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I want to calculate the head loss in the system?

There are couple of valves in the line.What data should I know to calculate the head loss in the system?

If you know the pressure drop, the difference in height, specific weight of the fluid, and are using an incompressible fluid it is a simple solution for the energy equation.

V1²/2·g + P1/gamma + z1 = V2²/2·g + P2/gamma + z2 + hf

Since the fluid is incompressible V1=V2 (velocity in the pipe is constant)
assuming a horizontal run z1 = z2 (height is the same)
P1 = pressure at point 1
P2 = pressure at point 2
gamma = fluid specific weight
hf = pipe head loss

you are left with P1/gamma = P2/gamma + hf
hf = P1/gamma – P2/gamma

Now, if you don’t know the pressure difference:

You need to know:
Velocity of the fluid, V
density of the fluid, rho
kinematic viscosity of the fluid, nu
Specific weight of the fluid, gamma
friction factor for your pipe, f
diameter of the pipe, D
head loss coefficient for each valve, K
pressure at one point, P
height, z
gravitational constant, g

Energy equation for pipe flow:

V1²/2·g + P1/gamma + z1 = V2²/2·g + P2/gamma + z2 + hf

The left side is for point 1 at the beginning of the pipe run (or before the valves) and the right side is for point 2 at the end of the pipe run (or after the valves).

hf = sum of all loss terms (friction from pipe and loss from 2 valves)
for a valve the loss term is K·V²/2·g
for the pipe the loss term is f·L·V²/D·2·g

You may need to solve iteratively.

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