Types of Pressure Vessels: An Engineering Guide by Himalaya Machinery

Technically reviewed by Rohan Shah, Co-Founder, Himalaya Machinery Pvt. Ltd. | ISO 9001:2015 certified by TUV SUD | 40+ years, 2500+ installations, 400+ repeat orders.

Pressure vessels are everywhere in heavy industry. Boilers, LPG bullets, distillation columns, reactors - the list runs long. But they all sit under a small set of engineering categories.

This guide sorts those categories out.

We've been at it a while. Himalaya Machinery has been shaping plate for pressure vessel builders since 1984. Tata, Reliance, BHEL, Godrej, Jindal, Toshiba, Indian Railways — most of the big names have run our machines at some point.

Below: how the industry classifies pressure vessels, what governs the design, and why the plate rolling step matters more than most drawings admit.

Quick answer

Pressure vessels are classified across five axes: shape (cylindrical, spherical, conical), orientation (horizontal or vertical), function (storage, process, heat exchanger, boiler, reactor), construction method (welded, forged, multilayer), and pressure or wall-thickness rating. They also get identified by their heads or dish ends, nozzles, and support systems. Almost every industrial pressure vessel gets designed to ASME Section VIII of the Boiler and Pressure Vessel Code, with IS 2825 in India and PED 2014/68/EU in Europe as regional equivalents.

What is a Pressure Vessel?

Any sealed container that holds gas or liquid at a pressure not equal to the surrounding air. Cross about 15 psig above ambient, and code jurisdiction kicks in. Below that, it's just a tank.

The examples are everywhere once you look. LPG bullets by tank farms. Boilers in sugar plants and power stations. Distillation columns 50 metres tall at refineries. Small air receivers on shop-floor compressors.

Each one has to hold that pressure difference safely for 20 to 30 years. Which is why every serious vessel gets designed to a code. Globally, that's ASME Section VIII of the Boiler and Pressure Vessel Code. India uses IS 2825. Europe uses PED 2014/68/EU. They overlap on the important safety math.

Types of Pressure Vessels by Shape

Shape decides pressure distribution across the wall. That decides thickness. Thickness decides cost. Three shapes cover most of the industry.

Cylindrical Pressure Vessels

Cylinders are the workhorse of pressure vessel fabrication. A cylindrical shell with a dished head at each end. Mounted horizontal or vertical.

The physics is simple. Hoop stress in a cylinder runs twice the longitudinal stress, so wall thickness gets sized against the hoop direction. That's why cylindrical shapes dominate storage tanks, separators, columns, and drums.

Spherical Pressure Vessels

Spheres are mechanically elegant. Pressure spreads equally across the surface, so wall thickness works out to roughly half of an equivalent cylinder.

The trade-off is fabrication cost. Spheres are built from curved petal segments (called orange-peel segments) that are pressed individually then welded together. Typical uses: LPG storage, LNG storage, ammonia storage, industrial gases — where the material saving is worth the extra fabrication.

Conical Pressure Vessels

Cones aren't storage vessels. They're transitions. Hopper bottoms on silos, cyclone separators, reducers between mismatched diameters.

Rolling a cone needs either a variable geometry machine or a specialised cone-rolling attachment. The seam has to track the theoretical apex line, and any drift compounds through the fabrication.

Types of Pressure Vessels by Orientation

Line one of any drawing: horizontal or vertical. Sounds trivial. Drives everything downstream — foundation loads, support type, mixing behaviour, rigging.

Horizontal Pressure Vessels

A cylinder on its side, cradled on two saddle supports. Cheaper to install, easier to inspect, and good for large liquid volumes.

LPG bullets on tank farms. Ammonia storage. Buffer drums in oil and gas. Where floor space is available but vertical clearance is tight, horizontal wins. Saddle spacing follows Zick's method for minimum bending moment in the shell.

Vertical Pressure Vessels

Upright, usually on a welded skirt. Gravity does part of the process work — distillation, catalyst beds, gas scrubbers, most reactors.

The engineering challenge with tall vertical vessels is roundness over length. A 40-metre column has to hold roundness to statutory limits along its full height. That tolerance carries through from the plate rolling stage. A shell that leaves the rolls barrelled can't be fixed downstream.

Types of Pressure Vessels by Function

Buyers usually think in terms of function, because that's what the P&ID specifies. Four categories cover most orders.

• Storage vessels: hold pressurised liquid or gas as inventory. LPG bullets, air receivers, ammonia tanks.

• Process vessels: where the process step actually happens. Reactors, columns, drums.

• Heat exchangers: transfer heat between two fluids across a wall. Shell-and-tube is the classic.

• Boilers: fired vessels that generate steam. Among the oldest and most heavily regulated pressure vessel types.

Types of Pressure Vessels by Construction Method

How the vessel is built decides structural integrity and MAWP (Maximum Allowable Working Pressure). ASME Section VIII covers each route.

• Welded construction: rolled plates joined by longitudinal and circumferential welds. The dominant method. High-criticality service needs full radiography of every weld.

• Forged construction: used for high-pressure vessels, hydrogen reactors, hydro-crackers. No weld seams in the shell removes one of the most common failure modes.

• Multilayer vessels: thin steel layers wrapped concentrically around an inner shell. A crack in one layer can't propagate through the full wall. Common in ammonia synthesis converters.

• Brazed or cast vessels: smaller units in refrigeration and hydraulic systems. Cast iron is heavily restricted by code because of low ductility.

Types of Pressure Vessels by Pressure and Wall Thickness

Two more axes that engineers use, especially when selecting the applicable ASME Division.

By pressure: low-pressure (under 1 barg), medium-pressure (1 to 40 barg), high-pressure (40 to 100 barg), ultra-high-pressure (above 100 barg). ASME Section VIII splits into Division 1 (design-by-rule), Division 2 (design-by-analysis), Division 3 (very high pressure).

By wall thickness: a thin-shell vessel has wall thickness less than 1/10 of shell diameter. A thick-shell vessel exceeds that ratio and needs different stress math (Lamé's equations, not the simple hoop formula). Most industrial vessels are thin-shell.

Types of Head or Dish End in a Pressure Vessel

The head, or dish end, closes off the vessel and absorbs a big share of the design pressure. Head shape is a structural decision driven by pressure and cost.

Head Type	Best For	Notes
Torispherical (F&D)	Low to moderate pressure	Most common, cheapest to form; knuckle stress concern
Ellipsoidal (2:1)	Medium to high pressure	Strong strength-to-cost ratio; widely specified
Hemispherical	Very high pressure	Highest strength, thinnest wall, most expensive to form
Conical	Transitions and flow ends	Geometry demands taper, not pressure
Toriconical	Cone with knuckle radius	Reduces stress at shell-to-cone joint
Flat	Low pressure only	Cheap and simple, weakest under pressure

The design rule most engineers follow: torispherical for lower-pressure storage and general service, ellipsoidal (2:1) for the bulk of medium and high-pressure industrial vessels, hemispherical when pressure demands it and the budget allows for the extra forming work.

Types of Nozzle in a Pressure Vessel

A nozzle is a short pipe section welded through the shell or a head. Every process line, instrument tap, drain, relief valve, and manway sits on a nozzle.

• Radial nozzle: points along the vessel radius. The default and most common.

• Hillside nozzle: offset from centreline, used when internal piping or trays demand it.

• Tangential nozzle: enters along the tangent for cyclonic flow. Common on cyclone separators and scrubbers.

• Angular nozzle: set at a designed angle for a specific process reason.

• Manway nozzle: 450 mm to 600 mm diameter typically, big enough for human access during inspection.

Every nozzle punches a hole in a pressure boundary. That hole needs reinforcement — either a repad or a thicker forged section. Inadequate nozzle reinforcement is one of the more common failures during hydrostatic testing.

Types of Support in a Pressure Vessel

Supports carry the operating weight, take wind and seismic loads, and allow the shell to breathe as it heats and cools. Support type follows orientation.

• Saddle support: two curved cradles under horizontal vessels. Simple, cost-effective, standard for horizontal cylindrical vessels.

• Skirt support: a cylindrical shell welded beneath a vertical vessel. Best resistance to overturning moment in seismic zones. Standard for tall columns.

• Leg support: two, three, or four legs for smaller vertical vessels. Common on ammonia bullets and workshop air receivers.

• Lug support: brackets welded on the shell that rest on a steel structure. Used when the vessel is mounted on a pipe rack or elevated platform.

Materials of Construction

Material selection follows design temperature, design pressure, and fluid corrosivity. The grades most often specified:

• Carbon steel SA-516 Gr. 70: the industry workhorse for moderate temperature and pressure service.

• Stainless steel SS 304 and SS 316: cryogenic service and corrosive chemical environments including pharma and food-grade work.

• Chrome-moly steel SA-387 Gr. 11 or 22: high-temperature reactors and hydrogen service where hydrogen attack is a concern.

• Clad plate: carbon steel base with a thin corrosion-resistant layer of Monel, titanium, or nickel alloy. Used when a full solid alloy build isn't economically justified.

• IS 2062 grades: commonly specified on Indian fabrication contracts working to IS 2825.

Design Formulas Engineers Use

Wall thickness for a thin-walled cylindrical pressure vessel is governed by two stress equations from ASME Section VIII Division 1.

Key stress and thickness formulas

Hoop (circumferential) stress:   σh = P × D / (2 × t)

Longitudinal stress:                 σl = P × D / (4 × t)

Required shell thickness:         t = P × R / (S × E − 0.6 × P)

Where P = design pressure, D = diameter, R = radius, t = wall thickness, S = allowable stress, E = joint efficiency.

Joint efficiency (E) ties directly to the extent of radiography. Full radiography gives E = 1.0 and lets you use a thinner shell. Spot radiography drops E to 0.85 and you pay for it in plate weight. The trade-off between inspection cost and material cost is standard project engineering.

Inspection and NDT Methods

Every serious vessel gets verified with Non-Destructive Testing (NDT) during fabrication and in service. API 510 and ASME Section V govern method selection.

NDT Method	What It Finds	Best Suited For
Radiography (RT)	Volumetric flaws: porosity, slag	Small cylindrical vessels; shop fabrication
Ultrasonic (UT / PAUT)	Planar flaws: cracks, lack of fusion	Thick-walled forged or welded reactors
Magnetic Particle (MT)	Surface and sub-surface cracks	Carbon steel storage vessels and saddles
Dye Penetrant (PT)	Surface-breaking defects	Stainless steel and non-ferrous alloys

How Plate Rolling Machines Shape Every Vessel

Here's the part nobody outside a fab shop pays much mind to. Every pressure vessel starts as a flat plate. A rolling machine bends it into a cylinder or a cone. Roundness, seam gap, and diameter tolerance all get decided at that step, well before any welder shows up.

Storage tanks and standard process vessels - the everyday work - get shaped efficiently by a 3 Plate Rolling Machine. For high-pressure vessels, high-volume production, or shells held to tight ASME tolerances, a 4 Roll Plate Bending Machine earns its cost because it pre-bends both ends of the plate in a single clamp and holds roundness across long runs.

Cones, heavy plate, and thick shells that push a fixed roll past its ratings - that's where a Variable Geometry Plate Bending Machine opens up geometry a conventional roll can't reach. Heavy pressure vessel fabricators and shipyards keep one on the floor for exactly that reason.

Match the machine to the plate and the code, and downstream fabrication runs clean. Mismatch it, and you fight roundness, seam gaps, and higher reject rates for the working life of the equipment.

Codes, Standards, and Selection Criteria

Nothing serious ships without a code stamp. The standards that come up most often:

• ASME BPVC Section VIII, Div 1: design-by-rule. The most common industrial code globally.

• ASME BPVC Section VIII, Div 2: design-by-analysis. Permits thinner walls with more rigorous stress work.

• ASME BPVC Section VIII, Div 3: ultra-high-pressure vessels above 100 barg.

• IS 2825: the Indian standard, broadly aligned with ASME.

• PED 2014/68/EU: the European Pressure Equipment Directive.

• API 510, API 620, API 650: service, inspection, and storage tank references.

Selection between vessel types comes down to four drivers: lethal-service classification (toxic fluids force full radiography plus post-weld heat treatment), cyclic loading (fatigue analysis under Division 2), coastal or seismic environment (heavier corrosion allowance, seismic-anchored supports), and code jurisdiction of the buyer.

Choosing the Right Type of Pressure Vessel for Your Fabrication

Every classification decision feeds the next one. Shape and orientation come from the process. Heads come from the pressure. Nozzles and supports come from the layout. Construction method comes from the code. And every one of them comes back to the plate rolling that turned raw steel into a shell.

At Himalaya Machinery, our plate rolling and bending machines have supported pressure vessel builders across India and export markets for over four decades. All our machines are ISO 9001:2015 certified by TUV SUD, backed by a service network built for the long service lives fabricators need.

If you're specifying plate rolling or bending machines for pressure vessel work, contact us and our engineering team will help you match the machine to your plate, your code, and your production volume.

Types of Pressure Vessels FAQs

What is a pressure vessel in simple words?

A sealed container built to hold gas or liquid at a pressure different from atmospheric. LPG cylinders, boilers, air receivers, and reactors all count.

What is the difference between ASME Section VIII Division 1 and Division 2?

Division 1 is design-by-rule with a higher safety factor built in. Division 2 is design-by-analysis, which permits thinner walls but demands rigorous stress calculations and often finite element analysis.

Why is a spherical vessel more efficient than a cylindrical one?

A sphere spreads pressure equally across the full surface. A cylinder carries twice as much hoop stress as longitudinal stress, so its wall works out to roughly double a sphere of the same diameter and pressure.

What are the types of head or dish end in a pressure vessel?

Torispherical (F&D), ellipsoidal (2:1), hemispherical, conical, toriconical, and flat. Torispherical and ellipsoidal handle most industrial vessels. Hemispherical gets used where pressure is highest.

What are the types of support in a pressure vessel?

Saddle supports for horizontal vessels, skirt supports for tall vertical ones, and leg or lug supports for smaller units. Skirts give the best overturning resistance in seismic zones.

What are the types of nozzle in a pressure vessel?

Radial, hillside, tangential, angular, and manway nozzles. Each carries process fluid, instruments, or access. Each needs a reinforcement pad around the opening.

Which materials are commonly used for pressure vessel fabrication?

Carbon steel SA-516 Gr. 70 for standard duty, SS 304 or 316 for corrosive or cryogenic service, SA-387 chrome-moly for high-temperature and hydrogen service, and clad plate where corrosion resistance matters more than base strength.

How do plate rolling machines affect pressure vessel quality?

The rolling machine sets the shell's roundness, seam gap, and diameter tolerance. All three carry through to weld quality and code inspection. A 4-roll machine holds tolerance better across long runs than a 3-roll, because it pre-bends both ends in a single clamp.