"Flowable" and "Non-flowable"
Pipeline and Hazardous Materials Safety Administration · Sep 23, 1999 · Reference No. 99-0175
Interprets: 49 CFR § 171.8
Reggie’s quick take
The letter answers a question about Sodium Sulfide (UN 1849) and whether it counts as a "flowable" or "non-flowable" solid under § 173.32c(g)(2) of the HMR. A solid is "flowable" if it moves smoothly and without interruption through tank valves, such as crystals or powder that flow freely. A solid in cake or other form that does not flow that way is "non-flowable." Sodium Sulfide in crystal or powder form is flowable only if it actually flows freely through the valves. If moisture causes clumping or solidification that stops free flow, it is non-flowable.
Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.
This is in response to your letter dated June 25, 1999, requesting clarification on determining whether your material, Sodium Sulfide, UN 1849, is a "flowable" or a "non-flowable" solid as it relates to provisions in § 173.32c (g)(2) of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180).
As used in § 173.32c(g)(2), a "flowable" solid is a solid material (e.g., crystal, powder, pellet, etc.) that moves in a smooth and uninterrupted movement through the valves of a tank. Sodium Sulfide in its crystal or powder form is considered a "flowable" solid if it flows freely through the valves. A "nonflowable" solid is a solid material in a cake or other solid form that does not flow in a smooth and uninterrupted movement through the tank valves. If moisture affects your material causing clumping or other solidification that interrupts free flow or movement through the valves of the tank, your material is considered a "non-flowable" solid.
A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.