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What is a Companion Flange?

Lori Kilchermann
Lori Kilchermann

A companion flange is a flange that corresponds to another flange. Having bolt holes that align with another flange is a trait of this type of flange. The companion flange is an important component of a pipeline or a plumbing system. In most situations, this flange is attached to the flange of another pipe and then the pipeline is assembled to fit tightly between the two flanges. A threaded companion flange is also used on a plastic pipe when adding a threaded iron pipe into the pipeline.

On a typical pipeline, changing pipe size and dimension is typically accomplished by placing a reducer into the line. When changing pipe size on a section of pipeline that might be removed, a companion flange is used to allow a simple dis-assembly of the pipe in the future. By placing a flange in the pipeline and using a gasket to seal the two sections, a joint is created that offers easy access in case of a plumbing emergency. The companion flange is also a convenient way to allow for expansion to the pipeline in case of future construction additions.


There are many types of companion flanges made out of many different materials. This makes it possible to blend two or more types of plumbing into a single system without having to replace the entire system. Copper can be added to a plastic system as well as iron pipe to a brass fitting, and so forth. The blending of the different materials requires only a matching flange and a gasket. Often, an anti-seize compound is used on the fasteners of two different material flanges. This is important to avoid oxidation and rust.

It is common practice for a plumber to install an access point into a plumbing line to make removal of a clog an easy matter. The installation of a flange to seal the inspection opening is also used to tie in future plumbing lines when the addition of more lines becomes necessary. It is for this reason that the companion flange on the access point is often a closed flange used to seal an open pipe. The closed flange can be changed out in the future should the requirement for it to be changed present itself. By placing flanges within a pipeline, the pipeline becomes a less confusing component to a building or system and makes future additions or subtractions an easy to perform task.

Discussion Comments


@JimmyT - I'm glad you answered that. I've been looking all around for the things and haven't found any. Now that you mentioned the gas meter, I know exactly what you're talking about.

Now that I know what one looks like, I'm pretty sure there is some sort of floor flange that connects the toilet to the sewer pipe. It's been a while since I've seen the bottom of a toilet, though, so maybe I'm imagining things.

Thanks so much for clearing that up.


@Izzy78 - I understand your confusion. I wasn't at first sure what a companion flange was. I think I have a couple good examples, though. As for the things under the sink, I have those too, but I'm not sure if those are considered flanges or not.

If you can, imagine a natural gas plant or chemical factory that has a ton of pipes running all around the building. The metal flanges are the rings that stick out slightly around the pipe and are bolted together. I found some decent pictures by searching for natural gas pipelines and underwater pipelines.

If your house uses natural gas, you can also look at the meter located outside. If yours is anything like mine, you will see what I am talking about. The companion flanges are what connect the pipe to the metering unit.

I hope this helps!


I'm still a little confused. I searched for pictures of companion flanges, but am still not sure I've seen one before. Are there any common places in your house that would have one?

I checked under my sinks. Under my kitchen sink, there are round, octagon shaped things that connect the sections of pipe. Are these PVC flanges or something else? The PVC pipe also connects to metal pipe, but again, there are no screws that hold the connecting piece together like the article describes.

If there are not any good examples in the normal home, where can I find a good picture of a companion flange, just so I know what it is?


I used to work in a laboratory that was built in the 1970s. For some reason, they chose to have all of the pipes in the building exposed and painted them different colors. They are kind of tacky 40 years later, but they offer a good view of the different types of flanges described in this article.

I've noticed a lot of the closed flanges that are described in the article and wondered what the purpose was. I guess I never thought that it would provide an easy way to alter the plumbing line.

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