Based on experience --- slightly pressurize to transfer works.
Once I have my current 30 and 55's drained enough, I will load them on the racks and then use a transfer pump in the future for filling the drums as I need more chemical.
Bubbles - John,
We used to get all our chemicals in 30 gallon drums. When we did we had a real nifty way of transferring without using a transfer pump. We just used less than 5 psi through one bung hole fitting while the other bung hole fitting would then transfer ... just from the tiny amount of air pressure. We had a 0-5 psi gauge that we watched really close but the tiny regulator was reliable. When filled we would change the position on the manual 3 way so the drum would stop the transfer & neutralize the pressure once again. It seems like the plastic threads would breakaway first before the whole drum would explode --- not that we had that kind of excitement!
Our drums & pails were always vertical so that could be a factor also. In your case it seems like overflow may not be an issue once the the chemical got down below the vertical draw tube.
This went on for about 20 years until about 5 years ago; then we went with the 5 gallon super concentrated Warsaw Micro products which the Dema tips are really small to allow only tiny amounts of super concentrated chemical per water dilution.
I am not sure if OSHA would have required some type of certified relief valve calibrated at less than a few psi ... for redundancy purposes though. Like driving down a heavy traffic 2 lane highway ... I always felt it was one of those tasks that required a person to not fall asleep or allow oneself to be distracted etc.
Just saying what worked good for us ... not necessarily the best fit for a place relying on untrained employees though. Second thought chemical handling needs precautions regardless of who.
mike