Dual-piston in-series

FIGURE 5 A schematic of a dual-piston in-series pump design. Diagram from HPLC equipment, CLC-30, reprinted with permission from Savant, Fullerton, CA. 

Blending accuracy Dual piston in-series design, high-speed... 

Modern pumps have more features and better reliability and performance than earlier models because of better designs in seals, pistons, and check valves as well as innovations such as dual-piston in-series and piston seal wash.2 Performance at low rates can be improved by variable stroke mechanism, micro pistons, or active check valves. The fluidic components in more inert pumps for bio-purification or ion-chromatography are often constructed from titanium or polyetheretherketone (PEEK). Low-pressure mixing quaternary pumps have become standard equipment in research laboratories whereas high-pressure mixing pumps are popular for LC/MS, HTS, and micro LC applications. 

Figure 3.19 Dual piston pumps with parallel pistons deliver a smooth flow, while those in series are easier to maintain, since they have two check valves instead of four.

The most important parts of the instruments are the pumps for maintaining constant, pulseless rates of flow, the column types for the molecular weight range of analysis, and the detector system see Table 3.12. Some of the popular types of solvent delivery pumps include syringe pumps, the plunger of which is actuated by a screw-fed stepper motor, and the dual piston reciprocating pumps, which may have the pistons in either series or parallel see Figure 3.19 (79). 

Dual-head reciprocating pumps offer lower solvent pulsation at the expense of mechanical complexity. Here two pistons fill and pump 180° out of phase and in theory provide pulseless flow. Dual-head pumps are more expensive and have either two check valves (series heads) or four check valves (parallel heads). Both designs generally provide for some type of pressure or flow feedback control to further compensate for minor flow variations during switching from one head to the next. A detailed discussion of the mechanics of these and other pumping systems is provided elsewhere (13). 

Rod position is sensed by a series of sealed glass reed swifches contained within a tube inside the drive piston. Two switches are spaced every 3 in. (76 mm) with each of the dual switches feeding a separate channel. These signals are multiplexed inside the containment and transmitted to the control room. The rod position information function decodes these data and makes them available to other parts of the RC I function, to the process computer and to the operator. The detection of an invalid inpuf caused by a failed reed switch is indicated. The status of the scram valves and accumulators on the hydraulic control unit is monitored, and these data are available to the operator and the computer. 

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