Load transient changes

Two factors may cause over-load on a gear set excessive load or speed transients. Many processes are subjected to radical changes in the process or production loads. These changes can have a serious effect on gear-set performance and reliability. 

Fig. 5.1 Idealized representation of the transient change in fiber and matrix stress that occurs during the isothermal tensile creep and creep recovery of a fiber-reinforced ceramic (the loading and unloading transients have been exaggerated for clarity). It is assumed that the fibers have a much higher creep resistance than the matrix. The matrix stress reaches a maximum at the end of the initial loading transient. After full application of the creep load, the matrix stress relaxes and the fiber stress increases. Upon specimen unloading, elastic contraction of the composite occurs, followed by a time-dependent decrease in fiber stress and increase in matrix stress. Overall, creep tends to increase the difference in stress between constituents and recovery tends to minimize the difference in stress. After Wu and Holmes.15...

Fig. 5.2 Comparison of creep behavior and time-dependent change in fiber and matrix stress predicted using a 1-D concentric cylinder model (ROM model) (solid lines) and a 2-D finite element analysis (dashed lines). In both approaches it was assumed that a unidirectional creep specimen was instantaneously loaded parallel to the fibers to a constant creep stress. The analyses, which assumed a creep temperature of 1200°C, were conducted assuming 40 vol.% SCS-6 SiC fibers in a hot-pressed SijN4 matrix. The constituents were assumed to undergo steady-state creep only, with perfect interfacial bonding. For the FEM analysis, Poisson s ratio was 0.17 for the fibers and 0.27 for the matrix, (a) Total composite strain (axial), (b) composite creep rate, and (c) transient redistribution in axial stress in the fibers and matrix (the initial loading transient has been ignored). Although the fibers and matrix were assumed to exhibit only steady-state creep behavior, the transient redistribution in stress gives rise to the transient creep response shown in parts (a) and (b). After Wu et al 1...

Figure 15. Transient changes of difflisivities and selectivity toward para-xyXtnQ of MFI type zeolite with coke loading,...

One issue that has not been addressed is the transient process that allows A to change from one value to another when the load is changed, while still maintaining the horizontal configuration as the final steady state. Because there is zero hydrodynamic force in the horizontal direction for any value of A in the horizontal configuration, the eccentricity cannot change from one steady state to another by simply shifting the inner cylinder in... 

Another critical aspect to consider for application of a membrane reactor in the FPS is the rapid transient capability. The requirement is to deliver required hydrogen in less than 5 s when a step change in the electrical load is changed from 10... 

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

Equipment loads include dead weight, restrained thermal expansion and dynamic effect such as pressure transients, changes in momentum, water and steam hammer in the equipment and earthquake. They also may include the effect of the restraint of attached piping. The effect of such phenomena must be considered in the design check. 

To be complete, then, the control computer should be programmed to maintain the process balance in the steady state and also in transient intervals between steady states. It must consist of both steady-state and dynamic components, like the process it is, in effect, a model of the process. If the steady-state calculations are correct, the controlled variable will be at the set point as long as the load is steady, whatever its current value. If the calculations are in error, an offset will result, which may change with load. If no dynamic calculations are made, or if they are incorrect, the measurement will deviate from the set point while the load is changing, and for some time thereafter, while new energy levels are being established in the process. If both the steady-state and dynamic calculations are perfect, the process will be continually in balance, and no deviation will be measurable at any time. This is the ultimate goal. 

The initial immediate response in steam flow to a change in grid frequency is not normally essential but it cannot be avoided when governors with a rapid response are used. By shaping the transient response of the governor so that it opens exponentially the dip in the load transient of Fig. 5 can be avoided. 

Overall, the TRACE and RELAP5-3D results for the complete loss of alternator load transient are similar. RELAP results are presented in Section 12.5.2.2. Both transients produce a peak power increase of 110 to 117% as illustrated in Figure 12-36 and Figure 12-138. As expected, the change in peak fuel temperature for the RELAP UO2 core is about twice that for the TRACE UN core. Relatively small differences are also seen in the calculated Brayton overspeed and resulting loop flow rates. 

Most of the microprocessor-based instruments are designed to handle steady-state vibration data. Few have the ability to reliably capture transient events such as rapid speed or load changes. As a result, their use is limited in situations where these occur. 

In this paper we report the effect of varying loads on a small size DMFC stack (10 cells with 9 cm active-area each). The transient responses of the stack voltage have been investigated upon variable current load conditions to obtain the information on the dynamic characteristics of the stack. Also, the transient responses of the stack current upon changing fuel flow rates have been monitored to obtain the optimal operating conditions for the staek. 

This equation provides us with a model-based rule as to how the manipulated variable should be adjusted when we either change the set point or face with a change in the load variable. Eq. (10-5) is the basis of what we call dynamic feedforward control because (10-4) has to be derived from a time-domain differential equation (a transient model). 3... 

Nelson I would like to return to what David Eisner mentioned about the plasma membrane determining the steady-state free Ca2+, and what Rick Paul said about sparks and long-conductance Ca2+-dependent K+ (BK) channels. We have looked at cerebral arteries from PLB knockout mice. The spark frequency and the associated transient BK current frequency are elevated by about a factor of three. SR load goes up, the membrane potential hyperpolarizes and the artery relaxes. It would be useful to measure membrane potential under all the conditions as well as determine the voltage dependence of tone, to make sure that your manipulations are not simply changing the membrane potential. 

---