Interpretation ID: nht75-3.36
DATE: 08/13/75
FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA
TO: Construction Machinery Company
TITLE: FMVSS INTERPRETATION
TEXT: This responds to your June 18, 1975, question whether a system which pressurizes a water tank on a concrete mixer by means of air from the truck's air brake system would violate the requirements of Standard No. 121, Air brake systems.
The answer to your question is no. Standard No. 121 does not contain a prohibition on the use of air pressure from the air brake system for powering auxiliary devices. The vehicle must of course conform to Standard No. 121 following installation of the device if the installation occurs prior to the first purchase in good faith for purposes other than resale.
Although not a requirement of the standard, the National Highway Traffic Safety Administration does consider it appropriate that a pressure protection valve be placed in the line to the auxiliary device so that a rupture of an auxiliary line does not cause depletion of air pressure in the brake system.
SINCERELY,
June 18, 1975
T. Herlichy, Chief Counsel National Highway Traffic Division
I've talked to Mr. Williams about the method we use to take air from the air system of a truck to pressurize the water tanks on truck mounted concrete mixers. He in turn suggested I write you for an opinion whether or not the system comes under the brake regulation 121 and, if it does, whether we comply with it.
Basically the design is shown on Page 31, which is enclosed. Valves 4 & 5 on Page 31, are the same as the valves shown on Page 22. An air line runs from Valve 4 up to a control valve that allows the operator to add air to the water tank.
To give you a better idea of how the entire operation goes, let me describe the sequence of events when loading, mixing, and hauling concrete. First of all, the truck is driven under the batching plant at the ready mix yard and loaded with concrete. At the same time the driver fills the water tank with water. After the truck is loaded, it is driven out from under the plant to a mixing area where air is added to the water tank and the operator washes off whatever concrete that has accumulated on the mixer during charging. After the mixer is cleaned and the concrete is mixed, the truck is ready to go on the highway.
At this point no further air is taken from the truck system because either the water tank is completely pressurized or the operator will shut the air off to the water tank. So, while the truck is traveling from the ready mix plant to the job site, no air is taken from the truck brake system.
While on the job site the operator will add water to the mix and washdown the truck after the mixer is empty. He will again be taking air from the truck system. But before he returns to the highway either the water tank will be completely pressurized or the air shut off so that while he is traveling on the road no air will be used from the truck system.
I believe I have given you all the information needed, but if not, let me know what else is required and I will forward it to you.
CONSTRUCTION MACHINERY COMPANY
James E. Johnson, Chief Engineer Vice President-Director of Engineering
PRESSURE WATER SYSTEM
(Graphics omitted)
The air valves in the pressure water system are adjusted and set at the factory. Further adjustment should not be required except for possible adjustment of the height of dial as shown in illustration of leveling of water tank. The pressure regulator (1) is set to maintain a maximum pressure of 40 p.s.i. In the interest of safety this setting should not be exceeded. The relief valve (2) is set to crack open at 75 p.s.i. affording added protection for the system. Control valve (3) in the position shown, is admitting air to the tank, its normal operation position. Turning the valve handle 90 degrees clockwise shuts off the air supply and exhausts the air from the tank. When the air stops escaping, the water tank can be filled. Then moving the valve back to its original position will charge the tank with air. Valve (4) is a pressure hold-back valve. This valve prevents air from passing from the supply tank of the truck when the supply tank pressure is below 65 p.s.i. thus keeping pressure available for operating the truck brakes. Valve (5) is main shut off for all air to mixer water tank and controls.
To operate the pressure water system, open Valve "A". To add water to the drum, open Valve "C". To drain the entire system in freezing weather, open all valves "A", "B", "C", "D" and "F".
AIR SUPPLY TANK HOOK UP
HOOK INTO THE TRUCK AIR SUPPLY, USING THE METHOD SHOWN WHENEVER AIR IS TAKEN FROM THE TRUCK AIR SYSTEM FOR ANY MIXER CONTROL.
(Graphics omitted)
WATER TANK INSTALLATION (Illegible Data)