Pneumatic amplifiers

Various accessories can be suppHed along with the control valves for special situations. Positioners ensure that the valve stem is accurately positioned following small or slowly changing control signals or where unbalanced valve forces exist. Boosters, which are actually pneumatic amplifiers, can increase the speed of response or provide adequate force in high pressure appHcations. Limit switches are sometimes included to provide remote verification that the valve stem has actually moved to a particular position. 

Figure H-7 is the schematic of a basic I/P transducer. The transducer shovvm is characterized bv (1) an input conversion that generates an angular displacement of the beam proportional to the input current, (2) a pneumatic amplifier stage that converts the resulting angii-...

The stiffness characteristic of the positioner/actuator varies with frequency. Figure 8-75Z indicates the stiffness of the positioner/actu-ator is high at low frequencies and is directly related to the locked-stem pressure gain provided by the positioner. As frequency increases, a dip in the stiffness curve results Trom dynamic gain attenuation in the pneumatic amplifiers in the positioner. The value at the bottom of the dip is the sum of the mechanical stiffness of the spring in the actu-... 

Figure 5.3 Different types of pusqps used in high pressure liquid chronatography. A, gas displacesent punp B, syringe punp C, pneumatic amplifier pump D, reciprocating punp.

Type constant flow twin reciprocating constant flow syringe type constant pressure pneumatic amplifier... 

The operating principle of the pneumatic amplifier pump is shown in Fig. 2.2b. 

Compared to syringe type or reciprocating pumps, pneumatic amplifier pumps are very cheap. They tend to be rather difficult to dismantle for repairs, and some types are very noisy in operation. Because they do not provide a constant flow of mobile phase, they are not used much in analytical hplc. They can, however, operate at high pressures and flow rates and so are used mainly for packing columns, where high pressures are needed and variations in the flow rate through the column do not matter. 

In order to obtain very low flow rates without pulsation (e.g. 1 pl/min), pumps based on the principle of high volume motorised syringes are used. The piston, activated by a pneumatic amplifier, moves at a constant linear velocity. These pumps are still in widespread use. 

The solvent is moved through the system by constant-flow or constant-pressure pumps which arc driven mechanically (screw-driven syringe or reciprocating) or by gas pressure with pneumatic amplifiers. For gradient elution Iwo pumps may be synchronised and programmed to provide a controlled, reproducible composition change. 

Pneumatic amplifier pump This pump is composed of two cylind that are different in piston cross-sectional area. The piston cros sectional area ratio between the two cylinders equals the pressure ampll fication factor from the low-pressure cylinder to the high-pressure i inder, and also equals the flow rate attenuation factor from th high-flow-rate cylinder to the low-low-rate (high-pressure) cylinder, practice, an area ratio of 5 10 is recommended for reasons such safety, reliability of ultrahigh-pressure seals and connectors, fluid com pressibility, and high-pressure cylinder volume. 

Pneumatic amplifier pumps are used for column packing and for applications where a pressure of over 500 bar is needed. With this design, a relatively low gas pressure activates the larger cross-sectional area of a piston which at its other end is in contact with the eluent over a much smaller area. The force is the same on both sides of the piston but the pressure is higher on the smaller area, according to the ratio of the two areas. The pressure amplification can be as high as 70-fold this means that a gas pressure of 10 bar can generate an eluent pressure of 700 bar. 

This overcomes the major disadvantage of pneumatic amplifier type pumps, and makes syringe pumps ideal for reproducibility of retention time data. However, the major problem encountered with the syringe pump is the design of a suitable refill mechanism. However, it is possible to use two pumps in concert giving limited gradient elution capabilities also the finite volume of the eluant chamber (250-500 ml) is not such a disadvantage with narrow bore columns with their small flow-rate requirements. 

Constant pressure pumps. Constant pressure pumps (Figure 6.11) deliver solvent via a small headed piston which is driven by a pneumatic amplifier. A gas acts on the relatively large piston area of the pneumatic actuator. This is coupled directly to a small piston which pushes the eluant through the column. Pressure amplification is achieved in direct ratio to the piston areas and thus for low inlet pressures (approximately lOOpsi (690 kPa)) it is possible to obtain large outlet pressures (lOOOOpsi (69 MPa)). 

However, due to their mode of operation, pneumatic amplifier pumps have certain disadvantages. They are constant pressure rather than constant flow and therefore, as the elution volume is proportional to flow, fluctuations in the latter—due to, for example, partial column blockage or temperature change—can lead to poor precision and accuracy of analysis. The flow-rate is also dependent on solvent viscosity and coliunn back pressure. 

Pneumatic pumps typically use a pressurized gas as the driving force to force the liquid through the column. The gas pressure can be either applied directly to the liquid (gas displacement pumps) or amplified so that the liquid pressure is greater than the gas pressure (pneumatic amplifier pumps). 

A piston moving in a cylinder can also be used to separate the liquid from the gas, as in the Morgan pump, but generally such a design is used for pneumatic amplifiers see the next section). 

A pneumatic amplifier pump uses the Pascal principle to amplify the pressure of a gas. It consists of two rigidly connected pistons with different surface areas, the piston with a large surface area (s ) being on the gas side and that with a small surface area (S ) on the liquid side. If is the gas pressure and P. the liquid pressure, then... 

Fig. 1. Diagram of a pneumatic amplifier pump, a = gas pressure controller, b, c = valves, d = gas piston, e = liquid piston, f = piston seal, g = solvent Chamber, h = column check valve, i = to column, j = reservoir check valve, k = reservoir.

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