The piping leading to a centrifugal pump must be at least one size larger than the inlet flange on the pump. Similarly, the discharge piping leading from the pump should be at least one size larger than the flange size on the discharge of the pump. For further details on this you can refer to my article "The Pitfalls of Pump Piping," which you can download from my Web site at http://www.practicalpumping.com/articles.php.