Interpretation ID: 2672o
Director, Government Technical Affairs
Freightliner Corporation
Charlotte Technical Center
9844 Southern Pine Boulevard
P.O. Box 7562
Charlotte, NC 282l7
Dear Mr. Rossow:
This responds to your letter requesting an interpretation of Standard No. l2l, Air Brake Systems. You asked whether a proposed design would meet the requirements of S5.l.2. Your question is responded to below.
By way of background information, the National Highway Traffic Safety Administration does not provide approvals of motor vehicles or motor vehicle equipment. Under the National Traffic and Motor Vehicle Safety Act, it is the responsibility of the manufacturer to ensure that its motor vehicles or equipment comply with applicable requirements. The following represents our opinion based on the facts provided in your letter.
Under section S5.l.2, trucks and buses are required to have the following equipment:
"Reservoirs. One or more service reservoir systems, from which air is delivered to the brake chambers, and either an automatic condensate drain valve for each service reservoir or a supply reservoir between the service reservoir system and the source of air pressure.
You stated that some of your existing air braked trucks utilize a supply reservoir or wet tank between the service reservoir system and the air compressor without using automatic condensate drain valves on the service reservoirs. You noted that the supply reservoir functions as a means of removing excess water vapor from the air supply to avoid water contamination of the braking system and works on thermodynamic principles whereby water condenses to a liquid as the hot compressed air cools.
Your proposed design would utilize an air dryer between the service reservoir system and the air compressor. According to your letter, the air dryer serves the same function as the supply reservoir in your existing system but works on a different principle. You stated that the moist, compressed air passes through a filter media contained in a small canister sized reservoir. The material, a desiccant, has a high chemical affinity for water. The water absorbs on the desiccant and is later purged by stored dry air. The air dryer would have an integral automatic condensate drain valve.
Since your proposed design would not include an automatic condensate drain valve for each service reservoir, the issue raised by your letter is whether it complies with S5.l.2's option for "a supply reservoir between the service reservoir system and the source of air pressure." You stated that you believe the air dryer with automatic condensate drain is the functional equivalent of the more generally accepted embodiment of a supply reservoir in the context of S5.l.2. You also noted that Standard No. l2l does not specify a separate volume for the supply reservoir, although it does require in S5.l.2.l that the combined volume of all service reservoirs and supply reservoirs be at least l2 times the total service brake chamber volume. You suggested that if the volume of the service reservoirs is l2 times the volume of the service brake chambers, it would appear that there is no requirement for a specific volume in the supply reservoir.
While Standard No. l2l does not include a definition for "supply reservoir," the term is one that is commonly understood. For example, you indicated in your letter that some of your current brake system designs utilize the "more generally accepted embodiment of a supply reservoir."
In considering whether a particular item of equipment can be considered a "supply reservoir," we believe that effect must be given to both "supply" and "reservoir." The dictionary defines "reservoir" as "a receptacle or chamber for holding a liquid or fluid, as oil or gas." The word "supply" is defined as "to furnish or provide." Random House Dictionary of the English Language (unabridged edition). The Society of Automotive Engineers (SAE) defines "air reservoir" as "(a) storage container for compressed air." SAE Recommended Practice J656g, "Automotive Brake Definitions and Nomenclature."
Thus, in order to qualify as a "supply reservoir," an item of equipment must hold or store air in order to furnish or provide the air to the rest of the brake system. The information provided with your letter does not provide sufficient information to determine whether your air dryer qualifies as a "supply reservoir." In particular, the information does not indicate whether the air dryer holds other than a de minimis amount of air. While your letter is correct that there is no requirement for a specific volume in the supply reservoir if the volume of the service reservoirs is l2 times the volume of the service brake chambers, an air dryer with a de minimis volume could not be considered to hold or store air in order to furnish or provide the air to the rest of the brake system. On the other hand, if a supply reservoir provides an air cleaning function as well as holding or storing air in order to furnish or provide the air to the rest of the brake system, it would still be a supply reservoir.
Sincerely,
Erika Z. Jones Chief Counsel
ref:121 d:2/18/88