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ASME B31.3 Fluid Service for NaOH

ASME B31.3 Fluid Service for NaOH

ASME B31.3 Fluid Service for NaOH

I am trying to justify the ASME B31.3 classification of Normal Fluid service for 50% NaOH.  The system design gauge pressure is below 150 psi and the design temperature is ambient.   Sodium Hydroxide does not produce systemic toxicity; its health effects are due to its corrosive nature.  However it does have a published LD50 of 100 mg/kg (oral Rat), which by OSHA/UFC definition makes it a toxic, leading to the possible classification of category M fluid service.  If anyone has experience with this and can provide a literature reference to back up either classification it would be appreciated.

RE: ASME B31.3 Fluid Service for NaOH


I think that the first thing to remember is that there is NO SUCH THING as a category "M" Fluid.   The B31.3 Fluid Service concept considers the fluid contained within the pipe and the particular set of operating circumstances as well as the safeguards that will be in place (see B31.3, Appendix "G").

If you look at the logic flow chart (Figure M300, Guide to Classifying Fluid Services)provided by B31.3 in Appendix "M", you will see a clear path of logic in determining that a "Toxic" fluid mad be contained in a piping system that is classified "Normal Fluid Service".  

Other references in print simply represent the opinions of the writers.  The determining factor is clear - "The Owner is also responsible for designating piping in certain fluid services and for determining if a certain Quality Service is to be Employed" (Paragraph 300(b) et. al.).

Best regards, John.

RE: ASME B31.3 Fluid Service for NaOH

As "JohnBreen" states NaOH is classified as "Normal Fluid Service" the last time I checked.  
We have 40% NaOH pipelines and and storage tanks all over the plant. We use C/S as is unless we heat the NaOH.  

RE: ASME B31.3 Fluid Service for NaOH

I bet this would be a first that caustic would be considered a lethal substance wrt B31.3.

RE: ASME B31.3 Fluid Service for NaOH

Just because a fluid contained in a pipe may be toxic or lethal, it does not make the fluid service Category M.  Look at the entire definition, all must be met.  e.g. ...... a single exposure to a very samll quantity of a toxic fluid,......... can produce serious irreversible harm to persons on breathing or boidly contact, even when prompt resotrative measures are taken."  Note "very small quantity."  For example, H2S is toxic, but the vast majority of owners (there are exceptions) do not classify piping with H2S as Category M Fluid Service. Caustic should be classified as Normal Fluid Service.

RE: ASME B31.3 Fluid Service for NaOH

CB4 is totally correct but the other key words in the B31.3 definition of Category M are "irreversible harm".  A small amount of acid will burn but with prompt restorative measures, the burns will heal.  So even acid is not normally considered Category M.  Of course this changes if you are in an 'alky' gasoline plant and the acid there keeps burning and burning even with prompt first aid measures.  Usually in these plants the acid systems are considered Category M.

RE: ASME B31.3 Fluid Service for NaOH


I would like to comment on the postings offered above on this thread.

Often the confusion stems from the notion that there is such a thing in the B1.3 rules as "Category M fluids" or "Lethal Service fluids" - there is no such thing.  When THE OWNER determines the "Fluid Service" that will prevail in the design, fabrication and testing of the PIPING SYSTEM, the fluid contained in the pipe is only one consideration.  Sodium Hydroxide is neither a "Category M fluid nor is it a "Normal Service" fluid.  By the way, "lethal service" is an ASME B&PV Code term, NOT a B31 term.  However, you will likely find that THE OWNER will decide to specify "Category M fluid service" for piping systems about as often as the pressure vessels in the system are specified for lethal service - not very often.

"Caustic should be classified as Normal Fluid Service" - well, no.  Again, caustic fluids are just fluids to B31.3.  When THE OWNED specifies the Fluid Service FOR THE PIPING SYSTEM, the fluid contained within the pipe AND the particular set of operating circumstances as well as the safeguards that will be in place must be considered.

A very important point - The ASME B31 Code for Pressure Piping, B31.3, Process Piping DOES NOT "classify" Sodium Hydroxide piping systems or any other piping systems.  ONLY THE OWNER can do that (using the guidance provided in the  B31.3 Code).

I cannot over emphasize the ABSOLUTE NEED to get THE OWNER involved in making this decision.

The opinions provided above are my opinions only and they do not necessarily reflect the opinions of ASME International, or the consensus opinions of the entire B31.3 Code Committee.

Best regards, and a happy holiday season, John

RE: ASME B31.3 Fluid Service for NaOH

Piping containing caustic does not fit the definition of Category M Fluid Service.  While it is correct that the owner is responsible for making fluid service classifications, it would be an incorrect (but conservatively so) decision by the owner to use the Category M Fluid Service rules for piping containing caustic.

RE: ASME B31.3 Fluid Service for NaOH


Well, Ok then I agree with the last posting.  It would be conservative (and probably overly so) for the owner to designate a piping system containing caustic as a "Category M Fluid Service piping system.  

Just going through the logic flow chart (Figure M300, Guide to Classifying Fluid Services)provided by B31.3 in Appendix "M", (while, as Chuck said in his first posting, paying attention to the definitions provided in the Code) will guide the owner in the right direction.

I think half of my reason for posting was simply to get y'all thinking towards the "fluid service" concept (i.e., not just the fluid). Yeah, I know I was being a bit of a pain in the butt over the semantics (holding your feet to the fire to make you put the words "piping system" in the phrase) but I think it might have been useful if it caused a little more thought. The other half was to reinforce the idea that the owner (NOT B31.3 and NOT the designer) was responsible for calling the shot.  So if the owner (against the council of his design consultant) were to take the minority decision to designate a piping system containing caustic as a Category M piping system then he would: be conservative; be voicing the minority opinion; and likely be wasting material.  But it is his money, and it is never the intention of the Code to put a limit on the rigor used in design, so he wouldn't be "wrong" (although I would disagree with his decision just like the rest of y'all).

Regards, John.

RE: ASME B31.3 Fluid Service for NaOH

To all,

I think that this is an important tread as it points out subtle yet significant differences in interpretation and understanding of the B31.3 piping code.

My complaint is that the two graduate engineers this thread have been involved with ASME and B31.1 code decisions and interpretations for many years......They can have a reasonable disagreement on terms and we still have a complex, user-unfriendly system to determine which systems should be Category M !!!

With all due respect, my opinion is that there should be a much better way for the casual engineer to determine fluid category than that complex flow chart in B31.3.

Remember gentlemen, in our fast-paced, fast track, hell-bent-for-leather world, todays mechanical piping designer will be tommorows structural engineer, electrical designer, tommorows process chemical speciallist. The MBAs of the 1990s have said so !!!

Is it fair or even reasonable to impose such a complex system on the piping designer "du-jour"? We all know that neither the Owner nor the MBA project manager will offer any guidance or take any resonsibility? This is the real reason why few systems end up as Category M....

My primary question, and I think it is a reasonable one, is cannot there be a simpler system of categorization ? How about a list where several "common" chemicals (acids, caustics, toxins etc)would be Category M when above certain pressure and/or temperature limits ?

How does the European engineering commumity address this issue ?

My opinion only


"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields  

RE: ASME B31.3 Fluid Service for NaOH

Sorry MJC, but I do not see a disagreement.

Looks to me that after a little fun with words, JB and CB4 are in agreement.  Several times I have had to provide my employer "the owner" (actually he is the chief engineer for a major university) with some guidance in determining piping system fluid services and I have never had a problem with the chart in Appendix M.  Hard for me to see how it could be more simple.

"......there should be a much better way for the casual engineer to determine (piping system) fluid (service) category than that complex flow chart in B31.3....."   Haven't had a problem advising my boss AND he knows it is his responsibility and he accepts it.

By the way MJC, I really appreciate all your postings too.  You have on several occasions answered my questions before I asked them.  I hope you and JB and CB4 all continue to find the time to post here.


RE: ASME B31.3 Fluid Service for NaOH

If I may offer my opinion on this matter.

One would expect that some form of risk assessment would be performed on a system. The Owner would need to convene and undertake the assessment. The engineer would contribute to the assessment as maybe perhaps insurers, regulators, corporates, the community, environmentalists etc etc. Depending upon the scale of the project. This matter is fundamental to a HAZOPS.

At the end of the day the risk lies with the Owner to determine in a qualitative sense the degree of risk, likelihood of an event and the consequences of failure.

The ASME B31.3 procedure is but one part of this process. If the Owner doesnt possess the necessary skills then the engineer has to impart knowledge. That engineer may be the contractor or some other consultant. If the engineer believes that the Owner's call is wrong he should walk away from the project.

THe engineering world has moved on from simply reading words in a Code and putting an interpretation on them to suit the day. As engineers we all manage risk in design, construction and operation.

RE: ASME B31.3 Fluid Service for NaOH

Agree caustric is not category M

Question more routinely comes up for H2S service, which again is not category M

There is an old Code Case for B31.3, don't recall the number off hand, but my interpretation was nerve gas might be considered category M.  Other than that, have not heard of a service that is routinely classified as M service.

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