Load Variation

The results of over 1 year of continuous, on-line acoustic emission (AE) structural surveillance of high temperature / high pressure steam headers, gained on 2 M-scale 600MW supercritical multifuel ENEL power units in normal operation, are presented in the paper. The influence of background noise, the correlation between plant operating conditions (steady load, load variations, startup / shutdown transients) and AE activity and the diagnostic evaluation of recorded AE events are also discussed. 

Different plant operating conditions (steady load, load variations, startups / shutdowns) have been encountered during the monitoring period. Electrical load, steam pressure and steam temperature values vs time have been acquired and stored during the entire period. At the same time, the RMS values of the acoustical background noise were have been continuously checked and stored, thus providing a quick check of proper instrumentation condition and a correlation between variations of plant parameters and the acoustical behaviour of the components. 

Localized AE sources appear during load variations, startups or shutdowns, but their positions are uniformly spread over the length of the two bodies of the header this can be seen from the histogram of the localized AE events for the front body (fig.S) and for the rear body (fig.9). 

During electrical load variations the background noise is low and constant along the SH headers, it increases with load variations. AE sources appear during load variations, but their position are again uniformly scattered over the length of the headers. 

An anaerobic digester is a no-recycle complete mix reactor. Thus, its performance is independent of organic loading but is controlled by hydraulic retention time (HRT). Based on kinetic theoiy and values of the pseudo constants for methane bac teria, a minimum HRT of 3 to 4 days is required. To provide a safety factor and compensate for load variation as indicated earlier, HRT is kept in the range 10 to 30 days. Thickening of feed sludge is used to reduce the tank volume required... 

Thrust Bearings Turboexpanders often have process upsets or ice plugging or the hke, which can cause serious thrust-bearing load variations. In applications above 506.6 to 1013.2 kPa (75 -148 psi), the best available thrust bearing usually is insufficient to protect against such high thrust loads. Various indicatious, such as the differential... 

The governor can be set to make the machine run at a constant speed, even on load variations, with extremely quick and almost instantaneous speed control, and thus maintain a near-zero AN. In a parallel operation they can also control load sharing automatically and accurately. Power grids, receiving power from different sources, are extremely susceptible to frequency variations. Even a small Af of the order of 0.5 Hz, may cause the system to trip. A fast-actuating governor with low response time (as low as 0.5 second) can overcome such a situation by quickly regulating the speed of the PM. 

The higher the droop, the higher will be the load variation. When there is a difference in the full-load speeds the load sharing during a parallel operation will not be equal and the generators will operate underutilized. 

Enhancing the stability of the system by ininimiz.ing the voltage tluctuations caused by load variations and... 

LT installations may be subject to frequent load variations and inductive switchings. It will be more desirable in such cases to provide them with an automatic p.f. correction scheme than manual correction. It may be... 

In rapidly changing loads it must be ensured that enough discharged capacitors are available in tbe circuit on every close command. To achieve this, sometimes it may be necessary to provide special discharge devices (Section 25.7) across the capacitor terminals or a few extra capacitor units to keep them ready for the next switching. It may require a system study on the pattern of load variations... 

Oscillating torque in the rotating machines as a result of load variations and harmonics present in the systetn. 

Likely expected load variations and p.f. of the load (which may be based on experience). 

Curve I without any compensation, the voltage profile sags on small load variations and is not capable of transferring even a natural load. 

A line can be theoretically loaded up to these levels. But at these levels, during a load variation, the far-end voltage may swing far beyond the desirable limits of 5% and the system may not remain stable. With the use of reactive control it is possible to transfer power at the optimum level (Pnias) hd yet maintain the far-end (or midpoint in symmetrical lines) voltage near to and also to have a near-flat voltage profile. 

For 10% load variation, to be on the side side, the capacitors must be rated for ... 

Since the load variation on an HT distribution network will he only nominal, anil the network will also have enough resistance, it should be possible to compensate the. system up to 70% or so. to turther improve the regulation of the network, say up to 5% of/ , without jeopardizing the level of stability. The application engineer can take a more judicious decision, knowing the condition of the network to be cotnpensated. 

The factors discussed in Section 23.5.2 give rise directly to the current drawn by the capacitor unit and indirectly add to its rating. The relevant Standards on this device recommend a continuous overload capacity of 30% to account for all such factors. A capacitor can have a tolerance of up to -t-15% in its capacitance value (Section 26.3.1(1)). All current-carrying components such as breakers, contactors, switches, fuses, cables and busbar systems associated with a capacitor unit or its banks, must therefore be rated for at least 1.3 x 1.15/,., i.e. 1.54. For circuits where higher amplitudes of harmonics are envisaged, for reasons of frequent load variations or more... 

For many years in meehanieal design, load variations have been masked by using faetors of safety in a deterministie approaeh, as shown below (Ullman, 1992) ... 

ZVS QR topologies appear to be the more popular of two methods of quasi-resonant technologies. This is mainly due to two reasons first, its typical variation in frequency over its input and load variations is 4 f as opposed to fO f for the ZCS topologies secondly, it has a better heavy load performance. Also, some of the more troublesome parasitic elements within the circuit can be more easily harnessed. 

As mentioned earlier, it is not feasible to generate test data for all possible combinations of load variations. However, there have been a number of experimental investigations of the problem and these have resulted in some very... 

The synchronous motor is a constant-speed machine. Unlike the induction motor which inherendy has slip from losses, the synchronous motor uses an excitation system to continually keep the rotor in synchronous speed with current flowing through the stator. Within its designed torque characteristics, it will operate at synchronous speed regardless of load variations. 

The calihration curve (three-phase induction motor) of Figure 14-5 is essential to determine the load change based on the current change for motors helow 200 hp or with load variations of more than 5-10%. 

Operating procedures should establish boundaries that limit the maximum load variations that can be used in normal operation. These limits should be well within the acceptable load rating of the gear set. 

Pressure reducing valves provide a steady pressure into a part of the system that operates at a pressure lower that normal system pressure. A reducing valve can normally be set for any desired downstream pressure within its design limits. Once the valve is set, the reduced pressure will be maintained regardless of changes in the supply pressure and system load variations. 

In actual practice, the narrowband filters should be somewhat greater than those in the example. Since constant-speed machines tend to have a slight variation in speed due to load variations, the narrowbands should be adjusted to compensate for these variations. In the example given previously, the limit of the lower filter should be decreased by 10 per cent and the upper limit rose by 5 per cent to compensate for speed variation. 

This change in vibration energy can be clearly observed in trend data acquired from machine-trains. A sawtooth trend is common to most predictive-maintenance programs, which can be directly attributed to variations in load. The only way to compensate for load variations is to track the actual load associated with each data set. 

It must he particularly noted that details should he entered at this time of amhient and load conditions, and any other factors which have an interface outside the plant itself. In this way, the relationship between amhient, load and plant can he estahlished as a guide to future seasonal and load variations. It is helpful to the user if some indication can he given of operating conditions under light load, since the plant may well work most of its life at less than full load and the operators might not he ahle to interpret the readings taken. 

Air/water vapor mixture, chart, 364,365 Air/water vapor, 359 Capacity at ejector suction, 369 Capacity for process vapor, 362 Evacuation time, 371, 380 Load for steam surface condenser, 367 Non-condensables, 362, 363 Size selection, 371 Steam pressure factor, 373 Steam requirements, 372 Steain/air mixture temperature, 361 Total weight saturated mixture, 362 Capacity, 358 Discharge, pressure, 358 Effect of excess steam pressure, 358 Effects of back pressure, 359 Effects of wet steam, 356 Inter-and-after condenser, 351 Load variation, 370 Materials of construction, 347 Molecular weight entrainment, chart, 360 Performance, 358, 370, 375 Relative comparison, 357... 

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