Pressure Vessel Design: Shapes, Materials, Standards and Inspection

Pressure Vessels: Shapes, Design and Standards

Vessel shapes (forms)
Cylindrical: most cases, easy to manufacture and transport, better use of material, tapered cylindrical transition between two bodies.
Spherical: in theory the best option, less thickness and less weight. However, it is expensive, difficult to manufacture and transport, and occupies more space. Only used for large dimensions.
Toroidal

Accessories: droplet eliminators, skirt, openings, inspection manhole

Thickness of Heads and Tubesheets

The greater of the following two values:
- Calculated for internal pressure: thickness + corrosion allowance
- Structural thickness to withstand own weight, assembly and collapse due to external loads
Recommended minimum structural thickness (minimum 4 mm):
t = 2.5 + 0.001 x Di + C
Di: diameter, mm
C: corrosion allowance for carbon and low-alloy steel:
- 1.5 mm for slightly corrosive media
- 3 mm for medium corrosion rate
- 4 to 6 mm for very corrosive fluids
- 0 for situations where corrosion is considered negligible
Note: for all types of corrosion?

Standards and Codes

• ASME Section I: Power Boilers
• ASME Section II (A, B and C): Materials
• ASME Section III: Nuclear Components
• ASME Section IV: Heating Boilers
• ASME Section V: Non-Destructive Examination
• ASME Section VIII (Divisions 1 and 2): Pressure Vessels
• ASME Section IX: Welding Qualification
• ASME Section X: Fiber-Reinforced Plastic Pressure Vessels

Project Stages

Definition of design data: definition of process, process design, thermal design, mechanical design, internal design, follow-up, issuance of purchase order, decision on the best proposal, supply of raw material, manufacturing design, manufacturing, inspection (quality control), assembly, special pre-tests and operation.

Design data definition:

• Local conditions (climate, subsoil, soil, etc.)
• Definition of design points
• Presentation for non-glass vessels only
• Definition of the standard (national, foreign or user's own)
• Desired service life
• Type of glass, applicable requirements
• Material requirements, weather, area limitation, transport
• Dimensions and weight
• Available utilities (water, energy, steam, etc.) and assembly facilities

Process Design (AR Design)

Calculation of general dimensions and equipment details and parts: internal format, length, diameter, type of cover, installation position, design pressure and temperature, diameter and position/elevation of nozzles.

Mechanical Design

• Selection and specification of materials (shell, head, flanges, tubesheets, internal bolts, plates, etc.)
• Definition of corrosion allowance
• Specification of coating type (internal, external or both)
• Final dimensions of the vessel
• Definition of construction and inspection standards
• Weld efficiencies, type and degree of inspection
• Full structural computation: plate thickness, reinforcement, internal and external parts, skirt, cradle, etc.
• Type and diameter of anchor bolts
• Final position of nozzles and internal parts and instruments
• MAWP and tests
• Approximate weights (empty, operating, test)
• Transport conditions
• Complete mechanical drawing, including accessories
• Diagram of loads on foundations

Procurement Request

Price request and commercial and technical proposals. The document should include:

• Scope of supply
• Drawings, data sheets, specifications, standards, etc.
• Place and delivery deadline of the proposal
• Delivery conditions of equipment
• Required documents (schedule, drawings, certificates, manuals, etc.)
• Presentation deadlines for documents
• Inspection and testing conditions
• Guarantees
• Search for at least 3 manufacturers not pre-qualified

Fabrication

Construction itself. Total or partial fabrication, including:

• External or internal coatings
• Thermal insulation
• Fabrication and installation of internal parts
• Packing and catalysts
• Radiographs and other NDE
• Heat treatments
• Spare parts
• Welding consumables and other assembly materials
• Shipping

Inspection (Quality Control)

• Raw material
• During manufacturing
• Assembly, when required or when parts are completed but not built locally
• Installation
• Qualification of welders and WPS
• Tests and other NDE
• Heat treatment
• Painting, thermal insulation and internal or external linings

Material Selection

Materials: carbon steel, low-alloy steel, stainless steel, plastics, internal/external coatings. Refer to ASME Section II.

Main influencing factors:

• Nature and concentration of the fluid, impurities, contaminants, presence of dissolved gases, suspended solids, temperature, pH, oxidizing or reducing properties, flammability, flash point, toxicity, explosiveness, compatibility with corrosion products, product contamination by corrosion (color, taste, etc.)
• Safety: avoiding accidents; most reliable materials for flammable, toxic, explosive fluids, high temperature and high pressures
• Product form: sheet, tube, forging, casting
• Manufacturing and assembly facilities: weldability, machinability, formability
• Service life under design conditions
• Previous experience and consultation of existing cases
• Dimensional stability: ability to withstand deformation due to mechanical loads, thermal expansion and corrosion without impairing function. Design experience is fundamental.

Heat Treatment of Carbon Steel Welds (ASME VIII, Div. 1)

Stress relief is required for vessels in carbon steel with radiography:

• Up to 50 mm: 24 min per 10 mm of thickness, minimum 15 min. Mandatory for thicknesses above 38 mm; mandatory above 32 mm when preheating is not applied above 200°C.
• Above 50 mm: 2 h, plus 6 min for each 10 mm above 50 mm.
• Radiography: mandatory total X-ray of welds for thicknesses above 32 mm.
• Heat treatment with a minimum temperature of 595°C. Above 12 mm, preheating of at least 100°C is also required.

Material Specifications

Standards issued by recognized normalization bodies, public individuals or manufacturers, containing:

• Description and purpose of the material
• Chemical composition
• Mechanical properties
• Required or recommended tests and examinations
• Acceptance, rejection and marking conditions
• Dimensional, physical and chemical properties
• Optional supplementary requirements

ASME VIII Division 1 Scope and Exclusions

Applies to all pressure vessels except:

• Vessels subject to flame
• Vessels for human occupancy
• Pressure from 0 to 1 kg/cm²
• Pressure above 200 kg/cm²
• Diameter below 6 in.
• Pressurized water (p < 20 kg/cm² and T < 99°C)
• Hot water (capacity up to 0.454 m³), T < 93°C and thermal load < 200,000 Btu

ASME VIII Division 1: Calculation Method

Membrane stress due to maximum circumferential internal pressure must not exceed:

• Below the creep temperature range: the lower of Sr/4 or Sy/1.5
• In the creep temperature range: the lower of:
  • Sr/4
  • Sy/1.5
  • Stress to cause 1% creep in 10,000 h
  • 2/3 of the average stress for creep rupture in 10,000 h
  • 80% of the minimum creep rupture stress in 10,000 h

Design Pressure and Temperature

Design pressure and temperature are established values, not calculated, and must be at least as severe as the service conditions (ASME VIII UG-21). Per ASME, the minimum design pressure is 1 kg/cm², even for vessels operating below that value. For vessels subject to external pressure, full vacuum is the design condition.

ASME VIII Division 2 considers design pressure as the set pressure of the safety valve. Example: vessels handling hydrocarbons with design pressure of 1.75 kg/cm² or higher, or design temperature above 150°C.

Safety Pressure Relief Devices

The safety valve set pressure is the value at which it opens in a relief situation. Overpressure, meaning pressure exceeding the design or MAWP, is protected by a safety device such as a rupture disk, safety valve, or both. Typically set at 10% above maximum operating pressure; with a pilot-operated valve, only 5%.

Vessels operating at high temperatures (above 150°C) should undergo hot calibration. Vessels designed for external pressure (tumblers) do not require a safety device, since vacuum is calculated as the total design condition.

Test Pressure

Test pressure verifies the strength of the material and welds established in design, after manufacturing or after repair. Conducted during manufacturing in the horizontal position; not tested at the original site position.

For internal pressure per ASME VIII Div. 1, paragraph UG-99:

• Q test = 1.5 x MAWP x Sc / Sh
• Q test = 1.5 x Design P x Sc / Sh when MAWP is not known

NR-13 establishes that any action compromising vessel integrity requires hydrostatic testing. Leak tests are performed to verify sealing conditions.

Maximum Allowable Working Pressure (MAWP)

MAWP is the lower of the values calculated for each component of the vessel (shell, head, flange, nozzle, etc.), which cannot exceed the allowable stress of the material at the operating temperature considered. It varies over the equipment's life due to thickness loss. It is calculated using the formulas for the shell and head thicknesses (corroded and nominal). For flanges, accessories and valves, it is the allowable pressure at the temperature considered, found in tables.

Loads Acting on Vessels

• Internal or external pressure
• Own weight (including accessories)
• Wind force
• Weight of fluid, packing or catalysts, or thermal insulation
• Weight of spare parts
• Weight of accessories (ladders, platforms, structures, etc.)
• Support reactions from connected piping
• Thermal expansion loads from vessel and piping
• Dynamic loads from flow or pressure variation
• Vibrations and impacts
• Snow weight

Normally only the first three are considered. Exceptions are nuclear vessels and larger vessels.

Openings

Openings include piping connections, inlets/outlets, instruments, drains, breathers, inspection manholes, and connections to other parts of the vessel.

Openings can be made with nozzles or in heads. They weaken the vessel wall and require reinforcement. Circular or elliptical openings are preferred; the weakening is greater with non-circular openings.

Internal and External Components

Internal: trays used in bubble columns or valves in distillation towers. Includes weirs and manways for personnel passage.

External: vacuum stiffeners, thermal insulation firmly secured by angle irons, wire, eyebolts or lifting lugs, heating coils, ladders, and platforms.

Calculation

Cylindrical shell: thin wall vs. thick wall

Thin wall: t ≤ R/2 or P ≤ 0.385 SE

t = (P × R) / (SE − 0.6P) + C

• t = minimum thickness under internal pressure
• P = internal design pressure, including hydrostatic head when applicable
• R = internal radius of the cylinder
• SE = allowable stress of the material per ASME VIII Div. 1 (see tables) and joint efficiency
• C = corrosion allowance

MAWP for cylindrical shell and nozzle neck

P = (SE × t) / (R + 0.6t)

Definition

Pressure vessels are equipment containing internal or external pressurized fluids.

Any product with a P × V product of 8 or greater:

• Vessel with product P × V less than 8 and containing Class A fluid
• P = maximum operating pressure (kPa)
• V = internal geometric volume (m³)

NR-13 does not apply to:

• Transportable cylinders and vessels transporting products
• Fire extinguishers
• Vessels for human occupancy
• Vessels that are integral parts of rotating machines or reciprocating equipment such as pumps, compressors, turbines, generators, engines, and pneumatic or hydraulic cylinders
• Ducts and piping for fluid conduction

Installation: Other Requirements

• Have a safety valve with opening pressure or equivalent value adjusted below MAWP
• Have a pressure indicating instrument
• Have a safety device against inadvertent blockage when the valve is not installed directly on the vessel
• Have an identification plate
• Have a visible location with painted or stamped identification of the equipment and its category

Documentation

• Safety Register
• Installation Project
• Alteration or Repair Projects
• Inspection Reports
• Operation Manual (for all boilers and vessels in Categories I or II)

Safety Inspection

Reasons making techniques or hydrostatic testing unfeasible:

• When technically unfeasible and recorded in the register by a qualified professional
• A hydrostatic test may be replaced by another technical or non-destructive test that provides equivalent safety.

Examples of these techniques:

• Ultrasonic testing
• Radiography
• Liquid penetrant testing
• Magnetic particle testing
• Leak testing, etc.