I was working in a Codeigniter project and when I hit refresh button (F5) continuously, My application session was expiring frequently. I googled for a while and found the simple solution.

A solution is to increase the amount of time for the session_time_to_update in codeigniter config.

Since I won’t be updating session frequently. I am setting it to much higher values such as 86400 (24 hours).

Here is my settings

$config['sess_cookie_name'] = 'app_session';  
$config['sess_expiration'] = 0;  
$config['sess_expire_on_close'] = FALSE;  
$config['sess_encrypt_cookie'] = FALSE;  
$config['sess_use_database'] = TRUE;  
$config['sess_table_name'] = 'app_sessions';  
$config['sess_match_ip'] = FALSE;  
$config['sess_match_useragent'] = TRUE;  
$config['sess_time_to_update'] = 86400; // 24 hours

Since then I haven't got this problem.

Reference : http://ellislab.com/forums/viewthread/182755/#864970