The demand for stationary accuracy often renders an integral part in the controller indispensable even if the controlled system itself already shows integral behavior. In this case the transfer function of the open control loop is, however, identified by a double pole in s=0 which has got quite an adverse effect on both the stability behavior of the modulating control and on the achievable control quality. By the example of the distillate level control for a multistage flash desalination plant where the aforesaid problems typically arise in the present contribution, a method is shown how these difficulties can be circumvented by a minor structural extension of a conventional PI controller. In spite of its extremely low realization effort, this so-called SPI controller facilitates a significant improvement of the achievable control quality which will be demonstrated by a realistic design example and a comparative simulation study.