In general, exposure to high heat has minimal impact on performance. “At elevated temperatures and for a long period of time, the urea in the water will decompose and the resulting mixture will change [the urea] concentration from the original 32.5%,” says Leprince. “SCR systems rely on a precise injection of DEF. If urea concentration is slightly lower, performance of the system will be slightly reduced.”
However, it can affect fluid consumption. According to Williamson, the average DEF usage rate is 2% to 4% of the diesel fuel usage rate — in other words, for every 100 gal. of diesel used, 2 to 4 gal. of DEF will be consumed. The percentage usage increases if DEF is exposed to high heat over an extended period.
“Even if the integrity of DEF is compromised from ambient conditions, it is still usable in SCR engine systems,” he states. “It will just be consumed at a faster rate.”
Freezing also has a minimal effect on DEF performance. DEF begins to crystallize and freeze at 12° F. “The 32.5% urea concentration is the ideal solution, as it provides the lowest freezing point,” Schroer states. “At 32.5% concentration, both the urea and water will freeze at the same rate, ensuring that as it thaws, the fluid does not become diluted or over concentrated. The freezing and thawing of DEF will not cause degradation of the product.”
Freezing is not a problem with onboard DEF tanks, since the SCR system is set up to provide heating to thaw DEF in the tank and supply lines. However, storage at temperatures below 12° F is not recommended for stationary tanks. “If exposed to these low temperatures, [DEF] would need to be heated in order to thaw,” says Leprince. This could require the addition of in-tank heating devices and/or potentially delay refilling of vehicles and equipment.
Maintaining the cleanliness and purity of DEF is important to ensure proper vehicle and equipment function.
“As vehicles with SCR systems become common in the construction market, contractors will need to handle DEF properly to keep their vehicles running as intended,” says Simons. “Both EPA and ARB have introduced rules requiring SCR systems to monitor the quality and quantity of DEF in vehicles. If the DEF quality doesn’t meet specified levels, the vehicle performance is supposed to be derated and the operator notified that there is a problem.”
But such safeguards aren’t foolproof. “Contamination of DEF by other liquids will impact performance of SCR systems,” Leprince indicates. “For example, dilution of DEF by water will reduce urea concentration from the normal 32.5%. SCR systems do not typically adapt their injection quantity based on changes in urea concentration.”
According to Williamson, there are no special handling procedures specified for DEF other than to ensure that it is strictly used in the DEF tank. “On the truck, there is a DEF tank — only fill it with DEF,” he emphasizes.
The risk of cross-contamination with diesel fuel is reduced via the use of a smaller nozzle diameter for dispensing DEF into onboard tanks. A blue tank cap also helps to distinguish the DEF tank from the diesel fuel tank.
DEF filters built into SCR systems also help to minimize ingress of solid contaminants. Of course, these filters must be maintained to remain effective. “[The DEF filter] needs to be cleaned every 3,800 to 6,400 hours (or 150,000 to 250,000 miles),” says Williamson.
Overall, the biggest risk of contamination stems from transfer of DEF from one container to another. “DEF can be easily transferred from large storage totes to smaller containers for easier transport,” says Simons. “However, the containers must be clean and used for DEF only.”
Because DEF is corrosive to many metals (e.g., copper, aluminum, mild steel), storage and transport containers must also be made of stainless steel or approved plastics (e.g., polyethylene or polypropylene).
“Although SCR systems include several filters in the DEF tank and supply pumps, those filters can become plugged if contamination is significant,” Simons cautions. “To keep the risk of contamination minimized, only use clean storage containers and prevent debris from migrating into DEF tanks when being filled or transported.”