JPS5912921B2 - How to operate a forced circulation boiler - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a forced circulation boiler, particularly a forced circulation waste heat boiler that recovers heat from a waste heat source, and is capable of coping with load fluctuations of the waste heat source over a wide range and allowing continuous operation of the boiler. This invention relates to an easy method of operating a forced circulation boiler.

A forced circulation boiler is a boiler with a short structure that does not allow for sufficient natural circulation of water in the can, in which the water in the can is constantly forced to circulate in order to prevent heat transfer tubes from burning out. be.

The water supply system to the evaporation water pipe of a conventional forced circulation boiler is
As shown in the figure, only a circulation pump is installed, and the flow rate of circulating water must constantly fluctuate in response to load fluctuations and accompanying changes in head loss H.

The reason why the circulating water flow rate changes is explained using Figure 2. In Figure 2, curve A shows the relationship between the circulating flow rate of the circulation pump, the static pressure head of the downcomer pipe, plus the total head of the circulation pump. There is.

When the load is high, the amount of steam generated in the evaporative water pipe increases, so the flow resistance of the air-water mixture flowing through the evaporative water pipe becomes
The circulating water flow rate increases as shown by curve B in the figure, and the circulating water flow rate increases as shown by curve A.
The flow rate at the intersection point A of curve B is Qi.

In addition, at low load, the amount of steam generated in the evaporative water pipe decreases, so the flow resistance of the air-water mixture flowing through the evaporative water pipe decreases as shown by curve C in Figure 2, and the circulating water flow rate decreases as shown by curve A. The flow rate at the intersection of curve C and curve C is 9 ports.

Therefore, when the load becomes lower than the case of curve C shown in Fig. 2, the curvilinear NPSH (effective suction head) curve rises rapidly. Water supply to the area may be interrupted.

Therefore, in order to cover load fluctuations above A to B,
In conventional circulation pumps, it is necessary to either increase the capacity or to limit the load range, for example, to a range of 42 ports.

Furthermore, due to the characteristics of the circulation pump, when the load on the boiler is large, the amount of circulating water decreases rapidly as shown in FIG. 2, and there is a problem in that the amount of circulating water decreases even though it is desired to cool the wall of the evaporative water pipe.

Therefore, when the capacity of the pump is increased, for example, as shown in the characteristic curve A' in Figure 2, the minimum circulating water amount is Q.
Q can be made larger than A, but on the other hand, when the boiler load is small, Q Nu is Q.

Therefore, the amount of circulating water is increased by the amount (Q-Q), which causes a loss of power, which is undesirable from the viewpoint of energy saving.

Another problem is that cavitation problems are likely to occur due to NFSH.

The purpose of this invention is to eliminate the above-mentioned drawbacks of the prior art,
This paper proposes an operating method for forced circulation waste heat boilers that enables continuous operation of forced circulation waste heat boilers over a wide range.

In short, this invention provides a method for operating a forced circulation boiler, in which a circulating water pump, a booster pump,
A circulating water flow rate control valve and a flow meter are placed in series, and the circulating water flow rate value is sent as a signal to a control box that stores the maximum head loss value and minimum head loss value, and the booster pump can be started and stopped by commands from the control box. To control the opening and opening of the flow rate control valve, the total head of the circulation pump used and the flow rate point midway between the usage section of the circulation flow rate characteristic curve are input and stored in the control box, and this is used as the set value (switching point), and this output is controlled. Control operations are performed using signals.

A method of operating a forced circulation boiler.

The present invention will be explained below with reference to the drawings.

FIG. 3 is a pipe system diagram showing the first embodiment, in which water is supplied to the boiler drum 1 from a water supply pump E via a pipe 11 and a water supply control valve 8.

Circulating water is supplied from the circulation pipe 5 to the circulation pump F1, the booster pump G1 located in series with this pipe, the circulating water flow rate control valve 9, and the circulating water flow meter 10 to the evaporation pipe 40 input rotary header 2a.
The air-water mixture is evaporated in the evaporation pipe 4 and then flows into the outlet header 2b.
It flows into drum 1 via.

The booster pump G is provided with a bypass line 11, and this line 11 is provided with a bypass valve 12.

A numerical value of the flow rate of circulating water is sent from the flow meter 10 to the control box 13 as a signal 14.

In Fig. 4, the relationship between head loss H and circulating water flow rate Q when circulation pump F without a booster pump is operated independently is curve F', and curve F' is the relationship between circulation pump F and booster pump G.
The H-G relationship when these are connected in series is shown by the FG' curve.

The R1 to R6 curves are resistance curves in this circulating water pipe.

According to this invention, the maximum head loss can be obtained at the point H on the FG' curve and can be varied along the FG' curve to the H point.

Here, by stopping the booster pump G, opening the bypass valve 12, and controlling the circulating water flow rate control valve 9, the water head loss can be changed to 2, and the water head can be changed to point C according to the 2 curves. can.

The starting or stopping point (E, 2) of the booster pump and the opening/opening point of the bypass valve depend on the characteristics of the booster pump and circulation pump (the relationship between Q and H) and the economy of the required driving power.
Select a suitable point Q between the gap and input it to the control box, thereby sending the signal from the flow meter 10 to the control box, switching the bypass valve (opening/closing valve) 12, starting and stopping the booster pump, By controlling the circulating water flow rate control valve 9 and the water supply control valve 8, the relationship between the head loss H and the circulating water flow rate Q can be changed according to the solid lines shown in FIG. .

That is, when the load on the circulating water pipe changes, a circulating water flow rate signal is sent to a control box that inputs and stores a circulating flow rate setting value located between the total head and the circulating flow rate characteristic curve for determining whether to start or stop the booster pump. By outputting a signal from the control box, when the circulation flow rate reaches the set value when the booster pump is stopped, the booster pump starts, the bypass valve closes, and when the circulation flow rate reaches the set value when the booster pump is operating. Controls booster pump stop and bypass valve opening.

FIG. 5 shows a second embodiment of the present invention, in which, in addition to the first embodiment shown in FIG. steam flow rate signal of main steam flow meter 15, circulation pump F
This is a case where a pressure signal of the suction pressure P is sent to the control box.

In this case, the pipe line 1 connecting the water supply pipe line 7 and the downcomer pipe 15
7 is provided, and a valve 18 provided in this conduit can be opened as necessary to prevent an increase in NPSH.

This makes it possible to perform more suitable circulating water flow rate control in addition to NPSH (effective suction head), thereby preventing burnout of the evaporator tube and saving power.

By implementing this invention, the amount of circulating water can be adjusted to match the load (evaporation amount), the safety of the boiler can be ensured, power can be saved, and the drum can be positioned higher to ensure NPSH. It has various effects, such as eliminating structural stress, automatically switching the booster pump's start/stop and controlling the control valves at set points, and saving the boiler's safety power.

[Brief explanation of the drawings] Fig. 1 is a conventional pipe system diagram, Fig. 2 is a diagram showing the relationship between head loss H and circulating water flow rate Q in the case of Fig. FIG. 4 is a pipe system diagram showing an embodiment, FIG. 4 is an HQ diagram in the case of FIG. 3, and FIG. 5 is a pipe system diagram showing a second embodiment of the present invention. 1... Boiler drum, F... Circulation pump, G... Booster pump, 4... Evaporation pipe, 8... Water supply control valve , 9... Circulating water flow rate control valve, 10... Flow meter, 11...
... Bypass pipe line, 12 ... Bypass valve, 13
...Control box, 15...Main steam flow meter.

Claims (1)

[Claims] 1. In a method of operating a wide forced circulation boiler during load fluctuations, a circulation pump, booster pump,
A flow meter is placed in series, and a bypass pipe with a bypass valve is connected to the booster pump, and the total head and circulating flow rate of the circulation pump are used to determine whether to start or stop the booster pump when the load on the circulating water pipe changes. By sending a circulating water flow rate signal to a control box that inputs and stores a circulating flow rate set value located in the middle of the characteristic curve, and outputting a signal from the control box, the circulating flow rate is adjusted to the set value when the booster pump is stopped. A method for operating a forced circulation boiler, characterized in that the booster pump is started and the bypass valve is closed when the set value is reached, and the booster pump is stopped and the bypass valve is opened when the circulating flow rate reaches the set value during operation of the booster pump.