The Difference Between U and UM Vessels

Pressure vessel fabricators who are ASME Certified, undergo a rigorous program to ensure compliance with the rules and regulations of the ASME Boiler and Pressure Vessel code, which covers all elements from design to final fabrication.

Within the ASME BPVC are specifics that pertain to the different aspects of boilers and pressure vessels and only those that correspond to the demanding standards of the code qualify for an ASME stamp. To signify specific standards of ASME codes, there are letter designations that appear on the unit itself.

Pressure vessels, in general, can be produced in a variety of sizes ranging from miniature units to larger, industrial scale units depending on their use. The stamps, while similar, are not exactly the same. To ensure these differUM Pressure Vesselsent units perform correctly and meet the safety specifications, there are specific requirements and stamps depending on the size and regulations set by ASME. ASME Section 8, Division 1, describes the designation for pressure vessels and mandates that large vessels receive the U stamp and miniature vessel receive the UM stamp.

Difference Between U and UM

A person could correctly assume that the difference between the U designation and the UM designation is related to size. However, this is not the only difference between the two. UM designated pressure vessels are not required to undergo the same inspection regimen as the larger, U stamped pressure vessels. Additionally, U vessels are required to have a 3rd party ASME inspector review and approve the calculations as well as witness the ASME hydro test while the UM vessels do not have either of these requirements. Companies with UM stamps receive an annual audit whereas those with U stamps undergo an inspection every three years.

This does not suggest that the quality of design or fabrication is less than the larger, U designated vessels, only that the inspection requirements are not needed as often as the smaller vessels. Naturally, this does not preclude any inspection program, only that the standards for inspections are different.

The main difference between U and UM is that UM vessels are smaller in volume than U vessels, even though both size vessels can have similar operating pressures.

Some categories for UM stamped pressure vessels as listed in the ASME BPVC U-1j subset, include:

Vessels 5 Cubic Foot of volume or smaller with pressures not exceeding 250psi.
Vessels 3 Cubic Foot of volume or smaller with pressures not exceeding 350psi.
Vessels 1.5 Cubic Foot of volume or smaller with pressures not exceeding 600psi.

Because pressure vessels operate in a variety of environments and processes, it is crucial that your pressure vessel be fabricated to the highest possible standards. This applies to both large and small pressure vessels. Failure to obtain an ASME vessel can and will put your business at risk and in many cases, ASME certification is required by local regulating agencies. The ASME code was established to ensure that all aspects from design to fabrication, to operation are conducted in a safe and quality manner.

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