I have added the below filter to allow span tag.. is any problem with that.
function myextensionTinyMCE($init) {
// Command separated string of extended elements
$ext = 'span[id|name|class|style]';
// Add to extended_valid_elements if it alreay exists
if ( isset( $init['extended_valid_elements'] ) ) {
$init['extended_valid_elements'] .= ',' . $ext;
} else {
$init['extended_valid_elements'] = $ext;
}
// Super important: return $init!
return $init;
}
add_filter('tiny_mce_before_init', 'myextensionTinyMCE' );