WO1990003496A1

WO1990003496A1 - Steam boiler system

Info

Publication number: WO1990003496A1
Application number: PCT/GB1989/001126
Authority: WO
Inventors: Stephen David Evans
Original assignee: Spirax Sarco Limited
Priority date: 1988-09-27
Filing date: 1989-09-26
Publication date: 1990-04-05
Also published as: GB8822584D0; ZA897036B; AU4322089A; ES2015235A6

Abstract

A method and apparatus for the control of the chemical dosing of boiler water wherein a steam boiler system includes means (60) for dosing the boiler feed water with an oxygen scavenger in dependence upon both the boiler feed water flowrate and boiler feed water temperature.

Description

STEAM BOILER SYSTEM

This invention relates to a steam boiler system, and in particular to an improved apparatus and method for the control of the chemical dosing of boiler water.

The function of a boiler is to transfer heat produced by the combustion of fuel to water confined within the boiler, in order to generate clean, dry steam under pressure. A boiler can continue to function efficiently only if the heat transfer surfaces, and the other waterways within the boiler system, are maintained in a clean and intact condition by proper control of boiler water quality.

It is known that impurities in the boiler water can produce deposits such as scale which may both restrict water circulation and retard the transfer of heat to the water, in both cases causing the metal of the heat transferring surface to be inadequately cooled, resulting in the metal finally becoming so hot and weakened that it can no longer withstand the operating pressure. Furthermore, impurities in the boiler water and in the boiler feed water can cause corrosion of the metal with which the water is in contact, one suggestion being that the metal is caused to react with the dissolved water constituents to result in part of the metal being taken into solution. Whilst some workers believe that with correctly adjusted water conditions the initial corrosion results in the formation of a protective oxide layer which prevents any further corrosion, they also warn that this beneficial situation may be aborted by insoluble deposits on the metal surface, such as the above-mentioned scale, which may actively encourage corrosion of the underlying metal by inhibiting the formation of or by destroying such protective oxide layer.

When water is evaporated within the boiler to form steam, the concentration both of the dissolved and of the suspended solids in the boiler water tends to increase. The concentration of (all or some of ) the impurities present in the boiler water has to be kept below a specified maximum by a so-called blowdown procedure whereby a proportion of the boiler water is removed, intermittently or continuously, and replaced by less-contaminated feed water. The common impurities in water are dissolved gases, dissolved mineral salts (usually leached out of the ground in concentrations depending upon the geological location), dissolved organic matter (likely to be fulvic and humic acids and their salts, arising from the decomposition of dead plant materials) and suspended matter.

Where possible, these impurities are removed or reduced by an external treatment in a treatment plant for the boiler make-up water, but usually it is also necessary to have an internal treatment regime within the boiler system, usually by injecting selected chemicals into the boiler feed water i . e . prior to the water entering the boiler. An internal treatment regime is nearly universal for boilers in which there is a large boiler water content relative to steam output. One known internal treatment is by carbonate control or phosphate control. Thus sodium carbonate (or sometimes sodium hydroxide) is added directly to the boiler water to maintain a controlled reserve of carbonate alkalinity, and to preserve the "hardness salts" such as calcium carbonate, maqnesium hydroxide or silicate in the form of a mobile non-adherent sludge. With the known phosphate control, sodium phosphate and sodium carbonate or hydroxide are fed directly into the boiler water; by maintaining a reserve of soluble phosphate in the boiler water together with a caustic alkalinity of between 10% and 15% of the total dissolved solids concentration, the calcium hardness is caused to precipitate as a calcium phosphate, and the magnesium as hydroxide or silicate.

It will be apparent that these "internal" treatments necessarily add to the total dissolved solids in the boiler water; furthermore there is no guarantee that the precipitate will not have scale-forming tendencies or that it will be adequately mobile, so that many operators will in addition inject a sludge conditioner such as an organic nolvmer.

All gases are to some extent soluble in water, though the most important gases in the context of water treatment for boilers are dissolved oxygen and dissolved carbon dioxide, as these play an important part in supporting corrosion of various boiler metals. The concentration of these gases in rain water depends upon the atmospheric partial pressure, so that if water at 20 degrees Celsius is in contact with air at a pressure of 1013 mbar, there is 9.2 mg/l of dissolved oxygen and 0.5 mg/l of dissolved carbon dioxide. However if the make-up up water is obtained from ground waters, often the carbon dioxide amount will be greater, notably from the decay of surface vegetation in the catchment area and from bacterial decomposition of organic matter within the soil. It is known to add sodium sulphite or hydrazine to the feed water to reduce the residual dissolved oxygen, and to assist the alkalinity of the boiler water in creating a protective oxide (magnetite) film on the surface of the boiler metal in contact with boiler water. Finally volatile amines such as morpholine or cyclohexylamine and ammonia may also be dosed into the feed water system, and since these are volatile and alkaline they are not only carried from the boiler with the steam but usually re-dissolve in the steam condensate, imparting alkaline properties to it, and so serving to protect the condensate system from corrosion due to any carbon dioxide and oxygen which may be present in the steam or have gained access to the condensate system.

Thus in the attempt to ensure steam purity and protection from corrosion, not only is it necessary properly to control the total dissolved solid content of the boiler water, but also to control the allowed quantities of important individual constituents. Furthermore, with the increasing use of hotwells to permit the re-use of condensate return from downstream of the boiler, it is necessary both to limit and to treat condensation contamination, which may occur by ingress of cooling water from process plant or condensers. In particular, boiler feed water from a contaminated hotwell can be either acid or excessively alkaline or with constituents which too greatly reduce the boiler water alkalinity.

Since most steam boiler systems draw their make-up water from a single source, and supply steam to local users who have a condensate control system known to the boiler operator, the additive quantities for an individual boiler system are often laid dovm by the boiler manager from past experience. The boiler operator is then only reguired to check the instantaneous flow rate of the boiler feed water (being added to the boiler to replace the water used for steam production or withdrawn upon blowdown) and to inject the various additives in proportion to that flow rate. There are however a number of disadvantages to this conventional procedure. Extra additive is often specified, or more additive is injected by the operator than strictly indicated as necessary from the instantaneous flow rate, in order that there will be enough in the boiler should a higher flow rate occur between flow meter readings; often too little (if any) allowance is made for the quantities of total dissolved solids withdrawn from the boiler during blowdown, since these are usually not known with sufficient accuracy. Thus the quantity of total dissolved solids in the boiler water is inflated, the blowdown requirement is increased, and the cost of boiler additives escalates.

A particular disadvantage which we have recognised is that the amount of dissolved gas e.g. oxygen, being fed towards the boiler depends not only upon the instantaneous flow rate of the boiler feed water but also upon the temperature of that feed water. There is the further complication we have recognised, that for those boilers receiving their feed water from a hotwell, wherein a greater or lesser volume (as the user conditions dictate) of condensate return from downstream of the boiler is topped up with respectively a lesser or greater volume of cooler fresh make-up water, usually externally chemically treated as described above, the measured temperature of water passing a selected position along the feed water conduit can rapidly change; furthermore, often the hotwell water is stratified into temperature zones and this is reflected in the feed water temperature.

We thus propose a steam boiler system which includes a steam boiler, a feed water conduit to the boiler, and dosing means adapted to inject a gas scavenger into the conduit, means to obtain the water flow rate in the conduit,and means to obtain the temperature of the feed water in the conduit, in which the dosing means is actuated to inject a calculated dose of the scavenger into the conduit, the dose being calculated automatically in response to the obtained flow rate and temperature of the water.

Preferably the stean boiler system will include a hotwell upstream of the boiler, and the feed waaer conduit will then be between the hotwell and the boiler. Usefully, the scavenger will be selected to treat at least dissolved oxygen.

We also propose a method of operating a steam boiler system comprising a steam boiler, a boiler feed water conduit, and dosing means adapted to inject a dose of a gas scavenger into the conduit, which includes the steps of obtaining a measurement of the flow rate and a measurement of the temperature of water in the conduit, automatically calculating the dose to be injected in response to those measurements and a pro-prepared alc-orithm, and injecting that dose into the conduit at an injection position.

The algorithm will indicate the dosage needed under different flow rates and temperatures, for specified gas concentrations.

Preferably there will be a hotwell upstream of the boiler, with the boiler feed water conduit between the hotwell and the feed water inlet to the boiler; and the scavenger will be an oxygen scavenger. Preferably the flow rate is measured downstream of the position at which the dose is injected, whilst the temperature is measured upstream of tha t position . Usefully the injection position is selected to allow at least 30 seconds flowtime before the dosed water enters the boiler.

Conveniently the dosing means includes a diaphragm pump injecting a predetermined volume of additive on each stroke, the number of pumped strokes over a given period being controlled in response to the flow rate and temperature of water in the boiler feed water conduit. However, a gravity fed or equivalent dosage control arrangement can be used, such as a solenoid operated on-off valve which is held open for different time periods, or a variable orifice valve automatically moveable to a position including and between its fully open and its fully closed, positions.

The invention will be further described by way of example with reference to the accompanying drawings in which:- Fig.1 is a schematic steam boiler system;

Fig.2 is a graph showing the solubility of oxygen

in water from air at various temperatures

and pressures (1.0 to 4.45 bar absolute); and

Fig.3 is a schematic view of an automatic

control unit.

Hotwell 10 is upstream of boiler 12 to which it is connected by feed water conduit 14. In an alternative arrangement, boiler 12 is one of an array of boilers, fed from a common hotwell. Boiler 1 2 is heated by furnace 30 .

Boiler 12 has a steam outlet 15, blowdown outlet 13 and sludge blowdown outlet 20. Valve 21 controlling the sludge blowdown outlet is opened at the operator's discretion at delayed intervals so that deposited sludge can bo allowed to discharge. Blowdown outlet 13 is automatically controlled by a solenoid-operated on-off valve 22, held ooen under the instructions of control unit 100 (Fig.2) for varying periods in order to maintain the boiler total dissolved solids below a predetermined maximum (e.g. 3000ppm), the varying periods being selected to provide a blowdown rate which is a calculated proportion of the flow rate of the boiler feed water as measured by flow meter 24. The quantity of total dissolved solids being fed into the boiler 12 in the feed water conduit 14 is measured by impurity sensor 28.

The temperature of the steam at steam outlet 16 is measured by thermometer 17 (mercury-in-glass), whilst the feedwater temperature in conduit 14 is measured by thermometer 74. In alternative embodiments the temperature is measured by alcohol-in-glass thermometers, by measuring the change of electrical resistance by a Wheatstone's Bridge, or by a thermocouple.

Hotwell 10 is fed with returned condensate from downstream of steam outlet 16 by way of condensate return conduit 34; it is also fed with make-up water from an external treatment plant (not shown) by way of make-up water conduit 33. The make-up water is added to the condensate return so that the water level in hotwell 10 remains substantially constant. Flowmeter 41 measures the rate of flow of make-up water in conduit 38.

The water in hotwell 10 is additionally heated by steam from the flash steam recovery chamber 42 by way of steam conduit 43 and multi-nozzle tube 44. Thus the water in hotwell 10 and so in feed water conduit 14 is pre-heated before entry into the boiler and so does not substantially cool the water in boiler 12. The flash steam recovery chamber 42 is fed with blowdown water under pressure by way of blowdown conduit 46. The hot condensate (water) from chamber 43 is led by way of water return conduit 50 to a heat exchanger 36 before passing to drain 39, to pre-heat the make-up water flowing in conduit 3R. By moans of the flash steam recovery chamber 42, the multi-nozzle tube or equivalent 44 and the heat exchanger 36, approximately 80% of the heat withdrawn from the boiler 12 in the blowdown water can be recovered. Peristaltic diaphragm pumps 62, 64 draw respective doses of selected additives from reservoirs 5-3, 70 and inject them through the wall of conduit 14 into the boiler feed water at a rate proportional to the flow rate of water in feed conduit 14, as measured by flow meter 24. Reservoir 70 contains a known alkali, whereas reservoir 68 contains a known sludge dispersant e.g. a phosphate. Thus pumps 62 and 64 are controlled to pulse at a speed directly proportional to the instantaneous rate of flow of the feed water in conduit 14.

Reservoir 65 contains a known oxygen scavenger; and diaphragm pump 60 is pulsed in accordance with the invention under the instruction of control unit 100 at a rate depending upon the instantaneous rate of flow of the feed water in conduit 14 and in accordance with the instantaneous temperature of that water, as measured in this embodiment by thermometer 74. The injection position 72 is selected so that the oxygen scavenger spends at least 30 seconds in conduit 14 before reaching boiler 12.

The temperature probe 74 is arranged by control unit 100 to check the temperature of the feed water in conduit 14 at 1 second intervals, though in alternative embodiments the temperature is checked at between 0.25 second intervals and 10 second intervals. When the diaphragm nump is arranged to inject a standard amount each pulse, typically the diaphram pump 60 will inject one such dose every 2 seconds, though the injection rate can range from a dose every second to one every 10 seconds (or even less frequently) depending upon the requirement for additive. For a dosage fed by a varying (one-way) orifice valve, the valve will usually be in an open condition but with the size of the orifice being regularly or intermittently changed.

Fig. 2 shows the solubility of oxygen in water from air at various temperatures and pressures. The lowermost graph is for a pressure of 1 bar absolute, and the graphs thereabove in sequence are at pressures (in bars absolute) of 1.17, 2.38, 3.07, 3.76, and 4.45. Since the pressure of the water in feed conduit 14 is usually known and constant, it is often only necessary to measure the feed water temperature to learn the amount of dissolved oxygen being introduced towards the boiler 12; though the water pressure can be fed into control unit 100 as and when required.

As seen in Fig. 3 the control unit receives information from thermometers 17 and 74, from flow meters 23 and 41, from impurity sensor 24 and from sludge blowdown valve 21. The control unit 100 is energised by mains electricity through line 102. From the readings of impurity sensor 24, and flow meters 23 and 41 the control unit sets the blowdown valve 22; from the reading of flowmeter 28, the control unit 100 sets the pumping rate of pumps 58,70. From the readings of flowmeter 23 and thermometer 74 the control unit 100 sets the pumping rate of pump 60.

Advantages of our invention are that an excess of the oxygen scavenger is not used, so reducing the total dissolved solids introduced into the boiler, reducing the frequency or amount of blowdown, and also reducing t.he cost of boiler operation. Boiler management can be greatly improved.

Claims

1. A method of operating a steam boiler system comprising a steam boiler (12), a boiler feed water conduit (14) and dosing means (60) adapted to inject a dose of a eras scavenger into the conduit, characterised by the steps of obtaining a measurement of the flow rate and a measurement of the temperature of water in the conduit, automatically calculating the dose to be injected in response to those measurements and a pre-preoared algorithm, and injecting that dose into the conduit (14) at an injection position (72).

2. A method according to claim 1 characterised in that the dose is of an oxygen scavenger.

3. A method according to claim 1 characterised in that the measurement of the flow rate is made downstream of the said injection position, and in that the measurement of the temperature of the water is made upstream of the said injection position.

4. A method according to claim 1 characterised in that the injection position is selected to permit at least 30 seconds flowtime before the dosed water enters the boiler.

5. A method according to claim 1 characterised in that the dose is injected with the assistance of a pump (60) feeding a predetermined volume each stroke, the frequency of the pump strokes being controlled to provide the calculated dose.

6. A steam boiler system which includes a steam boiler (12), a feed water conduit (14) to the boiler, and dosing means (60) adapted to inject a gas scavenger into the conduit, means to obtain the water flow rate in the conduit, and means to obtain the temperature of the feed water in the conduit, characterised in that the dosing means is actuated to inject a calculated dose of the scavenger into the conduit, the dose being calculated automatically in response to the obtained flow rate and temperature of the water.

7. A steam boiler system according to claim 6 characterised by a hotwell (10) upstream of the boiler.

8. A steam boiler system according to claim 6 characterised by a peristaltic pump (62, 64) adapted to inject doses of selective additives into the boiler feed water in accordance with the instantaneous flow rate of water in the boiler feed conduit (14).

9. A steam boiler system according to claim 6 characterised by means (100) to obtain the temperature of the feed water at intervals of no more than 10 seconds.

10. A steam boiler system according to claim 6 characterised by means (100) to effect injection of the scavenger every 2 seconds, the scavenger being an oxygen scavenger.