What’s the Difference Between Exhaust Flex Connectors and Bellows?

Did you know that 20ft of carbon steel exhaust pipe will “grow” approximately 3″ when its temperature rises from 90°F to a normal engine exhaust operating temperature of 900°F? Does your engine exhaust design have something in place to absorb this pipe growth? Are you specifying a flexible connector? Or have you selected a bellows? If you are designing the exhaust system for an emergency generator, please continue reading. This one will keep you from making one of the most common mistakes that I see in the field.

Every emergency generator application includes an exhaust system designed to route exhaust gases out of the engine room. These exhaust systems consist of piping, elbows and, very importantly, flexible sections that must be used to account for the engine’s vibration, and also for the pipe growth experienced as the engine runs and the exhaust gases quickly rise to 800°F and more. If the exhaust piping where to be installed rigidly, you can be sure that cracks will develop at the weakest point, usually at the engine’s outlet flanges, or at silencer inlet/outlet connections. Having a flexible connector between the engine and the silencer seems to be well understood. I have never seen an installation where the silencer was rigidly mounted to the engine, but I have seen many instances where no allowance was provided for pipe growth downstream of the engine.

In most cases, the exhaust piping seems to rely on the flexible connector to absorb all movements in the piping system. The flexible connector is not designed for this, and the consequences are almost always evident in premature gas leaks due to cracked flanges in the flexible connector. Not accounting for pipe growth with the correct type of device is the most common mistake that I see pertaining to engine exhaust piping.

Flexible Connector vs. Bellows

This leads me to the main point of this article: when should you use a flexible connector, and when should you use an expansion joint, aka “bellows”? These are different devices meant to perform specific functions.

Flex Connector
Flex Connector (shown as “Y” assembly)

Flexible connectors are intended to absorb or isolate high frequency, low amplitude engine vibration (fast but small movements) from the rest of the exhaust piping. They are NOT designed to compensate for or absorb pipe expansion, nor are they designed to “make up” for major mis-alignments in the exhaust piping. If the flexible connector between the engine and the silencer seems compressed or “s-shaped” in its idle state, it is not installed correctly. Flexible connectors are made up of flexible hose, which is typically constructed of a single ply of relatively small corrugations that allow the material to move with vibration. The corrugations aren’t designed to compress; over time the flex will fail if it is operated under compression. Likewise, the flexible connector’s inlet and outlet must be aligned. If installed in misaligned piping, it will fail after a short period of time. In general, the optimum length for a flex connector is 18”. Much shorter than this will result in less flex hose, and therefore less ability to absorb vibration. As a point of reference, a typical flex connector will allow approximately 0.25″ deflection on a 12″ length of flex hose.


bellows with flanged connections
Bellows with Flanged Connections

Bellows, or expansion joints, are much more flexible than flexible connectors. Bellows are designed to absorb pipe expansion. The corrugations are larger and therefore able to compress more. They are generally better able to handle lateral movement as well, so they can better tolerate pipe misalignment. Because they are more flexible, they can provide expansion absorption and vibration isolation in a shorter length than a flexible connector. A typical bellows section is 12″ long. This can be a major benefit when the silencer must be set low, as in installations where the generator room does not have much headroom above the engine.

Bellows come in single-ply and dual-ply versions, referring to the number of layers of steel forming the bellows. Dual ply bellows (constructed of two thinner layers of steel) are more flexible than single-ply, can absorb more expansion (typically 3″ vs. 1.5″ deflection) and vibration, and will have better durability than a single-ply bellow. Although dual-ply bellows carry a higher cost, they are my preferred selection for generator exhaust piping.

bellows with liner
Bellows (shown with internal liner)

Also worth noting… the corrugations in the flexible section of a flexible connector or a bellows create turbulence, which increases the pressure drop through the device. Bellows can be built with flow liners which smooth-out the flow and make the pressure drop closer to smooth pipe. The liner is welded on one end, with the other end floating, so that the bellows can still compress. Flow liners restrict lateral movement, so a bellows with a liner will not be able to tolerate misalignment or lateral deflection to the same degree as an unlined bellows.

Standard flexible connectors and bellows are manufactured from 321 grade stainless steel in the flexible section, with the connectors (flange, pipe, etc.) made of carbon steel. Since these items are exposed to high temperatures, and often cycle between ambient and operating temperatures in a short amount of time, I recommend 100% stainless steel construction to extend the life of either device. Here are some examples of flexible connectors and bellows.

So, my recommendations…

A simple plan for your exhaust piping design should include a flexible connector at the engine exhaust connection(s), and a section of bellows at any change of direction in the piping downstream. The flexible connector will absorb and isolate the engine vibration from the rest of the exhaust piping, and the bellows will account for any pipe growth that would otherwise push back on the engine connections. If there is a single straight section of pipe, and it is of substantial length (30ft+), I recommend the use of a bellows, especially if the pipe is rigidly braced somewhere along the pipe run.

I hope this article was helpful in pointing out when and how to specify these items. If you have a comment or question, please post below, and thanks for reading!

4 thoughts on “What’s the Difference Between Exhaust Flex Connectors and Bellows?”

  1. Hello David,

    Thank for your interesting article. I have two questions:
    1. My understanding from this article is, because we have both vibration and thermal expansion, we need to use both bellow and flexible?
    2. If we have combination of vertical and horizontal exhaust duct system, do we need to have bellows on both vertical and horizontal sections?

    • Bellows will be ideally-suited for expansion/contraction, but they will also address minor vibration issues. On your second question, yes, you should consider bellows for changes of direction that are subjected to stress due to pipe growth (and contraction).

  2. Thank you for the information. I have one question though:

    • You recommend to use a flexible connector at the engine exhaust, with bellows at any change of direction downstream. Why wouldn’t you use a bellow too at the engine exhaust instead of the flexible connector? After all, it has better properties to handle the expansion due to thermal variation and high amplitude vibrations, or am I wrong about the high amplitude vibrations?

    Thanks again.


    • Thanks for the question Ali. Flexible connectors usually form part of a relatively-short piping run between the engine and the silencer (with minimal concern for pipe growth). Although more costly, you may use a bellows at the engine connection, and sometimes a low engine room ceiling forces their use in place of flexible connectors. One word of caution – because bellows allow more movement, be mindful of potential problems caused by the bellows working against any vibration isolators installed at the base of the generator.


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