sess_expiration issue urgent

[eluser]Unknown[/eluser]
I am using these settings for session expire after 30min but but my session still loged in after 30min. I try this by others values but not works. any one help me...

$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 1800;
$config['sess_encrypt_cookie'] = FALSE;
$config['sess_use_database'] = TRUE;
$config['sess_table_name'] = 'ci_sessions';
$config['sess_match_ip'] = FALSE;
$config['sess_match_useragent'] = TRUE;
$config['sess_time_to_update'] = 300;