When does a pressure vessel not require a pressure relief device? (ASME 6-inch exception)

Since I began consulting I learned of something I thought I would never see… an ASME pressure vessel without a pressure relief device (PRD).  Typically when I have come across one of these vessels I have gotten the “grandfather” or “its too small” justification.  So let’s discuss these two items and see if they are actually legitimate exceptions to…

UG-125 GENERAL

(a) Other than unfired steam boilers, all pressure vessels within the scope of this Division, irrespective of size or pressure, shall be provided with overpressure protection in accordance with the requirements of UG-125 through UG-138, or with overpressure protection by system design in accordance with the requirements of UG-140, or a combination of the two.

Did you notice they mention “size” above?  So if they state “irrespective of size” then how did size become an exception?

Have you ever heard of the 6-inch exception or maybe the 6-cubic ft exception? 

DISCLAIMER:  I am not a mechanical engineer nor a Professional Engineer (PE).  This article is intended for safety professionals who have pressure vessels on their property which are managed by engineers.  It is written to explain how these exceptions are intended to be applied and they have been IMPROPERLY applied so they can better ensure a safe workplace in regards to pressure vessel safety.

The exemptions to a container having to be built to ASME Section VIII Division I are given in U-1(c)(2).  This was the original basis for the application of the “6-inch rule” (currently U-1(c)(2)(i)) and 15 psi (currently U-1(c)(2)(h)).  Over time, the Committee considered additional exclusions in paragraph U-1(c)(2).

Exemptions from Section VIII U-1(c)(2): (emphasis by me)

Based on the Committee’s consideration, the following classes of vessels are not included in the scope of this Division; however, any pressure vessel which meets all the applicable requirements of this division may be stamped with the Certification Mark with the U Designator.

1. 1. vessels within the Scope of other Sections;
   2. fired process tubular heaters
   3. pressure vessels integral with rotating machinery
   4. piping systems
   5. piping components
   6. vessels containing water under pressure
   7. hot water supply storage tank
   8. vessels under 15 psi design pressure
   9. vessels with ID less than 6 in.

These exemptions began in 1940’s to restrict the application of Code for vessels where the product of volume and pressure exceed 60 psi.  This was the original basis for the 6-inch rule and 15 psi limit.

So as we can see from above, size does matter, but it ONLY MATTERS when establishing if the pressure containment has an inside diameter of 6″ or less.  This exemption is often used for small air receivers and high-pressure accumulators.  With no limit on length or pressure, the stored energy can reach extremely high values, with severe consequences should there be a catastrophic failure.

But the “6-inch exception” has been REPLACED with:

U-1(c)(4)(b) 

vessels with a product of pressure times volume LESS THAN 90 psi-cu. ft are OUTSIDE the scope of this ASME Division 1

• Replaces the 6-inch rule
• Re-introduces a P x V = stored energy rule that was originally published in Section VIII in 1940
• This limit is independent of fluid or gas contained in vessel
• This limit is in the ballpark of the limit(s) used in the PED and Australian pressure vessel standard (AS 1228)

So some of you may have heard about the exception called the “6-cubic ft” exception.  What is this all about?

As stated above and in  U-1(c)(4)(b) the “6-cubic ft” exception replaced the “6-inch exception”.  Here is a nice table that ASME provides to demonstrate the application of the 6′ rule:

What Size of Vessel & Pressure Is at the 90 psi-ft³?

Note: Calculation based on P*V = 90 psi-ft³

Vessels with a P*V < 90 do NOT require ASME Code Construction

Using the table above, we can see it starts at 15 psi, as that is the pressure at which the vessel has to be built to a pressure vessel standard.  At the 15 psi mark, we can see the volume the vessel has to be is 6.0 cubic feet; anything smaller than this the vessel falls outside the scope of the standard.  The 6 cubic feet also translate into a 44.88-gallon vessel.  If this container was 6″ in diameter, then it could be NO MORE than 366.7″ in length – any longer it would have to be an ASME stamped container.

So here is the bottom line, if the object does NOT meet the definition of a “pressure vessel” then it does not fall under the ASME Code.  BUT PLEASE remember this… COMPLIANCE does NOT equal Safety!  We had a fatality here in the Cincinnati area where a “sample pot” which was pressurized but was NOT an ASME vessel, due to its size, catastrophically failed killing an operator who was catching a sample.

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