GB2137378A

GB2137378A - Dissolved oxygen control

Info

Publication number: GB2137378A
Application number: GB08302422A
Authority: GB
Inventors: Michael Raymond Hawkins
Original assignee: LTH ELECTRONICS Ltd
Current assignee: LTH ELECTRONICS Ltd
Priority date: 1983-01-28
Filing date: 1983-01-28
Publication date: 1984-10-03
Also published as: GB8302422D0; GB2137378B

Abstract

The dissolved oxygen level of circulating boiler water is controlled by sensing the temperature of water at a predetermined location 8 in the boiler circulation system and injecting a reducing agent at 7 into the boiler circulation system at a rate proportional to the difference between the sensed temperature and the reference temperature. <IMAGE>

Description

SPECIFICATION Dissolved oxygen control This invention is concerned with controlling the dissolved oxygen level of circulating boiler water.

Dissolved oxygen in boiler water is undesirable. It has previously been suggested to control dissolved oxygen in boiler water by the injection of a sodium bisulphite solution into the feed water by means of a pump. To allow for variations in water temperature and hence in the oxygen level, it has previously been suggested to supply the sodium bisulphite solution at a rate in excess of that which would be required under normal or average conditions.

Clearly this is wasteful and may lead to excessively reducing conditions being created in the circulating boiler water.

According to one aspect of the present invention, we provide apparatus for controlling the dissolved oxygen level of circulating boiler water, comprising: means for sensing the temperature of water at a predetermined location in the boiler circulation system; a metering pump adapted to inject a solution of a reducing agent into the boiler circulation system; and control means coupled to said sensing means and to said pump and adapted to control the metering rate of said pump in dependence on the difference between the sensed temperature and a reference temperature.

In a second and alternative aspect of this invention, we provide a method for controlling the dissolved oxygen level of circulating boiler water, comprising the steps of: sensing the temperature of water at a predetermined location in the boiler circulation system and injecting a reducing agent into the boiler circulation system at a rate proportional to the difference between the sensed temperature and a reference temperature.

In a preferred arrangement the control means include a relay coupled in series with the motor pump and a power source therefor, the relay receiving a pulsed signal whereby to connect the pump motor circuit for a time tin a repeating cycle of time T such that tfr is proportional to the said temperature difference.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a boiler circulation system illustrating the application of the invention thereto; and Figure 2 is a schematic circuit diagram for the preferred embodiment of control means for use in apparatus in accordance with the present invention.

In the schematic boiler circuit illustrated in Figure 1, the boiler 1 has a circulation path schematically illustrated at 2. Since the boiler suffers inevitable steam losses, schematically indicated at 3, it is necessary to supply make-up water from a supply 4 therefor.

A solution of a reducing agent, suitably sodium bisulphite from a supply 5 therefor is pumped by means of a pump 6 into the circulating feed water system at a point 7.

In the practice of the present invention, the temperature Ox of the circulating boiler water is sensed by means of a temperature probe 8 upstream of the location 7. The temperature Ox will vary with time dependent upon the amount of incoming cold make-up water from the supply 4 and the particular boiler loading. The maximum possible concentration of dissolved oxygen at this temperature Ox can be represented by the empirical relation: Cx = k(Om - Ox) (1) where Cx is the concentration of dissolved oxygen and k and Om are constants.

Control means, generally indicated 9 are coupled between the temperature probe 8 and the motor 10 for the pump 6. The control means is arranged to control the dosage rate of sulphite metered by the pump in proportion to (Om - Ox), whereby the concentration of sulphite in the feed water will generally follow the variation of oxygen concentration, or in other words that Cs = k'(Om - Ox) (2) where Cs is the concentration of sulphite and k' is a constant.

A A simplified circuit diagram for a preferred embodiment of control means 9 is illustrated in Figure 2. The resistance Rx of temperature probe 8 at temperature < 3x is connected in a bridge circuit 11 with resistors R1 R2 and R3. An amplifier, generally indicated 12, is connected to opposite terminals 13 and 14 of the bridge circuit 11. A DC voltage supply is connected across the remaining terminals 15 and 16 of bridge circuit 11.

A saw-tooth generator 17 adapted to provide a positive saw-tooth wave-form voltage VA is connected to the - input of comparator amplifier 18. Output voltage VB from amplifier 12 is connected to the + input of comparator amplifier 18. Amplifier 18 is shunted by a resistor R4 and its output is connected via a resistor R5 and a diode 19 to a transistor 20.

A positive voltage output from comparator 18 will cause transistor 20 to conduct so energizing coil 21 of a relay RL. Relay RL is connected in series with motor 10 for pump 6 and its voltage supply (not shown) so that when the relay coil 21 is energized, pump 6 operates to pump sodium bisulphite solution at a constant rate into the boiler water circulation.

As illustrated, a light emitting diode 22 with shunting resistance 23 may be connected in series with relay coil 21 and transistor 20.

Resistors R1, R2 and R3 of the bridge circuit 11 are chosen such that the bridge circuit 11 is balanced when the temperature at sensing probe 8 is Om. At temperature Ox less than Om, the amplified bridge voltage VB will be proportional to (Om - Ox) and is positive. This voltage VB appearing at the + input terminal of comparator 18 will cause transistor 19 to conduct and hence relay coil 21 to be energized until the rising saw-tooth wave-form VA exceeds Ve, where-upon the transistor 20 will cease to conduct, relay coil 21 will be de-energized, and the pump motor circuit disconnected until the next cycle of the saw-tooth wave form.It will be seen that the time tin each cycle T of the saw-tooth wave-form in which relay coil 21 is energized is proportional to VB and thus is proportional to (Om - flux) By adjustment of the period of the saw-tooth wave-form and/or selection of the concentration of sodium bisulphite in the supply tank 5 therefor and/or selection of the displacement of pump 6 in dependence on the constants k and k', the dosage of sodium bisulphite can be adjusted to accurately follow the concentration of dissolved oxygen so as to maintain the oxygen concentration level at a minimum without wastage of bisulphite.

Although the invention has been described hereinabove with particular reference to the use of sodium bisulphite, persons skilled in this art will readily appreciate that the teachings of the present invention are equally applicable to systems in which an alternative reducing agent is employed.

CLAIMS (Filed on 30/1/84) 1. Apparatus for controlling the dissolved oxygen level of circulating boiler water, comprising: means for sensing the temperature of water at a predetermined location in the boiler circulation system; a metering pump adapted to inject a solution of a reducing agent into the boiler circulation system; and control means coupled to said sensing means and to said pump and adapted to control the metering rate of said pump in dependence on the difference between the sensed temperature and a reference temperature.

2. Apparatus according to Claim 1, wherein the control means comprises a relay coupled in series with the motor pump and a power source therefor, and means for providing said relay with a pulsed signal whereby to connect the pump motor circuit for a time tin a repeating cycle of time T, and means for adjusting t and/or Tsuch that tl T is proportional to the said temperature difference.

3. Apparatus according to Claim 2, wherein said relay is coupled in series with a transistor, the said transistor being coupled to the output of a comparator amplifier adapted to cause said transistor to conduct whereby to close said relay when the voltage at one input to said comparator amplifier exceeds the voltage of the other said input; a saw tooth generator being coupled to one of the two said inputs of said comparator amplifier and the other of the two inputs to said comparator amplifier being coupled to a bridge circuit including said sensing means in one arm thereof, whereby operatively to provide to said second input of said comparator amplifier a signal dependent upon the difference between the sensed temperature and said reference temperature, said bridge circuit being balanced when said sensed temperature is equal to said reference temperature.

4. For controlling the dissolved oxygen level of circulating boiler water, apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

5. A boiler coupled to a circulation system for circulating boilerwater, and including an apparatus according to any preceding claims.

6. A method for controlling the dissolved oxygen level of circulating boiler water, comprising the steps of: sensing the temperature of water at a predetermined location in the boiler circulation system and injecting a reducing agent into the boiler circulation system at a rate proportional to the difference between the sensed temperature and a reference temperature.

7. A method according to Claim 6, wherein injection of said reducing agent is by a metering pump energized for a time tin a repeating cycle of time T, tand/or T being adjusted such that the ratio tl T is proportional to the said temperature difference.

8. A method according to Claim 7, wherein said metering pump is energized by means of a relay coupled to a transistor adapted to be rendered conductive when one of a saw tooth signal and an unbalance signal from a bridge circuit including means for sensing the temperature of water at said predetermined location in one arm thereof exceeds the other said signal.

9. For controlling the dissolved oxygen level of circulating boiler water, a method substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

into the boiler water circulation.

As illustrated, a light emitting diode 22 with shunting resistance 23 may be connected in series with relay coil 21 and transistor 20.

Resistors R1, R2 and R3 of the bridge circuit 11 are chosen such that the bridge circuit 11 is balanced when the temperature at sensing probe 8 is Om. At temperature Ox less than Om, the amplified bridge voltage VB will be proportional to (Om - Ox) and is positive. This voltage VB appearing at the + input terminal of comparator

18 will cause transistor 19 to conduct and hence relay coil 21 to be energized until the rising saw-tooth wave-form VA exceeds Ve, where-upon the transistor 20 will cease to conduct, relay coil 21 will be de-energized, and the pump motor circuit disconnected until the next cycle of the saw-tooth wave form.It will be seen that the time tin each cycle T of the saw-tooth wave-form in which relay coil 21 is energized is proportional to VB and thus is proportional to (Om - flux) By adjustment of the period of the saw-tooth wave-form and/or selection of the concentration of sodium bisulphite in the supply tank 5 therefor and/or selection of the displacement of pump 6 in dependence on the constants k and k', the dosage of sodium bisulphite can be adjusted to accurately follow the concentration of dissolved oxygen so as to maintain the oxygen concentration level at a minimum without wastage of bisulphite.

Although the invention has been described hereinabove with particular reference to the use of sodium bisulphite, persons skilled in this art will readily appreciate that the teachings of the present invention are equally applicable to systems in which an alternative reducing agent is employed.

CLAIMS (Filed on 30/1/84) 1. Apparatus for controlling the dissolved oxygen level of circulating boiler water, comprising: means for sensing the temperature of water at a predetermined location in the boiler circulation system; a metering pump adapted to inject a solution of a reducing agent into the boiler circulation system; and control means coupled to said sensing means and to said pump and adapted to control the metering rate of said pump in dependence on the difference between the sensed temperature and a reference temperature.
2. Apparatus according to Claim 1, wherein the control means comprises a relay coupled in series with the motor pump and a power source therefor, and means for providing said relay with a pulsed signal whereby to connect the pump motor circuit for a time tin a repeating cycle of time T, and means for adjusting t and/or Tsuch that tl T is proportional to the said temperature difference.
3. Apparatus according to Claim 2, wherein said relay is coupled in series with a transistor, the said transistor being coupled to the output of a comparator amplifier adapted to cause said transistor to conduct whereby to close said relay when the voltage at one input to said comparator amplifier exceeds the voltage of the other said input; a saw tooth generator being coupled to one of the two said inputs of said comparator amplifier and the other of the two inputs to said comparator amplifier being coupled to a bridge circuit including said sensing means in one arm thereof, whereby operatively to provide to said second input of said comparator amplifier a signal dependent upon the difference between the sensed temperature and said reference temperature, said bridge circuit being balanced when said sensed temperature is equal to said reference temperature.
4. For controlling the dissolved oxygen level of circulating boiler water, apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawing.
5. A boiler coupled to a circulation system for circulating boilerwater, and including an apparatus according to any preceding claims.
6. A method for controlling the dissolved oxygen level of circulating boiler water, comprising the steps of: sensing the temperature of water at a predetermined location in the boiler circulation system and injecting a reducing agent into the boiler circulation system at a rate proportional to the difference between the sensed temperature and a reference temperature.
7. A method according to Claim 6, wherein injection of said reducing agent is by a metering pump energized for a time tin a repeating cycle of time T, tand/or T being adjusted such that the ratio tl T is proportional to the said temperature difference.
8. A method according to Claim 7, wherein said metering pump is energized by means of a relay coupled to a transistor adapted to be rendered conductive when one of a saw tooth signal and an unbalance signal from a bridge circuit including means for sensing the temperature of water at said predetermined location in one arm thereof exceeds the other said signal.
9. For controlling the dissolved oxygen level of circulating boiler water, a method substantially as hereinbefore described with reference to the accompanying drawing.