The theoretical characteristic of a centrifugal pump is a parabola starting from the origin and proportional to the square of the speed.

The torque can be expressed as
T = k n2 (1)
where
T = torque (Nm, lbf ft)
k = constant
n = pump speed or velocity (rpm)
With the discharge valve closed the torque amounts to 30 - 50 % of the nominal torque at full speed.
Full Load Torque

The torque at full load can be calculated as
T = 30 P / (π n) (2)
where
T = torque (kN m)
P = power (kW)