What is effect of Restricted orifice plate on suction side of pump?

Hello Mr. Ghulam,

As per my knowledge and experience,I haven't seen any pump installed with a suction orifice.

To avoid cavitation,the pump suction is normally disigned larger in diameter than the discharge pipe.If you provide an orifice in suction,it will result in reduction of the diameter at that point in suction line,which will  result in pressure drop at orifice.This may cause the pressure of the fluid to fall below the vapour pressure at that perticular temperature and formation of vapur will induce,vapour locking/depriming/cavitation.