Ion Chromatography - Non-Return Valves
For efficient function, it is important that while the piston compresses the solvent to express it from the exit port, the flow is completely stopped at the inlet port.
Figure 14. The Design of a Typical Non-Return Valve
Conversely, when the pump draws fresh solvent into the cylinder during refill, the non-return valves must allow solvent to flow through the inlet valve but, any flow-back from the exit valve must be completely stopped. This is achieved by the use of efficient non-return valves. Most non-return valves are similar in design and the construction of a typical valve is shown in figure 14.
The critical part of the valve consists of a synthetic sapphire ball resting on a seat. The seat may be made of stainless steel, captured PTFE or, more commonly, of sapphire. When the flow is directed against the ball, the ball moves forward allowing the liquid to flow past it. When the direction of pressure changes resulting in potential flow-back through the valve, the ball is driven back onto its seat and stops the flow. With careful design and exacting construction these types of valve can be extremely efficient. In practice, to ensure the most effective performance, a single non-return valve assembly usually contains two non-return ball valves connected in series as shown in figure 14.
A twin headed pump manufactured by METROHM is shown in figure 15.
Figure 15. A Twin Headed Pump Manufactured by Mertohm for Ion Chromatography
The pump shown in figure 15 is available in stainless steel or in an inert material such as PEEK (polyetheretherketone) and and can operate at pressures up to 6000 lbs/sq.in.