Page 125 - Fluid Power Engineering

Hydraulic Pumps 99

FIGURE 4.9 The pulsation of fl ow in displacement pumps.

delivery stroke. Then, it decreases progressively until it becomes
null at the end of the delivery stroke. Therefore, the net pump flow
is pulsating, as illustrated in Fig. 4.9. The magnitude of flow pulsa-
tion is evaluated by the pulsation coefficient and is defined as

Q − Q
σ = max min × 100 % (4.16)
Q Q
m
where σ = Flow pulsation coefficient
Q
Q = Minimum value of pump flow rate, m /s
3
min
3
Q = Maximum value of pump flow rate, m /s
max
3
Q = V n = mean flow rate, m /s
m g
The pulsation of flow results in pressure oscillation and the non-
uniform motion of hydraulic cylinders and motors.
Considering the case of a throttled pump exit line (see Fig. 4.10)
and neglecting the fluid compressibility, the pressure at the pump
exit is given by
ρ
P = 2 2 Q 2 (4.17)
2 CA
d t
ρ
P = Q 2 (4.18)
m 2 2 m
2 CA
d t
ρ
P = Q 2 (4.19)
max 2 2 max
2 CA
d t
ρ
2
P min = 2 2 Q min (4.20)
2 CA
d t