Twin Headed Pump
Pumps used in liquid chromatography are required to deliver very constant liquid flows, free of pulses, at extremely high pressures. The early piston pumps had stainless steel bodies and carefully ground sapphire pistons. Their exit flow was controlled by a non-return valve and the pulses were reduced by employing carefully contoured cams that drove the pistons. The cams were cut so that the liquid was delivered at a relatively constant rate and, at the end of the piston stroke, the cam was cut to allow a relatively rapid return and refill. Although this system reduced the pulsing effect, there remained significant pulses in the exit flow from the pump. However, these pulses were further reduced by the introduction of the twin-headed pump. Twin headed pumps were constructed in a very similar form to the single piston pump but the cams were contoured so that the output of one pump always increased rapidly during the refill of the other pump. In this way, with the aid of a pulse dampener, pump pulses were virtually eliminated in the twin headed pump. Rapid-refill pumps also have two cylinders and pistons but they operate in an entirely different way. The cylinders are back to back and one cylinder is used to rapidly refill the other on its refill stroke. In general, the rapid refill pump is not as pulse-free as the twin headed pump.
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Basic-HPLC Pump Twin-Headed
twin pump
heads. During the operation of a two-headed pump, one cylinder is filled while
the other is delivering solvent to the column.
The Twin-Headed Pump.
The cylinders
and pistons of a two-headed pump are constructed in the same manner to the
single piston pump with sapphire pistons and stainless steel cylinders fitted
with non-return valves to both the inlet and outlet. The driving cams of both
pistons are carefully cut to provide an increase in flow from one pump while
the other pump is being filled. This compensate for the loss of delivery during
the refill process and the consequent fall in pressure. A diagram of a
twin-headed pump is shown in figure 9. It is seen that there is a common supply
of mobile phase from the solvent reservoir or solvent programmer to both pumps
and the output of each pump joins and the solvent then passes to the sample
valve and then to the column. In the diagram, a single cam drives both pistons,
but in practice, to minimize pressure pulses
HPLC Basic-HPLC Pump Twin-Headed
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Basic-HPLC Pump Twin-Headed
nbsp;
Figure 9. The Twin-Headed Pump
The cams are
carefully designed to produce a virtually pulse-free flow. The displacement
volume of each pump can vary from 20 or 30 microliters to over one milliliter
but the usual displacement volume for the typical pump is about 250 ml. The pump is driven by a stepping motor
and thus the delivery depends on the frequency of the supply fed to the motor.
As a consequence, the pump can have a very wide flow rate range from a few
microliters per minute to over 10 ml per minute
HPLC Basic-HPLC Pump Twin-Headed
