Make CDF field readonly on condition

Hii Guru,

I saw many thread of CDF handling.But didn't get success..

Is there any way to Make CDF field readable(on condition)!??

Scenario.
I have add some cdf field in "Purchase Requisition Line" Session
- In the "create" status of Purchase requisition system allow me to add value in cdf field.
But when i submit the requisition system should not be allow to edit that field.

Regards,
DDPatel