how to prevent node revision update when node is updated?
@camcam, have in mind that even when you are not using revisions (keeping separate node revisions for each update you make to your node content), the revisions table will still always have one record in the node_revisions table associated for each record in the node table that it keeps track of.  

On the other hand, if you have enabled revisioning, you will have multiple entries in your node_revision table associated with each node, corresponding, not just to the current revision, but all past revisions too.

My suggestion is to make sure you really want to mess with that functionality before you go ahead and do anything - because what you're seeing is actually standard behavior.  :-)

If you look at the code in node_save, you can see that a revision is created or updated in each instance of a node_save() call.

If this really doesn't work for you, I'd suggest using the hook_form() function to edit which function is called when your specific node content type is saved, and you can choose not to call the standard node_save() function (this should only be done if really necessary).  The api documentation, along with some code samples in the comments, can be found here: https://api.drupal.org/api/drupal/modules!node!node.api.php/function/hook_form/7.