There are two main types of pipe fittings commonly used in heating, ventilation and air conditioning systems; flanged and screwed. Screwed pipe fittings use a male and female thread combination and when screwed together tighten to form the joint. A flanged pipe fitting has a lip or extension projecting at a right angle to its surface. This lip is bolted to the facing lip of the adjacent fitting for added strength. Because of the strength of this fitting, flanged fittings are recommended for 4-inch pipe or larger.
Both of these pipe fittings are used to meke temporary joints because you can ultimately take them apart when needed. If the joint is ever brazed, soldered or welded the join is considered permanent as the join will not come apart. The advantage of these two temporary joints is that it can be easily disassembled for repairs down the road.
The typical pipe fittings can be divided on the basis of the following six functions:
Reducing or enlarging
Extension or joining
Shutoff or closing
Union or makeup
Reducing or Enlarging Fittings: Both bushing and reducers are examples of reducing or enlarging fittings. Thier function in pipe installations is to connect pipes of different sizes. The difference between a bushing and a reducer is distinguished by their construction. A reducer is a coupling device with female threads at both ends. A bushing has both male and female threads on each end.
Branching Fittings: A branching fitting is used to join a branch pipe to the main line. These fittings are used for Tees, Crosses, Elbows with side outlets, Y branches and Return bends.
Extension of Joining Fittings: Nipples, locknuts, couplings, offsets, joinst and unions are all examples of extension or joining fittings. With the possible exception of an offset, these fittings are designed to joiun and extend a length of a pipe.
Shutoff Fittings: Sometimes it is necessary to close the end of a pipe in your system. This is accomplished with a shutoff or closing fitting and the following two types are used for Plugs and Caps.
Union or Makeup Fittings: These are represented by union elbows and union tees. This type of fitting combines both a union and an elbow or tee in a single uni. They come in both male and female threads to attach where needed.
Directional Fitting: Direction fittings such as offsets elbows and return bends are used to change the direction of a pipe. Offsets may be considered to be a type of extension or joining fitting because of the overlap in function.
Pipe fittings are used in all aspects of new HVAC equipment and also in repairs. Understanding the difference between these fittings will help you on your next HVAC job.