Letting down

Cog enera.tion in a. Steam System. The value of energy in a process stream can always be estimated from the theoretical work potential, ie, the deterrnination of how much power can be obtained by miming an ideal cycle between the actual temperature and the rejection temperature. However, in a steam system a more tangible approach is possible, because steam at high pressure can be let down through a turbine for power. The shaft work developed by the turbine is sometimes referred to as by-product power, and the process is referred to as cogeneration. 

More details of operation in an actual study can be seen in Berty et al, (1982). In tliis work, a condenser and a liquid-gas separator were used in the product line before the pressure let-down. Keeping the products all in the vapor phase was difficult. Other improvements later included a continuous, four-component, feedgas make-up system with a compressor. 

If a liquid near its boiling point at one pressure is let down to a redueed pressure, vapour flashing... 

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 ... 

It is hoped that this volume will provide some guidance to operators, plant safety engineers and designers, and that some of the design practices are adopted particularly in older plant operations that do not apply proper pressure let down procedures and flaring practices. Many unsafe operations and even catastrophic failures have been observed by the author in overseas assignments to such countries as Ukraine and other Newly Independent States (NIS) in older plant designs. These incidents have prompted the preparation of this reference. 

A blockage occun ed downstream. The new let-down valve was leaking, and the downstream equipment was overpressured and burst. 

Figure 2-3. The second (lower) let-down valve was joined to the original line downstream of the check valve. When this let-down valve leaked, the downstream equipment was overpressured.

Expansion turbines are related in many design features to the centrifugal compressor. The key exception being that the turbine receives a high pressure gas for expansion and power recovery to a lower pressure and is usually accompanied by the recovery of the energy from the expansion. For example, applications can be (1) air separation plants (2) natural gas expansion and liquefaction (for gas let-down in pipeline transmission to replace throttle valves where no... 

In most units, about two-thirds of the flue gas pressure is let down via an orifice chamber or across an orifice chamber. The orifice chamber is a vessel containing a series of perforated plates designed to maintain a given backpressure upstream of the regenerator pressure control valve. 

If a liquid near its boiling point at one pressure is let down to a reduced pressure, vapour flashing will occur. This will cease when the liquid temperature is reduced, due to removal of the latent heat of vaporization, to a temperature below the saturation temperature at the new pressure. As a result ... 

If the motive power to drive the fluid through the pipe is available free, for instance when pressure is let down from one vessel to another or if there is sufficient head for gravity flow, the smallest pipe diameter that gives the required flow-rate would normally be used. 

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

Also shown in Figure 23.17 are let-down stations between the steam mains to control the mains pressures via a pressure control system. The let-down stations in Figure 23.17 also have de-superheaters. When steam is let down from a high to a low pressure under adiabatic conditions, the amount of superheat increases. Desuperheating is achieved by the injection of boiler feed-water under temperature control, which evaporates and reduces the superheat. There are two important factors determining the desirable amount of superheat in the steam mains. 

In other problems, the let-down stations might use desuperheating. Figure 23.25 illustrates a de-superheater. A material balance gives ... 

Figure 23.49 Steam turbine networks and let-down stations provide degrees of freedom for optimization. (From Varbanov PS, Doyle S and Smith R, 2004, Trans IChemE, 82A 561, reproduced by permission of the Institution of Chemical Engineers.)...

Thus, large let-down flows are usually seen to be a missed cogeneration opportunity but might provide a degree of freedom to bypass bottlenecks in the steam system. Also, flow through let-down stations allows the level of superheat in the lower-pressure mains to be controlled independently of the flow through the steam turbines. 

To develop "materials to render the induction of hypnosis easier and substances which will produce pure euphoria with n< subsequent let-down."... 

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