Control system for a forced circulation steam generator

Abstract

A control system for a forced circulation water tube steam generator, in which the system includes: a heat exchanger having an inlet and an outlet and arranged in use to be exposed to a heat source, such that steam can be generated from water passing through the heat exchanger; a steam separator having a steam outlet and a water outlet, and an inlet which is 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 water / steam emulsion to the inlet of the heat exchanger; a feed water injector connected between the outlet of the heat exchanger and said positive displacement pump; and a flowmeter, connected between the water outlet of the steam separator and the inlet of the heat exchanger.

Description

[0001] CONTROL SYSTEM FOR A FORCED CIRCULATION STEAM GENERATOR

[0002] Technical Field

[0003] 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 pumped around the system, as opposed to a “natural circulation” steam generator, which, broadly speaking, relies on water density to circulate water inside the steam generator.

[0004] Background Art

[0005] Steam generators fall into two main categories:- fire tube steam generators, in which hot combustion gas passes through tubes surrounded by water, to heat the water surrounding the tubes; and water tube steam generators in which water to be heated is passed through one or more tubes and heated gas circulates around the tubes to heat the water passing through the tubes.

[0006] Water tube steam generators can be further subdivided into natural circulation and forced circulation steam generators.

[0007] Forced circulation steam generators permit a greater latitude in the water tube heat exchanger design, and also allow a reduction in the volume of water and steam being contained under pressure in the steam generator.

[0008] The three most commonly used forced circulation designs are the once through design, the spillover design, and the recirculation design.

[0009] The once through design has the known disadvantages of difficulties in controlling the feed water and the outlet temperature under dynamic conditions in use, and also requires a supply of pure water, to prevent the rapid accumulation of deposits of impurities within the boiler.

[0010] The spillover design is a development of the once through design and deals with the difficulty of the accumulation of deposits from the water by providing a slight excess of water for carrying impurities out of the boiler. However, spillover type steam generators still suffer from feedwater control problems under dynamic conditions, and the need to spill excess water represents a thermodynamic loss. Neither the once through design nor the spillover design is well adapted to use with a multipath heat exchanger, because of the difficulty in evenly distributing the supply of feedwater and heat to each circuit. This limits the extent to which either design can be scaled up to large-scale steam generator use.

[0011] The recirculation design aims at solving the above drawbacks by providing a drum for containing the boiler water, and a circulating pump; a common design of this type is the La Mont type of boiler. In the La Mont design, feed water to the water tubes is controlled by monitoring the level of the boiler water contained within the drum. The circulating pump provides a water circulation ratio which typically exceeds five times the rate of evaporation. Usually, a multipath heat exchanger is used, and to prevent variations in the supply of water across the circuits, each circuit is provided with a metering orifice at the circuit inlet; this prevents instability in the heat exchanger and the “hunting” of water in each circuit, which would otherwise lead to cyclic overheating of the water tubes.

[0012] However, the La Mont design has the drawback that the drum needs to be of considerable volume, and where rapid load increases take place, the allowable variation of the water level within the drum must be large enough to accommodate the effective purging of water from the heating circuits.

[0013] Two further drawbacks of the La Mont design are that the automatic control of feedwater by measuring the water level in the drum requires delicate equipment which is prone to failure, especially under dynamic conditions, and that circulation of water within the heat exchanger must be provided while raising steam from cold, which is a disadvantage where the circulating pump is steam driven.

[0014] Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0015] Disclosure of Invention

[0016] An object of the present invention is the provision of a simple and reliable control system for a forced circulation water tube steam generator, which overcomes or reduces at least some of the above described drawbacks. Statement of Invention

[0017] The present invention provides a control system for a forced circulation water tube steam generator, in which the system includes:

[0018] • a heat exchanger having an inlet and an outlet and arranged in use to be exposed to a heat source, such that steam can be generated from water passing through the heat exchanger;

[0019] • a steam separator having a steam outlet and a water outlet, and an inlet which is connected to the outlet of the heat exchanger;

[0020] • a positive displacement pump connected to the water outlet of the steam separator, and arranged to supply water and / or a water / steam emulsion to the inlet of the heat exchanger;

[0021] • a feed water injector connected between the outlet of the heat exchanger and said positive displacement pump;

[0022] • a flowmeter, connected between the water outlet of the steam separator and the inlet of the heat exchanger.

[0023] The heat exchanger may be a single path heat exchanger or a multipath heat exchanger.

[0024] The flowmeter may be a Venturi type flowmeter or any of a range of suitable flowmeters, and preferably is connected between the feedwater injector and the pump or between the pump and the heat exchanger inlet.

[0025] The positive displacement pump may be a single pump or may be more than one pump.

[0026] Further, more than one steam separator may be used, with the separators connected in series or in parallel.

[0027] The present invention also provides a method of operating the above control system.

[0028] Brief Description of Drawings

[0029] By way of example only, preferred embodiments of the present invention are described in detail with reference to the accompanying drawings, in which: Figure 1 is a diagram of a control system in accordance with the present invention; and

[0030] Figure 2 is a diagram of a variant control system in accordance with the present invention.

[0031] Best Modes for Carrying Out the Invention

[0032] Referring to Figure 1 , a control system 10 for a forced circulation steam generator is shown diagrammatically. The objective of the control system is, broadly, to regulate the supply of feed water to the heat exchanger 14, so that the required volume of steam is produced, for supply through pipe 18, to the steam engine or other device requiring the steam.

[0033] As used herein, the term “boiler” means the whole of the steam generating system, including the control system.

[0034] The term “water level” is used to represent the quantity of water (as opposed to steam) in the boiler as measured by the control system.

[0035] 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.

[0036] In the diagram, for clarity, the heat exchanger 14 is shown as a single path heat exchanger, and indeed could be a single path heat exchanger, but more typically would be a multipath heat exchanger. Preferably, the heat exchanger is a multipath water tube heat exchanger which has multiple circuits of equal length where all of the heat exchanger circuits are arranged to be subjected to a similar average heat flux from a heat source H of any suitable type. All of the heat exchanger circuits are arranged to discharge into an outlet header 14a (Figure 2 only) at the heat exchanger outlet; this header is connected to the inlet pipe 13 to the separator 11.

[0037] The outlet of the pump 12 is connected to the inlet of the water tube heat exchanger 14 via a Venturi type flow meter 17 to which is connected a differential pressure gauge 16.

[0038] The steam separator 11 is of any suitable known type (e.g. a cyclonic separator) and therefore is not described in detail. The steam separator 11 receives a mixture of steam and hot water from the heat exchanger 14 via pipe 13; in the separator, this mixture is separated into steam, which leaves the separator through outlet pipe 18, for supply to an engine or other device, and hot water containing some steam which leaves the lower part of the separator through pipe 15.

[0039] A feedwater injector 20 of any suitable known type is connected into the pipe 15 between the separator 11 and the pump 12, to provide a controllable supply of fresh water into the system as needed. This injector may be arranged so that the momentum of both the incoming feedwater and any condensed steam is used to assist the pump 12.

[0040] The pump 12 is shown as a single pump, but in practice two or more pumps may be provided to allow for possible failure. The or each pump 12 is a positive displacement pump; it is believed that it might be possible to work this system using a regenerative turbine pump, but centrifugal pumps / velocity pumps will not work in the system. The or each pump 12 may be a rotary or oscillatory positive displacement pump, and must be suited to pumping both steam and water, separately and as an emulsion. The preferred method of operation is to operate the or each pump at a substantially constant volume flow rate, whether the pump is pumping water or steam or an emulsion of both water and steam.

[0041] The discharge from the or each pump 12 is arranged to pass through a Venturi type flow meter 17 and then into an outlet header 21 , from which the pumped fluid can enter the heat exchange circuit or, in the case of a multiple path heat exchanger, each of the heat exchange circuits.

[0042] In the case of a multi-path heat exchanger, the inlet of each circuit is not restricted by a metering nozzle, and it is believed that such metering nozzles may not be required in the fluid circuit of the present invention. However, it may prove necessary, or advantageous, to fit a metering nozzle (or other means of ensuring even distribution) into the inlet of each circuit in order to achieve even distribution of water and steam between the circuits.

[0043] A differential pressure gauge 16 is arranged to read the differential pressure across the Venturi flow meter 17. A Venturi type flow meter is preferred, but alternative forms of flow meter (e.g. orifice plate) could be used instead, provided that the flow meter is sensitive to any variation of the density of the fluid being measured. In the embodiment described with reference to Figure 1 , the flowmeter 17 and the pressure gauge 16 are shown located between the circulating pump 12 and the heat exchanger 14. Figure 2 shows an alternative arrangement, in which the flowmeter 17 and the pressure gauge 16 are located between the feed water injector 20 and the inlet of the circulating pump 12. It is believed that the arrangement shown in Figure 2 may be preferred. The reference numerals in Figure 2 are the same as for Figure 1 and refer to the same components.

[0044] The differential pressure gauge 16 is of known type and reads the difference in static pressure between the inlet to (or outlet from) the Venturi flow meter 17 compared to the pressure at the throat of the Venturi. This reading is then used to determine the quantity of water in the boiler at any given moment, or, the “water level.”

[0045] Because the or each pump 12 is pumping at a constant rate, the volume of flow through the flow meter remains relatively constant. However, the mass of the flow varies depending upon the amount of water in the flow, compared to the amount of steam:- the greater the proportion of water in the flow, the higher the mass. Since the dynamic pressure of the flow is directly dependent upon the density of the flow, and the density of the flow is equal to the mass divided by the volume (the volume being fixed), it follows that the reading of the differential pressure gauge 16 can be calibrated to indicate the proportions of water and steam in the flow.

[0046] Preferably, the system is set up so that the fluid entering the pump 12, from which the heat exchanger 14 is supplied via header 21 , is an emulsion of water and steam (between 10% and 90% water by volume). The reason for this is that in an emulsion of water and steam, the water is in a fully saturated condition (if this is not the case, the steam would condense) and therefore when the fluid enters the heating circuits in the heat exchanger 14, all of the energy supplied to the heat exchanger will go towards the generation of steam, rather than to increasing the fluid temperature to boiling point. By contrast, if the fluid entering the pump 12 is sub-saturated, it must first be heated to saturation temperature before steam is generated. Feeding more sub-saturated fluid to any one heating circuit in a multipath heat exchanger will lead to a reduction in the quantity of steam generated by that circuit. Such a condition can lead to instability in the heating circuits, because a reduction in steam generation in one circuit decreases the backpressure in that circuit, allowing even more sub-saturated fluid to flow into it, redirecting fluid from the other circuits, leading to those circuits overheating. As noted above, it is preferred (but not essential) to supply the heat exchanger with a fully saturated fluid. It follows from this that feed water should be injected into the system through the feed water injector 20 before the or each pump 12, and preferably close to the drain from the separator 11 , so that the feed water will be heated to saturation temperature by steam drawn out of the separator. The injection of feed water (which would normally be relatively cold) condenses a large quantity of steam in the fluid coming from the separator, so it should be located so that the flow of steam may reach it unimpeded. The separator drain is well placed to provide this flow of steam into the feed water.

[0047] A further advantage is that by causing a large quantity of steam to be drawn through the separator drain, the separator 11 is continuously evacuated of water and any foam that may be evolved as a result of specific water treatment regimes. This allows the use of a smaller, more compact separator than would otherwise be possible. Additionally, water drained from the separator carries the highest concentration of dissolved solids of any water in the boiler, and the combination of rapid heating of the feed water and immediate mixing with boiler water containing high dissolved solids improves the efficiency with which scale forming components are precipitated out of the feed water as non-adherent particles; this leads to the reduction or elimination of fouling of the heat exchanger, providing suitable chemical treatment is used.

[0048] It is preferred to arrange the flow meter 17 with reference to the maximum designed evaporation rate of the boiler. Preferably, when the flow of water through the flow meter is (for example) three times the maximum rate of evaporation from the boiler, the reading given by the differential pressure gauge 16 corresponds to the target “working water level” of the boiler.

[0049] It is an advantage of the control system of the present invention that it permits a relatively large fluctuation of the water level in the heat exchanger:- typically, at maximum evaporation rate, the ratio of the flow of water through the flow meter to the evaporation rate of the heat exchanger may be allowed to vary between 2: 1 and 4: 1 ; this variation in water level is accommodated within the comparatively large volume of the heat exchanger rather than within the volume of a separate drum, as in the La Mont design.

[0050] Because the amount of saturated fluid being fed into the heat exchanger is being measured and adjusted directly and the injection of feed water takes place immediately before the flow meter, there is little hysteresis and the operation of the control system is relatively straightforward. In effect, the control system, using the one flow meter (although multiple flow meters may be provided for redundancy, either in series or parallel), measures three values simultaneously, being the flow of excess water from the heat exchanger, the flow of feed water into the boiler and the flow of saturated fluid into the heat exchanger. Preferably the working water level is set as a function of the evaporation rate. In the case of a steam driven vehicle (e.g. locomotive) where the exhaust steam is employed to pump combustion gas through the boiler, the exhaust steam pressure corresponds directly to the boiler’s evaporation rate (but not in a linear manner).

[0051] Thus, the differential pressure developed by the flow meter 17 can be set to be within the same range as the exhaust steam pressure of the locomotive (measured by tapping into the exhaust chambers of the engine or by a pitot tube immediately before the exhaust nozzle) then the working water level of the boiler may be set to match the locomotive’s exhaust steam pressure above a pre-set minimum water level for idling / standby purposes (typically 25% of the maximum).

[0052] The result of the above arrangement is that the effective proportion of watersteam in the heat exchanger remains relatively constant, independent of the evaporation rate, which eliminates the problems associated with a rapid increase in the evaporation rate overwhelming the steam separator 11 and the circulating pump 12; this can result in water being carried over into the steam output of the engine which can rapidly cause substantial damage to the equipment being supplied with steam.

[0053] If the vehicle being driven by steam is being manually controlled, the operator can monitor the reading of both the differential pressure gauge 16 and the exhaust steam pressure and use this to adjust the feed water supply through the feed water injector 20. If the vehicle is being automatically controlled, the water level compensation system is included in the automatic control by arranging the water level setpoint as a function of exhaust steam pressure, or as a function of some alternative measure of the evaporation rate e.g. a flowmeter within the boiler’s main steam outlet. In this case, the control system would in effect measure four values in order to control the feed of water into the boiler through feed water injector 20. A further improvement is to arrange the working water level to be a function of the boiler steam pressure, in addition to being a function of the evaporation rate. This compensates for the variation of the density of the steam and water flowing through the flow meter as the static pressure of the boiler changes. This is particularly important during the warm up phase of the boiler’s operation, as it ensures that the water level remains relatively constant and thus prevents the boiler from becoming overfilled.

[0054] The control systems described with reference to Figures 1 and 2 essentially show the simplest forms of effective control systems, and may be varied for specific requirements and capacities. For example, more than one steam separator may be used, with two or more separators connected in series or in parallel.

[0055] Further, more than one pump may be incorporated in the circuit, either as a failsafe or if extra capacity is needed.

[0056] Another variation is the position at which the feedwater is introduced into the system it is preferred that the feedwater injector is connected between the water outlet of the steam separator and the positive displacement pump (as described above with reference to Figures 1 and 2), but it would be possible for the feedwater injector to be connected so as to inject directly into the steam separator or connected near the outlet of the heat exchanger. It would also be possible to inject feedwater at more than one point around the control system.

Claims

Claims1. A control system for a forced circulation water tube steam generator, in which the system includes:• a heat exchanger having an inlet and an outlet and arranged in use to be exposed to a heat source, such that steam can be generated from water passing through the heat exchanger;• a steam separator having a steam outlet and a water outlet, and an inlet which is 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 water / steam emulsion to the inlet of the heat exchanger;• a feed water injector connected between the outlet of the heat exchanger and said positive displacement pump;• a flowmeter, connected between the water outlet of the steam separator and the inlet of the heat exchanger.

2. The control system as claimed in Claim 1 , wherein the heat exchanger is a single path heat exchanger.

3. The control system as claimed in Claim 1 , wherein the heat exchanger is a multipath heat exchanger.

4. The control system as claimed in Claim 3, wherein the system further includes an outlet header which is connected to the heat exchanger outlet such that all of the heat exchanger circuits discharge into said header.

5. The control system as claimed in any one of the preceding claims wherein the feedwater injector is connected between the water outlet of the steam separator and said positive displacement pump.

6. The control system as claimed in Claim 5, wherein said flowmeter is connected between said feed water injector and said positive displacement pump.

7. The control system as claimed in Claim 5, wherein said flowmeter is connected between said positive displacement pump and said heat exchanger.

8. The control system as claimed in any one of the preceding claims, wherein said flowmeter is a Venturi type flowmeter.

9. The control system as claimed in any one of the preceding claims, further including a differential pressure gauge which is connected to said flowmeter.

10. The control system as claimed in any one of the preceding claims wherein an inlet header is connected immediately before the heat exchanger.

11. The control system as claimed in any one of the preceding claims wherein said positive displacement pump comprises two or more pumps.

12. The control system as claimed in any one of the preceding claims wherein said steam separator comprises two or more steam separators.

13. A method of operating the control system as claimed in Claim 1, wherein said positive displacement pump operates at a substantially constant volume flow rate.

14. The method as claimed in Claim 13, wherein the system is adjusted such that fluid entering said pump is an emulsion of water and steam.

15. The method as claimed in Claim 14, wherein the emulsion of water and steam is between 10% and 90% water by volume.

16. The method as claimed in any one of Claims 13-15, wherein the flowmeter is sensitive to any variation of the density of the fluid being measured.

17. The method as claimed in any one of Claims 13-16, wherein the control system includes a differential pressure gauge connected to said flowmeter, which is set up so that the reading of said flowmeter corresponds to the target working water level of theboiler when the flow of water through the flowmeter is three times the maximum rate of evaporation from the boiler.

18. The method as claimed in Claim 17, wherein the control system simultaneously measures values for the flow of excess water from the heat exchanger, the flow of feed water into the boiler, and the flow of water / steam emulsion into the heat exchanger.

19. The method as claimed in any one of Claims 13-18, wherein the control system is adjusted to arrange the working water level to be a function of both the boiler steam pressure and of the evaporation rate.

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