When designing a system or replacing a valve component, the question that commonly arises is “What’s the difference between a tube and a pipe?” Not a whole lot, but if you request a quote for a valve to connect to a 4″ tube when you really have a 4″ pipe, your valve is not going to fit.
Tube is used for powders, pellets, and granules, or in a dilute phase conveying system. The tube can be made from stainless steel, carbon steel, or aluminum. When you measure the outside diameter, otherwise identified by “O.D.”, you get the true measurement of the tube. A 4″ O.D. tube is exactly that, a tube that is 4 inches across the outside diameter.
Pipe, on the other hand, is used for abrasive materials or for dense phase conveying systems. Pipe can also be made from stainless steel, carbon steel, or aluminum. What makes it different from tube is how it is historically sized and the thickness of the pipe wall.
In the early 1900s, only three pipe thicknesses existed for the pipes that were casted in 2 pieces from wrought iron: STD (standard weight), which later became known as Schedule 40 pipe; XS (extra-strong), which later became known as Schedule 80 pipe; and XXS (double extra-strong), which later became known as Schedule 160 pipe. After WWII, pipes were being manufactured from stainless steel giving them a lighter weight and thinner wall thickness. Thus Schedule 5 pipe and Schedule 10 pipe were born.
The most common thicknesses in the dry bulk handling industry are Schedule 10, Schedule 40, and Schedule 80.
Pipe sizing is confusing because when you measure the outside diameter of a 4″pipe, it’s actually 4.5″. Every pipe is somewhat bigger than the labeled size until you reach 14″ and then the schedule pipe size is actually the outside diameter size. Originally, schedule pipe was labeled by the inside diameter size. But as the constructed material and wall thicknesses evolved, the inside diameter changed. The outside diameter stayed the same so it could mate with existing older pipe. Now, in order to have some form of consistency, pipe is ordered by the outside diameter measurement (example: 4″), which is a fixed value, then by schedule (example: Schedule 40), which determines the thickness of the wall.
If in doubt when you are ordering, measure the outside diameter of the tube/pipe and the inside diameter of the tube/pipe. Relay these measurements to your valve manufacturer who can help you determine if you have tube or pipe in your system. They can recommend the best size of valve for your system.