As summer yields inexorably to autumn and later, winter, that most common and familiar commodity we call water takes on a sinister edge.
Ice is great in a cool drink, or for skating. It’s not so much fun when it bungs up your air lines and starves your brakes of stopping power.
Even if you have a problem somewhere within your air system, you’ll probably never notice it in warmer weather. If the compressor is cycling on and off more frequently, would you even notice?
And just for laughs, when was the last time you drained your air tanks? Do you even know where the air tank drain valve is? Or more to the point, can you access it (them) with all the aero fairings on today’s trucks?
Compressed air always contains moisture. When air leaves the compressor, it’s hot. As it travels through the hose from the compressor to the air dryer, it cools, and that’s when the moisture in the air condenses to liquid.
A heavy truck’s air compressor puts out four to six ounces of water during an average day’s operation. But get this: less than a teaspoon of water can freeze and disable some portion of an air brake system — disabling the truck at the same time.
“Moisture in the system can condense and freeze in winter, heightening the risk of valve and brake malfunctions,” says Brian Screeton, technical training and service manager at Bendix. “The air dryer is a crucial part of defending the system against moisture year-round, but particularly in cold weather.”
Air dryers: The first line of defence
Air dryers are designed to prevent most of that moisture from reaching the first air tank in the system, the supply tank, or wet tank as it’s known for obvious reasons.
Warm, moist air from the compressor enters the air dryer, moisture becomes trapped within a desiccant cartridge filled with small, moisture-absorbing beads. Some air dryers use coalescing cartridges, which use a filter media to capture water droplets, dirt, and oil.
Air dryers require little attention or maintenance as a rule, so they tend to go largely unnoticed by drivers. Most air dryers have an electric heating element at the discharge or purge valve to prevent ice build-up around the valve. Ice can prevent the valve from closing properly after the moist air is expelled from the air dryer.
That’s obviously not a problem in summer, but a failed heater could cause some grief in the winter, leaving the valve open following a discharge cycle and the air free to leak out through the gap.
Check the service manual for your make and model of air dryer for inspection procedures and possible remedies of a non-functioning heater. This should be an item on all fall PM schedules.
Cartridge replacement
Air dryer suppliers, Bendix and Wabco, recommend the desiccant or coalescing cartridges be changed on one- to two-year cycle depending on the truck’s application and duty cycle. Trucks that consume a lot of air, such as an 8-axle B-train on air suspension, will probably need the cartridge replaced more frequently. Again, refer to the product literature for the recommended replacement interval.
Over time, they become fouled with oil and other contaminants coming from the compressor. If the need arises to replace the compressor due to oil blowby, replace the air dryer cartridge at the same time.
It all begins at the compressor
Compressors are generally reliable but can start slobbering as they age. Lubricating oil starts slipping past the piston rings as the cylinders wear over time.
That oil winds up in the air system and can cause trouble gumming up the tiny passages in your air brake valves — and even the air-activated solenoids that shift the gears in automated manual transmissions.
A working air dryer will keep most of that sludge out of the air system, but some will still get through. The best way to tell if you have an oil blow-by problem is to observe the discharge when draining the air tanks.
If it’s black and oily, guess what? It can also appear gray and foamy.
Aside from the possibility of contaminating various air valves, in cold weather this sludge thickens and can trap water which becomes ice crystals. These can form anywhere in the air system where there is a restriction to the air flow, such as a kinked air hose and perhaps a 90-degree elbow fitting that may have been improperly installed.
Right-angled air fittings are not recommended in air brake systems as they can restrict normal airflow and trap mucky sludge, possibly facilitating a freeze-up.
Recommended Practice RP 617A from the American Trucking Associations’ Technology & Maintenance Council (TMC) outlines a procedure for cleaning up an oily air system. It’s probably not something you want to tackle in your driveway as it can involve disassembling and cleaning air valves and other sensitive parts.
And you need a reliable supply of clean, dry compressed air.
You can also replace contaminated parts. TMC cautions it’s vital that replacement parts be of the exact same specifications — the exact same make, model, and part number as the old one. Deviations here can have an adverse effect on brake system function and performance.
You will have to replace the compressor too.
Telltale signs of trouble
Since the air system is more or less closed, spotting the telltale problems takes some deliberate effort.
The most obvious place to look is the discharge from the wet tank when you drain it. Look for oily residue in the water that drains from the tank. Hold the valve open for several seconds to strain out a quantity of water. If it’s clear, you’re fine. If there’s black or gray muck in there too, you may have a compressor problem.
You may also notice an oily residue around the purge valve on the air dryer. A fine black coating isn’t cause for alarm, but black stuff dripping from the discharge port should set the alarm bells ringing.
You can also get some clues about the condition of air system during the in-cab brake check during your pre-trip inspection.
A maximum allowable air pressure build-up is prescribed in the vehicle inspection rules. With the trailer supply valve closed, the maximum time allowed for the pressure to build from 85 psi to 100 psi must not exceed two minutes with the engine at idle.
That’s a really long time for the pressure to build by 15 psi. If your truck’s build-up time is in the upper range of that interval, it may signal a problem with the compressor.
If the time required to build up air pressure exceeds two minutes, the air brake system is defective — don’t drive it.
Also, the normal purge cycle of the air dryer will vary between trucks, but you should get used to yours after a while. It maybe five to 10 minutes or more between purges. If you notice a consistent change, purging too often or hardly ever, there may be a problem with the governor or the air dryer.
These aren’t necessarily safety issues, but excessive compressor cycling will push more moisture or water (oil too, maybe) into your air system, and that can lead to problems in cold weather.
Repurposed equipment
It’s worth mentioning that a truck originally spec’d for tandem-tandem operation that’s now hauling an eight-axle B-train may need an compressor/air dryer upgrade.
There are six more brake chambers to feed and possibly six more air springs to keep inflated. If the trailer is equipped with an automatic tire inflation system, demand could be well beyond what it was originally spec’d for.
Air dryer manufacturers provide application charts that include vocations, the number of axles, compressor duty cycles, etc., and recommendations for each application.
According to TMC, a linehaul application typically has a compressor duty-cycle of 10% or less, while a severe-service application can have a duty-cycle of 60% or more. That, too, pumps more water into the system.
And speaking of used trucks, check to make sure the braided air hose running from the air compressor to the air dryer doesn’t droop or have any low points where water could collect and freeze.
If the system has been previously repaired or modified, that air line may not be properly installed (it should slope continuously down from the compressor to the air dryer).
Air system de-icer
Trucks that spend a lot of time in the north, where it’s really cold most of the winter, will often be equipped with an alcohol evaporator bottle. These are installed between the air dryer and the wet tank.
Alcohol stored in the bottle evaporates as pressurized air passes through the unit. The alcohol vapor is carried down the line into the reservoirs and air lines where it works to prevent ice buildup.
It’s perfectly safe to spec’ such a unit, but sometimes an emergency de-icing is required. This involves adding a small amount of liquid de-icer (usually methyl alcohol) directly into the air system, usually via the trailer glad hands.
Bendix cautions that these de-icing chemicals can damage air system components like valves and O-rings.
“Drivers should try to limit the amount of the de-icing chemicals used to the affected area as much as possible,” Screeton advises. “And later on in the garage, make sure those parts are inspected closely for any signs of corrosion or weakness and the air brake system continues to work properly.”
System maintenance
Air systems require little regular maintenance but slacking off on even that can lead to freeze-up in winter. Wabco advises checking the exhaust from the air dryer on a weekly basis for signs of leaking water/oil, and to ensure air is not continually flowing from the device.
- Be aware of the condition of the discharge from the air dryer by observing the discharge when draining the air tanks;
- Plan on replacing the desiccant or coalescing cartridges every one or two years, depending of the severity of the duty cycle;
- Watch and listen for any unusual behavior from the air dryer and air compressor as this could indicate a potential freeze up is occurring in cold weather;
- Be alert for excessive cycling of the compressor and purges from the air dryer. This could indicate a problem, and it certainly will introduce more moisture to the brake system.
Credit: Source link
