Shan,

Which code is your manifold designed to ?

If it's ASME VIII or similar it's a vessel. If it's B31.3 or similar it's a pipe system. If it's a pipe, 70MPa is a perfectly reasonable figure for pipe, but you should check out any related flange stresses.

If the limit of 1.8 kNm refers to a vessel, speak to the vessel designer. It must be very heavy wall and in that case, 1.8 or even 8 kNm is nothing. Get the vessesl guys to check out local stresses with 8 kNm.