Sun,
We know that if the requirement qty is more then the available qty (Total stock + firm receipt) then system will not create planned order or PR. But how the purchase department will come to know the actual requirement quantity. This I am asking because purchase department create one PO for one year and then based on the requirement he inform vendor to Supply the materials.
Purchasing in general does not need to know the requirement quantity, they need to know the shortage quantity. This entire process is usually handled through MRP planning. The reason you are having difficulty is because you are not following Best Practices.
The business process you have described is normally satisfied by having your company place an outline agreement with the Vendor, such as a Purchasing Scheduling agreement, against these components, and not a "PO for one year". Here, you can have MRP plan the materials for you, without having to add a fixed receipt element that someone will have to monitor and adjust regularly..
Procurement Using Scheduling Agreements - Scheduling Agreement - SAP Library
In the case where there is also a business requirement to maintain buffer stock, it should also be included in your planning. The simplest method is to make an entry in the material Master for Safety Stock.
If you insist on bypassing any normal planning methods, and instead wish to use exploded demand to determine how much & when to order, you can create a custom report that contains all the data you think you need.
Best Regards & Good Luck,
DB49