query builder class help

Hello everybody,

I have a little question about the query builder class.
Using normal sql I'd do this

PHP Code:

Update table set val = val + 3 where id = 1; 


How can i do the same thing with the query class ? The code below doesn't work

PHP Code:

$this->db->set(array('val' => 'val+1'))->where(array('id' => 1))->update('table'); 


thank you for helping !