Flash Steam Systems

a variable-speed pump transfers the hot water from the production well into the steam separator. The speed of the pump is set by the tank level. (Variable-speed pump station controls are discussed in Section 2.17). The level control signal is corrected for steam pressure variations by multiplying the two. This is called a two-element feedwater system. 

The high-pressure steam from the separator is sent to the steam turbine under speed control, and the generated electricity is sent to the grid or to other users. The low-pressure steam is condensed by cooling it with cooling water. The flow of the cooling water is modulated by the turbine exhaust pressure controller. 

Geothermal flash steam power plant controls. 

Post- Oil Energy Technology After the Age of Fossil Fuels 

For additional details on turbine controls and optimization, refer to Section 2.19. 

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Vacuum Treatment. Milk can be exposed to a vacuum to remove low boiling substances, eg, onions, garlic, and some silage, which may impart off-flavors to the milk, particularly the fat portion. A three-stage vacuum unit, known as a vacreator, produces pressures of 17, 51—68, and 88—95 kPa (127, 381—508, and 660—711 mm Hg). A continuous vacuum unit in the HTST system may consist of one or two chambers and be heated by Hve steam, with an equivalent release of water by evaporation, or flash steam to carry off the volatiles. If Hve steam is used, it must be cuUnary steam which is produced by heating potable water with an indirect heat exchanger. Dry saturated steam is desired for food processing operations. 

The flashed steam method is less efficient and its requirements for steam properties—cleanliness, high temperature, and high pressure— are usually unavailable in most geothermal fields. The situation is different with the binary cycle system, which is quite efficient and widely used. This wet system involves the transfer of heat from the hot well stream into a more manageable boiling fluid to generate power through a turboexpander. 

Where Water Hammer Occurs. Water hammer can occur in any water supply line, hot or cold. Its effects can be even more pronounced in heterogeneous or biphase systems. Biphase systems carry water in two states, as a liquid and as a gas. Such a condition exists in a steam system where condensate coexists with live or flash steam in heat exchangers, tracer lines, steam mains, condensate return lines and, in some cases, pump discharge lines. 

Often in plant operations condensate at high pressures are let down to lower pressures. In such situations some low-pressure flash steam is produced, and the low-pressure condensate is either sent to a power plant or is cascaded to a lower pressure level. The following analysis solves the mass and heat balances that describe such a system, and can be used as an approximate calculation procedure. Refer to Figure 2 for a simplified view of the system and the basis for developing the mass and energy balances. We consider the condensate to be at pressure Pj and temperature tj, from whence it is let down to pressure 2. The saturation temperature at pressure Pj is tj. The vapor flow is defined as V Ibs/hr, and the condensate quality is defined as L Ibs/hr. The mass balance derived from Figure 2 is ... 

A 450 psig steam system discharges 9,425 lbs,/hr of condensate through traps into a return condensate line. The return header is to discharge into a flash tank held at 90... 

Because flashing steam-condensate lines represent two-phase flow, with the quantity of liquid phase depending on die system conditions, these can be designed following the previously described two-phase flow methods. An alternate by Ruskin [28] uses the concept but assumes a single homogeneous phase of fine liquid droplets dispersed in the flashed vapor. Pressure drop was calculated by the Darcy equation ... 

In an efficient and economical steam system, this so-called flash steam will be utilized on any load that can make use of low-pressure steam. Sometimes it can be simply piped into a low-pressure distribution main for general use. The ideal is to have a greater demand for low-pressure steam at all times than the available supply of flash steam. Only as a last resort should flash steam be vented to atmosphere and lost. 

Where high temperatures are required (e.g. for process work) and lower temperatures for space heating, it is desirable to use flash steam recovery from the high-temperature condensate to feed into the low-temperature system, augmented as required by reduced pressure live steam. 

Typically, a FW tank temperature of 180 to 190 °F (82-88 °C) is preferred and can be achieved by the direct application of live steam (from a LP steam supply or from a flash steam recovery system) through a perforated sparge pipe or by indirect heating via steam, gases of combustion, or electrical-resistance heaters. Each temperature rise of 10 °F (5.6 °C) results in at least a 0.3 to 0.4% fuel saving. Tanks should, of course, be properly designed and lagged. 

Further savings can be made by the use of a suitable blowdown (BD) water flash steam and heat recovery (FSHR) system. Such systems are standard equipment on large power boilers but are less common on smaller plant. However, the FS component (and often the HR component as well) can usually be justified for smaller systems because the capital cost payback of such equipment generally is less than 12 months and such systems continue to save fuel and high-quality water year after year. 

In most larger systems, the recovered flash steam is used in a deaerator, while the HR heat exchangers serve as FW preheaters for both HP and LP boiler plants. 

Automatic programmable systems may be designed to incorporate rapid action valves to provide flash steam to condensate tanks and/or HR for various industrial applications. 

Blowdown Water and Flash Steam and Heat Recovery Systems... 

Flash steam and heat recovery systems are almost never fitted into hydronic heating and LP steam boilers because of the very limited discharge of boiler water as blowdown. A schematic drawing of an FSHR system is shown in Figure 3.2. 

Flash steam and heat recovery systems provide a useful and simple means of recovering both heat energy and steam condensate that would otherwise be lost by BD. The capital cost for an average sized industrial boiler house is relatively low, and capital payback periods are usually within 12 to 24 months. 

Figure 3.2 Flash steam/heat recovery (FSHR) system.

Flash steam and heat recovery systems perform more efficiently if a continuous source of blowdown is provided. Depending on boiler pressure, the potential BW blowdown recovery is up to 25% of the blowdown volume recovered as flash steam and up to 75% of the heat content recovered. The flash steam can be passed to a LP steam line or sent back to a deaerator or feed tank, where it provides both FW heating and a replacement for MU water. 

In many ordinary industrial plants, the arrangement of the various boiler plant systems is not conducive to the optimum use of any flash steam generated. Recommendations to consider include ... 

Table 3.1 Flash steam recovery from a FSHR system...

Flash steam often may be usefully recovered by fitting a flash vessel either in a common condensate return system or after the steam traps, where large volumes of HP steam are used. Flash steam recovered in this way can be discharged to an LP steam delivery system. 

An alternative way to use flash steam is to provide several, small self-contained FSHR systems, keeping the system pipework to a minimum and lagging the pipes to avoid unnecessary heat losses. Flash steam forms at the point where the pressure drops, which is at the BD valve or the valve seat of the steam trap. From this point the flash steam and condensate travel together until the flash vessel is reached. The vessel then acts as a steam-water separator. 

The heat exchanger component of a boiler blowdown, flash steam, and heat recovery system (FSHR). 

Gestra (UK) and Highveld Instrumentation (South Africa). Boiler Blowdown and Flash Steam Recovery Systems. Technical and promotional literature. Gestra Aktiengesellschaft, Germany, 1990. 

FSHR, see Flash steam and heat recovery systems FT boiler designs, development of 30... 

Flash Steam Generation The most common type of hydroelectric power generation technique. Flash steam describes a system where a high temperature geothermal steam source can be used to directly drive a turbine. Also see Binary Cycle Generation. ... 

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