In process safety, we deal with a lot of pressure vessels and even some boilers. It took me years to understand the differences between the U-1, P-2, and P-3 forms and all the requirements for the placement and management of these pressure-retaining containers. This article is intended to help safety pros who may have to deal with these types of pressure-retaining containers; so this is a 30,000-foot view.
The U-1, P-2, and P-3 forms are standard reports used under the ASME Boiler and Pressure Vessel Code (BPVC) to document the design, construction, and certification of equipment. They serve as the “birth certificate” for a vessel, verifying that it was built and inspected in accordance with the Code.
ASME Form Comparison
| Form | Title | Primary Use |
| U-1 | Manufacturer’s Data Report for Pressure Vessels | Used for a complete pressure vessel (Section VIII, Division 1). It covers the entire unit, including shells, heads, and nozzles. |
| P-2 | Manufacturer’s Data Report for ALL Types of Boilers | Used for complete power boilers (Section I), except for electric boilers and those of miniature construction. |
| P-3 | Manufacturer’s Data Report for Watertube Boilers | Specifically used for watertube boilers, their superheaters, waterwalls, and economizers. |
- U-1 falls under ASME Section VIII (Pressure Vessels). This covers equipment like heat exchangers, air receivers, and chemical reactors.
- P-Series (P-2/P-3) falls under ASME Section I (Power Boilers). These are specifically for vessels used to generate steam or vapor at pressures exceeding 15 psig.
- The U-1 is a comprehensive document for a single finished vessel. If a vessel is manufactured in parts by different shops, those shops would provide a U-2 (Partial Data Report), which is then attached to the master U-1.
- The P-2 and P-3 are specific to the boiler industry. While a P-2 is more general for various boiler types, a P–3 is tailored to the complex piping and tube headers found specifically in watertube designs.
- U-1 forms are signed by the manufacturer and an Authorized Inspector (AI) and usually correspond with the “U” or “UM” stamp.
- P-2/P-3 forms correspond with the “S” (Power Boiler) or “A” (Assembly) stamps.
When reviewing these for a mechanical integrity program, the U-1 is what you will most commonly encounter for process equipment. The P-series will almost exclusively be found in the boiler house or power generation areas of a facility.