In United States codes, a pressure vessel is a closed container built to hold a gas or liquid at an internal or external pressure greater than 15 pounds per square inch gauge (psig). That single number is the pressure vessel definition that matters in practice: cross it, and the ASME Boiler and Pressure Vessel Code, OSHA workplace rules, and state operating permits all come into play. Stay below it, and the container is regulated, if at all, under lighter mechanical standards.
Why 15 psig Is the Line
Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code applies to “pressure vessels operating at either internal or external pressures exceeding 15 psig,” whether the pressure comes from an outside source, from applied heat, or from a combination of both.1American Society of Mechanical Engineers. Boiler and Pressure Vessel Code 2017 Brochure Fifteen psig is roughly 103 kilopascals. Below the line, a tank may still need to meet basic strength and testing rules, but it sits outside the major construction code. Above it, the full code applies.
The threshold works in both directions. A vessel operating under significant vacuum also qualifies, because vacuum creates an external pressure differential that stresses the walls inward. The direction of the force changes; the structural risk does not.
The 15 psig figure is specific to U.S. codes. The European Union’s Pressure Equipment Directive applies to equipment with a maximum allowable pressure above 0.5 bar, about 7.25 psig.2European Commission. Pressure Equipment Directive If a vessel will be installed abroad, the local threshold controls.
Maximum Allowable Working Pressure
Once a vessel crosses 15 psig, its design turns on a value called Maximum Allowable Working Pressure, or MAWP. This is the highest gauge pressure permitted at the top of the finished vessel at its designated operating temperature. Wall thickness, material selection, relief device set points, and hydrostatic test pressure all flow from MAWP. OSHA’s air receiver standard, for example, requires safety valves sized so pressure cannot exceed MAWP by more than 10 percent.3eCFR. 29 CFR 1910.169 – Air Receivers MAWP is stamped on the vessel’s nameplate and is the number a future inspector uses to judge whether the equipment can keep operating.
Fired and Unfired Pressure Vessels
Pressure vessels fall into two categories that affect which code sections and inspection rules apply. A fired pressure vessel receives heat from an external source, such as a burner or furnace, to generate pressure. Boilers are the common example. An unfired pressure vessel holds fluids already pressurized by an external supply, a chemical reaction, or another mechanism that does not involve heating the vessel itself. Air receivers, heat exchangers, and chemical reactors are typical unfired vessels.
The distinction matters because fired vessels face thermal stresses and corrosion patterns that unfired vessels do not. ASME devotes separate code sections to each category, and most state boiler laws impose stricter inspection schedules on fired equipment.
What Is Not a Pressure Vessel
Plenty of equipment holds pressure without meeting the code definition. The exclusions are risk-based: smaller or lower-pressure containers pose less danger and are handled under simpler standards or none at all.
The most common exemption covers small, low-pressure containers. Many jurisdictions exclude unfired vessels of five cubic feet or less at pressures up to 250 psig. Other cutoffs include three cubic feet at 350 psig and 1.5 cubic feet at 600 psig. These thresholds appear repeatedly in state boiler laws and in the ASME UM stamp program, which applies a streamlined process to vessels within those smaller size limits.
Several other categories sit outside the pressure vessel code:
- Piping systems designed to convey fluid between equipment are governed by separate piping codes.
- Containers operating at or below 15 psig fall outside Section VIII entirely, though other mechanical standards may still apply.
- Hot water supply heaters within certain temperature and volume limits are regulated under different sections of the boiler code.
- Equipment covered by its own regulatory program, such as DOT transport cylinders, nuclear reactor vessels under ASME Section III, and certain refrigeration components under ASHRAE standards, is excluded because it is already regulated elsewhere.
The ASME Boiler and Pressure Vessel Code
The ASME Boiler and Pressure Vessel Code (BPVC) is the primary construction standard in the United States. It has been incorporated into the laws of state and local jurisdictions across the country and in nine Canadian provinces.1American Society of Mechanical Engineers. Boiler and Pressure Vessel Code 2017 Brochure When a state law refers to a “code-built” vessel, it almost always means the ASME BPVC.
Section VIII governs pressure vessel construction and splits into three divisions:
- Division 1 is the prescriptive, design-by-rules path and the most commonly used division. It provides conservative formulas for wall thickness and nozzle reinforcement and is generally applied to vessels up to about 3,000 psi, though no hard upper limit is set.
- Division 2 allows both rule-based and analysis-based design. The more detailed engineering permits higher allowable stresses and thinner, lighter vessels for the same service, which pays off on large or expensive equipment.
- Division 3 is a pure analysis-based approach for high-pressure service, typically above 10,000 psi. It uses advanced stress analysis and high-strength materials for equipment such as autoclaves and research vessels.
Engineers choose the division based on operating pressure, the tradeoff between thicker walls and more engineering work, and required material properties. Division 1 covers the vast majority of industrial pressure vessels.
Stamping, Inspection, and Registration
Building to the code is not something a manufacturer can simply claim. Third-party oversight runs through the entire process, and the finished vessel carries permanent markings that prove compliance.
The ASME Certification Mark
Before stamping any vessel, a manufacturer must hold a valid ASME Certificate of Authorization. That involves writing a quality manual, training personnel to implement it, and passing an on-site assessment where an ASME review team verifies the quality program is functioning. The certificate is issued only after successful demonstration to the review team.4The American Society of Mechanical Engineers. ASME Certification Process
The most recognized symbol is the U stamp, which indicates a vessel built to Section VIII, Division 1. Smaller vessels within the miniature-vessel size limits may carry the UM designation instead. These letters, now part of a unified ASME certification mark, are physically applied to the nameplate and cannot be used without the matching certificate.
Authorized Inspection Agencies
A manufacturer cannot self-certify. The code requires an Authorized Inspection Agency (AIA), accredited by ASME under its QAI-1 standard, to inspect during manufacturing.5The American Society of Mechanical Engineers. Authorized Inspection Agency Accreditation The AIA assigns an Authorized Inspector who reviews design documents, conducts in-process inspections at key fabrication stages, witnesses pressure tests, and signs the Manufacturer’s Data Report. That inspector must hold a valid commission issued by the National Board of Boiler and Pressure Vessel Inspectors.6ASME. Form U-1 Manufacturer’s Data Report for Pressure Vessels
National Board Registration and the Nameplate
After a vessel passes its code inspections, the manufacturer files the Manufacturer’s Data Report (Form U-1 for Division 1 vessels) with the National Board of Boiler and Pressure Vessel Inspectors. Registration confirms that the vessel was designed and built to the ASME code, inspected by a commissioned inspector, and documented on permanent record. The vessel then carries an NB mark alongside the ASME certification mark. As of January 1, 2026, all data report forms must use the formats mandated by the 2025 ASME code revisions; the National Board returns outdated forms for correction.7The National Board of Boiler and Pressure Vessel Inspectors. National Board Registration
The nameplate attached to the vessel records the information a future inspector needs: the manufacturer’s name, the ASME certification mark with its designator, the National Board number, MAWP, minimum design metal temperature, and the serial number. That plate is the vessel’s legal identity. If the nameplate cannot be read, the operating limits cannot be confirmed, and most jurisdictions will not issue or renew a permit.
OSHA and Workplace Use
ASME governs how a pressure vessel is built. OSHA governs how it is used in the workplace. OSHA does not write its own design standards; it incorporates recognized codes by reference and enforces compliance through its general industry, maritime, and construction rules.8Occupational Safety and Health Administration. Pressure Vessels – Standards
The most directly applicable OSHA rule is 29 CFR 1910.169, which covers air receivers. All new air receivers must be built to ASME BPVC Section VIII. Every receiver must have a visible pressure gauge and spring-loaded safety valves tested frequently and at regular intervals. No shutoff valve may be placed between a receiver and its safety valve, and the valves must prevent pressure from exceeding MAWP by more than 10 percent.3eCFR. 29 CFR 1910.169 – Air Receivers Additional OSHA standards address specific compressed gases (acetylene, hydrogen, oxygen, nitrous oxide), flammable liquids, liquefied petroleum gas, and anhydrous ammonia storage, each with their own requirements for the pressure equipment involved.8Occupational Safety and Health Administration. Pressure Vessels – Standards
OSHA’s Technical Manual guides inspectors in evaluating pressure vessel hazards on site. The manual notes that most vessels in U.S. service were designed under either the ASME code or API Standard 620 for lower-pressure storage vessels, and that certification can only be performed by trained inspectors qualified for each code, with written examinations and practical experience required.9Occupational Safety and Health Administration. OSHA Technical Manual – Section IV Chapter 3 Pressure Vessel Guidelines Operating a vessel without functional relief capacity violates both the ASME code and OSHA workplace safety standards.10Occupational Safety and Health Administration. Pressure Vessels – Overview
Inspection After a Vessel Enters Service
A pressure vessel does not stop being regulated once it starts operating. Ongoing inspection is how operators catch corrosion, cracking, and other degradation before something fails. The primary standard for in-service inspection of refining and chemical process vessels is API 510, the Pressure Vessel Inspection Code.
API 510 ties inspection frequency to remaining useful life. Internal or on-stream inspections cannot be spaced more than half the vessel’s calculated remaining life or 10 years apart, whichever is shorter. When remaining life drops below four years, the interval can extend to the full remaining life, up to a maximum of two years. External visual inspections default to every five years, or at the next required internal inspection if that comes first. Pressure relief devices in typical process service should be tested at intervals not exceeding five years; in clean, nonfouling, noncorrosive service, that interval extends to ten years.
Before a new vessel enters service, it undergoes a pressure test to verify integrity beyond normal conditions. The standard method is a hydrostatic test, filling the vessel with water and pressurizing it to 1.3 times MAWP. Water is nearly incompressible, so a failure releases little stored energy. Pneumatic testing with nitrogen or another gas is reserved for vessels that cannot tolerate moisture, and it carries additional safety precautions because a pneumatic failure can produce an explosion rather than a leak.
State Operating Permits
Most states require pressure vessels to carry a current operating certificate or permit issued by the state’s boiler inspection division, usually housed within a labor or public safety department. These permits are separate from the ASME construction stamp and National Board registration. Where the stamp proves the vessel was built correctly, the state permit proves it has been inspected in place and remains safe to operate.
Annual fees, inspection intervals, and the penalties for operating with an expired certificate vary by jurisdiction. Enforcement is consistent: states employ or commission inspectors, often required to hold National Board commissions, who perform periodic inspections. Operating a vessel with a lapsed certificate can bring fines, shutdown orders, or both. If you are responsible for pressure vessels at a facility, your state boiler office is the starting point for local obligations.