In an interpretation dated 6 October 2026, issued as reference 26-0062, the agency said neither figure represents a general restriction under the Hazardous Materials Regulations (HMR). However, the clarification does not remove other requirements governing tank selection, filling levels, chemical compatibility or the transport of hazardous residues.
The response addresses two operational questions raised by North Little Rock, Arkansas-based Fortrex Solutions, relevant to chemical road tankers, ISO tank operators and tank depots handling equipment after unloading.
Organic peroxide filling restrictions
Fortrex asked whether shipments of UN3119, a temperature-controlled Type F liquid organic peroxide, were subject to a maximum load of 40,000 lb when transported in UN portable tanks. The enquiry concerned a peroxyacetic acid formulation.
PHMSA confirmed there is no general weight limit of this kind for the material in question but highlighted an existing filling restriction under 49 CFR 173.225(h)(3)(xi), which limits the degree of filling to 90% at 15°C (59°F). The agency said this provision governs the filling level of the portable tanks covered by the rule and should not be interpreted as a universal payload limit or as blanket authorisation to carry any organic peroxide in any tank.
The applicable provisions of Section 173.225 specify which formulations may be transported and under what conditions, including temperature-control requirements. Type F formulations not listed in the Organic Peroxide Portable Tank Table require approval from PHMSA’s associate administrator before they can be carried in portable tanks.
Other restrictions may apply to individual products. For example, the table specifies a maximum of 2,000 kg per portable tank for a particular solid dicumyl peroxide formulation classified as UN3110. Although this concerns a different substance, it illustrates why the absence of a general 40,000 lb limit does not mean all quantity restrictions have been removed.
Tank operators must also comply with requirements for authorised tank types, external inspections before filling and the maximum allowable gross mass stated on the tank’s identification plate. Restrictions on partial filling may also apply depending on the tank design and the properties of the liquid being carried.
Caustic soda residues
The second issue concerned residual sodium hydroxide solution (caustic soda), classified as UN1824, Class 8, at concentrations between 32% and 50%.
Fortrex sought clarification on whether regulations restricted the amount remaining after unloading to 925 gallons or another specified volume, in either cargo tanks or portable tanks.
PHMSA said the HMR do not establish the claimed general limit. It referred instead to Section 173.29, which governs empty packagings containing hazardous material residues. Unless an applicable exception is available, such packagings must be offered for transport and transported in accordance with the requirements that applied when they contained a greater quantity of the hazardous material.