Vassili,
If you create an object inside a private DS, then the DS will live as long as the object still exists. It sounds like you need some additional cleanup code to get rid of any internal object refs you are creating into the word object and make sure you shutdown word by calling it's QUit() method and then nulling out your oWord object reference.
>If anybody knows reasons that can prevent destroying private data session during form unloading? Form creates WinWord instance and fills word document.