In process piping support allowable stress is higher than in power piping or MSS-SP-58(2009).

As per B31.3 clause 321.1.3, pipe support allowable stress in tension is as per B31.3 para 302.3.1 (Su/3 at room temp and Sy/1.5, whichever minimum)
In B31.1 & MSS-SP-58 2009), at room temp allowable stress in tension is Su/3.5 at room temp and Sy/1.5, whichever minimum .

Is my above understanding right that in B31.3 we can go for higher allowable than in power piping ?

reg,
sam

Yes, power piping (B31.1) and process piping (B31.3) have similar criteria but power piping has more strict limits on calculated stress.