It is possible to choose several types of liquid valves. Each has advantages as well as disadvantages. In general, a two-way valve is selected for applications that require the stopping and starting of flow. A three-way valve is useful where it is necessary to select one liquid or another, or to mix two liquids.

In poppet solenoid valves, the liquid flows around the plunger, spring, and the internals of the body of the valve. It is therefore a valve that has a high internal volume and is not well swept, meaning there could be significant carryover. Poppet valves are good at handling crystals, and they seal at high pressures. When made from the right grades of stainless steel, the valves can be highly resistant to bleach and other chemicals. When an application calls for no carryover, it is best to choose an isolation valve.

Diaphragm isolation valves often look like a poppet valve, but instead of the fluid flowing through the body, spring, and actuator, a diaphragm is placed such that the poppet pushes on one side while the fluid is on the other side, thereby isolating the fluid. The valves thus have very low carryover, and the components on the nonliquid side of the valve do not have to be chemically compatible with the fluid. The valves can have very small internal volumes and typically do not handle high pressures well. They also tend to have slow response times because the diaphragm, which has low inertia, has to be moved. Diaphragms are typically made out of Teflon, EPDM, or FKM.

Another kind of isolation valve is the rocker isolation valve. In this type of valve construction, a solenoid acts on a rocker assembly that pivots, sealing first one seat and then the other seat. It has more internal volume than a diaphragm isolation valve but less than a poppet valve. It is typically less well swept than a diaphragm isolation valve. The valve is small, so the design is more compact, which is an advantage in some situations. However, the valves tend to have lower pressure ratings compared with poppet valves and tend to have more carryover than diaphragm isolation valves.

Pinch valves allow no contact between the valve and the fluid. Tubing containing the fluid is placed in a channel in the valve, and the valve pinches the tubing to stop flow. The valves are inexpensive and have no carryover. However, over time the tubing begins to shed particles which can clog the system or contaminate the fluid. Time fatigue can cause the tubing to restrict flow. The tubing does not open completely when the valve opens. The tubing also wears out quickly, which means that pinch valves have higher maintenance costs.

Donald S. McNeil, BA., MBA, is senior product manager for Parker Hannifin Corp.’s Precision Fluidics Division. He has more than 25 years of experience developing laboratory instruments for clinical diagnostics and analytical chemistry. Previous positions include product management at Beckman Coulter, Bio-Rad Laboratories and Veeco Instruments. E-mail him at [email protected].