Fittings are a vital part of any laboratory system. Although these components are not particuarly complicated, a solid understanding of their application is essential to avoid system problems. Fittings are used anywhere tubing or piping need to be attached, adapted, or adjusted. Their primary functions include preventing a liquid or gas from leaking out of a flow path and holding tubing in place against opposing pressure.
A fitting is composed of a nut and a ferrule. The nut provides the driving force that causes the ferrule to seal. For more information on nuts, please refer to the Nuts Selection Guide. The ferrule supplies the holding power.
Most laboratory fitting systems work through external compression onto the tubing's outer wall, accomplished by the ferrule's tapered nose. Ferrules come in many shapes and sizes.
Flanged fitting—A fitting used in low-to-moderate pressure applications. The fitting requires flanging—or spreading the end of the tubing—before use.
Flangeless—A specialized fitting designed to replace flanged fittings, where through the use of a nut and a unique ferrule, a seal can be made on tubing in areas where flanged fittings were traditionally necessary.
Flat-bottomed—In flat-bottom configurations, as the port threads run out, the bottom of the port becomes flat across the diameter with a small through-hole in the center of the flat bottom. In this port, the ferrules have tapered noses facing toward the nut and away from the flat bottom of the receiving port. This is commonly used with flanged tubing to allow the tubing itself to create the seal in the port.
Coned—In high pressure systems a coned port is best used. In this design, the port has an internal taper just past the threaded portion of the port and the tapered nose faces away from the nut and toward the port. There is often a tubing pocket just beyond the internal taper. The "tubing pocket" is nearly the same diameter as the tubing that extends into it.
- Finger-tightened—Fingered-tightened fittings were first developed in 1983. These fittings remove the need for a wrench or other tools when tightening a fitting into place. The fitting is made from durable and chemically-resistant plastic and is able to hold high pressures. Finger-tightened fittings are more popular in laboratories because they are biocompatible, universal, and interchangeable. They can be repositioned as needed.
Wrench-tightened—Wrench-tightened fittings must be tightened with a wrench. They can be used for applications such as swaging (fixing a fitting to a tube permanently).
Threads of the Receiving Port
Threads can be external or internal on a nut. Most nuts have external threading. The thread is described by two numbers. The first describes the diameter of the thread and the second number describes how close together the threads are. The denser the threads (the higher the second number), the more resistance to opposing pressure being created inside the port. This reduces the chance that the nut will fail.
Ferrules are often manufactured from materials such as stainless steel, PEEK, Tefzel (ETFE), polypropylene and Kel-F® (PCTFE). PEEK and stainless steel are best for high-pressure applications, while ETFE and polypropylene are used most commonly in low-pressure applications. Polymer fittings can be used with many types of tubing, in more types of environments (i.e. different chemicals and pressures), and can be changed within the system more frequently than stainless steel fittings. However, stainless steel fittings are a good option if the tubing will be connected to a receiving port and rarely taken out. Always consider chemical environment when selecting a fitting.
Amount of Pressure Expected
There is no standard number for what constitutes high pressure or low pressure, however the pressure expected in the system is very important when selecting a fitting. If the system is expected to have 'low' pressure then a flat-bottom configuration should be used. High-pressure systems are most succesful with coned-shaped ports. For super high-pressure applications, most prefer to use metal nuts and ferrules. But for low, moderate, and standard high-pressure applications, there is usually a polymer option that matches the application.
DIN 12918-2—This specification covers laboratory equipment for valves used for fuel gases.
AND 10059—This standard is used for fittings that are pipe threaded and flared.
ASTM 01.01—Volume 01.01 features standards for various types of steel pipe that specify requirements for high-temperature service, ordinary use, and special applications, such as fire protection.