Liquid 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. 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, 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 of similar design and the construction of a typical valve is shown in figure 5.
The critical part of the valve consists of a synthetic sapphire ball resting on a seat. The seat may be of stainless steel, captured PTFE or, more commonly, also 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.
Figure 5. The Design of a Typical Non-Return Valve