Mountain Equipment Buyer's Guide

Mountain Equipment is frequently asked what "ASME" means and why it is important for a pressure vessel to have a "U" stamp. Simply put, an ASME U stamp (and its accompanying U1A data form) show the pressure vessel was built according to the standards and specifications defined by the American Society of Mechanical Engineers (ASME). More importantly, it indicates the vessel was inspected by a qualified and certified ASME inspector before being approved and stamped.

Not all vessels are ASME coded and may not comply with ASME standards, and should be avoided. Vessels that are missing their tags, whether lost or removed, should also be avoided. Here is a quick breakdown of the most common ASME stamps:

ASME "U" Vessel Stamp

U or U1: Vessel was manufactured following the standards set forth for Section VIII Division 1 by the American Society of Mechanical Engineers. ›› ASME Overview

ASME "U2" Vessel Stamp

U2: Vessel was manufactured following the standards set forth for Section VIII Division 2 Alternative Rules, by the American Society of Mechanical Engineers. Also applies to vessels up to 3000# psi. ›› ASME Overview

ASME "U3" Vessel Stamp

U3: Vessel was manufactured following the standards set forth for Section VIII Division 3 High Pressure Vessels, by the American Society of Mechanical Engineers. Typically for vessels rated 10,000# psi or higher. ›› ASME Overview

National Board "NB" Stamp

Like the American Society of Mechanical Engineers, the National Board issues certification stamps. The most common one seen is the “NB” stamp. Many vessels, but not all, have a National Board (NB) #. The serial number after the actual NB stamp indicates the vessel was registered with the National Board of Boiler and Pressure Vessel Inspectors (NBBI) by the manufacturer. Registrations means a report, commonly called a “U1A”, can be obtained at a nominal cost for review before purchase of a used vessel.

When shopping for a new or used pressure vessel like a separator, look for a picture of the original manufacturer’s tag. At a minimum the tag should show:

  • Maximum working pressure and working temperature
  • Year of Manufacture
  • Name of Manufacturer
  • National Board number or drawing number (if available)
  • Any ASME and National Board stamps
  • Additional stamped certifications such as HT (Heat Treated) or RT-1 (Radiological Testing, level 1)

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