Pulsation damping systems

As seen above, the flow of sinusoidal drive reciprocating pumps is greatly pulsed. Even when a triplex pump is used, a residual pulsation remains (Fig. 9b). Most detectors in general use, especially differential refractoineters, are flow sensitive, the pulses increasing the baseline noise, resulting in a poorer detection limit. Under certain conditions, the pulses can even affect the profile of the eluted peaks and thus the resolution, as the HETP is not a linear function of the actual flow-rate. For these reasons, pulse damping is often necessary. 

The conditions required for a good damping device were indicated about 10 years 

Flow-through devices are generally of the RC type they contain an expandable 

Derivation devices can also be Bourdon tubes or capillary tubes closed at one end. When the fluid is pressurized the tube tends to uncoil. Spring-loaded bellows give good pulse damping but can be used only in a narrow pressure range. 

A gas capacitor (hydrodynamic accumulator) can be used on a wider pressure range and at higher pressures. 

---

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. 

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. 

Pulsation seldom poses a problem in FI separation and/or preconcentration systems in which packed columns or long reactors are arranged on-line, or where gas-liquid separators are used. On-line separators often function as an effective pulse-damping system, so that pulsations are noticeable only when pump rotation speeds are extremely low, a condition which in any case should be avoided in FI operations. 

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. 

Figure 2b. Schematic diagram of TIRF flow apparatus. The surge system damps out the pulsations of the peristaltic pump. Helium is used to purge the system of fluorescence-quenching oxygen. The 4-way valve is placed close to the flow-cell to reduce priming volumes to 0.1 ml. The flow-cell/prism assembly is mounted vertically so that fluid enters from the bottom and exists at the top of the chamber. (Reproduced with permission from Ref. 17. Copyright 1983 Academic Press.)...

For brevity, this naming scheme follows the usual distinction between the emitter of a wave, the oscillator, and what is emitted, the wave. The system natural pulsation features the oscillator, whereas the wave pulsation is one of the variables of a wave. Without entering in too many details, it will be just said that the latter is a state variable, more precisely an effort of an energy variety present in waves called phase or wave energy. It becomes equal to the system natural pulsation (Oq only in peculiar cases of matching between emitter and emitted signal, as in the conservative harmonic oscillator treated here, but in a damped oscillator (see case study H7 in Chapter 11) or in case of forced oscillations for instance, the two variables will not coincide. 

It is only when the oscillator is conservative that the pulsation of the associated wave corresponds exactly to the natural pulsation of the oscillator. In the present case of a damped oscillator, the wave pulsation is lower than the natural pulsation, with the rule that the higher the damping factor the lower the wave pulsation. With an upper limit that is the natural pulsation, because over this threshold the diversion of energy into the conductance does not leave enough energy for maintaining the oscillations, which are disappearing. In this case, the system undertakes a simple relaxation. 

The general definitions are given in terms of operators, but to simplify the discussion, linear operators are considered from now and the two damping factors are assumed to be lower than their respective thresholds (natnral pulsation and space vector) for allowing oscillations to exist. As the modeling of this system proceeds according to the previously detailed models, only the final steps are developed here. 

The flexible shaft will act as a heavily damped spring in the system to absorb the shock of sudden machinery stops and starts. It will also tend to smooth out vibration in the drive system. This heavy internal damping can cause a temperature rise in the shaft if it is used with a continuously pulsating driver, such as a single-cylinder engine. To reduce the heating, the shaft should be oversized in applications like this. 

Assuming that solid flow fluctuations can be damped in an expansion section in the same manner as gas pulsation in compressor systems, determine the length of 0.0508-m-ID pipe needed to damp 25% of the incoming solid flow fluctuations. The solids are coal particles with a density of 1280 kg/m and an average diameter of 150 jum, and they are flowing in a 0.0254-m-ID pipe in a gas stream with a density 1.2 kg/m and velocity of 15.25 m/sec. The solid flow fluctuation is 10 cps and the particle velocity can be found by use of Hinkle s correlation. 

---