Steady loads

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. 

Figure 6 shows the histogram of localized AE events vs axial position for the same time period as in fig.5. The location of the AE source corresponds, within source location errors (< 10-15 cm), to one of the welds under surveillance. The weld was known by ultrasonic examination to be affected by internal discontinuities. However, the position of the source could also correspond to one of the hangers. The steps observed in EA event accumulation have taken place during steady load operation, which normally corresponds to very low background noise conditions. This type of event, however, has not been observed afterwards. 

Sticking of valve stems is common if solids are present in the steam. The steam mast be without sohds. (Note comments later under loss of power.) It is important that units operating on a steady load for long periods be checked for sticking stems at regular intervals. The records show that in several cases deposits have caused the stem of both the governor valve and the trip valve to stick when there was a loss of load. The effect of the loss of load was destructive overspeed. 

Isometric data from the creep curves may also be superimposed on the creep rupture data in order to give an indication of the magnitudes of the strains involved. Most plastics behave in a ductile manner under the action of a steady load. The most notable exceptions are polystyrene, injection moulding grade acrylic and glass-filled nylon. However, even those materials which are ductile at short times tend to become embrittled at long times. This can cause... 

When a load is initially applied to a specimen, there is an instantaneous strain or elongation. Subsequent to this, there is the time-dependent part of the strain (creep), which results from the continuation of the constant stress at a constant temperature. In terms of design, creep means changing dimensions and deterioration of product strength when the product is subjected to a steady load over a prolonged period of time. 

To analyse bond breakage under steady loading, we take advantage of the enormous gap in time scale between the ultrafast Brownian diffusion (r 10 — 10 s) and the time frame of laboratory experiments ( 10 s to min). This means that the slowly increasing force in laboratory experiments is essentially stationary on the scale of the ultrafast kinetics. Thus, dissociation rate merely becomes a function of the instantaneous force and the distribution of rupture times can be described in the limit of large statistics by a first-order (Markov) process with time-dependent rate constants. As force rises above the thermal force scale, i.e. rj-t> k T/x, the forward transition... 

An electric motor under steady load draws 9.7 amperes at 110 volts, deliveringO.93 kW of mechanical energy. What is the rate of heat transferfrom the motor, in kW ... 

Where part of the equipment requires large drafts of fluid (as for instance the flanging presses in a boiler shop) it is permissible to use a dual system with only a slight difference in pressures. The higher pressure fluid goes to supply the steady load and the surplus is vented to the low-pressure system. 

Most high voltage motors used in the oil industry operate at steady loads between 75% and 90% of their rated power capability. Should the motor suddenly find itself underloaded then it is possible that the driven machine has inadvertently lost its load, e.g. a pump loses liquid at its suction port. This... 

Denitrification is typically the dominant pathway of nitrate ranoval in wetlands. Thus, the nitrate reduction rates presented in Tables 8.10 and 8.11 represent denitrification rates. Rates are presented on areal basis (Table 8.10) and as first-order rate constants (Table 8.11). Nitrate reduction rates reported for constructed wetlands are in the range of 3-1,020 mg N m day In most wetlands, denitrification rates are limited not only by nitrate concentration but also by hydraulic retention time (or contact time of nitrate with anaerobic zones) and diffusion/mass flow of nitrate from aerobic zones to anaerobic sites (Martin and Reddy, 1997). Denitrification rates are usually higher in soils receiving steady loading of nitrate than in soils receiving low or negligible nitrate levels (Cooper, 1990 Gale et al., 1993). 

Use of colloidal silica in the so-called microfilled composites allows these resins to be polished, so that less wear occurs and less plaque accumulates. It is more difficult, however, to make these with a high fraction of filler. All the dental composites exhibit creep. The stiffness changes by a factor of 2.5 to 4 (depending on the particular material) over a time period from 10 sec to 3 h under steady load [Papadogianis et al, 1985]. This creep may result in indentation of the restoration, but wear seems to be a greater problem. 

For steady loads, the vessel must support these loads more or less continuously during its useful life. As a result, the stresses produced from these loads must be maintained to an acceptable level. 

Steady loads—Long-term duration, continuous. 

Tlie term static fatigue is used to refer to slow crack growth under long-term steady loading. There appears to be a threshold value of Ki below which no crack growth occurs above this limit, sub-critical crack growth can be represented by ah empirical power law relating the applied Ki and (dn/dt) ... 

Figure 15.18 shows crack speed measurements on a model joint pulled with a steady load. The crack initiated at the comer of the joint as expected. It then speeded up steadily, until it penetrated about one joint thickness along the interface. Then it travelled at constant speed in accord with Equation (15.5). From these results, the initiation force was found to be 1.39 times higher than the propagation load. This result fitted an approximate theory based on the idea that the strain energy in the shaded region of Fig. 15.18 ranained constant as the crack initiated. This gave the initiation force... 

However, when the crack approached an interface where the elastic modulus changed, in this case because stiff fibers were embedded in the rubber, the crack speed was seen to change substantially, as shown in Fig. 16.7(b). At first the crack traveled at constant speed under the steady load, as expected from Equation (16.1). Then, at the interface with the stiffer material, the crack slowed down by a factor of 100. Subsequently, as the crack penetrated the stiff material, it speeded up to regain its original constant speed after 15mm of further travel. 

In power generating practice, during steady-load operation, the scope for ingress of air is limited to leakage into those parts of the plant working at less than ambient pressure, i.e. the low pressure turbine and condenser, along with the air present in the small amount (about I of make-up water. 

The capacity of a cell to store charge is a measure of the total quantity of electricity which it can cause to be displaced around a circuit after being fully charged. It is stated in ampere-hours, abbreviation Ah, and calculated at the 10-hour rate which is the steady load current which would completely discharge the battery in 10 hours. Therefore, a 50 Ah battery will provide a steady current of 5A for 10 hours. 

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