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A pipe union is a secure and semi-permanent connection between two pipes. Most pipe unions consist of two pipes connected together via a third piece. All three are threaded to make a firm connection. While unions are similar to couplings, they are generally easier to take apart and allow pipes of different metals to come together safely. This joining method is common in household and commercial pipe systems.
At first glance, a pipe coupling and a pipe union seem very similar. They both connect two pipes together using a male and female threading system. The main difference between the systems comes from the actual method of joining. Two coupled pipes screw together directly, one inside the other. To take the pipes apart, every pipe that connects to the coupled pipe needs to turn. In a full pipe system, this would mean the entire system needs to come apart to remove one pipe.
With a pipe union, the two pipe ends don’t screw into one another—they each screw into a third piece. When one pipe needs to come apart from the other, the union piece simply screws onto one of the two pipes completely. If both ends of a pipe are attached using a union, the pipe may be removed on its own without unscrewing the other pipes in the system.
The second use for a pipe union is a protective system. These unions, often made of plastic, keep pipes made of different metals from touching each other. When two metals touch the same conductive material, it creates an effect called galvanic corrosion. Any conductive liquid, such as water, will cause electrons to move from one material to the other. This will cause one of the pipes to slowly dissolve and the other to accrete material.
These pipe unions generally create a physical separation between the two pipes. When one pipe screws into the union, there is a small plastic barrier that completely covers the metal. The second pipe screws into the system up to the barrier, and they are prevented from making physical contact. This type of pipe union will not completely halt galvanic corrosion, but it will slow it down dramatically.
Pipe unions are common in all sorts of basic plumbing situations. Houses and businesses typically use these pipe systems because they are easy to maintain and repair. On the downside, since they have more threaded parts, they are more likely to leak than standard couplings. As a result, dangerous or high-pressure liquids are often moved through pipes joined via other methods.