Pressurised tank

[firavia]

Main Question or Discussion Point

I have a system that consist of a water tank a pump that is pumping into a pressuirsed tank at 100 GPM and another pump at the outlet of the pressurized tank that is pumping to the 6th floor , my question is can I consider the 2 pumps as 2 pumps in series ? , my second question is : is there a continuity of flow ? and by conitnuity I mean the flow rate at the outlet of pressurised tank should be 100 gpm as the flow entering the tank ?? or it can be less or more , if it can be less or more does that mean there is no conservation of flowrate ?

- can a pressurized system have an inflow rate less or more thn its outflow rate ??? using 2 pumps one before and another at the outlet ??? is that posssible or there must be a conservation of flow which means the 2 pumps must pump at the same flow rate ???
Please help , thank u :)-Please check the picture attached.

 

[russ_watters]

What's in the tank? If the tank is filled solid with water (like a water heater tank), it's just a big pipe and continuity of flow applies. If it's partly water an partly air (like an expansion tank), then the tank level can go up and down.

Either way, the two pumps are in series and the outlet pressure of the first pump is just about equal to the inlet pressure of the second (caveat: unless thee are flow control devices like valves between them or if there is a lot of pipe friction loss). If the flow is steady, then a dynamic equilibrium will be reached whereby the pressure inside the tank is the same for the completely full case as it is for the not completely full case. But if the flow rate is not steady, the tank level and pressure will go up and down. For example, if the tank is half full and pressurized and you turn off or slow down the second pump, water (this is water, right?) will flow in but won't flow out (it could flow in from both sides! unless you have back-flow preventers) until the pressure gets high enough for the first pump to do all the work(if it can).

 

[firavia]

what if the pressurised tank is fulll and the 2 pumps are not identical and 1 of them is selected to have 100 gpm as flow rate and the 2nd to have 150 gpm ? when the work together will the water level in the pressurised tank decrease or the 2 pumps with adjust and reach an equilibrumm state where both flow at the same flowrate even if our selection was as above ? would their system curve cross on a point where both of them will flow at the same flowrate ?