Dual-head pumps

The noise due to the reciprocating action of the dual-headed pump in the 150C ALC/GPC has a constant frequency pattern and can be reduced by hydraulic filters as shown in Figure 3. A 60% reduction in the peak-to-peak noise is achieved by using a Mark II dampener (Laboratory Data Control, River Beach, Florida), while the M-45 filters under same conditions have reduced the noise by 90%. 

My preference today would be for a high-pressure mixing system if I had to run very complex mixtures on a routine basis because these systems give the best reproducible gradients as a rule. As a routine research instrument or a methods development system, I would prefer a low-pressure, four-solvent dynamically mixed system using a dual-headed pump. I would use the deoxygenation apparatus (Fig. 6.4) to degas my solvents with helium and run them under a helium demand valve to conserve helium. 

Reciprocating Pumps—Single- and dual-headed pumps that use a piston and check valves to pump solvent from a reservoir into the system. 

In single-headed reciprocating pumps, the eluent is delivered to the column for only half of the pumping cycle. A pulse dampener is used to soften the spike of pressure at the peak of the pumping cycle and to provide a eluent flow when the pump is refilling. Use of a dual head pump is better because heads are operated 180° out of phase with each other. One pump head pumps while the other is filling and vice versa. 

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). 

Recently, dual-head pumps with a special piston movement designed so that the Sum of the flow-rates delivered by the two heads is constant, have become commercially available. The principle of operation is shown in Fig. 9c for the Waters Model 6000 and 6000A pumps. During the period when one piston moves forwards at constant speed, the other moves backwards for re-filling. During the intermediary period, the second piston is uniformly accelerated forwards while the first is uniformly decelerated forwards at the same acceleration rate. 

For dual-head pumps with a special piston movement, the flow-rate is set by using an electronic device that controls the piston cycle frequency through a variable-speed motor (often a stepping motor), the distance travelled by the piston being constant. Hence the flow-rate can be ad.iusted over the range 1-100 or more... 

Fig. 9. Waters Model 660 solvent programmer, a, a = solvent reservoirs, b, b = dual-head pumps, c, c = pressure sensors, d = solvent programming manifold, e = high-pressure noise filter, f = to column.

Some pump manufacturers use a different taetie to expand the operating window of their products. This is why the dual ohite pump exists. The dual volute pump is designed to operate over a wide range of flows and heads. 

Linearly polarized, near-diffraction-hmited, mode-locked 1319 and 1064 nm pulse trains are generated in separate dual-head, diode-pumped resonators. Each 2-rod resonator incorporates fiber-coupled diode lasers to end-pump the rods, and features intracavity birefringence compensation. The pulses are stabilized to a 1 GHz bandwidth. Timing jitter is actively controlled to < 150 ps. Models indicate that for the mode-locked pulses, relative timing jitter of 200 ps between the lasers causes <5% reduction in SFG conversion efficiency. 

Pump characteristic Simple single- head Single- head smooth pulse Simple dual-head Dual-head, compressibility-corrected, smooth pulse Dual-head, closed loop flow control Triple head low-volume Syringe- type Hydraulic amplifier Simple Amplifier Amplifier with flow control... 

Modifier Pump. The first feature in our adapted design is the introduction of a liquid pump via an instrument controlled VALCO (Model E04, Valeo Instruments, Houston, TX), four position selection valve. We have used an LKB Model 2150, dual piston pump for pumping modifier and entrainer fluids (LKB-Produkter AB, Bromma, Sweden). However, any suitable liquid pump could be substituted. Only pure fluids such as carbon dioxide have been introduced with the Suprex system syringe pump. With the addition of this second pump to deliver liquids, modifier is introduced directly into the extraction vessel. A wide range of alternative fluids and fluid mixtures can be rapidly selected with this dual pumping option. The criteria for selection of a modifier pump include the ability of the pump heads to withstand pressures in the range of 100 to 300 atm and interfacing capabilities, i.e. the ability to be turned on and off by the Suprex contact closure controls. 

The next step was the electronically compensated pump. All pumps speed the motor as resistance increases to maintain a constant solvent slow. These pumps also add a major plunger speed-up during refill and repressurization. With this modification, a pump with a single pump head and a pulse dampener could give 90% of the performance of a two-headed pump for 50% of the cost. An overall dramatic price reduction for the dual-pump HPLC system resulted. 

Figure 3.6 Cam-driven, dual-head reciprocating-piston pump capable of delivering constant flow with relatively low pulsation. Flow rate is controlled by the cam rotation frequency. (Reprinted from Ref. 3 with permission.)...

Figure 9.24. (a) Schematic and (b) flow profiles of a dual-head reciprocating pump. Reprinted from Yost, Ettre, and Conlon, Practical Liquid Chromatography, 1980 by courtesy of Perkin-Elmer. 

The apparatus consists of a flow through extraction system that can be operated at pressures up to 400 bar and temperatures up to 200°C. This apparatus was described elsewhere (2). Its main piece is a 19 cm3 reactor, where 4-5 g of 40-60 mesh milled cork were placed between two G3 fritted glass discs. The reaction mixture is expanded into a series of three 35 cm3 precipitation traps. A dual-head high pressure liquid pump was used to compress the solvent. One pump head was cooled with ice to pump liquid C02 while the other pump head was used for 1,4-dioxane. 

Fig, 9. Piston position and flow in reciprocating pomps. In each figure the upper curve gives the position of the piston and the lower curve the flow-rate profile as a function of time, (a) Single sinusoidal piston movement head A = backward stroke, B = forward stroke, (b) Three sinusoidal piston movement heads. In the lower diagram the thick solid line indicates the total flow-rate, (c) Dual-head, special-piston movement pump. 1 piston A accelerates 2 piston A moves at constant speed 3 piston A decelerates 4 piston A retracts 1 to 4 similar movements of piston B. The broken line indicates the total flow-rate. 

Single-head sinusoidal drive pumps can be obtained at modest cost, but dual--head, special-drive pumps are generally expensive. 

As indicated earlier, the flow-rate of a dual-head, special-drive reciprocating pump depends on the downstream back-pressure because of the compressibility of the liquid and the elasticity of the chambers. In the Haters Model 6000 pump, the output Of the pressure signal amplifier is fed to the motor speed control circuit to increase the pump speed slightly and keep constant the flow-rate at high pressure. The amount of compensation is adjusted by a potentiometer to allow for the difference in the compressibilities of the solvents. 

In order to damp almost completely the pulses generated at high pressure in a dual-head, special-drive piston reciprocating pump, the electronic correction circuitry in the Altex Model 100 pump calculates the average back-pressure and then corrects the speed of the pump motor so as to maintain the actual pressure close to the average calculated pressure. The effect of this correction is to speed up the motor momentarily when pumping is switched from one piston to the... 

Other. A calibration for changes in solvent compressibility can be made manually by using a screw-driver. Basically, this system is not very different from those operating at constant pressure, with dual-head, special-drive pumps. The main difference is that in one instance the reference pressure is calculated as a function of the selected flow-rate while in the other the reference pressure itself is selected. 

Gradients generated with dual-head, special-drive reciprocating pumps 56... 

Fig. 8. Gradient system using a dual-head sinusoidal-drive counter-piston diaphragm reciprocating pump, a, a = containers of solvents A and B, b, b, = pump heads, c = pump motor, d, d = variable-speed or stepping motor to adjust the course of the counter-piston, e, e = pulse damping systems and flow-rate sensors, f = mixing chamber, g = to column, h = programmer.

In these four systems with two dual-head, special-drive reciprocating pumps, the speed of each pump motor is varied in an attempt to keep constant the sum of the two flow-rates. 

Type of pump Dual head reciprocating Reciprocating Pneumatic amplifier Pneumatic amplifier Dual head reciprocating... 

Type of pump dual head reciprocating dual hydraulii reciprocating... 

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