Steam flow control

Efficiency is related to turbine loading and the response to changes in operation. Key factors in part-load turbine performance are the operational methods employed to control steam flow to the turbine and the actions taken to reduce boiler cycling, low-load conditions, and boiler water carryover. 

Figure 8.5 Condensate backup controls steam flow.

The intercept valves control steam flow to the low-pressure turbine cylinders. There are six of them, located in the hot reheat lines at the inlet to the low-pressure turbine cylinders. During normal operation of the turbine, automatic action progressively adjusts the intercept valve opening as turbine load changes. There is a reheat stop valve in series with each intercept valve. The reheat stop and intercept valves all close completely on a turbine trip. 

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

Feedforward control can also be applied by multiplying the liquid flow measurement—after dynamic compensation—by the output of the temperature controller, the result used to set steam flow in cascade. Feedforward is capable of a reduction in integrated error as much as a hundredfold but requires the use of a steam-flow loop and dynamic compensator to approach this. 

The accuracy of the system depends on controlling heat flow therefore, if steam pressure varies, compensation must be applied to cor-... 

FIG. 8 54 Controlling evaporators requires matching steam flow and evaporative load, here using feedforward control. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

The speed control operates the governing valve to maintain steam flow commensurate with load demand while holding speed essentially constant. For sudden load changes there will be a short-time overshoot, and a special case is the instantaneous loss of load, load dump at full load. The usual specification states that the overshoot on load dump must not exceed 9 to 10 percent of rated speed. The settled speed rise will of course be equal to the regulation, 4 or 6 percent for a NEMA Class C or B governor and less than I percent for Class D. 

The initial moisture content is a determinant factor in the rate of heat transfer to the center of the core mat [226,227]. At short press closing times the rapid temperature rise occurs at the same time for both lower and higher moisture content percentages indicating that the steam condensation front reaches the core at the same rate and that this is then determined more by local permeability rather than local moisture content. The slope of the rise is similar as it is the balance of horizontal and vertical permeability which controls the rate of steam flow to the core layer. Furthermore, water remains in the surface layer in a quantity such as... 

Consider a test procedure to periodically check the operation of. in important steam flow control valve and the position of its associated bypass s jJve... 

One of the oldest energy control systems is the steam engine speed control device, developed by James Watt, consisting in the regulation of motor speed through input steam flow. This device is purely mechanical, and its physical principle is shown in Figure 1. 

Reasonable factors of safety should be applied to the various loads in order to insure adequate capacity. Excess ejector capacity can be handled by pressure control and some adjustment in steam flow and pressure, but insufficient capacity may require ejector replacement. Factors of 2.0 to 3.0 are not uncommon, depending upon the particular qqte of system and knowledge of similar system operauons. 

A governor valve controls the flow of steam to the turbine. This valve is actuated by the governor, which is operated by the speed of the machine. When the speed exceeds the set value of the governor, a trip-valve is actuated to completely shut off the steam supply. The trip valve may... 

The steam enters the nozzles of the turbine and expands to the buckets or blades. Hand-operated or automatic valves are usually available to control the flow to groups of these nozzles. This allows more efficient operation at reduced loads. 

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. 

Water-level controls continuously monitor the level of water in a steam boiler in order to control the flow of feedwater into the boiler and to protect against a low water condition which may expose the heating surfaces with consequent damage. The controls may be either float operated or conductivity probes. 

Steam flow to vessels can be controlled by motorized valves, allowing timed control of the valve. Such valves can also be made to modulate rather than on/off operation. 

Changing the control system so stripping steam flow is proportional to LCO stripper product. 

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