The Tri Clamp fittings can be confusing for those not familiar with them, but if you remember these few simple guidelines working with and sizing them will be a much easier task:
Sanitary clamp end connections were commonly known as "Tri-Clover," a brand and registered trademark of Alfa-Laval, the major manufacturer of these types of connections for over 50 years. Later, Ladish Co. took over the brand "Tri-Clover" and rebranded them as "Tri Clamp." Tri Clamp specifically, was originally used to describe the Tri-Clover 3-segment clamps, but now the name "Tri Clamp" is universally used as a way of referring to these types of sanitary clamps across the industry.
The most important factor to remember is that Tri Clamp Size is determined by the outside diameter (OD) of the tubing, NOT the outside diameter of the flange. The flange size, however, follows the Tri Clamp standard for the flange diameter. On average the flange diameter is approximately ½" larger than the tubing diameter. Because of this, a common mistake is to use the flange diameter to determine the Tri Clamp size, and this will result in over sizing a fitting. For example: The flange diameter of a ½" Tri Clamp ferrule is 0.984"; being so close to 1" in diameter, it can be easily mistaken for a 1" Tri Clamp ferrule.
Another major confusion point; because of the smaller sizes being so close together, they combined ½" - ¾" to use the same clamp and 1" - 1.5" to use the same clamp. So you can join a ½" ferrule to a ¾" ferrule using the same clamp, and same with the 1" and 1.5".
Because you can directly join these smaller sizes, it is important to remember the effect this has on the material flowing through the tubing. On larger tubing you would need a reducer to change sizes. This is accomplished with either an end cap reducer that has an abrupt size change, or a concentric/eccentric reducer that gradually changes the size. When joining ½" directly to ¾" and 1" directly to 1.5", remember the effects that an abrupt change can have on flow that is similar to an end cap reducer, and when a smoother cleaner transition is needed use a concentric/eccentric reducer.
Another factor found on larger tubing is you have appropriately sized gaskets for all sizes, and when clamped they have a minimal intrusion into the flow of material. But when joining a larger ferrule directly to a smaller ferrule of a compatible clamp size, you must choose the gasket to match the larger ferrule size. For example: When joining a 1.5" ferrule to a 1" ferrule, if a 1" gasket is used, you run risk of unsanitary conditions around the unclamped section of a 1" gasket loosely held against the face of the 1" flange.
Tri Clamp clamps come in two main types, 2-segment (single hinge) and 3-segment (double hinge), with a high pressure bolted variation of the 2-segment. The ½" - ¾" sized clamp ONLY come in 2-segment type. The 1" - 1.5" and higher size clamps come in 2-segment and 3-segment which are more expensive and are generally considered as heavy duty (though they may or may not be rated for higher pressure). The main benefit of the 3-segment type is they are a bit easier to install in tight spaces but it is up to the end user to choose the type they prefer. Most people choose 2-segment clamps as suitable and cost-effective for most applications and as such, we have a limited selection of the 3-segment.
The high pressure bolted version is not hinged but instead come as 2 segments that are bolted together. These are more suited to a permanent or semi-permanent application where frequent service is not needed or high pressure safety supersedes ease of service. These clamps are only available down to the 1" - 1.5" size.