Unstable operation

Several parameters come into the relation between density and equivalence ratio. Generally, the variations act in the following sense a too-dense motor fuel results in too lean a mixture causing a potential unstable operation a motor fuel that is too light causes a rich mixture that generates greater pollution from unburned material. These problems are usually minimized by the widespread use of closed loop fuel-air ratio control systems installed on new vehicles with catalytic converters. 

Either of the two stable operating conditions can be selected by adjusting the positions of the curves so that only one intersection is obtained. In a plant, long-time unstable operation is unlikely because of imprecise temperature control. 

The performance of a centrifugal compressor (discharge to suction-pressure ratio vs. the flow rate) for different speeds is shown in Fig. 11-87. Lines of constant efficiencies show the maximum efficiency. Unstable operation sequence, called surging, occurs when compressors fails to operate in the range left of the surge envelope. It is characterized by noise and wide fluctuations of load on the compressor and the motor. The period of the cycle is usually 2 to 5 s, depending upon the size of the installation. 

Compressor surge is a phenomenon of considerable interest, yet it is not fully understood. It is a form of unstable operation and should be avoided in both design and operation. Surge has been traditionally defined as the lower limit of stable operation in a compressor and involves the reversal of flow. This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually a part of the compressor is the cause of the aerodynamic instability, although it is possible that the system arrangement could be capable of augmenting this instability. Figure 6-36 shows a typical... 

Surge-deteetion deviees may be divided into two groups (1) statie deviees, and (2) dynamie deviees. To date, statie surge-deteetion deviees have been widely used more researeh work needs to be done before dynamie deteetion deviees are generally used. A dynamie deviee will probably meet the requirements and hopes of many engineers for a eontrol deviee that ean antieipate stall and surge, and prevent it. Obviously, deteetion deviees must be linked to a eontrol deviee that ean prevent the unstable operation of a eompressor. 

Reboiler return line sized too small. Poor separation limited capacity. Unstable operation. Expansion design error. 

Holes spaced closer than twice the hole diameter lead to unstable operation. The recommended spacing is 2.5 do to 5 do with 3.8 do being preferable [42]. Holes are usually placed on 60° equilateral triangular pitch with the liquid flowing norrrially to the rows. Holes should not be greater than 2.5-3 in. apart for effective tray action. 

There exists an optimum jacket temperature for maximizing conversion at a given average molecular weight product. The study further suggests that an unstable operating region exists where wide conversion fluctuations result from attempts to increase the reactor conversion by minor adjustments in initiator amount or jacket temperature. 

Light emission from a multiple inlet combustor under (a) stable and (b) unstable operation. (From Poinsot, T.J., Trouve, A.C., Veynante, D.P., Candel, S.M., and Esposito, E.J., /. Fluid Mech., 177,265,1987. With permission.)... 

OS 39] ]R 25] ]P 27] A 24 h run of a pilot-scale micro reactor for azo pigment production was performed using a diazo suspension [55], At the end of this period, the pressure loss of the micro reactor increased exponentially. Special means were developed to prevent clogging and unstable operation. By partial removal of the deposits, the pressure loss was brought back to normal. 

Regions of stable and unstable operation determined by numerical simulation of mass and heat balances equations first- and second-order, autocatalytic, and product-inhibited kinetics graphically presented boundaries in co-ordinates in practice. safe operation if l/5e>2. Equality of heat generation and heat removal rates Semenov approach modified for first-order kinetics. 

While this method is very simple it can be quite time consuming in terms of number of trials required and especially when the process dynamics are slow. In addition, it may be hazardous to experimentally force the system into unstable operation. 

The PULSAR units are high efficiency static aerators that have been developed for municipal wastewater treatment plants and have successfully been used over extended periods of time without any operational problems such as unstable operation or plugging up during intermittent operation of the air pumps (Chourda-kis, 1999). Data have been collected from a pilot plant unit at the Wastewater Treatment plant of the Industrial Park (Herakleion, Crete). A series of experiments were conducted for the determination of the mass transfer coefficient (kLa) and are shown in Figure 17.4. The data are also available in tabular form as part of the parameter estimation input files provided with the enclosed CD. 

With this arrangement, the output of one controller is used to adjust the set point of another. Cascade control can give smoother control in situations where direct control of the variable would lead to unstable operation. The slave controller can be used to compensate for any short-term variations in, say, a service stream flow, which would upset the controlled variable the primary (master) controller controlling long-term variations. Typical examples are shown in Figure 5.22c (see p. 235) and 5.23 (see p. 235). 

It is suspected that this nonlinear rate form, which has a maximum value, may cause certain regions of unstable operation with multiple steady states. How should the operation be conducted to ensure unique steady conditions ... 

Impure Feeds - Impure feeds cause Claus and Stretford plant problems (3,16). The most troublesome impurities are NH3, HCN, and hydrocarbons. These can cause catalyst fouling, plugging, chemical losses, and unstable operations. Remedies include catalytic conversion of impurities, use of special burners (16), and the use of different AGR processes to reduce the amounts of these compounds in acid gases. 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

Supervision and diagnosis. For example, when it is necessary to supervise a microorganism concentration in a biofilm to avoid unstable operating conditions. 

Table 21.1 Average flow conditions during naturally unstable operations ...

Of equal importance and directly related to size consist is the proper selection of velocities for transport. The velocity cannot be excessive so as to cause ahrasion of pipe wall and inordinately high pressure drops. Conversely, the velocity should not be so low as to cause heterogeneous flow, with resultant excessive wear in the pipe bollotn or bed formation which will cause unstable operation. 

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