B31.3 Class M

Hello Mr. Wright,

It is the owner's responsibility to determine the fluid service category. If I were the owner, I would opt to make the system "Normal Fluid Service".

Referring to B31.3, Appendix M, Figure M300, we can follow the logic that would be used to determine the system fluid service. "Can a single exposure, caused by leakage, to (of) a very small quantity of the fluid produce serious irreversible harm as defined under Category M Fluid service?" The answer is NO. At the concentration you have, the presence of the gas would be obvious (causing a prudent person to move to clear air) and any harm would not be irreversible.

You will find that the frequency of systems being correctly labeled "Category M Fluid Service" under B31.3 is about the same frequency as you will see "Lethal Service" vessels under the ASME B&PV Code.

Of course, the above is just my opinion and not necessarily that of ASME or any Code Committee.

Best regards, John.

John,

Thanks for this.

I have asked around and have been unable to find any UK oil and gas systems which have been categorised as category M. Typically, what sort of systems are generally category M?

Regards,

Dave Wright

Hi there.

I'm not mechanical engineer expert and not even mechanical engineer, but I've work on this fluid, with advise of my mechanical engineer last time.

When I refer back to my latest B31.3 copy, and my previous work, one of the classification of class M fluid is having toxic gas in the system.

This is happen in one of my offshore project, where the gas is very sour (as define in NACE MR 0175), which contains high amount of H2S at a very high pressure (reservoir wellhead pressure).

Once the fluid is categorised under Class M fluid, such fluid can not follow clause 302.2.4 in B31.3, ie the overpressure limitation.

Hope this help

Rusman
Process Engineer
Kuala Lumpur

sorry djwright

Since your H2S is high (0.45 mole %) (NB: my previous H2S content is only 3500 ppm(v) and pressure is bit high than yours), then I would say that your fluid/gas service is fall under Class M fluid as per B31.3.

Thanks

Rusman,

Thanks for this.

It seems that categorization of system (either Normal or M) in B31.3 is a quite subjective.

Does anybody know if this topic is addressed in Casti's book on design of piping in accordance with B31.3?

Regards,

Dave Wright

Hi Dave,

The determination of Fluid Service Categories (Category "D", Normal Fluid Service, or Category "M&quot is really not that difficult. Simply go to Appendix "M" of B31.3 and refer to Figure M300 which is a logic flow chart (“hopper chart”). This Appendix is provided to assist the owner in determining the fluid service category.

The topic IS discussed in Chapter 8 of the CASTI book by G. Woods and G. Baguley, as far as designing for Category "M" is concerned. But the first issue is to determine the Fluid Service Category. I think the most common problem is the perception of some engineers (and others) that there is such a thing as a Category "M" FLUID, per se. THERE IS NO SUCH THING.

In paragraph 300.2 Definitions, we find:

Fluid Service. A general term concerning the APPLICATION of a piping system, considering the combination of fluid properties, operating conditions, and OTHER FACTORS which establish the basis for design of the piping system. See Appendix M. (the upper case letters are mine).

Further in 300.2:

(b) Category M FLUID SERVICE: a fluid service in which the potential for personal exposure is judged to be significant AND in which a single exposure to a very small quantity of a toxic fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken. (the upper case letters are mine).

Further, if you look in B31.3, Process Piping, Chapter VIII, Piping for Category M Fluid Service you find paragraph M300 General Statements. Paragraph M300, subparagraph (d) states, “Consideration shall be given to the possible need for engineered safeguards (see Appendix G, paragraph G300.3 in addition to the safeguards already provided (paragraphs G300.1 and G300.3)”.

It is possible to design a piping system with the appropriate level of safeguarding which would allow the owner to determine that the system would be Normal Fluid Service even if the toxic fluid contained within the piping were one "in which a single exposure to a very small quantity of the toxic fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken".

The owner is not determining that the fluid is or is not a risk to personnel. The owner IS determining if THE DESIGN, THE LOCATION, THE OPERATION (and etc.) OF THE PIPING SYSTEM WILL RESULT IN A POTENTIAL FOR PERSONAL EXPOSURE AND (AND AND AND) the fluid in the piping is such that a single exposure to a very small quantity of a toxic fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken. There is a BIG difference.

If Appendix “G”, Safeguarding, is followed there will be VERY FEW cases where it is necessary for the owner to determine that a piping system will be Category “M” Fluid Service.

If you have a specific case for which you would like to have the B31.3 Committee provide an interpretation, sent them an “inquiry”. See B31.3 Appendix “Z”.

And, as always, the above is just my opinion and not necessarily that of ASME or any Code Committee.

Best regards, John.

John,

Thanks for this. It has been most helpful.

Regards,

Dave Wright