Provided that O-rings and gaskets are all rubbers, and they are all sheet gaskets, I would like to suggest FKM, FFKM, EPDM, Teflon.

Based on your budget, contact your usual gasket supplier and ask their price.

The materials that I mentioned will withstand caustic soda service very well.

For NaOH - Sodium Hydroxide, the best choice is EPDM (Ethylene Propylene Diene Monomer - assuming temperature is OK.

That's right, and EPDM is cheap.

But if I remember right, at one time GRP wasn't considered suitable for NaOH, as it slowly attacks the glass fibres. Perhaps it's special glass? By the same token, glass-reinforcing of concrete was a no-no some years back, as cement is quite alkaline, but I believe suitable glass is available now. Anybody comment?

Cheers.........Codey

Viton or EPDM will be a better choice

Try out this link from Parker Seals division:

http://www.parker.com/portal/site/PARKER/menuitem.62418d8588e44a30c2ff4c10237ad1ca/?vgnextoid=95d60967b4fd4110VgnVCM10000048021dacRCRD&vgnextfmt=default&application_url=http://Divapps.parker.com/divapps/org/fcg/fcg.asp&vcmId=b126981ef31a0110VgnVCM10000048021dacRCRD

Vitone or silicon is suitable to any corrosive & High temp

u should go for metallic braded steam packing joint or teflon ( ptfe).

steam joint would be enough for u