Is it safe to install an U-shaped PVC pipe on top of roof vent pipe to prevent rain water getting in the vent pipe?

Do you get snow there? One decent snowfall can block the vent.

There is a reason why stack pipes have to have the opening so many inches/feet above the roofline.

Instead of a U shape, just install a long radius 90 on it.

Or use the correct fitting.

why , you cant get water in you house from a vent anyway , its part of you sewerage system , fit a hood on it .Any plumbing store sells them anyway .

I wouldn't it could cause issues with the sewer gas venting from air getting trapped.   What little bit of rain goes down the pipe just ends up in the sewer if it makes it that far.

It's not necessary, for a sewer vent, but I can't immediately put my finger on a code section that prohibits it.  Your local inspectors might drive around the neighborhood looking for things like that and come knock on your door.

doing so is pointless ... and might encourage small birds to nest there, blocking the vent.  this pipe [called the 'stack'] leads directly to the sewer pipe so a bit of added water or dirt doesn't matter  -- grampa

You dont put nothing on top the vent pipe because theres already water at the bottom of the pipe...its just an air vent for your drain water...you could pour water down that pipe and it would only go down the drain with the rest of your water...

what is the vent for? for a plumbing vent you don’t need the bends

it looks like a lot of reducers right before the bend

The reason ALL vent pipes are installed open and vertical is for SAFETY and you just defeated it.

Put it back the way it SHOULD be or you will likely have a huge explosion some day

You mean you made a plastic hat for the roof vent. Nothing wrong with doing that won't hurt a thing.