in the attachment you will find the nessassary information to get the SIL CL 1, 2 or 3
It's dependent on the HFT (hardware fault tolerenz), the DC (diagnostic coverage) and the SFF (safe failure fraction)
Electro mechanical components eg. position switches, or contactors have a HFT 0. If you use two components then you have a HFT 1. 
To according SIL 2 or 3 in combination with electro mechanically components you should use two components.

Electronic components have normally a fix SIL CL valua. 
You get this value from the manufacturer.
her are a link to all Siemens SIMATIC failsafe components:
http://support.automation.siemens.com/WW/view/de/27832836 
 
All Siemens F-PLC's have a SIL CLAIM limit from 3 (SIL CL).
This means that you can use a SIL 3 SIMATIC F-PLC in application for SIL 1, 2 or 3
A Hardware with SIL 1 can you only use in SIL 1 application. 
But you can achieve the SIL CL. Limit
for example use two componets (redundancy) or a higher SFF
This is easy for elektro mechanically components but not for electronic.

I hope we could help you

best regards

our SET-TEAM