Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at

July 15, 1994

Mr. Charles Knudsen
2835 24th Avenue
Marion, Iowa 52302

Dear Mr. Knudsen:

This is in response to your April 12 letter, requesting interpretation of the Process Safety Management (PSM) of Highly Hazardous Chemicals standard, 29 CFR 1910.119 with respect to a parts cleaning station (PCS) involving flammable liquids as described in the scenario below. Please accept our apology for the delay in responding. Your question and our reply follow.


A flammable solvent mixture is used at the PCS to clean ink from small parts. The PCS is connected to two flammable solvent storage tanks. The flammable solvent mixture is composed of the following

    Components              CAS Number           % By Weight

    n-Proponol              71-23-8              25-30
    Ethanol                 64-17-5              20-35
    N-Proponal Acetate      109-60-4             30-40
    Isabutyl Acetate        110-19-0             0-3
    VM&P Naphtha            8032-34-4            0-2

The PCS is connected by pipeline to two outdoor above ground storage tanks each containing a maximum of 5000 gallons (18,925 liters) or 35,600 pounds (16,147.8 kg). The piping from each of the two storage tanks meets at an outside electric ball valve which directs delivery from only one tank at a time. The PCS system also contains a pneumatic ball valve system to control the filling and emptying of the PCS wash tank.

Spent solvent is pumped directly from the PCS wash tank to an outdoor waste solvent storage tank which has a maximum capacity of 8000 gallons (30,280 liters) or 57,760 pounds (26,199.4 kg). An in-line reverse check valve prevents backflow in this line. The typical volume of flammable solvent mixture in the PCS wash tank and process lines (during normal use) is 70 gallons (265 liters) or 499 pounds (226.3 kg).

The PCS wash tank and outdoor flammable solvent mixture storage tanks are not chilled or refrigerated to keep the solvent mixture below the boiling point of 208 degrees Fahrenheit (97.8 degrees Centigrade). The two storage tanks and the waste solvent storage tank operate at atmospheric pressure.

Question: Is the preceding workplace scenario covered by the PSM standard?

Reply: Yes. By the definition of "process" at 1910.119(b), the two flammable liquid storage tanks, the wash tank, the waste solvent storage tank and interconnecting piping are considered a single process which contains a threshold quantity (TQ) or greater amount of flammable liquid at any one point in time. By 1910.119(a)(1)(ii) and as further delineated in the following clarification, this is a covered process. Please note that the TQ or greater amount of flammable liquid contained in a process covered by the PSM standard is based on the aggregate amount of all flammable liquids contained in a process at any one point in time.

For the purposes of the "process" definition in 1910.119(b), "any group of vessels which are interconnected and separate vessels which are located such that a highly hazardous chemical could be involved in a potential release shall be considered a single process".

Also, by the definition of "process", your covered process includes the activities of not only storage and associated transfer but also, on-site movement and use. The on-site movement and use activities of your process cancel the exception at 1910.119(a)(1)(ii)(B) which allows only storage and associated transfer as excepted activities. Please note that the exception at 1910.119(a)(1)(ii)(B) applies to atmospheric tanks as defined in 1910.119(b). As such, atmospheric tanks must be designed (emphasis added) to operate at pressures from atmospheric through 0.5 pounds per square inch gauge (PSIG) or 3.45 kilo pascals (KPA). Atmospheric tank design requirements are contained in 1910.106(b).

Thank you for your interest in occupational safety and health. If we can be of further assistance, please contact us.


John B. Miles, Jr., Director
Directorate of Compliance Programs