Control system for forced circulation steam generator

Abstract

A control system for a forced circulation water pipe steam generator, the control system including: a heat exchanger having an inlet and an outlet, the heat exchanger being positioned to be exposed to a heat source in use, the heat exchanger being capable of generating steam from water passing through the heat exchanger; a steam separator having a steam outlet, a water outlet, and an inlet connected to the outlet of the heat exchanger; a positive displacement pump connected to the water outlet of the steam separator and arranged to supply water and / or a mixture of water and steam to the inlet of the heat exchanger; a feedwater injector connected between the outlet of the heat exchanger and the positive displacement pump; and a flow meter connected between the water outlet of the steam separator and the inlet of the heat exchanger.

Description

[Technical Field]

[0001] The present invention relates to a control system for a forced circulation water tube steam generator, i.e., a steam generator in which water is circulated through the system by a pump, as opposed to a "natural circulation" steam generator. [Background technology]

[0002] Steam generators fall into two main categories: fire-tube steam generators, in which hot combustion gases pass through tubes surrounded by water, heating the water around the tubes, and water-tube steam generators, in which heated water passes through one or more tubes, and the heated gases circulate around the tubes, heating the water passing through the tubes.

[0003] Water tube steam generators are further divided into natural circulation steam generators and forced circulation steam generators.

[0004] Forced circulation steam generators allow greater flexibility in the design of water-and-tube heat exchangers and also allow for a reduction in the volume of water and steam under pressure within the steam generator.

[0005] The three most commonly used forced circulation designs are once-through, spillover, and recirculation.

[0006] The once-through system has known drawbacks such as difficulty in controlling the feedwater and outlet temperatures under dynamic conditions during use, and requires a supply of pure water to prevent rapid buildup of impurities inside the boiler.

[0007] The spillover system is an improvement over the once-through system, and addresses the problem of impurity buildup in the boiler water by supplying a small amount of excess water to drain the impurities out of the boiler. However, spillover steam generators still face the problem of feedwater control under dynamic conditions during use, and the need to drain excess water results in thermodynamic losses.

[0008] Neither the once-through nor the spillover type is suitable for use with a multi-pass heat exchanger because it is difficult to distribute the feedwater and heat evenly among the circuits. This limits the applicability of either type to large-scale steam generators.

[0009] Recirculation systems aim to overcome these drawbacks and include a drum containing boiler water and a circulating pump. A common design of this type is the Lamont boiler. In the Lamont design, the water supply to the water tubes is controlled by monitoring the boiler water level in the drum. The circulating pump provides a water circulation ratio that is typically greater than five times the evaporation rate. A multi-pass heat exchanger is typically used, with flow-regulating orifices at the inlet of each circuit to prevent uneven water supply to each circuit. This prevents unstable operation within the heat exchanger and "hunting" within each circuit, which can result in periodic overheating of the water tubes.

[0010] However, the Lamont design has the disadvantage that it requires a large capacity drum and that the water level in the drum must be allowed to fluctuate sufficiently to effectively drain the water from the heating circuit in the event of a sudden load increase.

[0011] Further disadvantages of the Lamont design are that the automatic control of the water supply by measuring the water level in the drum requires delicate equipment that is prone to failure, especially under dynamic conditions, and the need to circulate water through the heat exchanger while heating the steam from a low temperature makes it disadvantageous when the circulating pump is steam-driven.

[0012] The discussion of prior art throughout the specification is not an admission that such prior art is widely known or forms part of the general knowledge in the art. Summary of the Invention [Problem to be solved by the invention]

[0013] It is an object of the present invention to provide a simple and reliable control system for a forced circulation water pipe steam generator, which control system overcomes or reduces at least some of the above-mentioned disadvantages. [Means for solving the problem]

[0014] The present invention provides a control system for a forced circulation water pipe steam generator, the control system including: a heat exchanger having an inlet and an outlet, the heat exchanger being arranged to be exposed to a heat source in use, wherein the heat exchanger is capable of generating steam from water passing through the heat exchanger; a steam separator having a steam outlet, a water outlet, and an inlet connected to the outlet of the heat exchanger; a positive displacement pump connected to the water outlet of the steam separator and arranged to supply water and / or a mixture of water and steam to the inlet of the heat exchanger; a feedwater injector connected between the outlet of the heat exchanger and the positive displacement pump; and a flow meter connected between the water outlet of the steam separator and the inlet of the heat exchanger.

[0015] The heat exchanger may be a single-pass or a multi-pass heat exchanger.

[0016] The flow meter may be a Venturi type flow meter or any of a variety of suitable flow meters and is preferably connected between the feedwater injector and the pump or between the pump and the inlet of the heat exchanger.

[0017] The positive displacement pump may be a single pump or multiple pumps.

[0018] Additionally, multiple steam separators may be used, with the separators connected in series or in parallel.

[0019] The present invention also provides a method of operating the above control system. [Brief explanation of the drawings]

[0020] Preferred embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: [Figure 1] FIG. 1 is a block diagram showing a control system according to the present invention. [Figure 2] FIG. 2 is a block diagram showing a modified example of the control system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] Referring to Figure 1, there is shown a schematic diagram of a forced circulation steam generator control system 10. The purpose of the control system is generally to regulate the water supply to a heat exchanger 14 to produce the required volume of steam for delivery through a line 18 to a steam engine or other device requiring steam.

[0022] As used herein, the term "boiler" refers to the entire steam generating system, including the control system.

[0023] The term "water level" is used to describe the amount of water (as opposed to steam) in the boiler as measured by the control system.

[0024] In this system, a steam separator 11 is connected to the inlet of a positive displacement pump 12 via a pipe 15 and to the outlet of a water-tube heat exchanger 14 via a pipe 13 .

[0025] In the figures, for clarity, heat exchanger 14 is shown as a single-pass heat exchanger, and although it may in fact be a single-pass heat exchanger, it would more commonly be a multi-pass heat exchanger. Preferably, the heat exchanger is a multi-pass water-tube heat exchanger, with multiple circuits of equal length, all arranged to receive a similar average heat flux from any suitable type of heat source H. All heat exchanger circuits are arranged to discharge at their heat exchanger outlets into an outlet manifold 14a (FIG. 2 only), which is connected to the inlet pipes 13 to the separator 11.

[0026] The outlet of the pump 12 is connected to the inlet of the water-tube heat exchanger 14 via a Venturi flowmeter 17 to which a differential pressure gauge 16 is connected.

[0027] Steam separator 11 may be of any suitable known type (e.g., a cyclone separator) and will therefore not be described in detail. Steam separator 11 receives a mixture of steam and hot water from heat exchanger 14 via pipe 13, where the mixture is separated into steam which exits the separator via outlet pipe 18 for supply to an engine or other device, and hot water containing some steam which exits the bottom of the separator via pipe 15.

[0028] A feedwater injector 20 of any suitable known type is connected to the pipe 15 between the separator 11 and the pump 12 to provide a controllable supply of fresh water to the system as required. The injector can be positioned so that the momentum of both the incoming feedwater and the condensed steam is used to assist the pump 12.

[0029] Although pump 12 is shown as a single pump, in practice, two or more pumps may be provided to allow for possible failures. Alternatively, each pump 12 may be a positive displacement pump. While it is conceivable to operate the system using a regenerative turbine pump, centrifugal / speed pumps will not work. Alternatively, each pump 12 may be a rotary or oscillating positive displacement pump, provided it is suitable for pumping both steam and water, both separately and as a mixture. The preferred method of operation is to operate the pump at a constant volumetric flow rate, whether pumping water or steam, or a mixture of water and steam.

[0030] The discharge from the or each pump 12 is arranged to pass through a Venturi flow meter 17 before entering an outlet manifold 21 from which the pumped fluid can enter a heat exchange circuit, or in the case of a multi-path heat exchanger, each heat exchange circuit.

[0031] In the case of a multi-path heat exchanger, the fluid circuits of the present invention may not require the provision of metering nozzles, since the inlets of each circuit are not restricted by metering nozzles. However, in order to achieve equal distribution of water and steam between each circuit, it may be necessary or advantageous to provide metering nozzles (or other means of ensuring equal distribution) at the inlets of each circuit.

[0032] A differential pressure gauge 16 is positioned to read the differential pressure across a Venturi flow meter 17. Although a Venturi flow meter is preferred, other types of flow meters (e.g., orifice plates) could be used instead, provided the flow meter is sensitive to changes in the density of the fluid being measured.

[0033] In the embodiment described with reference to Figure 1, the flow meter 17 and pressure gauge 16 are located between the circulation pump 12 and the heat exchanger 14. Figure 2 shows an alternative arrangement in which the flow meter 17 and pressure gauge 16 are located between the feedwater injector 20 and the inlet of the circulation pump 12. The arrangement shown in Figure 2 is considered preferred. The same reference numerals are used in Figure 2 as in Figure 1 to refer to the same components.

[0034] The differential pressure gauge 16 is known and reads the difference in static pressure between the inlet (or outlet) to the venturi flow meter 17 and the pressure at the throat of the venturi. This measurement is used to determine the amount of water, or "level," in the boiler at any given time.

[0035] Because each pump 12 pumps at a constant volumetric flow rate, the volume passing through the flow meter remains relatively constant. However, the mass of the fluid varies depending on the amount of water in the fluid relative to the amount of steam. The greater the proportion of water in the fluid, the greater the mass. Because the dynamic pressure of a fluid is directly dependent on the density of the fluid, which is equal to mass divided by volume (volume being fixed), the reading on the differential pressure gauge 16 can be made to indicate the proportion of water and steam in the fluid.

[0036] Preferably, the system is configured so that the fluid entering pump 12, supplied by heat exchanger 14 through outlet manifold 21, is a mixture of water and steam (ranging from 10% to 90% water by volume). This is because in a water-steam mixture, the water is fully saturated (otherwise, the steam would condense). Therefore, when the fluid enters the heating circuit of heat exchanger 14, all of the energy supplied to the heat exchanger is directed toward generating steam rather than raising the fluid's temperature to its boiling point. On the other hand, if the fluid entering pump 12 is subsaturated, it must first be heated to its saturation temperature before steam can be generated. Supplying more subsaturated fluid to any one heating circuit of a multi-path heat exchanger reduces the amount of steam generated in that circuit. This condition can lead to instability in the heating circuit. This is because a decrease in steam generation in one circuit reduces the backpressure in that circuit, drawing more subsaturated fluid into that circuit and diverting fluid from other circuits, leading to overheating of those circuits.

[0037] As mentioned above, it is preferable (although not essential) to supply the heat exchanger with a fully saturated fluid. For this reason, feedwater should be injected into the system through feedwater injector 20 before pump 12, preferably near the drain from separator 11, so that it is heated to saturation temperature by the steam extracted from the separator. The injection of feedwater water (which is usually at a relatively low temperature) condenses a large amount of steam in the fluid coming from the separator, so consideration should be given to ensuring that the steam flow reaches it unimpeded. The steam separator drain is well positioned to feed this steam flow into the feedwater.

[0038] An additional benefit is that by drawing a large volume of steam through the separator drain, the separator 11 is continuously purged of water and any bubbles that may form as a result of certain water treatment techniques. This allows for the use of a smaller, more compact separator. Furthermore, the water exiting the separator contains a higher concentration of dissolved solids than any water in the boiler, and the rapid heating of the feedwater and its immediate mixing with the boiler water, which contains a high concentration of dissolved solids, effectively precipitates scale-forming compounds from the feedwater as unadherent particles. This, when properly treated with chemicals, reduces or eliminates heat exchanger fouling.

[0039] The flow meter 17 is preferably positioned relative to the maximum evaporation rate for which the boiler is designed. Preferably, when the flow rate of water through the flow meter is (for example) three times the maximum evaporation rate from the boiler, the reading provided by the differential pressure gauge 16 corresponds to the target "operating level" of the boiler.

[0040] An advantage of the control system of the present invention is that it allows for relatively large variations in the water level in the heat exchanger. Typically, at maximum evaporation rates, the ratio of water flow through the flow meter to evaporation rate in the heat exchanger is allowed to vary between 2:1 and 4:1.

[0041] Because the rate at which saturated fluid is supplied to the heat exchanger is measured and regulated directly, and the injection of feedwater occurs immediately before the flow meter, there is little hysteresis and the control system is relatively simple to operate. In effect, the control system simultaneously measures three values ​​using a single flow meter (although multiple flow meters may be provided in series or parallel for redundancy): the flow rate of excess water from the heat exchanger, the flow rate of feedwater to the boiler, and the flow rate of saturated fluid to the heat exchanger. Preferably, the operating water level is set as a function of the evaporation rate. In the case of a steam-powered vehicle (e.g., a locomotive) where exhaust steam is used to pump combustion gases through the boiler, the exhaust steam pressure corresponds directly (but not linearly) to the evaporation rate of the boiler.

[0042] The differential pressure developed at flow meter 17 can therefore be set within the same range as the locomotive exhaust steam pressure (measured by a tap in the engine exhaust duct or by a Pitot tube just before the exhaust nozzle). In this case, the boiler operating water level can be set to match the locomotive exhaust steam pressure above the minimum water level (usually 25% of maximum) set for idle / standby applications.

[0043] As a result of the above arrangement, the effective rate of water vapor in the heat exchanger remains relatively constant regardless of the evaporation rate, eliminating the problem of sudden increases in evaporation rate overwhelming the vapor separator 11 and circulation pump 12.

[0044] If the steam-powered vehicle is being controlled manually, the operator can monitor both the differential pressure gauge 16 and the exhaust steam pressure measurements and use the results to adjust the amount of feedwater through the feedwater injector 20. If the vehicle is being controlled automatically, the water level correction system provides automatic control by utilizing a water level setpoint as a function of the exhaust steam pressure or as a function of a surrogate measure of evaporation rate, such as a flow meter in the main steam outlet of the boiler. In this case, the control system effectively measures four values ​​when controlling the amount of feedwater to the boiler through the feedwater injector 20.

[0045] A further improvement is to make the operating water level a function of the boiler steam pressure in addition to the evaporation rate. This compensates for changes in the density of the steam and water flowing through the flowmeter as the boiler static pressure changes. This keeps the water level relatively constant, preventing the boiler from overfilling, which is especially important during the warm-up phase of boiler operation.

[0046] The control system described with reference to Figures 1 and 2 is essentially the simplest form of an effective control system and may be modified depending on particular requirements and capabilities. For example, two or more steam separators may be used, connected in series or in parallel.

[0047] Additionally, multiple pumps can be incorporated into the circuit as a fail-safe or if extra capacity is required.

[0048] Another variation is to vary the location where the feedwater is introduced into the system. While the feedwater injector is preferably connected between the water outlet of the steam separator and the positive displacement pump (as described above in connection with Figures 1 and 2), the feedwater injector could be connected for direct injection into the steam separator or near the outlet of the heat exchanger. Also, feedwater injection at multiple points around the control system is possible.

Claims

1. A control system for a forced circulation water tube steam generator, comprising: a heat exchanger having an inlet and an outlet, the heat exchanger being positioned in use so as to be exposed to a heat source, the heat exchanger being capable of generating steam from water passing through the heat exchanger; a steam separator having a steam outlet, a water outlet, and an inlet connected to the outlet of the heat exchanger; a positive displacement pump connected to the water outlet of the steam separator and arranged to supply water and / or a mixture of water and steam to the inlet of the heat exchanger; a feedwater injector connected between the outlet of the heat exchanger and the positive displacement pump; a flow meter connected between the water outlet of the steam separator and the water inlet of the heat exchanger; Including, A control system comprising:

2. the heat exchanger is a single-pass heat exchanger; 2. The control system of claim 1 .

3. the heat exchanger is a multi-pass heat exchanger; 2. The control system of claim 1 .

4. an outlet manifold connected to the outlet of the heat exchanger such that all circuits of the heat exchanger discharge into the outlet manifold; 4. The control system according to claim 3, wherein:

5. the feedwater injector is connected between the water outlet of the steam separator and the positive displacement pump. A control system according to any one of claims 1 to 4, characterized in that

6. the flow meter is connected between the feedwater injector and the positive displacement pump; 6. The control system according to claim 5, wherein:

7. the flow meter is connected between the positive displacement pump and the heat exchanger; 6. The control system according to claim 5, wherein:

8. The flow meter is a Venturi flow meter. A control system according to any one of claims 1 to 7, characterized in that

9. further comprising a differential pressure meter connected to the flow meter. A control system according to any one of claims 1 to 8, characterized in that

10. an inlet manifold connected immediately before the heat exchanger; A control system according to any one of claims 1 to 9, characterized in that

11. The positive displacement pump comprises two or more pumps. A control system according to any one of claims 1 to 10, characterized in that

12. The steam separator comprises two or more steam separators. A control system according to any one of claims 1 to 11, characterized in that

13. 10. A method of operating the control system of claim 1, comprising: the positive displacement pump operates at a substantially constant volumetric flow rate; A method characterized by:

14. the system is conditioned so that the fluid entering the positive displacement pump is a mixture of water and steam; 14. The method according to claim 13.

15. The water and steam mixture ranges from 10% to 90% water by volume.

15. The method of claim 14.

16. the flow meter being sensitive to any change in density of the fluid being measured; 16. The method according to any one of claims 13 to 15, characterized in that

17. the control system includes a differential pressure meter connected to the flow meter, the differential pressure meter being set so that a reading of the flow meter corresponds to a target operating water level of the boiler when the flow rate of water passing through the flow meter is three times the maximum evaporation rate from the boiler.

17. The method according to any one of claims 13 to 16, characterized in that

18. the control system simultaneously measures the flow rate of excess water from the heat exchanger, the flow rate of feedwater to the boiler, and the flow rate of the water and steam mixture to the heat exchanger; 18. The method of claim 17.

19. the control system adjusts the operating water level to be a function of both the steam pressure and evaporation rate of the boiler; 19. The method according to any one of claims 13 to 18, characterized in that

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