Emmanuel wrote:just plunge the inlet tube of the rotary pump into a water tank. The water tank can be disposed under or above the coffee machine. that's all ... correct??

Yes. I prefer under or next to the machine for convenience reasons and so the tank can't empty itself onto my machine from above.

Emmanuel wrote: Can all the coffee machine with rotary pump do that? If not >> why (rotary pump work too hard to give 9bar when start from 0bar and therefore are rapidly damaged?? rotary pump can not start without a minimum inlet pressure ??...) ???

This gets tricky...

The concern with a choked water supply on the inlet of any pump is cavitation and starvation. Having a positive pressure supply with sufficient flow eliminates the risk of both, but in contrast, dropping a tube into an open top water tank does not cause either issue on its own. Running the tank dry can starve the pump and running the pump dry is bad for it because the water serves as the lubrication for the vanes inside the pump. So make sure you keep your water tank topped up to prevent starvation of the pump. Cavitation is harder to nail down, but the short answer is to run a 1/2" line that's no longer than it needs to be to reach the bucket that's no lower than it needs to be (like not in the basement, under a cabinet is fine) and don't restrict it's inlet with anything other than a free-flowing foot valve at the end to keep the pump primed and you should be fine. Also don't seal the top of your water tank. It needs to breathe. Cavitation is caused when the suction pressure at the inlet of the pump dips below the vapor pressure of the water and the water starts to boil. As long as you heed the advice above, this can't really happen under normal circumstances, but there is inherently more risk when bottle feeding than when plumbed to a pressurized and high-flowing supply.

Emmanuel wrote: I know that Some rocket machine are constructed with the 2 options : water tank or water line. It seems that going from one option to the other one is as simple as described by Jake. But could we generalize to the other coffee machine shaped with rotary pump (except lever coffee machine)??

In short, yes. Almost all the rotary pumps are essentially the same. Machines with internal tanks have open tops, short hoses and no restrictions on the lines, although you can get away with a softener on the end of the line in the tank.

Emmanuel wrote: In my coffee machine I found the nameplate where it is specified that the inlet pressure should be between 2 to 5bar >> Jake (or other) when you used water tank supply on coffee machine instead of water line supply, have you done it on coffee machine with same kind of inlet water pressure prescription on the nameplate ?

This is a pretty standard recommended pressure range and the manufacturer assumes all customers will plumb. If you plumb it, follow these guidelines. Reducing the supply pressure below 2 bar often restricts the available flow to the machine simply because of the design of most pressure regulators. You set them to say 1 bar with no flow, and since they have such a high pressure differential across them, the outlet pressure can begin dropping when the pump kicks on, which can lead to cavitation. There exist high flow, low pressure regulators that could provide a safe and low operating pressure but they are not what is typically available, so manufacturers stock with conservative guidelines.

Sorry for the long responses!

And Marcel, I hope you didn't take my first reply as an indictment on you in any way. You're far from the first person to suggest feeding a rotary pump driven machine with an external rotary pump. I meant no disrespect in my questioning of the idea!

Cheers!

- Jake