Steam pressure, reduction

A design of turbine in which a partial reduction in steam pressure takes place in fixed nozzles (vanes) and a further steam pressure reduction takes place in nozzles created by moving rotor blades. A reactive force is generated that results in the rotation of the turbine shaft. 

Useful work from steam pressure reduction... 

Cool inlet air to gas turbines (Green and Perry, 2008) Use steam turbines as drivers for pumps/compressors and also for steam pressure reduction Use gas turbine exhaust gases to supply waste heat to the fired heaters (Branan, 2012)... 

For Steam Pressure Reduction Valve Letdown valves (PRV) are used to supplement steam demands at lower pressures for operational flexibility. Modeling of PRV is based on the valve characteristics, which are expressed in the general form... 

Figure 6.25a shows the same grand composite curve with two levels of saturated steam used as a hot utility. The steam system in Fig. 6.25a shows the low-pressure steam being desuperheated by injection of boiler feedwater after pressure reduction to maintain saturated conditions. Figure 6.256 shows again the same grand composite curve but with hot oil used as a hot utility. 

Among the measures which have successfully prevented metal dusting are the use of additives (steam, and compounds of S, As, Sb, and P) in the feed, reduction of pressure, reduction of temperature, and material change. The most common additives are sulfur compounds and steam. Susceptibility can be reduced by using a material in which the total percent of Cr plus two times the percent of Si is in excess of 22 percent. In some environments, a. small amount of a sulfur compound will stop the dusting. When sulfur compounds cannot be tolerated in the process stream, a combination of steam and an alloy with a Cr equivalent of over 22 percent may be most desirable. 

With steam generated at or close to the boiler design pressure it is inevitable that some of the steam-using equipment will have to be supplied at a lower pressure. In some cases the plant items themselves have only been designed to withstand a relatively low pressure. Sometimes a reaction will only proceed when the steam is at a temperature below a certain level or an unwanted reaction will occur above a certain level. For these and similar reasons, steam often is distributed at a relatively high pressure which must then be lowered, close to the point of use. Pressure-reduction stations incorporating pressure-reducing valves are fitted to perform this function. 

With such a reduction in steam flow the steam pressure within the exchanger will fall. A balance is reached when the steam pressure is such that the steam temperature gives an appropriately lowered temperature difference between the steam and the mean temperature of the heated fluid. Under no-load conditions, if the flow of the fluid being heated were to cease or if the fluid were already at the required temperature as it entered the exchanger then this temperature difference would have to be zero and the steam at the same temperature as the fluid leaving the exchanger. 

Sudden reductions in steam pressure. Soot blowing or sudden, very high process steam demands can cause carryover. 

Erosion processes also may take place, especially where high pressure steam is discharged into low pressure CR lines. The resulting flashing and pressure reduction effects encourage steam impingement around the point of discharge. 

If the temperature of dry saturated steam is increased, then, in the absence of entrained moisture, the relative humidity or degree of saturation is reduced and the steam becomes superheated (Fig. 20.5). During sterilization this can arise in a number ofways, for example by overheating the steamjacket (see section 4.2.2), by using too dry a steam supply, by excessive pressure reduction during passage of steam from the boiler to the sterilizer chamber, and by evolution of heat of hydration when steaming... 

Fig. 5. In a double-flash plant for producing electricity from hydrothermal water, superheated water is delivered from the well, A, to an initial flashing unit, B, where the pressure is reduced to release steam which drives a turbine, D. The liquid fraction is then delivered to a second flashing unit, C, where further pressure reduction produces more steam which is introduced to the turbine at an intermediate stage. The waste fluid from the second flashing stage, E, may contain very high concentrations of dissolved or suspended solids, presenting significant disposal problems. The spent steam can be recondensed and...

When any vapor expands, due to a pressure reduction (other than H2 and C02), it cools off. This is called a Joule-Thompson expansion. The reduction in temperature of the steam is called a reduction in sensible-heat content. The sensible heat of the steam is converted to latent heat of condensation. Does this mean that the latent heat of condensation of 10-psig steam is much higher than that of 450 psig steam Let s see ... 

Power generation using steam or gas turbines is now well established, however power recovery by the pressure reduction of process fluids is more difficult and less common. In general, the equipment is not considered to be particularly reliable. Rankine cycle heat engines have been developed/adapted to use relatively low-grade waste-heat sources (particularly from organic fluids) to generate power in the form of electricity or direct drives. They tend to be used when the heat source... 

The Carnit process for production of concentrated AN solutions requires no external heat supply. The reaction of ammonia and nitric acid occurs in a recycle flow loop where the pressure is higher than the vapor pressure of the solution. The recycle solution, which is slightly ammoniacal, supplies heat for the final concentration and for production of export steam. The free ammonia in the production off-take is neutralized before pressure reduction and subsequent concentration steps227. 

In the pressure reduction from the digester to the blow tank, 14 wt % of the stream leaving the digester is lost as steam released in the blow step. 

These results are extremely informative, but it may well be that even simpler and more direct correlations can be made by relating the effective expansion process to the temperature above the melt temperature for any mix, since this correlates directly with the steam pressure and with thermal reduction of the rheology of the melt. The TJ NatsT content line should ideally be derived for material at the die exit rather than for the raw materials, since the effects of dextrinisation or particulate dispersion in the barrel would not be present in the raw materials. This can be done quite simply by measuring pretreated mixes. However, the fact that measurements of two state transitions (glassy and melting) of a formulation, one prior to extmsion and the second after it, are sufficient to predict raw material performance is remarkable, and deserves to be tested widely on other materials. 

The uniqueness of this process results from the fact that the steam provides the total heat necessary for the primary cracking stage. It is not therefore a simple vapo-cracking. This process intervenes simultaneously while suppressing to the maximum the formation of tars and coke by dilution of steam and partial reduction of the steam pressure resulting from the heavy hydrocarbons produced. Additionally, this process is very flexible and it is suitable for the treatment of highly variable feed rates of waste. 

In general, with decreasing hydrocarbon partial pressure, unsaturated components such as acetylene, ethylene, propylene, and butadiene increase whereas BTX, pyrolysis fuel oil, and saturated components such as methane, ethane, and propane decrease. Low hydrocarbon partial pressure can be attained either by high steam dilution or by low absolute pressure in the cracking coil, which is determined by furnace outlet pressure and pressure drop in the cracking coil. For each specific case there is an optimum steam dilution. Reduction of steam dilution influences yields, utilities, running times and, in the case of a new ethylene plant, of course, investment costs—but in different ways, either positive or negative. Thus, an optimization has to be carried out to identify the most economic steam dilution. 

Two or three steam ejectors (similar to laboratory water aspirators) operating in series with interstage condensers provide the pressure reduction necessary and are connected to the top end of the vacuum distilling column (Fig. 18.5). Each take-off point on the main vacuum column is also passed... 

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