in line water storage

100 psi is .68 Mpa

But there is no may in hell that a drum is going to hold 300 Mpa. If so, we could use 55 gallon drums in place of ASME pressure vessels for air compressors.

The UN rating, which is law in the US for hazmat in drums, has a long code on it that specifies the maximum specific gravity (ie it may be OK for water, but not mercury), the wall thickness, the manufacturer, date, and a hydro value. This hydro value,I believe is in kPa, and only refers to a one time hydro test, not that the drum may be operated, moved or transported at that pressure. I think I'm used to seeing a 100 there. 100 kPa =15 psi.

 

I went to the website on the picture in your original post, those are the type of barrels I suspected them to be. I would not use those barrels for anything other than water storage, unless you want them to burst. However, your stored water can be pumped or gravity feed back into the plumbing system and then pressurized. Having your water storage elevated to create head pressure is the most desirable means of pressurizing your plumbing. Gravity is free.

Please do read the thread melbo linked in his post.