Our client piping class advises sdss monoflange valves for all DSS piping class. All other valves and fittings remain DSS. Is there any specific reason and what is the general practice ??
Not knowing your operating conditions I can only speak to the material differences. It could be that a more crevice-corrosion resistant material (SDSS) is wanted for the flange (crevice is present), while DSS is suitable for the piping (no/less crevices).