Watson-Marlow SP40 - Manual (Page 15)

DESCRIPTION
12
4.3 Operation of a hose pump
The heart of the pumphead consists of a specially
constructed pump hose which lies contorted against the
inside of the pump housing. Both ends of the hose are
connected to the suction and discharge lines by means
of a flange construction. A bearing-mounted rotor with
two facing pressing shoes is in the center of the
pumphead.
In phase 1 the lower pressing shoe compressed the
pump hose by the rotational movement of the rotor,
forcing the fluid through the hose. As soon as the
pressing shoe has passed, the hose recovers to its
original shape due to the mechanical properties of the
material.
In phase 2 the product is drawn into the hose by the
(continuous) turning motion of the rotor.
In phase 3, the second pressing shoe will subsequently
compress the pump hose. Due to the continuous
rotating movement of the rotor not only new product is
sucked in, but also the already present product is
pressed out by the pressing shoe. When the first
pressing shoe runs from the pump hose, the second
pressing shoe has already closed the pump hose and
the product is prevented from flowing back. This
method of liquid displacement is also known as the
"positive displacement principle".