Why eccentric reducers are used in suction lines and not concentric? where does it say that eccentric reducers in suctions lines are mandatoy and why?

it was needed for cavitaıons  thats why we are usıng  for suctıons sıde eccentric reducer . Because due to cavitatıons problem.

for  Preventing  of any cavitation, normally we are using Eccentric reducer to remove entrap air in the suction line  

to check on pump cavitation

Eccentric reducers are used at the suction side of pumps to ensure that air does not accumulate in the pipe. In a concentric reducer, air gradually accumulates and form a large bubble that could eventually cause the pump to stall or cause cavitation when drawn into the pump.

Suction piping design recommendations are listed in ANSI/HI 9.8 shall be be referred while designing pump suction piping. for e.g;  straight pipe run, fluid velocity limit, etc.

eccentric reducer are used at pump suction .

to avid the pump cavtation. 

Cavitation & reduce the pressure drop of liquid

Eccentric reducers are used to avoid cavitation.

1-due to cavitation problems 

in eccentric reduser liquid flow is direct into impeller eye as compare to concentric, and cavitations problems comes if concentric reducer use at section side.