Leonard and all,
I think that Mike Cronin’s point is well made – there can be materials contained in the piping that make the piping systems CANDIDATES for Category M fluid service but the fluid alone will not be the determining factor in determining the piping system to be Category M.
As an example, let us take a piping system that contains (in the true sense of that word) methyl isocyanate.
Entering the logic chart in Figure M300 of B31.3 Appendix M, we get to column 3 “Is the fluid toxic”? Well, yes methyl isocyanate is truly toxic so we move down column 3 where we see the next question: “Can a single exposure, CAUSED BY LEAKAGE, to a VERY SMALL QUANTITY of the fluid, produce serious IRREVERSIBLE HARM as defined under Category M fluid service? (see fluid service paragraph 300.2).
First of all the exposure must be due to leakage in the piping system of a very small quantity. The question would be “is the exposure due to leakage in the piping system”? The next question would be “would the exposure to this very small amount cause irreversible harm”? The key words here are “small” and “irreversible”. Will a teaspoon of methyl isocyanate cause irreversible harm. The harm might not be irreversible if timely medical care is received (the owner might have counter measures on site to treat exposures). A negative answer to either one of these questions would “bump” us over to Column 4 and send us to the Base Code for design. But let us assume for this example that the owner answers in the affirmative and continue down to the next “box” in column 3.
“Considering EXPERIENCE, SERVICE CONDITIONS, and LOCATION INVOLVED, will design per the Base Code or Chapter VII sufficiently protect personnel from exposure to very small quantities of the fluid in the environment”? The point here is that piping systems may be designed by applying the rules of the Base Code to protect against leakage of small amounts of fluid. Also, if the service conditions are similar (in say, pressure and temperature) to Category D service conditions the Base Code might (by the owner’s experience) provide adequate protection. Significant benefits can be realized by safeguarding in accordance with Appendix G. “Safeguarding by Plant Layout and Operation” (Appendix G, paragraph G300.2) can result in an operating location where personnel are generally not present (restricted). Isolating operating areas by including buffer zones can also be effective.
These are all considerations that must be evaluated when the owner determines if a piping system that contains methyl isocyanate is to be classified as a Category M piping system. Again, it is not the contained fluid by itself that determines the category. And again, the is NO SUCH THING as a Category M fluid.
Regards, John.