Process instability

Compressor Configuration To propeny design a centrifugal compressor, one must know the operating conditions—the type of gas, its pressure, temperature, and molecular weight. One must also know the corrosive properties of the gas so that proper metallurgical selection can be made. Gas fluctuations due to process instabilities must be pinpointed so that the compressor can operate without surging. 

The cantilever design is more susceptible to process instability than centerline centrifugal compressors. Figure 36.1 illustrates a typical cantilever design. 

Analysis parameters should be established to monitor the key indices of the compressor s dynamics and failure modes. These indices should include bearings, gear mesh, rotor passing frequencies, and mnning speed. However, because of its sensitivity to process instability and the normal tendency to thmst, the most critical monitoring parameter is axial movement of the male and female rotors. 

Screw compressors use both Babbitt and rolling-element bearings. Because of the thrust created by process instability and the normal dynamics of the two rotors, all screw compressors use heavy-duty thmst bearings. In most cases, they are located on the outboard end of the two rotors, but some designs place them on the inboard end. The actual location of the thmst bearings must be known and used as a primary measurement-point location. 

The gear set should be monitored for axial thrusting. Because of the compressor s sensitivity to process instability, the gears are subjected to extreme variations in induced axial loading. Coupled with the helical gear s normal tendency to thrust, the change in axial vibration is an early indicator of incipient problems. 

The sections to follow describe the most common machine-train failure modes critical speeds, imbalance, mechanical looseness, misalignment, modulations, process instability, and resonance. 

Normally associated with bladed or vaned machinery such as fans and pumps, process instability creates an unbalanced condition within the machine. In most cases, it excites the fundamental (lx) and bladepass/vanepass frequency components. Unlike true mechanical imbalance. 

In most cases, this failure mode also excites the third (3x) harmonic frequency and creates strong axial vibration. Depending on the severity of the instability and the design of the machine, process instability also can create a variety of shaft-mode shapes. In turn, this excites the lx, 2x, and 3x radial vibration components. 

Polymer Processing Instabilities Control and Understanding, edited by Savvas Hatzikiriakos and Kalman B. Migler... 

The design of a die for an appiication is a very complex process. The design must be such that regions with long residence times do not exist, and it must provide uniform flow at the exit. Moreover, coextrusion dies must be designed with iayer uniformity and without process instabilities known as the wave instability and the zig-zag instability [i2, i3]. These instabilities are beyond the scope of this book and thus are not discussed further. Like transfer lines, the velocities in a die shouid be at least i.5 cm/s for thermally stable resins (with antioxidant systems) and at least 3 cm/s for resins that are not thermally stable. 

Screw and barrel wear can reduce the performance of the extruder by causing the specihc rate to decrease and the discharge temperature to increase. Screw wear is discussed in Sections 13.2 and 13.4.1. Although extremely rare, wear in the feed casing can lead to a rate reduction and process instabilities. This case study is presented in Section 12.7.7. 

Grulke, E. 1993. Introduction to Polymer Processing. Prentice-Hall, Englewood Cliffs, NH. Hatziliriakos, S.G. and Migler, K.B. 2005. Polymer Processing Instabilities. Taylor Francis, Boca Raton, EL. 

Nevertheless, the pattern was unresponsive to our standard tests for process instability, and individual batch results were well within established control limits for this product (180 to 220 mg). The grand mean of 99.0% is 2.0 mg below the theoretical tablet potency, probably because of below-target purity of... 

Fig. 13.21 Schematic representation of the surface defects that appear periodically and alternate in location on opposite sides of a flat mold. [Reprinted by permission from A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, Tiger Stripes Instabilities in Injection Molding, in Polymer Processing Instabilities, S. G. Hatzikiriakos, and K. B. Migler, Eds., Marcel Dekker, New York, 2005.]...

The large variations in viscosity during reactive extrusion can lead to process instabilities as a result of competition between die effects and pressure build-up abilities of the screws. In general, two stable working points can be distinguished - one at low conversion with a short, fully filled length, and one at high conversion with a... 

To control access to the cabinet, a solenoid lock and thumbwheel can be installed on the door of the field cabinet and wired to input/output points on the controller. The access code is specified by the system programmer and should be a pair of combinations that will be easy for the maintenance workers to remember. Upon entry of the correct code or a process instability, the solenoid will be de-energized to permit entry. This arrangement makes it unnecessary for production personnel to know the access code, and the unlock algorithm detailed in the figure uses a 10-s timer to frustrate clever operators. 

Effect of Oxyethylene Content on Stability. The processing instability of TPU elastomers based on PPG 2000 can be overcome by the use of ethylene oxide "tipped" PPG polyols, especially those containing high levels of oxyethylene groups, e.g., 30% to 45% by weight. The improved stability of the polymers based on such polyols is demonstrated in Table 2. 

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