Because the drop time of hammer body is very short, the hydraulic oil first flows
into the low-pressure accumulator near the hydraulic cylinder. At this time, the
pressure loss of the return pipeline is large, and it is too late to flow back to the tank
through the return pipeline. Therefore, the low-pressure accumulator has been in the
state of oil filling during the drop process.
(4) Flow equation of long return pipeline is
Q p ¼
pd
2
p
4
v p
ð12:42Þ
where
d p Long return pipe diameter;
v p Average flow rate of hydraulic oil in long return pipeline.
(5) Pressure loss of long oil return pipeline is composed of pressure loss along the
pipeline and local pressure loss, and its calculation equation is as follows:
Dp 1 ¼
X k
L
d p
qv
2
p
2
þ
X n
qv
2
p
2
ð12:43Þ
where
k Pressure loss coefficient along pipeline. When the flow state is laminar, k ¼ Re
and Re is Reynolds number;
n Local pressure loss coefficient;
q Density of hydraulic oil, q ¼ 890 kg=m
3 .
(6) Considering the volume elasticity of the oil in the lower chamber of the
hydraulic hammer, the flow continuity equation of the oil is as follows:
A d
dy
dt
À
V
b
dp d
dt
¼ Q a þ Q p
ð12:44Þ
where
V Oil volume in lower chamber of hydraulic hammer;
b Volumetric modulus of elasticity of hydraulic oil.
302
12 Pneumatic–Hydraulic Pile Driving Hammer