Reverse rotation risk
Slow roll turning gear speed
References
Gas composition (condensation / hydrate / risk)
Ask supplier

After vendor proposal how can i decide ?

page 6

https://www.dresser-rand.com/literature/turbo/8518...

5 min. google search...

page 4/6 typo

Im trying to find a general criteria about this matter. But in my search i found dresser rand recommendation. By the way i appreciate your concern

I don't understand what you want to say.

quote
Unidirectional seal groove patterns are known to provide greater gas film thickness and thus better reliability than bidirectional groove patterns.

For a given gap, unidirectional groove patterns provide three to five times greater gas film stiffness than bidirectional groove patterns.

as we’ve shown, unidirectional seals offer superior performance over bidirectional seals in all cases except where there is an absolute necessity for rotating the machine in either direction. If reverse rotation cannot be avoided, then other elements (bearings, for instance) in the compressor train (including the driver and possibly the gear) will also have to accommodate that requirement.

unquote

Means there is price for everything : you cannot have reverse rotation feature without scarifying some of the lifting capability of the seal which is something to pay attention when conditions of operations are more stringent.

So it is up to the designer to evaluate the risk of reverse rotation for compressor with respect to the train and process configuration involved against the probability of having the seal facing non favorable conditions for the specific compression application (which I mentioned in my first post) which would require some contingencies of the seal performance.

If I knew that paper from D-R and their view was not a common view of the technical community in the subject, I would not have posted it.

Furthermore you original question : "Which one is better? Uni-Directional DGS or Bi-Directional DGS?"
Which one is Better is not a proper way to address seriously a technical matter from my perspective on complex subject.

Looking for general criteria is not a recommended approach neither. There is a risk to oversimplify things for which people dedicated decades to build an understanding.

That's why I advised to ask the suppliers, which you should still do consider having gained any amount of (false) confidence in the subject.