How to find total head for primary or secondary Pump in the Chilled water systems? Give any thumb rules to select head

There is no thumb rule for this you can only calculate it based on friction losses in pipe by equivalent length method and for fittings k factors from the various supplier of the fittings ashrae k factors are old they tend to change according to various suppliers.

All suppliers for the fittings have to have the k factors according to prove there manufacturing according to designated area code of practice.

head for primary pump

= chiller friction losses + pipe losses for longest pass + fitting losses 

head for secondary pump

= pipe losses for longest pass + fitting losses + last equipment in the longest pass ( FCU or AHU )

there is tow way to calculate the pump head 

1- equivalent length method and you may found it in carrier handbook.

2- k factor you will find it in ASHRAE.

the main concept for calculation and sizing 

pipe size < = 2" -----> Max velocity = 4 fps

pipe size > 2" --------> max friction losses 4ft wg / 100 ft

there are many programs for pump head calculation like 

H-sym (Elite) , Trane pipes , pipe flow expert