I have a few questions regarding boiler feed water; please offer some advice, fellow sailors.
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1. If a frequency converter is installed on the boiler feed water pump, frequency conversion is achieved by adjusting the speed. The flow rate of a centrifugal pump is proportional to the cube of its rotational speed ； Head is the square of the rotational speed. So the question arises: if the boiler load decreases, for example, if the flow rate drops to 80% of the rated value, then the speed of the feed water pump must also be reduced to 80% of its rated speed. As a result, the head generated by the pump will be only 0.8*0.8=0.64 times the original value. How can the pressure in the boiler be maintained constant in such a situation? 2. The cooling water for the temperature reduction valve comes from the outlet of the feed water pump; as the pressure at the outlet of the feed water pump decreases, the pressure of the cooling water also decreases. How can it be ensured that, even after the feed water pump operates at variable frequency, the pressure of the cooling water remains higher than the pressure of the steam? 3. What does the pressure of superheated steam and the drum depend on? Is it only related to the head of the feed water pump? (Without pressure buildup from superheated steam), is then the heating surface of the boiler in a constant-pressure process? 4. The regulation of the feedwater flow rate in our waste heat boiler is achieved through control valves and recirculation valves; the pump operates at the power frequency – is it necessary to make it variable frequency as well? The recirculation valve is manual – is it necessary to make it a control valve? You asked too many questions at once; you’re a bit greedy, haha. Thank you to all the sea friends! !
I’m here to learn* as well; it’s just that ordinary people don’t participate much in this kind of issue.
Do you mean the economizer recirculation or the feedwater pump recirculation when you talk about recirculation? After saying all this, the main concern is still regarding changes in frequency
The head of the feed water pump has little impact on the boiler pressure, right? With variable frequency drive, does it suffice to ensure that water can still be supplied to the steam drum at the lowest speed?
For temperature and pressure reduction, using a single circulation pump makes control easier
1. Boiler feedwater regulation is aimed at maintaining the level of the drum; it primarily employs a cascade three-input control method, with the steam flow rate signal being used as a feedforward signal. The feedwater flow rate is adjusted to achieve the desired control effect. During this process, the opening degree of the feedwater control valve is changed in order to regulate the feedwater flow rate. Variable frequency drives are used to reduce the throttling losses associated with the feedwater control valve, thereby enabling the control objectives to be achieved at the lowest possible feedwater pressure (before the valve). Therefore, it is not necessary to stick to a constant feedwater pressure – it should be understood that the boiler pressure is not determined by the feedwater pressure, but rather by the amount of fuel used (internal disturbances) and changes in external load (external disturbances). When these two factors are in balance, the boiler pressure remains constant. 2. Generally, the frequency conversion range for variable-speed feed water pumps is not lower than 85%. The regulation of the cooling water flow rate in temperature reduction devices is accomplished by adjusting the opening degree of the cooling water control valve; sufficient margin is provided for such valves during design, so that a decrease in the outlet pressure of the feed water pump does not result in an inability to regulate the cooling water flow rate to meet the requirements regarding steam temperature. 3. Superheated steam has no relation to the drum pressure; rather, it is the pressure of the superheated steam that is related to the drum pressure. The difference between these two pressures represents the resistance in the superheater system. When operating at a constant pressure and the load increases, the pressure difference also increases, and this has nothing to do with the head of the feed water pump. When the boiler operates at a constant pressure, it is undoubtedly true that all of its heating surfaces also operate at a constant pressure. 4. When the boiler is operating normally, the feedwater flow rate is adjusted primarily by changing the opening degree of the feedwater control valve. The recirculation valve is mainly activated at low load levels, in order to prevent cavitation in the feedwater pump when it operates under such conditions. Therefore, at normal load levels (usually above 30% of the maximum load), the recirculation valve should not be opened, as this is not economical. Ultimately, this situation arises due to improper selection of the feedwater control valve or the head of the feedwater pump; once the appropriate head for the feedwater pump and the suitable feedwater control valve have been chosen, there is no need to use variable frequency speed control. The recirculation valves for the power plant feed water pumps are all electric (or pneumatic) control valves, and they generally include a minimum flow control for the feed water pump; when the feed water pump operates at its minimum flow rate, the corresponding minimum flow control valve opens automatically.
The frequency conversion control of the feed water pump is mainly intended to address the significant fluctuations in steam load