Continuity residual not converging
 

I can't tell why my multiphase(Eularian) CFD simulation for a water/sand system is not converging. I have tried both presure out and outflow for my outlet boundry conditions with a velocity inlet. The continuity residual gets down to 0.00197, then it starts to go up while the other residuals flattens out eventhough they have reached their convergence criteria. Can anyone help me with this? Is there something I'm overlooking in defining my solver. It's a simple case of pipeflow (straigth and coiled tubing) i have tried second order and it gave similar results, Thanks.