Vaadin session in liferay

Hi

I have a liferay page with multiple vaadin portlets added. Every time the page is loaded or refreshed, multiple extend_session requests are send (see screenshots). I tried to set -1 in web.xml of each portlet but it s not working. Is there any solution to keep the extend_session from being requested?

Thanks.

P.S:
Vaadin version: 6.8.8
Liferay version: 6.1.1 CE GA2
12825.jpg