• Thomas Veraar

    May 16, 2023 at 12:38

    That depends on the dimensions, liquid/gas, pressure and the type of the cylinder. Do you have more specifics?

  • Roger Penn

    May 16, 2023 at 16:16

    Hi Christiaan, different stacks have different compression levels depending on the type of compression system, the materials used, number of cells in the stack, type of application, and stack requirements (e.g. performance, shock and vibration, etc.).

    A typical range of stack compression forces is 18 kN – 42 kN (but some special designs may be outside this range – a stationary power application may be <18 kN and a rugged application may be >42 kN). So the tie rods, or straps, used to hold the stack together must withstand this stack compression force (depending on design, may have 4 to 8 tie rods or straps per stack).

    Kind regards,


  • Christiaan van der Blij

    August 18, 2023 at 09:29

    Hi Thomas and Roger,

    my apologies, I did not notice your replies and just came back here after a long time. Thanks a lot for your input. Customers have indicated interest in insulating, non-corrosive composite threaded rods for tensioning, but I couldn’t answer the question of tensile stress.

    Best regards,


  • Ace Fujiwara

    August 28, 2023 at 09:05

    This is a matter of design compliance which I notice is lacking in a lot of the manufacturers at the moment. The tensile strength of the rods cannot be taken as a fixed value because of the operating variation of the stack over its lifespan will change.

    End-users or vendors when assembling these stacks do not apply ASME flange tightening procedures carefully, as a result the stack is not mechanically loaded correctly.

    A lot of the end bolt-nuts don’t have any markings to indicate otherwise.

    As a result, some bolt-nuts section are tightened and in compression at the wrong value.

    • Roger Penn

      September 22, 2023 at 20:43

      Many stacks have a compression system with springs inboard of the location the straps or rods connect to. So any stack length variations due to temperature, pressure, and especially hydration are taken up by the spring packs/towers. The straps/rods must take the overall force, but variations due to compression set or thermal expansion of the stack should be accommodated by the springs in the compression system.

      Tie rods and bolts are not state-of-the-art and even lab stacks don’t use them anymore. In many cases the stack has metal straps that are welded instead of rods with nuts (the flat straps save packaging space), or another arrangement that doesn’t require a rod or a nut. Some electrolysis stacks still use tie rods, but most fuel cell stacks in production have gone away from tie rods and nuts.

  • Ace Fujiwara

    August 28, 2023 at 09:08

    Sorry, I forgot to add, the corrosion of the thread-nut is because at the bolt-nut cap the washer material is important, and the end nut assembly should have a galvanic protection anode in place. Very simple technical fix which is often over-looked.

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