A pump running two different fluids does not automatically mean a seal change. The seal only meets the process at a short list of wetted parts: faces, elastomer, and metal hardware. Compatibility is judged part by part. The elastomer is the usual weak point, since SiC, tungsten carbide, and 316 hardware resist most chemicals that destroy rubber. When both fluids fall inside the same material envelope, the seal stays. When one fluid leaves that envelope, the elastomer or the whole seal changes. Temperature follows the hotter service, and residual fluid left in the stuffing box can react with the next fluid, so a chamber flush between services matters. A double seal shifts the question to the barrier fluid.