GB2294748A

GB2294748A - Improvements in and relating to the regulation of the level of water in a pressurised boiler

Info

Publication number: GB2294748A
Application number: GB9426117A
Authority: GB
Inventors: George Robert Walker
Original assignee: BOILERHOUSE SERVICES Ltd
Current assignee: BOILERHOUSE SERVICES Ltd
Priority date: 1994-12-23
Filing date: 1994-12-23
Publication date: 1996-05-08
Anticipated expiration: 2014-12-23
Also published as: GB2294748B; GB9426117D0

Abstract

A system for regulating the level of water in a pressurised boiler (10), preferably a steam boiler, comprising level detection means (18), a control unit (24) and a pump (26) having a variable speed motor located in a water supply line to the pressurised boiler (10). According to an embodiment of the invention, the speed of the motor (26) is controlled by the control unit (24) so that it is proportional to the distance between the level of water (28) in the pressurised boiler (10) and a nominal level. The speed of the motor dictates the rate of flow of water into the boiler and thereby the level of water may be regulated. <IMAGE>

Description

Title: Improvements in and relating to the regulation of the level of water in a pressurised boiler.

DESCRIPTION This invention relates to the regulation of the level of water in a pressurised boiler and, in particular, to the regulation of the water level in a pressurised steam boiler.

It is known to provide an automatic system for the regulation of the level of water in a boiler in order to replace water lost through evaporation or drained from the boiler through normal use.

One such system comprises a pump located in an infeed line which connects a feeder tank to a boiler and means for detecting a drop in the level of water in the boiler. In use, the detection of a drop in the level of water below a predetermined level results in the pump switching into operation and pumping cold water into the boiler in order to restore the level of water. The pump is switched off once the level of water has reached its normal position.

A problem associated with this system is that a large volume of cold water is pumped into the boiler in a short period of time and consequently the temperature of the water will drop below the required temperature.

This is of particular concern for pressurised steam boilers wherein the inflow of a large supply of cold water is likely to result in the boiler being unable to produce steam until the cold water has been heated.

Another system comprises a float having an inductance coil which is mounted on the boiler shell, a control unit connected to a modulating valve located in an infeed line connecting a feeder tank to the boiler and a pump which continuously pumps water through the infeed line. A change in the water level in the boiler will result in a change of the inductance value of the coil and this change is transmitted to the control unit which modulates the valve accordingly in order to counteract the change in the water level, ea. if the water level drops then the control unit will cause the valve to open further.

This system does reduce the problem experienced with the former system since the level of water is monitored more closely and as a result the volume of cold water flowing into the boiler in a given time period is less. However, the valve does take time to respond to a signal from the control unit and as a result the level of water in the boiler tends to fluctuate between positions above and below the required nominal position.

A further disadvantage of this system is that the pump is required to operate continuously and at its optimum speed.

An aim of the present invention is to provide an improved system for the regulation of the level of water in a pressurised boiler.

In accordance with the present invention there is provided a system for regulating the level of water in a pressurised boiler comprising level detection means, a control unit and a pump having a variable speed motor located in a water supply line to the pressurised boiler wherein said control unit controls the speed of the motor according to a signal received from the level detection means thereby enabling the flow of water into the pressurised boiler to be regulated.

The level detection means may measure the change between a nominal level and the actual level of water in the pressurised boiler. Alternatively, the level detection means may simply detect a change in the actual level.

It is preferred that the pump is controlled such that the rate of flow of water into the pressurised boiler is proportional to the distant between a nominal water level and the actual level ie the greater the distance the greater the rate of flow of water into the pressurised boiler.

The level detection means may comprise a moveable component wherein said component may move in tandem with the level of water. Alternatively, said level detection means may comprise a fixed component having a property which varies with the water level.

Said property may be resistance or may be capacitance.

The level detection means may comprise a float and may further comprise one or more induction coils but, more preferably, comprises a capacitance probe.

The capacitance probe may be located in a chamber outside the pressurised boiler or may, alternatively, be located in said pressurised boiler. The level detection means may comprise a transmitter such that a change in the capacitance of the probe as a result of a change in the water level in the pressurised boiler results in a corresponding signal being transmitted to the control unit.

The transmitter may send a signal of a predetermined current value corresponding to a given distance of the level of liquid in the boiler below a predetermined nominal level.

The level detection means preferably has a poer input and a signal output line. The output signal from said detection means is preferably variable and may vary in accordance with the level of water in the boiler.

The current value of the output signal may be variable. The current value of said signal may increase as the level of water falls. Said current value may increase from a minimum to a maximum value. The minimum value may correspond to the level of the water being at a predetermined nominal position and the maximum value may correspond to the level of water being at or below a predetermined minimum position.

The increase in said current value of the signal may be proportional to the position of the level of water below said nominal position.

The output signal may have a current value in the range of 1 to 40mA. Preferably, said current value is in the range of 4 to 20mA. Alternatively, said output signal is a voltage and may be in the range of 1 to 10 volts.

Preferably, the control unit is calibrated such that a received signal results in the control unit regulating the speed of the pump a corresponding amount in order to counteract the change in the level of water.

For example, if the current value of the received signal is at a maximum then the control unit controls the pump such that it operates at its maximum speed.

The speed of the motor of said pump may be controlled by varying the frequency of its power supply.

Preferably, the speed of the motor is controlled by the control unit depending upon the difference between the level of water in the pressurised boiler and the predetermined nominal level of water.

In a preferred embodiment of the invention, said speed of the motor is proportional to said difference.

The speed may increase in proportion to the distance of the level of water below said nominal level.

In a preferred embodiment of the invention said pump operates continuously at least at a minimum speed.

The pump may operate in the range of 1,000 to 3,000 r.p.m. Preferably, the pump operates at its minimum speed when the level of water in the pressurised boiler is at its predetermined nominal level. With the pump operating at its minimum speed it is preferred that the pressure of water being pumped through the water supply line is less than the pressure in the boiler.

Preferably, a bleeder line is connected to the water supply line at a position between said pump and boiler.

The system may have a high water level alarm.

The alarm may activate upon the detection of water above a predetermined level and preferably results in the control unit operating the pump at its lowest speed.

A valve may be incorporated in the water supply line to the boiler and said valve may be closed upon the activation of said alarm.

A low level alarm may also be provided whereby the detection of water below a predetermined level preferably results in the control unit operating the pump at its maximum speed thereby ensuring the maximum possible flow of water into the boiler.

The pressurised boiler is preferably a pressurised steam boiler.

Referring to the accompanying schematic drawing, an embodiment of the present invention will now be described by way of example only.

A system for regulating the level of water in a pressurised steam boiler is shown in the accompanying drawing. A pressurised steam boiler 10 has a water feed tank 12 and a line 14 connecting said tank to the boiler 10 with a non-return value 16 located in the line.

The regulation system comprises a capacitance probe 18, power unit 20, electrical variable speed controller 24 and pump 26. A bleeder line 27 is connected to line 14 and at a second position located between pump 26 and tank 12.

The capacitance probe 18 is capable of sending a signal to the controller 24 having a current value in the range of 4-20mA.

The current value of the signal is dependent upon the water level 28 in boiler 10. With the water level at a predetermined nominal position the current value of the signal is at its lowest value, ie 4mA.

The value of the current of the signal increases as the level of water in the boiler falls until a maximum value of current is reached ie 20mA, which corresponds to a predetermined minimum level of water.

For example, the greater the distance of the actual level of water below the predetermined nominal position the greater the current value of the output signal assuming that the predetermined minimum level has not been reached.

The speed of the motor of the pump operates in the range 1,700 r.p.m to 2,800 r.p.m and is proportional to the frequency of the pump's voltage supply. The speed can be continuously increased or decreased by varying the frequency of the voltage input in the range of 30 to 50 HZ. Hence, an increase in the frequency of the voltage input will result in an increase in the speed of the pump. For example, the top speed of the motor is obtained by applying a voltage with a frequency of approximately 50HZ to the input terminals of the motor, whilst the minimum speed is obtained by applying a frequency of 30 HZ.

A mains supply (not shown) is fed to the variable speed controller 24 and the frequency of the output voltage supplied to the pump can be varied in order to vary the speed of the motor. The frequency of the output voltage is proportion to the current input to the controller 24 which is itself, as mentioned above, dependent upon the level of water in the boiler.

The pump is selected such that the pressure of water in line 14, between the pump and boiler, generated by the pump operating at its lowest speed is less than the pressure generated in the boiler. In order to pump water into the boiler it is necessary for the pressure of the water in the line to be greater than that in the boiler and consequently no water will flow into the boiler with the pump operating at its lowest speed.

The operation of the system will now be described with initially the level of water in the pressurised steam boiler at its predetermined nominal position.

Since the level of the water is at its predetermined nominal position the current value of the signal from the capacitance probe 18 will be 4mA ie the lowest possible current value. The signal is subsequently received by the speed controller 24 which detects the current value of the signal and outputs a voltage having a frequency which is proportional thereto which in this case will be 30HZ ie the lowest frequency value, which will drive the pump 26 at its lowest speed. At this speed of operation, the pump is unable to generate a sufficient pressure to allow water in line 14 to enter the boiler. The non-return value 16 prevents water from the boiler flowing back into line 14. The bleeder line 27 ensures that although water cannot be pumped into boiler 10 there is still water being pumped through the line.

A sudden drop in the water level due to a large drain of steam from the boiler will result in a large increase in the current value of the signal from the capacitance probe to the variable speed controller 24.

The increase in the current value will result in a corresponding increase in the frequency of the output voltage and consequently the speed of the pump will increase proportionally. The resulting speed of the pump is sufficient to generate a pressure in the line which is greater then the pressure in the boiler and hence pump 26 is able to pump water into the boiler. As the water level increases in the boiler as a result of the inflow of water, the current value of the signal will decrease progressively and hence the speed of the motor will also decrease in a correspondingly progressive manner with the result that the rate of flow of water into the boiler will decrease until the water level reaches its nominal level.At the nominal level the frequency of the output voltage will have reached its lowest value resulting in the pump operating at its lowest speed which effectively switches off the water supply to the boiler.

The system comprises a high and low level alarm whereby if the level of water in the boiler extends above or below predetermined levels, an alarm is activated.

The above described system results in a fine control of the water level within the boiler and since the pump is not always working at its optimum speed there is a significant saving in power compared to the above mentioned prior art system wherein the pump is working continuously at its full speed. Since the pump of the present invention operates at lower speeds there is also a reduction in the wear and tear of the motor and hence the life of the pump is extended.

Due to the fact that the pump is operating continuously, at least at a low speed, the response time of the system to a change in the level of water is fast and hence the problem associated with the water level fluctuating above and below a predetermined normal level is not experienced to the same extent as that of the prior art systems.

Claims

1. A system for regulating the level of water in a pressurised boiler comprising level detection means, a control unit and a pump having a variable speed motor located in a water supply line to the pressurised boiler wherein said control unit controls the speed of the motor according to a signal received from the level detection means thereby enabling the flow of water into the pressurised boiler to be regulated.

2. A system as claimed in claim 1 wherein the speed of the motor and thereby the rate of flow of water into the pressurised boiler is dependent upon the difference between the level of water in the pressurised boiler and a predetermined nominal level of water.

3. A system as claimed in claim 2 wherein the speed of the motor is proportional to said difference.

4. A system as claimed in claim 3 wherein said speed increases in proportion to the distance of the level of water below said nominal level.

5. A system as claimed in any one of claims 1 to 4 wherein said motor operates at a maximum speed when the level of water is at or below a predetermined minimum level.

6. A system as claimed in any one of the preceding claims wherein said pump operates continuously at least at a minimum speed.

7. A system as claimed in claim 6 wherein the pump operates at its minimum speed when the level of water in the pressurised boiler is at its predetermined nominal position.

8. A system as claimed in claim 6 or 7 wherein when the pump operates at its minimum speed the pressure of water being pumped through the water supply line is less than the pressure in the boiler.

9. A system as claimed in any one of the preceding claims wherein a bleeder line is connected to the water supply line at a position between said pump and boiler.

10. A system for regulating the level of water in a pressurised boiler substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

10. A system as claimed in any one of the preceding claims wherein the pressurised boiler is a pressurised steam boiler.

11. A system for regulating the level of water in a pressurised boiler substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

Amendments to the claims have been filed as follows 1. A system for regulating the level of water in a pressurised boiler comprising level detection means, a control unit and a pump having a variable speed motor located in a water supply line to the pressurised boiler wherein the pump operates continually at least at a minimum speed wherein at said speed there is no flow of water into the boiler from the water supply line and water is pumped around a feedback loop connected to said line, and wherein said control unit controls the speed of the motor according to a signal received from the level detection means thereby enabling the flow of water into the pressurised boiler to be regulated.

2. A system as claimed in claim 1 wherein with the pump operating at said minimum speed the pressure of the water pumped through said water supply line and feedback loop is less than the pressure in the boiler.

3. A system as claimed in claim 1 or 2 wherein a non-return valve is located in the water supply line between the pressurised boiler and pump thereby preventing the flow of water from said pressurised boiler into the water supply line.

4. A system as claimed in claim 1, 2 or 3 wherein the speed of the motor and thereby the rate of flow of water into the pressurised boiler is dependent upon the difference between the level of water in the pressurised boiler and a predetermined nominal level of water.

5. A system as claimed in claim 4 wherein the speed of the motor is proportional to said difference.

6. A system as claimed in claim 5 wherein said speed increases in proportion to the distance of the level of water below said nominal level.

7. A system as claimed in any one of claims 1 to 6 wherein said motor operates at a maximum speed when the level of water in the pressurised boiler is at or below a predetermined minimum level.

8. A system as claimed in any one of claims 4 to 7 wherein the pump operates at its minimum speed when the level of water in the pressurised boiler is at its predetermined nominal position.

9. A system as claimed in any one of the preceding claims wherein the pressurised boiler is a pressurised steam boiler.