You ask why you "must" use a self priming pump for unloading concentrated HCL. The answer is that it's not essential in any pumping application for the pump to be self-priming. Self-priming pumps certainly make plant operators' jobs safer and more tolerable. If operators had to go out a prime every pump, you'd have to be prepared for an extremely high operator turnover rate due to job dissatisfaction, extremely high overhead cost due to inefficient design, and an extremely high number of injuries due to personnel exposure to chemicals. Back to the case at hand; we're talking about concentrated HCL. Think about what is necessary to prime a pump (how priming is done) and consider whether that's an tolerable practice with this particular fluid. I think the answer to your question will be obvious.