there is a lot diference between a tank  size 10 meter high and 1 m surface, and other 1 meter high and 10 meter surface, please remember that allways you need to state both: flow and pressure or head pressure.

Pardal

your tank drainage calculation will be the result of a
period of time from second 1 to 1 hour.

the differential pressures
when the tank is full
and the pressure when the tank is emptied
are the main concern here also the orifice diameter and any vortex obstruction created durring draining the tank are at issue.
the pipe lenght and pipe diameter and pipe material and pipe fittings and the number of vectors that are taken durring the draining process are also an issue.
the viscosity of the water and the temperature of the water.
and the density of the water.
there are numerous determining factors or elements to include when determining the correct pipe size to use in a systems design.
if you would please be more specific as to your system there are many highly qualified individuals who will be happy to assist you in this forum , this I have learned from experience.
 



 

a simple solution is

Economic diameter for a rate of flow
of 10 m³/h:

Small viscosity (ca.10^-6 m²/s)
Optiaml pipe diameter about: 43 mm
Possible diameter: DN 40 ==> Velocity 2.21 m/s
Possible diameter: DN 50 ==> Velocity 1.41 m/s

High viscosity (100*10^-6 m²/s)
Optimal pipe diameter about: 63 mm
Possible diameter: DN 50 ==> Velocity 1.41 m/s
Possible diameter: DN 65 ==> Velocity 0.84 m/s

Pardal

The flow rate will depend on level of water and will change with time. So you will have to adopt step by step method or area-integration method to compute discharge.

Suppose, you start draining the tank and at time 't' seconds after, the head in tank is 'Ht'.

Net head (driving head) Hd =Ht-headloss in pipe=
 Hd = Ht- f*l*v*v/2/g/d   ...........(A)
 f=friction factor; l =length of pipe; v= vel in pipe; g= gravitation cont; d= dia of pipe;

Velocity haed in pipe =Hd = v*v/2/g  ........(B)

For any particular dia and time step dt at time t you will get v for any t and compute discharge
Dq = v*pi*d8d/4*dt

Perform computations for t=0 to 3600 secs and sum up discharge volumes (dq). The dia which gives you the volume of 10 m3 is the most eco dia.

Hope I it will make some sense.