I think that would depend entirely on the manner of how the unloading is setup and the pump used, so if you have some sketches and details to share that would help get you better answers. In general, if a pump isn't primed it won't be able to properly move the desired fluid because the air that's in the system hasn't been displaced.
If the inlet to the pump is below the liquid level in the tanker and the pipework doesn't have any high points above that liquid level then the pump should be able to self prime due to gravity flow.
But the devil is in the detail so if you sketch out your system with elevations and lengths then we might get somewhere.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
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.
