What is a Pressure Vessel? Types, Design Code, Materials & Uses

Pressure Vessel Definition

A pressure vessel is a leak-tight container that holds a gas or liquid at a pressure different from the surrounding atmosphere. The pressure can be positive (compressed air receiver, steam drum, separator) or negative, as in a vacuum vessel. Because the contained fluid stores energy, a pressure vessel is one of the most heavily regulated pieces of equipment in industry, and its design, welding, inspection and testing follow published codes rather than the maker's own judgement.

You meet pressure vessels everywhere in a process plant: separators, knock-out drums, flash drums, surge drums, accumulators, air receivers, reactors, columns and heat exchanger shells are all pressure vessels. If the duty is severe, see our dedicated guide on the high-pressure vessel.

Main Components of a Pressure Vessel

Heads deserve special mention: a hemispherical head is strongest and thinnest for a given pressure but expensive to form, a 2:1 ellipsoidal head is the usual economical choice for moderate pressure, and a torispherical head is cheaper still for low pressure.

Types of Pressure Vessels

By orientation

Vertical vessels stand on a skirt or legs, take up little floor space, and suit separators, knock-out drums and flash vessels. Horizontal vessels lie on saddles and suit surge drums, condensate receivers and two-phase separators because they offer a long, wide liquid surface.

By function

• Storage vessels: store pressurised gas or liquefied gas (air receivers, nitrogen buffer tanks).
• Process vessels: separators, flash drums, accumulators, reflux drums, receivers and reactors.
• Heat transfer vessels: shells of heat exchangers and reboilers.
• Mass transfer vessels: distillation columns and absorbers.

By pressure

Internal pressure vessels, vacuum vessels and combined (pressure and vacuum) vessels each have their own design cases. Many chemical vessels see full vacuum during cleaning or solvent recovery, so external-pressure design is checked even when the main duty is positive pressure.

Pressure Vessel Design Codes and Standards

The code decides how the vessel is calculated, welded, examined and stamped. The ones you will meet in India are:

• ASME Boiler and Pressure Vessel Code, Section VIII Division 1: the most widely used design code for process vessels. Creative Engineering holds the ASME U stamp (new vessels) and R stamp (repairs).
• ASME Section IX: qualification of welding procedures and welders.
• ASME B16.5 and B16.47: flange dimensions and ratings.
• IBR (Indian Boiler Regulations): mandatory for steam and boiler-related duty in India; the company is IBR approved.
• TEMA: for the shells of tubular heat exchangers.
• National Board (NB): registration and inspection of stamped vessels.

Using a manufacturer that already holds these stamps and approvals shortens approvals on your side, because the quality system, welding procedures and inspection plans are already accepted.

Basic Design Principles: How Wall Thickness Is Decided

For a thin-walled cylindrical shell the required thickness rises in direct proportion to the design pressure and the internal diameter, and falls as the allowable stress of the material goes up. In practice, ASME Section VIII Division 1 adds three further factors: the weld joint efficiency (a fully radiographed seam earns a higher efficiency than a spot-examined one), the corrosion allowance added for the planned service life, and the minimum thickness needed for handling and fabrication.

This means two vessels with the same volume can have quite different plate thickness, simply because of pressure, diameter, material grade and how much NDT is specified. A larger diameter shell needs thicker plate than a slender one at the same pressure, which is one reason tall, narrow vessels are economical for high-pressure separation.

Heads follow a similar logic. A hemispherical head carries pressure with roughly half the thickness of a cylinder, a 2:1 ellipsoidal head needs about the same thickness as the shell, and a torispherical head needs more. Designers weigh head thickness against forming cost and available height when choosing the head type.

Pressure Vessel vs Boiler vs Storage Tank

These three terms are often confused, but they differ in purpose, code and risk.

Creative Engineering fabricates pressure vessels and also atmospheric and low-pressure storage tanks, so the right equipment can be specified against the correct code from one supplier.

Inspection, Maintenance and Service Life

A well-built vessel can run for decades if it is inspected and maintained. A practical in-service programme includes:

• External visual inspection for leaks, corrosion under insulation, coating damage and support condition.
• Thickness measurement (ultrasonic) at intervals to track corrosion rate and remaining life.
• Relief valve testing and calibration at the interval set by plant procedure or regulation.
• Internal inspection through the manway during shutdowns, looking for pitting, cracking and fouling.
• Re-test or repair when damage is found; repairs to stamped vessels should follow the code, which is why an R stamp holder is valuable.

Keep the manufacturer's documentation: the drawings, design calculation, MTCs and data report are essential if you ever need to re-rate, repair or move the vessel.

Common Mistakes When Buying a Pressure Vessel

1. Specifying only volume. Without pressure, temperature and fluid the quotation cannot be compared.
2. Choosing the cheapest plate grade. A saving on material can be lost many times over in corrosion repairs.
3. Ignoring vacuum. Vessels that see steam-out or vacuum need external-pressure design.
4. Forgetting nozzle loads and orientation. Late changes to nozzle positions are expensive once plate is cut.
5. Skipping documentation requirements. State the dossier, third-party inspection and certificates you need at enquiry stage.
6. Not asking about weld NDT extent. It directly changes joint efficiency, thickness and price.

Materials Used for Pressure Vessels

Material is picked for corrosion first, then temperature and strength, then cost. Where a corrosion-resistant alloy is only needed on the wetted surface, a cladded or lined vessel can cut cost compared with a solid alloy vessel.

How a Pressure Vessel Is Manufactured and Tested

1. Design and drawings: thickness calculation, nozzle reinforcement, support design and General Arrangement drawing.
2. Material receipt: plate, pipe, flanges and fittings checked against mill test certificates and, where needed, PMI.
3. Cutting, rolling and forming: shell courses rolled and heads formed.
4. Welding: qualified procedures (GTAW, SMAW, SAW, FCAW) by qualified welders, with weld numbers recorded.
5. NDT: radiography, ultrasonic, dye penetrant and magnetic particle testing as specified.
6. Heat treatment and finishing: PWHT where required, blasting, painting or polishing.
7. Hydrostatic test: pressure test at the code multiple of design pressure with inspection for leaks.
8. Documentation: MTCs, NDT reports, hydro-test certificate and the code data report.

Creative Engineering combines NDT Level II engineers, calibrated QC equipment and full traceability of raw material, weld maps and test records.

Where Pressure Vessels Are Used

• Refineries and petrochemicals: separators, drums, accumulators.
• Chemical and specialty chemical plants: process vessels and receivers.
• Pharmaceutical and API plants: SS 316L pressure vessels with polished interiors.
• Power and utility: air and steam receivers.
• Oil and gas: production separators, slug catchers and skid units.

Industry-specific information is available on our pages for oil and gas, petrochemicals, pharma equipment and specialty chemicals.

How to Specify a Pressure Vessel for a Quotation

Provide a data sheet containing:

• Design pressure, operating pressure, vacuum condition and design temperature.
• Fluid and corrosion details.
• Volume or dimensions, orientation and head type.
• Material, corrosion allowance and surface finish.
• Code, inspection agency and documentation level.
• Nozzle schedule, internals and accessories (demister, vortex breaker, safety valve).
• Delivery location and schedule.

Send it via the contact page or to sales@creative-engineering.in. Enquiries are answered within 24 hours.

Frequently Asked Questions