By: Frank Caprio | On: April 26, 2021
The U.S. has about 3,600 asphalt production sites and produced about 420 million tons of asphalt mixture in 2019. Over a half-million workers are exposed to fumes from asphalt, and this number is likely to increase with plans to make improvements to our roads and bridges. Exposure to fumes (and the solvents they contain) are hazardous, but a greater danger exists if these workers come into contact with hot tar or asphalt products.
While tar and asphalt are different products with distinct characteristics, they both require a great deal of caution during transfer or loading applications. These products must be kept heated in order to flow freely, and the heat presents several safety concerns when selecting the best hose for the application. Let’s look at some safety issues for tar and asphalt transfer, and discuss how Hose Master has designed innovative hose solutions for these challenging service conditions.
Tar and Asphalt Are Not the Same
Tar and asphalt are often referred to collectively because of their common uses, but the two products have very different origins. A simplified explanation is that tar is derived from coal (often a residual from coke production in the steel industry), while asphalt is made using heavy residuals from the oil refining process. A quick test to determine whether you have asphalt or tar is to take a small sample and dissolve it in paint thinner; asphalt will turn the solution cloudy black while tar turns the liquid a clear, greenish color. This is important in the roofing industry as tar is generally not recommended for use with asphalt-based roofing materials.
Selecting the Best Hose for Tar and Asphalt Transfer
There are many types of hoses that are specifically designed for tar and asphalt transfer. Rubber hoses are common, and are made using elastomers that are resistant to the media. They are generally rated to a maximum working pressure of 150 psi and a maximum temperature of 300 or 350°F., which is sufficient for most applications.
While rubber hoses are a cost-effective option, several limitations must be considered.
- Temperature/pressure combinations: Be sure to have the vendor verify the maximum allowable working pressure of the hose and fittings at the maximum operating temperature, as derating factors may apply in high-temperature operations.
- Vacuum service: Not all tar and asphalt hoses are rated for vacuum/suction service.
- Clean out hose after each use: Residual media can cause performance and safety issues.
- Coupling retention: Avoid potentially dangerous coupling separation by using interlocking type clamps, which grab onto the fitting and can be re-tightened periodically to ensure positive fitting retention.
- Weathering or cracking: Rubber hoses should be inspected for any signs of cracking, damage, or degradation before each use, and should be retested on a regular basis.
Metal hoses are also used for tar and asphalt transfer, and come in two main styles: Interlocked and corrugated metal hose.
Interlocked (aka “stripwound”) metal hoses are widely used for tar and asphalt transfer, but must be handled properly to avoid damage to the hose during use. While stripwound hoses are weather-resistant and durable, they are vulnerable to leakage if twisted during handling. Stripwound tar and asphalt hoses also should not be used for less viscous media, as leakage may occur. As with rubber hoses, stripwound hoses should be drained thoroughly after each use.
Corrugated metal hose is another popular option for tar and asphalt transfer, as it is gas-tight and resistant to weathering and high temperatures, and offers positive fitting retention. However, the corrugations allow media to collect inside the hose, which can harden once the hose cools down. This solidified media will damage the corrugations if the hose is flexed before the media is re-melted. Some operators will actually attempt to re-melt the trapped media by applying a blowtorch to the exterior of the assembly prior to using the hose! This practice is not recommended, as the excessive heat from a direct flame can anneal the stainless steel over time, decreasing the hose’s resistance to metal fatigue. The corrugations then lose their elasticity, and subsequent fractures and leakage can occur. A better practice is to keep the hose in a heated cabinet between uses to prevent media from hardening inside the hose, but this is not always practical.
Another option we have developed provides a unique solution by integrating a product already in use at asphalt plants: hot oil systems. Most asphalt plants use heated oil to reduce the viscosity of the asphalt in their tanks and pipes, and this thermal fluid can also be used to reheat a corrugated metal transfer hose. Hose Master has extensive experience in fabricating metal hose assemblies which are specially designed for the safe and efficient transfer of tar and asphalt. The hot oil circulation radiates sufficient heat to re-liquefy the trapped media so the hose can be safely placed into service without any flow restrictions. Our unique design uses an over-sized transfer hose, which provides unrestricted flow of asphalt through the traced assembly, thus decreasing unloading times.
This novel design is just one of many ways we provide customized solutions to everyday problems, improving our customers’ efficiency and safety.
Whatever hose style is best suited for your application, it should be noted that the end connections should be threaded or flanged, but quick disconnects should be avoided on tar & asphalt hoses. While cam & groove fittings facilitate coupling and uncoupling, the potential for an accident and the safety hazards posed are just not worth the risk. If you have any questions about which hose is best-suited for your application, contact us today and we’ll be happy to discuss the features and benefits of each option in more detail.