What material group would N.G. odorant be?

[KentAT]

I am looking over our existing installations for odorant injection systems for our natural gas pipeline and compressor station, and I am wondering if Group D is the correct material group for the areas containg the raw odorant mixture such as the pumps and injection lines. (The bulk storage is typically an underground storage tank outside of the injection building). Just curious, that's all.

The odorant mixture used would be similar to Scentinel N, here is MSDS: http://www.cpchem.com/enu/msds_unsecured/Import_99720_MSDS_O_ENGLISH_A_ENGLISH_A_N.pdf

I'm curious because I think we use Group D, but there is no list I could find that includes the ingredients of this mixture, and the manufacturer will not specify the NFPA material group to be used for haz area classification:

Tertiary Butyl Mercaptan
Isopropyl Mercaptan
n-Propyl Mercaptan
sec-Butyl Mercaptan

Plain old Butyl Mercaptan is on the materials list of NFPA 497 as Group C.

Another curious item is that we typically use APIRP500 drawings for a "lighter than air" vapor. However, the odorant liquid itself produces vapor having a density >1 per this MSDS, and also that of another odorant product we might use indicates a vapor density of 3.0, although they are both indicated as having "stable" reactivity at normal temperatures and pressure.

I would tend to think that the liquid would vaporize and produce a heavier-than-air vapor.

Any thoughts on whether a Group C or D would apply?
Would an area need to be classified based on the existence of the raw odorant pump (1000-1500 psig discharge) itself if the gas streams being injected into were not present?

Kent

 

[don_resqcapt19]

Does it change anything if it is one or the other of those two groups? I know that went you get to Groups A and B, there are changes, but off the top of my head, I can't think of any installation differences between Groups C and D, and I don't think I have ever seen any listed devices or equipment that were not listed for both Groups C and D.

 

[KentAT]

I can't think of any installation differences between Groups C and D, and I don't think I have ever seen any listed devices or equipment that were not listed for both Groups C and D.

I noticed the same thing, since I have only looked for Group D listings because my material has always been natural gas.

 

[rbalex]

My understanding is that Methyl Mercaptan is the most commonly used odorant because it is already a gas at standard temperature and pressure and has a vapor density fairly close to air. That makes it “hang around” long enough to get noticed. Most of the other mercaptans are significantly heaver and tend to hug the ground and may be more difficult to smell.

In any case, the significant issue, if any, would be T-rating. It wasn’t that long ago that it was a primary differentiator between Group C and the other Groups. Before what is now Table 500.8(C) was introduced, the presumed autoignition temperature of Group C was 180C (T3A); it was 280C (T2A) for the other Groups.

As both you and Don noted, it probably would make no difference in the actual design and installation. The amount stored in individual containers (bottles?), their pressures and the delivery method to the natural gas lines might affect it, but I suspect that effectively there would be no classification generated before mixing.

There is a method in NFPA 497, Appendix B to determine the effective Group once they are mixed. Again, I suspect it would still be Group D.

 

[KentAT]

Many thanks to all.