Addition of URFs for continuity and momentum to unity?
 

I have seen it in forums or mentioned during my discussions with CFD engineers many times that sum of URFs for continuity and momentum for SIMPLE algorithm should be 1, however I can't find it written anywhere in ANSYS documentation. How true is it? Can I change URFs independently depending on how the residuals are behaving or should they always sum to 1?

It's not continuity and momentum, it's actually pressure and velocity/momentum.

In an ideal setting where you don't have divergence issues due to another reason it is a very good rule of thumb because that's the optimal behavior of the SIMPLE algorithm. The idea behind SIMPLE is to assume pressure, calculate velocity, correct pressure, and correct velocity. The more you change velocity, the less you change pressure and vice versa. And that's why you want the urf's to sum to 1. Off the top of my head, I don't recall the particular paper where this was shown.

You don't want the urf's to sum to more than 1, that makes the algorithm unstable.

You certainly can have the urf's sum to less than 1, this is true under-relaxation. Having the urf's sum to 1 is more like no under-relaxation. But if you can't run with the urf's summing to 1, it is a hint that there is a problem with your simulation. Usually it is some complexity in the problem due to non-linear coupling between other equations.