Flow pulsations

Delivery of LC solvent in one stroke without pressure or flow pulsation induced by piston refill strokes... 

Electrolysis of mobile phase constituents will cause a continuous detector response (background current) resulting in a chromatographic baseline level that differs from the electrical detector zero-response level. The difference, baseline- offset, is an important analysis parameter, because baseline fluctuations (noise, drift) due to fluctuations in electrolysis conditions (potential, mobile phase flow rate, temperature) are proportional to baseline offset. See Figure 2-5 for an example of the influence of flow pulsation at different baseline offset... 

Influence of flow pulsation on baseline for varying background current... 

Electrochemical detectors are more sensitive to flow pulsation than UV-detectors and usually an efficient pulse dampener is required to minimize the contribution to baseline noise at high sensitivity. 

Other Flow Disturbances Other examples of deviations from fully developed, single-phase newtonian flow include nonnewtonian flow, pulsating flow, cavitation, multiphase flow, boundary layer flows, and nonisothermal flows. See Sec. 6. 

In the presence of flow pulsations, the indications of head meters such as orifices, nozzles, and venturis will often be undependable for several reasons. First, the measured pressure differential will tend to be high, since the pressure differential is proportional to the square of flow rate for a head meter, and the square root of the mean differential pressure is always greater than the mean of the square roots of the differential pressures. Second, there is a phase shift as the wave passes through... 

Pipe vibration is caused by f. Internal flow (pulsation)... 

The most sensitive detector is the differential UV photometer, which is appropriate for a polymer with a significant UV absorbance with a nonabsorbing eluent. This detector is not appreciably affected by flow pulsations, flow rate changes, and temperature fluctuations. 

In general, gas and liquid flows pulsate, with a particular perforation passing both gas and liquid intermittently, but seldom simultaneously. In large-diameter (>2.5-m, or 8-ft) dual-flow trays, the pulsations sometimes develop into sloshing, instability, and vibrations. The Ripple Tray is a proprietary variation in which the tray floor is corrugated to minimize this instability. 

This analysis is performed using a Dionex Model 12 ion chromatograph with 200pL sample loop, an eluent flow rate of 2.3ml min 1 (30% of full capacity) and a sample pump flow rate of 3.8ml min 1 (50% of full capacity). Pulse damper is installed just before the injection valve to reduce flow pulsation. A Dionex amperometric detector... 

FIGURE I Expanded-bed affinity chromatography (EBAC) setup Tl and T2, tanks for homogenate, washing, elution, and reequilibration buffers P, peristaltic pump C, expanded-bed column W, wastes collector E, eluent collector VI, V2, V3, and V4, three-way valves V5, four-way valve K, knitted silicone tubing used to reduce flow pulsation. 

Figure 1 shows a triplex plunger pump and its flow pulsation at a volumetric efficiency of 95 %. The volumetric efficiency, representing the ratio of the actual volume flow Q to the theoretical flow (equation (1)), depends essentially on the fluid and working chamber elasticity... 

Orifice plates are often applied to damp resonant oscillations in existing installations. Their effect is based on frictional losses. Therefore orifice plates shall be placed at pipe positions with a maximum of flow pulsation. 

A disadvantage of damping flow pulsations by orifices is the additional pressure loss brought into the system. So the obtainable reduction in pressure pulsation is quite limited. 

To minimize the pulsation in the plant, a reduction of pressure pulsations by orifice plates was investigated. Therefore an orifice with an inner diameter of 13,5 mm (pipe diameter 80 mm) was placed at the inlet of the reactor. This is the position where maximum flow pulsation arises (open ended pipe). 

Gupta BB, Blanpain P, and Jaffrin MY, Permeate flux enhancement by pressure and flow pulsations in microfiltration with mineral membranes, J. Membr. Sci. 1992 70 257-266. 

Flow rate. The flow rate from the pump should be accurate, regardless of the system in which the pump is used. This means that the flow rate actually produced by the pump is the same as that dialled up on the front panel, and that this should not be affected by the rest of the HPLC system (for constant flow pumps, at least). The flow rate should be reproducible and practically free of pulsations. Pulsatile flow can limit the sensitivity of HPLC assays, resulting in a rhythmic variation in the apparent refractive index of the mobile phase flowing through the detector, which ultimately manifests itself in the chromatogram as baseline noise. Various means are employed to attempt to eliminate flow pulsation (see section 5.3). 

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