Watson-Marlow SP Series - Manual (Page 11)

11
4.3 Operation of the 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”.