Steam flow increase

The amount of steam generated is determined by reactor power level. As power level Increases water temperature in the steam generators decreases and steam flow increases. That is, the rise in power level opens the steam valves which Increases steam flow and decreases steam pressure. 

In Figure 17-1, what happens when the steam flow increases ... 

To quantify the demand for steam in the plant, the controller relies on the steam flow and pressure. If the steam demand increases, then the steam flow increases and the pressure decreases, calling for more generation of steam. This increase in firing rate demand increases the amount of fuel burned and maintains the thermal energy of the boiler during an increase in demand for steam. 

A comparable statement can be made with regard to the power apphed to a mechanical recompression evaporator.) In summary, the steam flow required to increase the sohd content of the feed from Xq to X. is... 

Increasing the steam temperature at a given steam pressure lowers the steam output of the steam turbine slightly. This occurs because of two contradictory effects first the increase in enthalpy drop, which increases the output and second the decrease in flow, which causes a loss in steam turbine output. The second effect is more predominant, which accounts for the lower steam turbine amount. Lowering the temperature of the steam also increases the moisture content. 

A major cause of pulsing in flare systems is flow surging in the water seal drum. One of several reasons why it is important to eliminate pulsing is to reduce flare noise. Combustion flare noise has been shown to increase as the steam rate increases. Since the amount of steam required to suppress smoke in a flare is set by the flaring rate, flow surges will require a higher steam rate than for a steady flow. 

Newby et al. found that increasing the PO turbine pressure resulted in higher steam flow (for a given pinch point temperature difference in the HRSG), increased PO turbine power and overall plant efficiency. However, at the highest pressure of 100 bar attempts to increase the steam flow further resulted in incomplete combustion in the main combustor and the overall thermal efficiency did not increase substantially at this pressure level. 

For a given ejector, an increase in steam pressure over the design value will increase the steam flow through the nozzle in direct proportion to the increase in absolute... 

In some cases, the ground contours are such that a steam main can only be run uphill. This will mean that the drain points should be closer together over the uphill section (say, 15 m (50 ft) apart) and the size of the main increased so that the steam velocity is not more than about 15m/s (50ft/s). The lower steam velocity may then allow condensate to drain in the direction opposite to the steam flow. 

If the pressure drop across the valve is to be more than 42 per cent of the inlet absolute pressure the valve selection is the same as if the pressure drop were only 42 per cent. With this pressure ratio the steam flow through the valve reaches a critical limit, with the steam flowing at sonic velocity, and lowering the downstream pressure below 58 per cent of the inlet absolute pressure gives no increase in flow rate. When the heater needs a higher pressure, or when the pressure required in the heater is not known, it is safer to allow a smaller pressure drop across the control valve. If the necessary heater pressure is not known, a pressure drop across the control valve of 10-25 per cent of the absolute inlet pressure usually ensures sufficient pressure within the heater. Of course, in the case of pressure-reducing valves the downstream pressure will be specified. 

Where localized corrosion occurs, the rate of hydrogen production grows with increase in heat flux, and the dissolved hydrogen concentration rises with increase in steam flow. 

The proportional controller is reverse-acting so that the control valve throttles down to reduce steam flow as the hot water outlet temperature increases the control valve will open further to increase steam flow as the water temperature decreases. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

With the cascade control system, the steam flow controller will immediately see the increase in steam flow and will pinch back on the steam valve to return the steam flow rate to its setpoint. Thus the reboiler and the column are only slightly affected by the steam supply-pressure disturbance. 

Changes in steam flow are achieved by increasing or decreasing the area used for condensing steam in the reboiler. This variable-area flooded reboiler is used in some processes because it permits the use of lower-pressure steam. However, as you will show in your calculations (I hope), the dynamic performance of this configuration is distinctly poorer than direct manipulation of steam flow. 

There are two ways to answer this question. Let s first look at the reboiler. As the tower-top temperature shown in Fig. 4.1 goes down, more of the lighter, lower-boiling-point alcohol is refluxed down the tower. The tower-bottom temperature begins to drop, and the steam flow to the reboiler is automatically increased by the action of the temperature recorder controller (TRC). As the steam flow to the reboiler increases, so does the reboiler duty (or energy injected into the tower in the form of heat). Almost all the reboiler heat or duty is converted to vaporization. We will prove this statement mathematically later in this chapter. The increased vapor leaving the reboiler then bubbles up through the trays, and hence the flow of vapor is seen to increase, as the reflux rate is raised. 

Let us assume that both the reflux rate and the overhead propane product rate are constant. This means that the total heat flow into the tower is constant. Or, the sum of the reboiler duty, plus the feed preheater duty, is constant. If the steam flow to the feed preheater is increased, then it follows that the reboiler duty will fall. How does this increase in feed preheat affect the flow of vapor through the trays and the fractionation efficiency of the trays ... 

The steam enters through the top of the channel head of the reboiler. Any superheat in the steam is quickly lost to the tubes. Superheated steam does very little in increasing heat-transfer rates in a reboiler. Actually, when considering the temperature difference between the steam and the process fluid, it is best to use the saturated steam temperature, as the real temperature at which all the heat in the steam, is available. For example, assume the following steam flow to a reboiler ... 

To consider a third case, we wish to maintain the original 240°F shell-side temperature, but to increase the steam flow from 10,000 to 15,000 lb/h. This will force the steam inlet control valve to open. As the control valve opens, the pressure in the channel head rises from 100 psig to the full steam header pressure of 160 psig. At this pressure, steam condenses at 360°F. The new AT is then (360°F - 240°F) = 120°F. This new temperature driving force is 50 percent greater than the case one driving force of 80 percent. Hence the rate of steam condensation also increases by 50 percent, from 10,000 to 15,000 lb/h. 

It is better not to use a steam inlet control valve when using low-pressure steam. The channel head pressure will then always equal the steam header supply pressure. The flow of steam to the reboiler can then be controlled only by raising or lowering the water level in the channel head, as shown in Fig. 8.5. This sort of control scheme will work perfectly well until the water level drops to the bottom of the channel head. If the condensate drain control valve then opens further, in an attempt to increase steam flow into the reboiler, the condensate seal is blown, and the reboiler heat duty drops. 

On the surface, this story sounds crazy. But, let s see what happened. This deaerator had been designed for a much smaller flow of 160°F BFW, and a much larger flow of hot-steam condensate, than are current operations. The cold BFW feed line had been oversized, but the steam line was of marginal size. As the demand for hot BFW increased, the cold-BFW level-control valve opened. This reduced the temperature and pressure in the deaerator drum. In response, the steam pressure-control valve also opened. But when the cold-BFW level-control valve was 40 percent open, the steam pressure-control valve was 100 percent. Steam flow was now maxed out. 

The pressure inside the deaerator started to drop, as there was not enough steam flow to keep the water in the drum at its boiling point. The reduction in the deaerator pressure increased the volume of steam flow through the bottom tray of the stripping tower. Why ... 

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