Air Supply

I mostly used my truck's air compressor and sometimes used the customer "factory air" system to air-off corrosives. I never saw any details of the factory air compressors. As long as it can create 30-35 psi, I wouldn't know what volume is required. I hauled corrosives exclusively. Obviously, you can't air-off flammables, etc. In my experience, a steeper angle for the trailer to be parked speeds up the unloading time considerably. My 5,000 gallon tanker would typically unload in 45 minutes, using truck air and a nearly level unloading pad. One Chicago customer provided massive factory air source and a sunken unloading pit maybe 3 feet below grade. I remember my trailer landing gear, when fully up, almost touched the pavement when the trailer was in the pit. 5,000 gallons could be unloaded in 20 minutes. There is probably a bigger benefit to a sunken unloading dock/pad for rear-unload trailers than factory air, since trailers may not accept more air pressure than 30-35 PSI.

As for making it driver friendly, a covered area near the trailer's unloading port, if not a "shack" that allows the driver to escape the weather and noise is nice, or a rubber mat to stand on if no seating is possible is also nice. About 25-40% of my customers unloaded my trailer and allowed me to stay in the truck (preferred) or required me to stay in a company break-room while they unloaded the product I delivered. In my ideal world, I would drop the trailer in the unloading position, bobtail to a parking area within eyesight of the unloading trailer and I could stay in my truck using my APU for HVAC and electricity while the company employees did everything, except maybe open my trailer valves, if that is customer's company policy. For a HazMat driver this gets rid of the need to wear the rubber suit. Personally, I'd prefer to stay in my truck than sit in the company break room, since my truck is my home away from home. Some drivers may prefer to choose between a company break-room and restrooms or the truck as a waiting option. Just my 2 cents. I only did HM tanker for 3 years. Tanker customers are the best in trucking.

 

Having the shortest, straightest, most level plumbing run possible, of adequate internal diameter piping, will decrease unload time by reducing backpressure.

As far as the optimum air pressure and flow rate in your air system, you could try looking at the data plates on the delivering trailers. Ideally you would want to have a regulator on your system to keep pressure just below the point where the trailer's pressure relief valve will open.

And as far as making the unloading process driver friendly, you're already on the right track by seeking out ways to decrease unload time. So kudos to you! It's also nice if the drivers have a clean restroom, vending machines (or at least a water fountain), and an air-conditioned break room.

 

Is your product hydroscopic? We used a sullair screw compresor with twin dessicant units [one switched on while the other dried out] to produce high scfm of dry air in a cleanig rack.
It eliminated N2 purging of trailers for transformer oil by using a digital hydrometer to purge the trailer to a dew point of -50 degrees...

Screw compressor with a large tank [2" distribution lines] with a regulated 35# - 45# psig should provide enough air to unload multiple trailers and operate multiple air diaphram pumps throughout the plant at a steady rate.

 

Most tankers have a blowoff valve that normally lets go at around 32 PSI.
Hopefully it is in good shape and resets after the air pressure goes down, otherwise you are screwed.