What is the difference between a union and a flange?

Definition and Basic Design

• Union
• A union is a type of pipe fitting that allows for the connection and disconnection of pipes without the need to cut or otherwise damage the pipes. It typically consists of three main parts: a nut, a female end, and a male end. The female end has internal threads, while the male end has external threads. When the nut is tightened, it compresses a gasket or seal between the two ends, creating a tight seal. The union is designed to be easily disassembled and reassembled, which makes it useful for situations where maintenance or replacement of a section of the pipe is required.
• Flange
• A flange is a plate or ring that is attached to the end of a pipe or a pipe fitting. It usually has a flat surface with bolt holes around its circumference. Flanges are used to connect pipes, valves, pumps, and other equipment in a piping system. They are typically connected by bolting the flanges together with gaskets in between to ensure a leak - tight seal. Flanges come in various types, such as blind flanges (which are used to close off a pipe end), slip - on flanges (which are slipped over the pipe and then welded), and threaded flanges (which have threads for connection to pipes with threads).

Connection Method

• Union
• The connection method for a union is by threading. The male and female threaded ends are screwed together and the nut is tightened to secure the connection. This threaded connection is relatively easy to install and disassemble, but it may not be as strong or as leak - free as a flanged connection in high - pressure or high - temperature applications. The seal in a union relies on the compression of a gasket or the threads themselves to prevent leaks.
• Flange
• Flanges are connected by bolting. The flanges are placed together with a gasket in between and then secured with bolts. This method provides a very strong and reliable connection, especially for high - pressure and high - temperature applications. The gasket is compressed between the flanges by the bolts, creating a tight seal. Flanged connections are more complex to install than unions because they require precise alignment and tightening of the bolts.

Applications

• Union
• Unions are commonly used in situations where frequent disassembly and reassembly of the piping system are required. For example, in residential plumbing, unions might be used to connect water supply pipes to appliances such as washing machines or dishwashers. This allows for easy removal and replacement of the appliance without having to cut or replace the pipes. They are also used in some industrial applications where maintenance and inspection of the piping system are necessary.
• Flange
• Flanges are widely used in industrial and commercial piping systems, particularly in applications involving high - pressure fluids, gases, or steam. They are used to connect large - diameter pipes, valves, pumps, and other equipment. For example, in oil refineries, chemical plants, and power plants, flanged connections are used extensively to ensure the integrity of the piping system under high - pressure and high - temperature conditions. Flanges are also used in fire protection systems and in the construction of large - scale water distribution networks.

Strength and Pressure Rating

• Union
• Unions generally have a lower pressure rating compared to flanges. This is because the threaded connection is not as strong as a flanged connection. The pressure rating of a union depends on its material (such as brass, steel, or plastic) and its design. However, they are still suitable for many low - to - medium - pressure applications, such as residential water supply systems and some light - duty industrial applications.
• Flange
• Flanges are designed to handle high - pressure and high - temperature conditions. They have a much higher pressure rating than unions. The strength of a flanged connection comes from the bolts and the gasket, which can withstand significant pressure and temperature variations. Flanges are available in different pressure classes, such as Class 150, Class 300, etc., which indicate their ability to handle different levels of pressure.

Cost

• Union
• Unions are generally less expensive than flanges. They are simpler in design and require fewer components. This makes them a more cost - effective option for applications where frequent disassembly is needed but high - pressure or high - temperature conditions are not a concern.
• Flange
• Flanges are more expensive than unions. They require more material and have a more complex design. The cost of a flange also depends on its size, material (such as carbon steel, stainless steel, or alloy), and pressure rating. The additional cost is justified by their ability to provide a strong and reliable connection in demanding applications.