Twin wall pipes are one option that can be used on a concrete boom pump truck. When correctly installed, inspected and maintained, twin wall pipes often have a longer operating life than single wall pipes.
Twin wall pipe is a pipe within another pipe. These two pipes have distinctive characteristics in how they function, which is essential to the pipes’ performance. The outer layer of twin wall pipe is a high strength steel engineered to hold the pressure for which it is designed. While the steel for this outer layer is strong, it is not designed to resist the abrasive wear that the inner wall is exposed to during concrete pumping.
The inner pipe is a high carbon steel. The higher carbon content allows the steel to be processed (heat treated) to get an extremely hard surface. When heat treating this type of steel, the entire thickness of the material is through-hardened. This means the steel’s entire thickness has the same hardness from the first millimeter of the thickness to the last.
While improving abrasion resistance, the through-hardening process makes the material more brittle and sensitive to heat. The brittleness of the inner liner is the reason twin wall manufacturers state not to hit it with a hammer. The hammer blows could damage the inner liner by cracking it or even breaking out sections of the inner pipe.
Heat treating the inner liner lines up the molecules in the steel to form a better “framework” inside the steel. The pipe is immediately water quenched to lock in this “framework.” This process gives the material the hardness and consistent wear we see in twin wall pipe. However, welding on twin wall pipe introduces heat to a material that has already been heat treated. Introducing heat from welding can cause the inner liner to have its “framework” disturbed and will create soft spots where the heat of the weld contacts the inner liner. This welding needs to be done in a controlled manner to not affect the heat treatment of the inner liner.
It is important to note that some manufacturers of twin wall pipe will have proprietary materials, equipment and techniques to greatly reduce loss of hardness from welding. Many manufacturers recommend that welding be completed in a controlled environment to ensure the pipe does not suffer heat treat loss.
A precision fit of the inner liner and outer pipe is vital. If the fit is not precise, a problematic air gap may exist between these two layers. The inner liner is not designed to hold the pressure of a standard concrete pump (85 bar/ 1233 psi). If the pipe is exposed to high pressure in a plugging situation, the inner liner could crack or shatter. These cracks can cause sections of the inner liner to peel or fall out during pumping, exposing the outer lining of this pipe to abrasive wear for which it is not designed.
The inner liners of twin wall pipes can be built in various wall thicknesses. This difference in thickness will affect the weight and performance of the pipe. The thicker the liner, the longer it will last. It is important to inquire into the thickness of the inner liner to better understand how it will hold up in your pumping applications. Different product offerings could be available that better suit the needs of your specific boom pump truck.
Gross vehicle weight and the boom’s weight-per-foot tolerance can be critical considerations based on your pump and boom specifications. Some manufacturers offer lighter weight twin wall pipes. This pipe is specifically designed with a thinner inner layer to lower the total weight per foot of the pipe and still offer the longer life of twin wall versus single wall pipe.
Inspecting twin wall pipe requires special calipers that can extend around the pipe weld end to directly measure the pipe wall thickness. To use these calipers, the pipe or elbow next to the one being tested needs to be removed. The calipers are inserted into the pipe past the weld end to measure the pipe wall thickness; the pipe is measured in several spots to get a more accurate reading of its wear. The lowest measurement on any given pipe should always be used.
The minimum pipe thickness of twin wall pipes varies depending on materials, construction and manufacturer. Considering the variety of combinations in the market, it is vital to ensure your pipe is CPMA-certified. A quality pipe supplier can provide data on how wall thickness wear impacts resistance to pumping pressures. Always consult the manufacturer for best practices in handling and inspecting their products.
Visual inspections of the system should be performed frequently to look for dents and obvious issues. Any flaws or damage to the exterior pipe wall warrants a more thorough inspection and may require seeking a qualified person for their recommendation. Use a flashlight and look down into the pipe’s inner wall. If cracks are seen in the liner, consult your pipe supplier or manufacturer for guidance on how wear may affect safe pumping pressures. If part of the inner liner is observed to be missing, that pipe should be removed from service and replaced.
Periodic inspections should be performed on a schedule set by the operators, based on manufacturer’s guidance. Periodic inspection should also include a manual check of the pipe mouth just past the weld end. If a depression or worn spot is felt, measure the wall in this area to make sure it is within the safe operating specifications provided by the pipe manufacturer. If the mouth of the pipe is worn in one area more than another, the pipe wall thickness should be measured to confirm that it meets a minimum thickness for safe use. The pipe may also need to be rotated to yield better wear. Each manufacturer has a specific wear inspection for their product; owners and operators need to understand when to replace their system.
Inspection intervals of twin wall pipe are generally set by the owner/management of the concrete boom truck. The ASME B30.27 and the CSA Z151 safety standards define delivery system components (boom and deck pipe) as a frequent inspection item. “Frequent” is defined as daily to monthly. These standards should be considered when evaluating the current inspection intervals set by your company.
