GB2168153A - Liquid level control and indication - Google Patents

Abstract

A liquid level controller for use in, for example, a boiler has a vertically disposed heated probe (15, 16) adapted for location in a steam boiler or an associated water level measuring chamber (13) so as to be maintained relatively cool by the surrounding water in the boiler or chamber and connected to a temperature sensor adapted upon predetermined temperature rise in the probe caused by the probe no longer being surrounded by the water (i.e. fall in water level) to initiate level control and/or alarm and shutdown signals and functions. The probes (15, 16) is vertically disposed within the boiler or chamber extending upwards from an underside, or downwards from a topside. The two probes provide both low level, and high level analogue output signals. Compensation for varying boiling point may be provided. <IMAGE>

Description

SPECIFICATION Liquid level controller This invention relates to apparatus for primarily detecting and controlling the level of a liquid in a vessel, especially but not exclusively the level of water in a boiler. Secondarily, the apparatus is for initiating alarm and shut-down signals.

Such apparatus has many industrial applications and an example of a major industrial application lies in steam boiler where not only is the continuous and automatic detection and control of the water level important but the alarm and shut-down functions are also particularly significant since a fall of the water level in the boiler sufficient to uncover the heated surface can, without these functions being present, result in damage to the boiler structure which, at worst, can lead to a disasterous explosion.

Such apparatus for convenience is hereinafter referred to as a "liquid level controller" but it is to be clearly understood that the apparatus functions to detect, indicate and control liquid levels in a vessel, or, where circumstances require it, to detect and indicate liquid levels and to initiate alarm and shutdown signals and functions.

Also for convenience reference will hereinafter be made to "water" and to "boilers" but such references are not to be construed as in any way restricting the scope of the invention.

Liquid level controllers have been in use for many years to perform the functions of control, alarm, and shut-down and are now universally used. Most controllers are based on the principle of an electrical switch operated, often by magnetic coupling, by the movement of a float in the boiler water. It is frequently convenient to place the float in a chamber external to the boiler and connected to it by pipes to ensure that the level of water in the chamber follows that in the boiler. In other cases, the float can be contained in the boiler itself, but in those cases the controller is subject to considerable disturbance by the movement of the water, which can be quite violent.

With both the internal and external float arrangements, the essential mechanical movement of the controller can be vitiated by deposits arising from the water in the boiler and thereby fail to operate correctly which could result in structural failure of the boiler. To reduce the probability of this happening, it is necessary for the water level in the boiler to be reduced by "blowdown" to verify that the controller is acting correctly. With floats in external chambers the blowdown process is necessary also to remove any sludge which may have accumulated in the chamber or in the pipes connecting it to the boiler, which would prevent the level of water in the chamber being in equilibrium with that in the boiler.

Blowing down the chamber is a vitally important exercise and its neglect has resulted in many cases of boiler failure, in some cases involving fatalities.

Another approach to liquid level control and detection is to use the leakage current from an electrode which terminates at the water level. This is not subject to the failure, described above, due to impedance of movement of a float, but the leakage current which is used to produce the control or safety signals can be altered by the occurrence on the electrode of conducting or non-conducting scale or deposit, in the first case giving a false "safe" signal, and in the latter a false "unsafe" signal.

It is an object of the present invention to provide a liquid level controller which obviates or mitigates the drawbacks of existing liquid level controllers as hereinbefore described and which at the same time provides an analogue output signal.

According to the present invention there is provided a liquid level controller comprising a heated probe adapted for location in a steam boiler or associated water level measuring chamber so as to be maintained relatively cool by the surrounding water in the boiler or chamber and connected to a temperature sensor adapted upon predetermined temperature rise in the probe caused by the probe no longer being surrounded by the water (i.e. fall in water level) to initiate level control and/or alarm and shut-down signals and functions, the liquid level controller being characterised in that the heated probe is vertically disposed within the steam boiler or associated water level measuring chamber extending upwards from the underside, or downwards from the topside, of the steam boiler or associated water level measuring chamber to provide either a low level, or a high level, analogue output signal.

Preferably, there are two such heated probes provided vertically disposed as aforesaid to give both a low level and a high level analogue output signal.

More specifically, a liquid level controller comprises a pair of substantially vertical probes mounted side-by-side extending one from the underside and one from the topside of a boiler or associated measuring chamber, a heater element within each probe, and a temperature sensor or detector mounted within, and in thermal contact with, each probe adjacent to the heater element or being incorporated or inherent in the heater element and adapted to provide an analogue output which, in predetermined temperature rise (water level fall) conditions, initiate all or selected signals and/or functions of level control, level indication, alarm and shut down.

The heater element is preferably electrically heated by means of a low voltage current suitable for the boiler or chamber within which it is to be used.

The sensor output may be conducted to relays or solid state devices which code the output into signals which, for example operate a pump or valve controlling the water supply to the boiler so as to maintain the water level substantially constant. The signals may alternatively or additionally cause actuation of audible and/or visual alarm means and/or indicate the water level in the boiler.

Preferably the liquid level controller incorporates a water temperature compensation facility to accommodate varying boiler water temperatures.

Such a facility may incorporate a conductor associated with a probe having a heat store, the conductor extending into the water the level of which is to be detected (high level or low level), there being electrically coupled to the probe a thermocouple for controlling the probe heating element temperature to a preset value, and a pair of thermocouples electrically associated with the conductor, one in the water and one out, and serving respectively to sense the water temperature and the conductor temperature above the water level.

Where there are top and bottom probes to provide low and high level sensing the probes may be connected by a tube, thermocouples being provided (a) to control the heating element of the top probe to a maximum pre-set temperature value; (b) to sense the tube temperature above water level; and (c) to sense the water temperature and to control and maintain a minimum pre-set temperature value.

The temperature sensor can be a thermocouple, thermistor, resistance thermometer, or thermal expansion device, or the heating element may be its own temperature sensor by making use of its inherent resistance/temperature coefficient, in which case the electrical circuits are arranged to monitor continuously the resistance of the heating element, e.g. by using the well known Wheatstone's Bridge circuit. The liquid level controller, and its associated electrical or fluidic circuitry will cause the boiler to shut-down under the following cir cumstances:- 1. The temperature sensor detecting a high probe temperature due to water level or scale formation of sufficient thermal resistance on the external surfaces of the probe.

2. Accidental detachment of the temperature sensor from the probe surface.

3. Short circuit of the temperature sensor (if electrical), or of the heater element.

4. Open circuit of the temperature sensor or heater element.

5. Failure of electrical or other energy supply.

The liquid level controller will therefore, under all possible cdircumstances fail safe, and distinct from the other controllers referred to above which can faii to danger. The controller of this invention is, in fact, a model boiler, and factors which endanger the actual boiler react on the model beforehand and are thus prevented from endangering the boiler.

A liquid level controller according to the invention, therefore, has none of the drawbacks of a "float" controller. Also, it has the advantage that any scale or deposit occurring on the probe will not result in a false "safe" signal. In fact any scaling taking place will result in the probe temperature increasing leading eventually to an "unsafe" signal and to shut-down of the boiler. This gives warning of another unsafe condition, namely the formation of scale, which has been the cause of many boiler failures, (quite apart from a low water condition).

It is important in industrial applications where liquid level controllers are employed that these should be highly reliable and be capable of withstanding repetitious operation at intervals of, say, only a few minutes between each operation. Because of these requirements the present invention provides a liquid level controller which is considered to be substantially more robust and reliable than existing controllers and which uses thermoelectric means rather than mechanical means to detect and react to temperature changes in the probe or probes.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of the present invention with the liquid level control ler having both low level and high level sensing; Figure 2 is a diagrammatic view of the low level sensing; Figure 3 is a corresponding diagrammatic view of the high level sensing; Figure 4 is a numerical presentation of thermocouple (temperature sensor) outputs; Figure 5 is a presentation of high and low level thermocouple outputs connected to provide an analogue output from an amplifier; Figure 6 is a diagrammatic view of a liquid level controller with high level sensing according to the invention with water temperature compensation to accommodate varying boiler water temperature; and Figure 7 is a diagrammatic view of a liquid level controller with high and low level sensing and water temperature compensation to accommodate varying boiler water temperature.

In Figs. 1 to 3, the boiler is indicated at 10 and communicates through pipes 11 and valves 12 to an external chamber or electrode tube assembly 13. The water level in both the boiler 10 and chamber 13 is indicated by the reference 14.

In the chamber 13 are mounted two vertical heated thermocouple sensors 15 and 16 flanged mounted in the chamber 13 or elec trode tube assembly as indicated at 17. They may, it will be appreciated, be directly mounted into the boiler 10 in any convenient way and they may be separated by an internal partition 13A (see Fig. 1).

The heated thermocouple sensor 15 is for low level sensing and the thermocouple is indicated at 15A and the heater at 15B (see Fig. 2). It is mounted vertically from the underside of the chamber 13 and such mounting is important since it permits an analogue output signal to be received from the probe. The heated thermocouple sensor 16 is for high level sensing and similarly its thermocouple is indicated at 16A and its heater at 16B (see Fig. 3). Probe 16 is mounted vertically from the topside of the chamber 13 against to ensure an analogue output signal.

It will be manifest that the chamber or electrode tube assembly 13 and partition 13A is formed of a suitable material or materials and is constructed to withstand the pressures and temperatures generated in the boiler 10.

Fig. 2 shows, as aforesaid, the lower thermal probe 15 mounted vertically upwards in the water for low level sensing. At level 'A' the thermocouple 15A is clear of the water, the heater 15B being held at the water temperature and consequently the thermocouple output being held at the water temperature.

At level 'B' the thermocouple 1 5A and a section of the heater element 15B have emerged clear of the water. Since this section of the heater element 15B is no longer being cooled by the water surrounding it, then the thermocouple output increases and stabilises in proportion to the amount of heater element 15B now exposed in the free space, generating a temperature above the water temperature.

This progression can be continued until the cooling action of the water is lost to the heater element 15B. A temperature controller (not shown), preferably of the proportional band type, set at the maximum heater element temperature is provided to prevent damage to the heater element when the cooling action of the water is lost.

Fig. 3 shows, as aforesaid, the upper thermal probe 16 mounted vertically downwards in the liquid for high level sensing. At level 'C' the thermocouple 1 6A is clear of the water together with a section of the heater element 16B. The thermocouple 16A will have rected as previously described for low water sensing at level 'B'. As the water rises towards level 'D' the water will cool the heater element 16B and cause the output from the thermocouple 16A to decrease until level 'D' is reached when the output will match the temperature of the water not the heater element 16B.

Fig. 4 shows numerically the thermocouple outputs at normal water level 'A-C'. The low water sensing probe thermocouple output will be zero, while the high water sensing probe thermocouple output is '7'. Should the water level fall to 'B' then the low water sensing probe thermocouple output would increase, the high water sensing probe remaining at 7 due to the action of its temperature controller stabilising its heater element temperature.

Should the water level now rise to 'D', the low water sensing probe thermocouple output would return to zero and the high water sensing probe thermocouple output would decrease to zero.

Fig. 5 shows the high and low water sensing probe thermocouple outputs electrically connected to give the analogue output from the amplifier related to the water level falling from point 'D' to point 'B' (numericlly 14).

The analogue output is connected to electronic, solid state or otherwise, equipment for the purpose of liquid level indication and to initiate alarm and control functions at any desired output value for the purpose of controlling the actual liquid level by means of operating automatically controlled valves, pumps and alarm devices.

In Fig. 6, a heater element, say 16, is enclosed in a heat store 18 and is coupled to a conductor 19 extending into the water. To accommodate varying boiler water temperatures three thermocouples are provided. One thermocouple 20 on the heater element 16 controls the heat store temperature to a preset value while thermocouple 21 senses the temperature of the conductor 19 above water level and the thermocouple 22 senses the temperature of the water.

In Fig. 7, heater elements 15 and 16 are connected by a tube 23 with, on heater element 16, a thermocouple 24 serving to control the upper heating element to a maximum pre-set temperature; and, on the tube 23 above water level, a thermocouple 25 for sensing tube temperature; and at the junction of the tube 23 and the heater element 15 a thermocouple 26 serving to control the lower heater element to a minimum pre-set value.

Claims (11)

1. A liquid level controller comprising a heated probe adapted for location in a steam boiler or associated water level measuring chamber so as to be maintained relatively cool by the surrounding water in the boiler or chamber and connected to a temperature sensor adapted upon predetermined temperature rise in the probe caused by the probe no longer being surrounded by the water (i.e. fall in water level) to initiate level control and/or alarm and shut-down signals and functions, the liquid level controller being characterised in that the heated probe is vertically disposed within the steam boiler or associated water level measuring chamber extending upwards from the underside, or downwards from the topside, of the steam boiler or associated water level measuring chamber to provide either a low level, or a high level, analogue output signal.

2. A liquid level controller as claimed in claim 1, comprising two heated probes provided vertially disposed to give both a low level and a high level analogue output signal.

3. A liquid level controller as claimed in claim 1, comprising a pair of substantially vertical probes mounted side by side extending one from the underside and one from the topside of a boiler or associated measuring chamber, a heater element within each probe, and a temperature sensor or detector mounted within, and in thermal contact with, each probe adjacent to the heater element or being incorporated or inherent in the heater element and adapted to provide an analogue output which, in predetermined temperature rise (water level fall) conditions, initiates all or selected signals and/or functions of level control, level indication, alarm and shut down.

4. A liquid level controller as claimed in claim 3, in which the heater element is electrically heated by means of a low voltage current suitable for the boiler or chamber within which it is to be used.

5. A liquid level controller as claimed in claim 3 or 4, in which the sensor output is conducted to relays or solid state devices which code the output into signals which operate a pump or valve controlling the water supply to the boiler so as to maintain the water level substantially constant.

6. A liquid level controller as claimed in claim 5, in which the sensor output alternatively or additionally causes actuation of audible and/or visual alarm means and/or boiler shut-down and/or indicate the water level in the boiler.

7. A liquid level controller as claimed in any one of claims 1 to 6, comprising a water temperature compensation facility to accommodate varying boiler water temperatures.

8. A liquid level controller as claimed in claim 7, in which the water temperature compensation facility incorporates a conductor associated with a probe having a heat store, the conductor extending into the water the level of which is to be detected (high level or low level), there being electrrically coupled to the probe a thermocouple for controlling the probe heating element temperature to a pre-set value, and a pair of thermocouples electrically associated with the conductor, one in the water and one out, and serving respectively to sense the water temperature and the conductor temperature above the water level.

9. A liquid level controller as claimed in any one of claims 2 to 8, in which the top and bottom probes to provide low and high level sensing are connected by a tube, thermocouples being provided (a) to control the heating element of the top probe to a maximum pre-set temperature value; (b) to sense the tube temperature above water level; and (c) to sense the water temperature and to control and maintain a minimum pre-set temperature value.

10. A liquid level controller as claimed in any one of claims 3 to 9, in which the temperature sensor is a thermocouple, thermistor, resistance thermometer, or thermal expansion device, or the heating element may be its own temperature sensor by making use of its inherent resistance/temperature coefficient, in which case the electrical circuits are arranged to monitor continuously the resistance of the heating element, e.g. by using a Wheatstone's Bridge circuit.

11. A liquid level controller substantially as hereinbefore described with reference to the accompanying drawings.