2.2 Supply and Service Tanks, Contamination, and Draining
Key Takeaways
- The first tank to receive air is the supply tank (also called the wet or purge tank); supply is the formal term, and it captures most of the contamination.
- Air tanks are hydrostatically tested to at least five times the compressor cut-out pressure or 500 PSI (3,448 kPa), whichever is greater.
- Air tank volume must supply at least 12 full service brake applications for power units and 8 for trailers.
- Pull the supply tank lanyard often (pre-trip, en-route inspections, fuelling); if moisture is accumulating, drain the tanks completely at the end of the shift, because tanks do not drain properly until the air pressure is depleted.
- The easiest way to identify the supply tank on a truck is to find the safety valve.
Supply and Service Tanks, Contamination, and Draining
Quick Answer: Compressed air goes first to the supply tank, often called the wet tank because it catches most of the water and oil, and sometimes called the purge tank. From there it passes through one-way check valves into the service tanks for each circuit. Tanks must be hydrostatically tested to at least 5 times the compressor cut-out pressure or 500 PSI (3,448 kPa), whichever is greater. They must hold enough air for at least 12 full service applications on a power unit and 8 on a trailer. Alberta teaches drivers to pull the supply tank drain lanyard often. If moisture is building up, drain the tanks completely at the end of the shift, because a tank does not drain properly until its pressure is gone.
The tanks in a typical tractor
| Tank | Also called | Fed by | Feeds |
|---|---|---|---|
| Supply tank | Wet tank, purge tank | Compressor through the air dryer | The service tanks, through one-way check valves |
| Primary service tank | Primary tank | Supply tank | The primary (green) service circuit, normally the drive axles |
| Secondary service tank | Secondary tank | Supply tank | The secondary (red) service circuit, normally the steering axle |
| Trailer tanks | Trailer service tank(s) | The tractor's trailer supply line | The trailer's service and spring brakes |
The formal name is supply tank. "Wet tank" is common because it captures the bulk of the contamination. "Purge tank" is often used as well, but in fact most air dryers manage their own purge air, so the supply tank rarely does that job. Some vehicles combine tanks. For example, one driver-side tank may be divided into two compartments, with the supply tank in the front portion.
How to find the supply tank on a real truck: look for the safety valve. The supply tank usually carries the safety valve, the drain lanyard underneath, the incoming line from the air dryer, and the small line running to the governor in the engine bay.
Standards for tank strength and capacity
The course gives two standards worth memorizing:
- Strength. Air tanks are hydrostatically tested to handle at least five times the compressor cut-out pressure or 500 PSI (3,448 kPa), whichever is greater.
- Capacity. Each service tank is sized for the brake chambers it supplies. The total tank volume must provide at least 12 full service brake applications for power units and 8 full service applications for trailers.
These reserves are why a driver who notices a problem early still has several full applications available. They are also why pumping the brakes wastes the reserve so quickly (see Section 3.3).
Why tanks collect water and oil
Outside air always contains water vapour. Compressing air to about 120 PSI squeezes roughly nine volumes of outside air into one volume, and it heats the air as well. As that hot, humid air cools in the lines and tanks, the water vapour condenses into liquid. It collects at the bottom of the tanks, mixed with any oil carried over from the compressor. The supply tank, as the first and coolest stop, collects most of it.
Contamination causes three kinds of problems:
- Freezing. In an Alberta winter, water in tanks, lines, or valves can freeze. Ice can block a valve so that air cannot reach the chambers, or trap air so that brakes stay applied and drag.
- Corrosion. Water sitting in steel tanks rusts them from the inside, where you cannot see it.
- Sludge. Oil and water mix into a sticky sludge that fouls valves and attacks rubber parts.
Drain valves
| Type | How it works | Notes |
|---|---|---|
| Manual drain valve | A petcock or valve at the bottom of the tank, opened by hand | Common on service tanks |
| Drain lanyard (pull cable) | A spring-loaded valve opened by pulling a cable routed to the side of the vehicle | Most supply tanks have one; lets you drain without crawling under the truck |
| Automatic drain valve | Opens briefly on its own as pressure cycles; often heated | Can freeze or clog, so still confirm it is working |
How Alberta teaches draining
The course separates checking for contamination from fully draining a tank:
- Pull the supply tank lanyard often during the day: at the pre-trip inspection, during en-route inspections, and when fuelling. A quick pull under pressure does not properly drain the tank, but it shows you whether contamination is accumulating.
- If moisture is accumulating, drain the tanks completely at the end of your shift. Tanks do not drain properly until the air pressure has been depleted and the contamination settles and starts to drip out.
- If necessary, lanyard pulls can be held open with a tarp strap after the shift so the tank drains as the pressure bleeds away. Close them again before the next trip.
- Start with the supply tank, which holds most of the contamination. If you drain a service tank while the supply tank is still charged, air flows from the supply tank through the one-way check valve into the service tank.
- Always stand clear of the discharge, and wear eye protection.
On vehicles with an AD-IS air dryer module, daily manual draining is no longer necessary. It is still recommended that the lanyards are pulled regularly to confirm the dryer is working properly (see Section 2.3). If you find a lot of water or oil in a tank, report it: the air dryer is not doing its job, or the compressor is passing oil.
According to the Alberta course, why should a driver fully drain the air tanks only after the air pressure has been depleted?
To what minimum standard are air brake tanks hydrostatically tested?
You are looking under a truck and need to pick out the supply (wet) tank. What is the most reliable visual clue taught in the course?