Hello ..
I would like to know which is better gaskets EPDM or synthatic fiber rubber? as I have ductie iron flanges pipes and flanges fittings, I want fix gaskets between flanges for sealing.
The medium is potable water, max. pressure 25 bars, pipe dia 500mm, temprature 40-50 C degree.

At 25bar you are liable to have leaks with EPDM or synthetic rubber. Gasket material should be a mineral fibre. i would not specify rubber gaskets for pressure > 700kpag.

Contact the gasket people such as Klinger, Novus etc.

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Agree with Stanier. Unreinforced elastomer gaskets should be used at pressures <100 psig. Fiber reinforced non-asbestos gaskets are a good choice as long as you have adequate bolting. B7 for instance. With flat face cast iron flanges you'll need full face gaskets and that's a lot of area to compress, hence the high strength bolting. Expanded PTFE is much more expensive, yet it seats at lower stresses and thus lower bolt loads. Look also at reduced area gasket designs where you get full face coverage with 1/2 the required bolt load of conventional full face gaskets.