This study is founded by numerical analysis to (H-Q) curve for centrifugal device pump with radial and axial flow fitted firstly with natural spline interpolation and secondly using reserved cubic interpolation spline. Two case studies are used, first one analyzed the (H-Q) curve for axial flow with high specific speed, NS, when the H-Q curve having undesirable dip due to incorrect design of the impeller. The second case study is the parallel operation for two devices (pumps) for different (H-Q) curves, the joint the curves of combination which having important point, until the suitable and working point which reached, the first one doesn’t transfer the flow. The second pump can begin to fund to the flow, it is for this reason that in such a case, the second pump should be started first and then the first pump, locate this point provides us with energy and maintenance costs. The results shown that the normal cubic interpolation which will useful to interpolate these types of curves to obtain the undesirable overshooting points than constrained cubic spline.