Pulsation Damping

Figure 5 gives the flow scheme of a feed oil system. A high pressure plunger pump feeds the reactor with oil at a pressure of about 75 bar. The mass-flow of feed oil is measured by three Coriolis-Flowmeters. The second pump is out of line. The feed oil flow is adjusted by pump speed which is variable in a range of 25 to 110 rpm. Two resonators are used for pulsation damping. 

In this expression, r and ro are, respectively, the instantaneous and equilibrium (i.e., when no sound field is acting on the liquid) values of the bubble radius and f and r represent, respectively, the first and second order time derivatives of the instantaneous bubble radius p is the liquid density y is the polytropic exponent of the gas inside the bubble (i.e., the ratio of heat capacities, Cp/Cv) Pa is the acoustic pressure amplitude Poo is the hydrostatic (ambient) pressure b is the bubble pulsation damping term that accounts for thermal, viscous, and radiation effects cr is the liquid surface tension t is time and coj. is the resonance frequency of the bubble, which is defined by the equation below ... 

The effect of pulsating flow on pitot-tube accuracy is treated by Ower et al., op. cit., pp. 310-312. For sinusoidal velocity fluctuations, the ratio of indicated velocity to actual mean velocity is given by the factor /l + AV2, where X is the velocity excursion as a fraction of the mean velocity. Thus, the indicated velocity would be about 6 percent high for velocity fluctuations of 50 percent, and pulsations greater than 20 percent should be damped to avoid errors greater than 1 percent. Tne error increases as the frequency of flow oscillations approaches the natural frequency of the pitot tube and the density of the measuring fluid approaches the density of the process fluid [see Horlock and Daneshyar, y. Mech. Eng. Sci, 15, 144-152 (1973)]. 

The large variety of displacement-type flmd-transport devices makes it difficult to list characteristics common to each. However, for most types it is correct to state that (1) they are adaptable to high-pressure operation, (2) the flow rate through the pump is variable (auxiliary damping systems may be employed to reduce the magnitude of pressure pulsation and flow variation), (3) mechanical considerations limit maximum throughputs, and (4) the devices are capable of efficient performance at extremely low-volume throughput rates. 

Stationary engines (historic), damp pulsations Spoked hub, steel rim Massive, low-speed rotors, belt or shaft mechanical connection to application... 

Since discrete quantities are trapped and transferred, the delivery pressure and flow varies, as shown in Figure 32.21, which also illustrates how increasing the number of cylinders in a reciprocating pump reduces fluctuations. In the case of lobe and gear pumps the fluctuations are minimized by speed of rotation and increasing tooth number, but where, for control or process reasons, the ripple in pressure is still excessive, means of damping pulsations has to be fitted. Often a damper to cope with this and pressure pulses due to valve closure is fitted, and two types are shown in Figure 32.22. 

This design is sometimes a twin-piston design, in which a second piston is 180° out of phase with the first. This means that when one piston is in its forward stroke, the other is in its backward stroke. The result is a flow that is free of pulsations. With the single-piston design, a pulse-damping device positioned in the flow path following the pump is used. 

According to Baird and Cheema [Can. J. Chem. Eng., 47, 226—232 (1969)], the presence of square-wave pulsations can cause a rotameter to overread by as much as 100 percent. The higher the pulsation frequency, the less the float oscillation, although the error can still be appreciable even when the frequency is high enough so that the float is virtually stationary. Use of a damping chamber between the pulsation source and the rotameter will reduce the error. 

Pump pulsation may significantly impact on mixing quality and in this way the whole reaction performance [108]. In order to minimize this detrimental effect, commercially available syringe pumps were modified in order to suppress temporary pulsations. By this measure, pulsation spikes could be damped after installation of an additional switching valve in combination with a microcontroller for the individual control of both pistons. 

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