A large variety of powdered materials are delivered to end users by bulk trucks using Pressure Differential (PD) trailers. This technology has been around for years, and a truck is a truck, right? Not exactly, and it’s worth taking a few minutes to discuss various truck-mounted bulk delivery systems and what makes each unique.
The PD truck design parameters used in North America are significantly different than those common in Europe. North American PD trailers utilize design standards with operating pressures up to 1 bar (15 PSIG). That is 15 PSIG above normal ambient air pressure at sea level. On the other hand, European PD trailers are designed with operating pressures up to 2 bar (28 PSIG). This pressure difference is significant because any “vessel” operated above 1 bar is considered a “pressure vessel”, requiring completely different (and more expensive) design standards.
Pressure vessels operating above 1 bar are subject to the design standards of the American National Standards Institute (ANSI) which add additional complexity and extra expense to build the trailer. Once in operation, these pressure vessels require periodic testing and permanent record keeping. Operating such a truck trailer also requires higher skilled truck drivers and operators. While common for certain liquids and most gases, high pressure trailers are rarely used for bulk powdered materials in North America. There are pros and cons for each type of truck trailer design – those that operate below or above 1 bar. Let’s dive into the detail.
PD Truck Standards in the United States
Truck transport and unloading systems in North America are generally consistent for a wide variety of powdered materials used by most US industries. With lower upfront costs and lower operating costs, North American bulk truck operations prefer lower operating pressures than are common in Europe. With a 40,000-pound capacity, a single truckload can provide several days of supply for a typical manufacturing plant. Truck delivery is also logistically simpler and cheaper than railroad based bulk delivery.
Along with an operating pressure of 1 bar, US PD trucks are commonly equipped with standard 4-inch pipe connections for unloading. Loading the trailer is accomplished by filling through manways located on the top of the trailer. Trailers are generally designed to include multiple bins rather than a single, large bin. This design facilitates the flexibility of carrying multiple ingredients in a single load and improved discharge design that facilitates completely emptying each bin. Most truck unloading systems require about 2 hours to completely empty a full truckload.
US PD trucks normally include a truck mounted blower to supply compressed air to unload the product. Many small bulk material customers have limited in-house compressed air infrastructure and are therefore dependent on a truck mounted blower to unload the delivery. Unfortunately, using truck blowers to empty the trailer can be problematic. Onboard truck blowers produce outlet temperatures that could be in excess of 200 degrees Fahrenheit. Extended exposure to such high air temperatures can damage or degrade some powdered materials. Sugar, for example, exposed to high temperatures can soften or even melt, which can stick to pipes, valves, or silos. In extreme cases, melted sugar can form a glaze layer in convey line piping, which can eventually build up to limit convey rates or even plug the line.
The alternative to using a truck mounted blower is an in-house truck unloading system. This system includes a stand-alone blower and related piping that is specifically designed for the ingredients used at that location. To protect against high temperature air, an in-house truck unloading system should include an aftercooler to reduce the outlet temperature of the compressed air used to unload the truck. An on-site truck unloading system can also provide additional control safeguards which monitor the unloading process and adjust air flow automatically if needed. These safeguards can include automatic shutdown to prevent a silo from being overfilled. This automation is useful when the truck driver is away from his truck or otherwise distracted.
While Canadian PD trailers are similar to US trailers in terms of operation, there is a significant difference in the trailer weight limits. In the US, truck weight limits are regulated by both the Federal and State governments. As mentioned above, the US net weight (the weight of the goods carried) limit for PD trucks is 40,000 pounds. This standard is the Federal standard and for the majority of State standards. This is completely different in Canada, where truck net weight limits are a function of the number of axles on the trailer. Tandem (2) axle trailers have a 35,000-to-39,000-pound weight limit, about the same as the US. There is a second standard for tridem (3) axle trailers. These larger trailers have a net weight limit of 44,000-to-60,000 pounds. Tridem trailers are very common in Canada given the extra net weight capacity and should be considered when specifying and designing truck unloading systems in Canada.
PD Truck Standards in Europe
PD Tanker trucks in Europe are generally designed and built to operate with higher pressures than PD trucks in the US. European PD trucks can be pressurized up to 2 bar (28 PSIG) for unloading. The higher pressure reduces the unloading time compared to US trucks, but the higher pressure requires special design considerations as they are considered “pressure vessels”. European PD trucks operate at twice the pressure commonly used for PD trucks in the US. Because of this, European trucks generally do not have truck-mounted blowers, so a customer-owner blower is required to unload the truck. While the upfront cost of a European PD trailer is higher than its US counterpart, its higher operating pressure has advantages. It can be unloaded faster and it requires smaller diameter convey lines from the truck to the storage silo.
Heavy duty PD trailers in Europe generally utilize three axles and have a load capacity of approximately 25,000 Kg.’s or about 55,000 pounds. Smaller PD trucks, without separate trailers, are more common in Europe than in the US so there is a wider variety of PD truck sizes in Europe and no design can be considered an industry standard.
Another design standard common in Europe is a built-in lifting capability. Tilting one end of the tank toward the outlet end facilitates the flow of material out of the truck. The lifting can be accomplished with either a self-lifting truck frames (same principle as a dump truck) or with a lifting platform built into the customer’s truck unloading area. Either way, lifting the truck trailer toward the outlet end facilitates better product flow. This self-lifting option is not incorporated into US PD truck designs nor do US customers generally use lifting platforms for unloading powdered materials from a PD truck.
PD Truck Unloading Considerations
In the US, efficient unloading of PD trucks, in the US, requires a material appropriate air pressure set point (0 – 12 PSIG for sugar) which ensures steady material flow. The maximum amount of air is usually not the best set point as the faster convey speeds can degrade the material or overfill the line which can lead to line plugging. The best set point allows an efficient operation that maximizes product flow and minimizes damage and surging. This optimal flow rate is also influenced by factors unrelated to the truck operation. The physical layout of the pneumatic convey lines from the truck to the silo also impacts operation. These factors include the connections between the truck and plant convey lines, the pipe size of the convey lines, the distance from the truck to the silo., valve arrangement in the convey lines, and of course the number of elbows in the convey line.
Also, the truck driver’s operating experience (or lack thereof) and his knowledge of both the truck and its cargo can have a huge impact on the efficiency of the unloading process. So, while several factors can impact the unloading rate of a PD truck, the lower operating pressure of American trucks will always extend the time needed to empty the load compared to the much higher operating pressures of European “pressure vessel” trailers.
Conclusion
The PD trucks may look similar in the US and Europe, but their design and operation are completely different. The difference in US to European operating pressures raises the upfront cost of European trailers but results in much faster unloading than the standard US design. The use of customer-owned, standalone blowers, used instead of truck-mounted blowers, offers better control, lower air temperatures, and greater air volume capacity. Irrespective of the trailer design, the time to unload a truckload is also governed by the physical layout and length of the pneumatic conveying lines from the truck to the storage silo.
As a provider of bulk ingredient handling systems, AZO understands the unique challenges of truck delivery of bulk powdered material. Backed by engineering-centric design and industry experts, you can be rest assured our truck unloading systems will get your materials unloaded and into storage quickly and efficiently. Contact us to learn more about our equipment and systems, and how we can support you.
