division 1 formula for a cylindrical shell under internal pressure is:

t = PR / (SE − 0.6P)

where P is design pressure, R inside radius, S the allowable stress at design temperature and E the joint efficiency. Corrosion allowance is added on top.

E is where fabrication meets design. For a double-welded butt joint (or equivalent):

Dropping from full to spot radiography makes the shell about 18% thicker; dropping to none makes it about 43% thicker. On a thick shell, full radiography is almost always cheaper than the extra steel. Radiography is also mandatory in many cases regardless of E — for example on carbon steel butt welds above 32 mm thick, and in lethal service. Ultrasonic examination, including phased array and TOFD, can replace radiography under defined code conditions, and is often preferred on thick sections.

The chalked "RT-OK" on the shell courses in the photograph records exactly this step: the longitudinal seam has been examined and accepted before the course goes on to assembly.

Rolling Shell Courses

1. Cut and bevel. The plate is cut to the developed length at the mean diameter, with the weld gap allowed for, and the edges are prepared for the longitudinal seam.
2. Pre-bend the ends. Plate rolls cannot curve the last part of each edge. Pre-bending in a press or on four-roll machines avoids flats either side of the seam.
3. Roll. Three-roll and four-roll machines roll the course in several passes, checked with templates.
4. Tack and weld the seam. Run-on and run-off tabs move weld starts and stops off the course. Thick seams are welded by submerged arc; production impact test plates are welded as an extension of the seam where impact testing of welds is required.
5. Re-round. After welding, the course is re-rolled if needed to bring it back within tolerance.
6. Examine. Radiography or ultrasonic examination of the seam.

Tolerances that matter

• Out-of-roundness. ASME limits the difference between the maximum and minimum inside diameters at any section to 1% of the nominal diameter. For a 1,800 mm shell that is 18 mm — far too much for a bundle. TEMA's tighter shell tolerances govern for heat exchangers, because baffles are only a few millimetres smaller than the shell.
• Weld alignment. ASME limits the offset between plates at longitudinal and circumferential seams, with tighter limits on longitudinal seams. Mismatched head and shell circumferences show up here.
• Weld reinforcement inside. Internal weld caps are ground flush wherever the bundle passes. A proud bead stops the bundle.
• Seam staggering. Longitudinal seams of adjacent courses are offset by at least five times the thickness of the thicker plate, unless the intersection is radiographed, so seams never form a continuous line.
• Straightness and length. Checked over the full shell before the bundle is inserted.

Circumferential seams

Courses, body flanges, cones and heads are joined by circumferential seams on turning rolls, which rotate the shell under a fixed welding head so the weld is made in the flat position — as in the photograph of the 013 shell. Submerged arc welding is standard for thick circumferential seams.

Cones and Conical Transitions

Where cones appear in heat exchangers

• Kettle reboilers — the shell enlarges from the bundle diameter to a larger kettle shell that provides vapour-disengagement space. The transition is usually eccentric, keeping the bottom of the shell in line.
• Reducers between a channel and a shell of different diameters
• Vertical exchangers and vessels — conical bottoms for drainage
• Inlet and outlet transitions on large-diameter connections

How they are made

A cone is cut from a developed blank — an annular sector whose radii follow from the cone's diameters and height — and rolled on the plate bending machine with the rolls set to produce the taper. Large cones are rolled in segments and welded. The longitudinal seam is then welded and examined, as on the 006 cone in the photograph.

Design: the junction is the hard part

The basic ASME thickness formula for a cone is:

t = PD / (2 cos α (SE − 0.6P))

where α is the half-apex angle. The formula is valid up to α = 30°. Two complications:

1. Junctions. Where the cone meets a cylinder, the wall changes direction abruptly. Under internal pressure the cone pulls the junction inward at the large end and pushes it outward at the small end, creating a compressive ring load at the large end, a tensile one at the small end and local bending at both — and ASME requires a separate reinforcement check at both ends for half-apex angles up to 30°. Reinforcement is provided by thickening the cone and cylinder locally or by adding a reinforcing ring.
2. Steep cones. Above 30°, the simple rule no longer applies. Either a knuckle is added — a toriconical transition — or the junction is designed by more detailed analysis.

The practical rule: keep half-apex angles at or below 30° unless there is a good process reason, and expect the junction check, not the cone wall, to set the thickness.

Nozzles

Types

• Pipe neck with weld-neck flange — the standard process nozzle, set into the shell and welded with full penetration
• Long weld neck (LWN) — a forged neck and flange in one piece, thicker than pipe, often self-reinforcing
• Self-reinforced forged nozzles — heavy forgings that provide the reinforcement themselves, avoiding pads
• Set-in versus set-on — set-in nozzles pass through the shell wall and are welded inside and out; set-on nozzles sit on the shell surface. Set-in is preferred where full-penetration welds and examination matter
• Couplings and small weld-neck nozzles — for vents, drains, instruments and thermowells

Reinforcement

An opening removes metal that was carrying load. ASME's area-replacement rule requires the removed area — essentially the opening diameter times the required shell thickness — to be replaced within defined limits around the opening from:

• excess thickness in the shell beyond what the pressure requires
• excess thickness in the nozzle neck
• weld metal
• a reinforcing pad

An illustration, simplified: an 8 in. Schedule 80 nozzle in a 1,200 mm shell at 20 bar, SA-516 Grade 70, with a 14 mm shell and 3 mm corrosion allowance, removes roughly 1,750 mm² of required area. The excess shell and neck thickness supply only about half of that. The rest has to come from a reinforcing pad or a heavier neck.

Reinforcing pads are welded around the nozzle on the outside of the shell. Each pad has a small tell-tale hole so that a leak through the nozzle weld can be detected, the space can vent during welding and heat treatment, and the pad weld can be pneumatically tested. Where pads are undesirable — in cyclic, high-temperature or hydrogen service — a thicker shell section or a forged self-reinforced nozzle is used instead.

Minimum neck thickness. Separately from reinforcement, the nozzle neck itself must meet a minimum thickness, and that minimum often exceeds the pressure requirement for small nozzles, which is why small connections are usually heavy-wall.

Nozzle loads

Piping pushes and twists the nozzles it is connected to. Those loads create local stresses in the shell that pressure calculations do not see. They are checked by local stress methods — the WRC bulletins are the usual references — and petroleum specifications such as API 660 set minimum nozzle load requirements for exchangers. Agree the nozzle loads with the piping designer before the shell thickness is frozen; adding a pad after the piping study is far more expensive than designing for the load.

Orientation and projection

Every nozzle is set out to the orientation (angle around the shell) and projection (distance from the shell centreline to the flange face) shown on the general arrangement drawing. Both are classic inspection points. Piping is fabricated to the drawing, not to the exchanger, so a nozzle 15° out or 20 mm short is a site problem.

Body Flanges

The flanges that join the channel to the shell, and the shell to its cover, are girth flanges — not catalogue flanges. They are designed for the specific diameter, pressure, temperature and gasket under the ASME rules for bolted flanges, with TEMA adding heat-exchanger requirements on bolting and gasket design. A girth flange has to:

• seat the gasket and hold it seated at test and operating pressure
• stay stiff enough not to rotate and unload the gasket — ASME includes a flange rigidity check
• tolerate thermal cycling without relaxation leaks

The forged rings in the photograph are body flange forgings for the four units, rough-machined and wrapped for storage. Final machining produces the gasket face, with a surface finish matched to the gasket type, and the bolt holes.

Gaskets for exchanger girth joints are commonly double-jacketed, spiral-wound with inner and outer rings, or kammprofile (grooved metal with soft facing), with pass partition ribs where the channel has multiple passes. See our heat exchanger gaskets. Bolting is tightened to a controlled procedure; our guide to flange bolt torque and assembly to ASME PCC-1 covers the method.

Materials and Minimum Design Metal Temperature

The minimum design metal temperature (MDMT) is the lowest temperature at which the shell may see significant pressure — cold ambient on start-up, auto-refrigeration on depressurisation, or cold process fluid. Carbon steel becomes brittle at low temperature, and MDMT decides whether the material and welds need impact testing.

ASME Division 1 assigns carbon steels to impact-test exemption curves, with the exemption depending on material, heat treatment and thickness. SA-516 Grade 70 normalized, for example, sits on a more favourable curve than the same plate as-rolled. When the MDMT falls below the exemption, impact testing is required:

• Plate — SA-516 normalized with Charpy testing at MDMT as a supplementary requirement
• Pipe — SA-333 Grade 6, impact tested at −46°C (−50°F)
• Forgings — SA-350 LF2 Class 1, impact tested at −46°C (−50°F); LF2 Class 2 is tested only at −18°C (0°F), so specify the class
• Welds and heat-affected zones — impact tests in the procedure qualification, and production test plates where required

The nozzle-neck pipe pieces in the photograph show why traceability matters: each carries its grade and heat number, so its impact test results on the certificate can be tied to the piece in the shell.

Cold climates make this a real issue, not a paper one. Plants in the Caspian region, Central Asia and northern Europe routinely see winter ambient temperatures that put MDMT well below the exemption for thick carbon steel.

Supports and Attachments

• Saddles for horizontal exchangers, one fixed and one sliding — the sliding saddle has slotted bolt holes so the shell can grow thermally without loading the nozzles
• Stacked units need intermediate supports designed for the weight of the upper shell and its contents
• Lifting lugs designed for the empty weight with an impact factor, located so the unit lifts level
• Earthing bosses, nameplate bracket, insulation supports

Attachments welded to the pressure shell are part of the code design: their welds are examined, and temporary attachments are removed, ground flush and examined by magnetic particle or dye penetrant.

Common Specification Mistakes

1. Spot radiography specified to save inspection cost. The 0.85 joint efficiency adds about 18% to shell thickness, and the extra steel costs more than the radiography saved.

   Prevention: Compare full and spot radiography on total cost — steel, welding and examination — for each shell.

2. Pipe shell with mill ovality and a close-fitting bundle. The bundle will not slide in.

   Prevention: Specify a shell ovality limit that suits the baffle clearance, or use rolled plate.

3. No requirement to grind internal welds flush. A proud bead blocks the bundle.

   Prevention: State on the drawing that internal weld reinforcement is ground flush along the bundle path, and witness a gauge-disc pull-through.

4. Cone half-apex angle above 30° without a knuckle or analysis. The simple formula is outside its validity.

   Prevention: Keep α at or below 30°, or specify a toriconical transition and a junction analysis.

5. Cone-to-cylinder junction reinforcement forgotten. The cone wall is adequate; the junction is not.

   Prevention: Require the junction reinforcement check at both ends as part of the calculation package.

6. Nozzle loads agreed after the shell is designed. A reinforcing pad or thicker section is added late.

   Prevention: Agree nozzle loads with the piping designer before freezing shell and nozzle design.

7. Reinforcing pads in hydrogen or severe cyclic service. The crevice and the pad weld become the weak point.

   Prevention: Use integral reinforcement — thicker shell or forged self-reinforced nozzles — in those services.

8. No MDMT, or MDMT copied from operating conditions. Material bought without impact testing is found unsuitable at certificate review.

   Prevention: Define MDMT from the lowest credible metal temperature under pressure, including start-up and depressurisation.

9. Girth flange treated as a catalogue item. A standard flange substituted for a designed one rotates and leaks.

   Prevention: Design girth flanges for the actual gasket, bolting and temperature, including the flange rigidity check.

10. Temporary attachments left on or removed without examination. Weld spots become crack initiation sites.

    Prevention: Require removal, grinding flush and surface crack examination of all temporary attachment locations.

Supply from Kasko Makine

Heat exchangers

Complete tubular heat exchangers — U-tube, floating-head, fixed tube plate and tube-in-tube — predominantly to TEMA Class R, designed to ASME, AD 2000-Merkblätter, EN 13445, ČSN 69 0010 or STN 69 0010.

Shells, cones and pressure parts

• Rolled and welded shells, with radiography or ultrasonic examination to the specified level
• Concentric and eccentric cones, and toriconical transitions
• Nozzles: pipe neck and weld-neck flange, long weld neck and self-reinforced forged designs
• Girth flanges and body flanges from forgings, machined to drawing
• Saddles, lifting lugs and attachments

Materials

Carbon steels including impact-tested low-temperature grades, alloy steels for high temperature and hydrogen service, and austenitic stainless steels, with clad and weld-overlay options.

Associated supply

• Heat exchanger gaskets for girth joints and pass partitions
• Stud bolts and nuts for girth and nozzle flanges
• Expansion joints for fixed-tubesheet shells

Engineering support

Send the general arrangement drawing or data sheet with design pressure and temperature, MDMT, corrosion allowance, nozzle loads and the code of construction. We will check shell, cone and nozzle design against the code, flag radiography and reinforcement choices that drive cost, and quote.

Certification

Material certificates to EN 10204 3.1 or 3.2 with impact test results where required, WPS and PQR records, radiography or UT reports, surface examination reports, PWHT charts and hydrostatic test certificates.

Send your drawing and design conditions for a fabrication review and quotation within five working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Heat Exchanger Fabrication Series

• Heat Exchanger Manufacturing Process: Step by Step
• Pressure Vessel Heads: 2:1 Ellipsoidal vs Torispherical vs Hemispherical
• Tubesheets, Baffles and Tube Bundles
• Flange Bolt Torque and Assembly: ASME PCC-1

Frequently Asked Questions

Q: How does radiography affect heat exchanger shell thickness?
A: Under ASME Section VIII Division 1 the radiography level sets the joint efficiency E in the shell thickness formula t = PR/(SE − 0.6P). For a double-welded butt joint E is 1.0 with full radiography, 0.85 with spot radiography and 0.70 with none. Spot radiography makes the shell about 18% thicker than full radiography, so on thick shells full radiography is usually cheaper overall.

Q: What is the out-of-roundness tolerance for a pressure vessel shell?
A: ASME Section VIII Division 1 limits the difference between the maximum and minimum inside diameters at any cross section to 1% of the nominal diameter. For heat exchangers this is not tight enough, because the baffles are only a few millimetres smaller than the shell, so TEMA's tighter shell tolerances govern and internal welds must be ground flush along the bundle path.

Q: What is the maximum cone angle in ASME Section VIII Division 1?
A: The basic conical shell formula, t = PD/(2cosα(SE − 0.6P)), applies up to a half-apex angle α of 30°. Cone-to-cylinder junctions also need a separate reinforcement check at both the large and small ends. For half-apex angles above 30°, a knuckle — a toriconical transition — or a more detailed analysis of the junction is required.

Q: How is nozzle reinforcement calculated?
A: ASME uses area replacement: the metal removed by the opening, essentially the opening diameter multiplied by the required shell thickness, must be replaced within defined limits around the opening. Excess shell thickness, excess nozzle neck thickness, weld metal and reinforcing pads count towards it. Where pads are unsuitable, a thicker shell section or a forged self-reinforced nozzle provides integral reinforcement.

Q: Why do reinforcing pads have a tell-tale hole?
A: The small threaded hole in a reinforcing pad lets the space between pad and shell vent during welding and heat treatment, allows the pad welds to be pneumatically tested, and shows a leak if the nozzle-to-shell weld beneath the pad starts leaking in service. Without it a leak could pressurise the pad and go undetected.

Q: What is MDMT and when is impact testing required?
A: The minimum design metal temperature is the lowest metal temperature at which the vessel may see significant pressure, including cold start-up and depressurisation. ASME assigns carbon steels to impact-test exemption curves based on material, heat treatment and thickness; below the exemption, material and welds must be impact tested. Low-temperature grades such as SA-333 Grade 6 pipe and SA-350 LF2 Class 1 forgings are tested at −46°C.

Q: What gaskets are used on heat exchanger girth flanges?
A: Girth flanges joining channels and shells commonly use double-jacketed gaskets, spiral-wound gaskets with inner and outer rings, or kammprofile gaskets with soft facings, often with integral ribs to seal pass partitions in multi-pass channels. The gasket type determines the required flange face finish, the bolt load and the flange design, so it should be fixed before the girth flange is designed.