Page 111 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

4. Hf - friction head is the friction losses in the system expressed in
feet of head. The Hf is the measure of the friction between the
pumped liquid and the internal walls of the pipe, valves,
connections and accessories in the suction and discharge piping.
Because the Hv and the Hf are energies lost in the system, this
energy would never reach the final point where it is needed.
Therefore these heads must be calculated and added to the pump at
the moment of design and specification. Also it’s necessary to know
these values, especially when they’re significant, at the moment of
analyzing a problem in the pump. The Hf and the Hv can be
measured with pressure gauges in an existing system (see the Bachus
& Custodio formula in this chapter). If the system is in planning
and design stage and does not physically exist, the Hf and Hv can
be estimated with pipe friction tables (ahead in this chapter). The
Hf formula for pipe is:

Kfx L
Hf = ~
100

where: Kf = friction constant for every 100 fi of pipe derived from
tables L = actual length of pipe in the system measured in feet.

The Hf formula for valves and fittings is:
Hf =KxHv

V2
where: K = friction constant derived from tables, and Hv = -
44
The sum of these four heads is called the total dynamic head, TDH.

TDH = HS + Hp + Hf + Hv
The reason that we use the term ‘dynamic’ is because when the system
and the pump is running, the elevations, pressures, velocities, and
friction losses begin to change. In other words, they’re dynamic.

When the system is designed, the engineer tries to find a pump that‘s BEP is equal to I
or close to the system’s TDH (the system’s TDH E BEP of the pump). But once the
I static BEP.
pump is started, the system becomes very dynamic, leaving the poor pump with a

M 94 !