Hi Adi,
Suggest to use Forecast based planning for all the spare parts materials. In FBP system will consider the materials consumption for the past period and forecast the requirements. After forecasting once MRP is run system will generate purchase requsitions.
Forecasting period can be monthly/year etc as required for business. Generating purchase requsitions based on forecast can be monthly or one single PR for year etc.
Refer the below SAP Help for details.
Forecast-Based Planning - MRP Procedures - SAP Library
Also refer to the below document posted by me on forecast based planning.
http://scn.sap.com/docs/DOC-53063
Thanks & Regards,
Ramagiri