Is there any way to get the user-id/login-id of the person who approves the particular PO? I checked the PO History Table tdpur451 but couldnt get the exact information.

Have you input the logon code in bpmdm0101m100?

In the Purchase Order History (tdpur4550m000, table tdpur450) you can see who did approve the order.
On line level the intake posting is made on moment of approving the line.
HtH
Regards, Adriaan

Have you input the logon code in bpmdm0101m100?

Sorry,,there's nothing to do in here..it's related with my approval customization..
just ignore this :p