The invention relates to pressurised steam boilers and their control, in particular to to a method and apparatus for assessing the mass flow of steam from a steam boiler.

Operators of pressurised steam boilers frequently purchase steam flow meters to measure the steam flows in the steam exit lines from each of the boilers. A frequent reason for installing such meters is for auditing purposes, to enable the amount of steam exported from the boiler to be compared to the amount of fuel used by the boiler. Such meters are, however, expensive.

Document JP-A-09 119 602 (and corresponding Patent Abstracts of Japan vol. 1997, no. 09, 30 September 1997) describes a steam load analysis apparatus for a boiler that calculates the steam usage by measuring increases and decreases in steam pressure.

It is an object of the invention to provide a method and apparatus for assessing the mass flow of steam from a pressurised steam boiler without resorting to a steam flow meter.

According to the invention there is provided a method of assessing in a control unit the mass flow of steam from the boiler by processing of input signals including ones enabling assessments to be made of:

• a) the heat generated by combustion in the burner
• b) the temperature and pressure of the steam generated by the boiler
• c) the heat dissipated other than in the steam,

It should be understood that a designer is able to make some selections as to how accurate the assessments of a) to c) above are to be and therefore how many variables are to be measured and how accurately they are to be measured. For example, in order to assess the heat dissipated other than in steam an operator might merely measure the temperature of the combustion products and assume a certain further dissipation of heat by other means such as conduction, convection and radiation from the boiler housing.

By making an assessment of the mass flow of steam from measurements of other variables, the need for an expensive steam flow meter is avoided. Although it may appear that the measurement of several other variables in order to assess the steam flow is unnecessarily expensive and complicated, that need not be so because the other variables may be mainly or entirely ones that are being measured anyway for the purpose of controlling the operation of the pressurised steam boiler and burner.

Where reference is made to monitoring a variable, it should be understood that the variable itself may not be directly sensed but rather one or more other variables, from which the variable being monitored can be calculated, may be sensed. For example, the firing rate of the burner need not be directly sensed and the pressure of the water in the boiler may be sensed to indicate the pressure of the steam.

Variables measured to assess the heat generated by combustion in the burner may include the rate of feeding of fuel to the burner, and/or the composition of the combustion products.

Variables measured to assess the heat dissipated other than in the steam may include the temperature of the combustion products and/or the rate of feeding fuel to the burner.

In GB 2169726A, a fuel burner control system is described which includes flue gas sampling and analysing apparatus and which also includes a burner controller which is the subject of GB 2138610A. That control system already receives inputs relating to the rate of feeding fuel to the burner, the composition of the exhaust gases and the temperature of the exhaust gases. Furthermore it is common for a pressurised steam boiler control system to include sensors for measuring the temperature and pressure of the steam generated by the boiler. Thus it can be seen that all the variables required for the assessment of the mass flow of steam from the boiler may already be available without any extra sensors being required. If desired, however, one or more extra sensors may be provided. For example, a sensor for measuring the temperature of the water being fed into the boiler may be provided.

The assessment of the mass flow of steam from the boiler may be used only as a measure of the flow at a moment in time, or it may also or alternatively be used to provide an assessment of the aggregate amount of steam generated over a certain extended period of time. In the latter case, it may be necessary to allow for other losses within the system, when making the assessment, for example it may be appropriate to assume that a certain percentage of heat is lost during blow down of a boiler. For example an overall loss of 6 per cent might be allowed for.

Although the invention has been defined above with reference to a method, it will be understood that it is also embodied in an apparatus comprising a pressurised steam boiler.

The present invention still further provides a pressurised steam boiler including:

• a boiler housing for containing water in the boiler,
• a burner for heating water in the boiler and converting the water into steam,
• a pressure detector for detecting the pressure of steam in the boiler,
• a temperature detector for detecting the temperature of steam in the boiler,
• a fuel flow detector for measuring the flow rate of fuel into the burner,
• a further temperature detector for detecting the temperature of the exhaust gases,
• a control unit for receiving and processing input signals from all of said detectors and for assessing indirectly the mass flow of steam from the boiler, the boiler further including a still further temperature detector for detecting the temperature of water at an inlet to the boiler, the control unit being arranged to receive and process also an input signal from the still further temperature detector for assessing indirectly the mass flow of steam from the boiler.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, of which:

Fig 1

is a schematic drawing of a burner and a pressurised steam boiler and of a control unit for controlling the burner and steam boiler,

Fig 2

is a schematic drawing of the pressurised steam boiler of Fig 1,

Fig 3

is a sectional view of one of a pair of capacitance probe assemblies employed in the pressurised steam boiler shown in Fig 2, and

Fig 4

is a block circuit diagram of the signal control and processing arrangement provided in each capacitance probe assembly.

Referring first to Fig 1, there is shown a burner 20 having a burner head 21, a combustion chamber 22 and a duct 23 for combustion products which comprise exhaust gases. As will be described below the duct 23 passes through a pressurized steam boiler; thereafter the exhaust gases are vented through a flue.

Air is fed to the burner head 21 from an air inlet 24, through a centrifugal fan 26 and then through an outlet damper 27. The burner head 21 is able to operate with either gas or oil as the fuel; gas is fed to the burner head from an inlet 28 via a valve 29 whilst oil is fed to the burner head from an inlet 30 via a valve 31.

A control unit 1 is provided for controlling the operation of the burner and boiler. The control unit 1 has a display 2, a proximity sensor 3 for detecting that a person is nearby, and a set of keys 5 enabling an operator to enter instructions to the control unit. The purpose of the proximity sensor is not relevant to the present invention and will not be described further herein; its purpose is described in GB2335736A.

The control unit 1 is connected to various sensing devices and drive devices, as shown in the drawing. More particularly the unit is connected via an exhaust gas analyser 37 to an exhaust gas analysis probe 38 (which includes a temperature sensor), and to a flame detection unit 40 at the burner head. The control unit 1 is also connected via an inverter interface unit 41 and an inverter 42 to the motor of the fan 26 (with interface unit 41 receiving a feed back signal from a tachometer 26A associated with the fan 26), via an air servo motor 44 to the air outlet damper 27, to an air pressure sensing device 45 provided in the air supply duct downstream of the outlet damper 27, via fuel servo motors 46 to the fuel valves 29, 31 and to a further servo motor 47 for adjusting the configuration of the burner head 21.

The connections described above relate to the control of the burner 20 by the control unit 1. The control unit 1 is, however, also connected, via an RS485 link 48 to a further controller 49, which is shown in Fig 2 and whose functions are described below.

The combustion chamber 22 of the burner 20 is arranged inside a boiler 50 in a conventional manner. In Fig 1 the boiler 50 is shown schematically in chain dotted outline. Although Fig 1 suggests that the combustion chamber leads directly to the exhaust duct 23, it will be understood by those skilled in the art that in practice the gaseous products of combustion follow a serpentine path passing through the boiler 50 a few times before reaching the exhaust duct 23 and being exhausted to atmosphere.

Fig 2 provides a schematic representation of the boiler and shows a boiler housing 51 which in normal use is filled to approximately the height shown by dotted line L1 in Fig 2. It will be appreciated that the combustion chamber and ducting for the exhaust gases are not shown in Fig 2.

A water pipe 52 feeds water into the bottom of the boiler at a rate determined by settings of a variable speed pump 53 and via a motorized control valve 54. A temperature detector 59 senses the temperature of the water as it enters the boiler.

A steam outlet pipe 55 takes steam under pressure from the top of the boiler 51. The pressure of the steam taken from the boiler housing 51 is sensed by a pressure detector 56 while its temperature is sensed by a temperature detector 57. Mounted in the top of the boiler housing 51 are a pair of capacitance probe assemblies 58A and 58B. The capacitance probe assemblies are identical to one another and one is described below with reference to Figs 3 and 4.

The further controller 49 receives input signals from the following (excluding the connection via the RS485 link 48 to the control unit 1):

• a) each of the capacitance probe assemblies 58A and 58B;
• b) the steam temperature detector 57;
• c) the inlet water temperature detector 59;
• d) the control valve 54 (a feedback signal indicating the degree of opening of the control valve 54); and
• e) the pump 53 (a feedback signal indicating the setting of the pump).

In addition a signal from the pressure detector 56 is passed back along a line 60 (not shown in Fig 1) to the control unit 1 where it provides an input signal representing demand to the control unit.

The further controller 49 provides output signals to the following (excluding the connection via the RS485 link 48 to the control unit 1):

• i) the control valve 54 (to adjust the degree of opening of the valve);
• ii) the pump 53 (to adjust the setting of the pump);
• iii) a warning light and audible alarm 61A, 61B, respectively, which are activated when the water level falls to a first low water level below its normal operating range "first low");
• iv) a warning light and audible alarm 62A, 62B, respectively, which are activated when the water level falls to a second low water level below the first water level ("second low"); and
• v) a warning light and audible alarm 63A, 63B, respectively, which are activated when the water level rises to a high water level above its normal operating range.

It will be understood that the particular warning light and audible alarms that are employed may be varied from one application to another according to what is required.

In Fig 2, the dotted line L1 indicates the centre of the normal operating range of water level in the boiler. Also shown is a dotted line L2 marking the "first low", a dotted line L3 marking the "second low" and a dotted line L4 marking the high water level.

Referring now also to Fig 3, it can be seen that each capacitance probe assembly 58A, 58B includes a main body 70 and an elongate probe 71 which projects downwardly into the interior of the boiler and extends through the high water level (L4), the normal operating level (L1), the "first low" (L2) and the "second low" (L3). Since boilers vary in size the probes 71 are manufactured in various lengths and an appropriate length of probe is chosen for each boiler. For example, the probes may be available in lengths of about 0.5m, 1.0m and 1.5m.

Each probe 71 is formed from a central steel bar 72 surrounded by a sleeve 73 of dielectric material. Also a plug 74 of dielectric material is provided at the free end of the sleeve 73 to seal that end of the probe. Thus, in a manner that is know per se, the probe 71 forms together with the medium surrounding the sleeve 73 a variable capacitance. Since the capacitance is very dependent on whether the medium is water or steam the value of the capacitance is dependent upon how great a length of the probe is surrounded by water rather than steam. Thus, the capacitance of the probe provides an indication of the level of water in the boiler, for all levels between, and including, L3 and L4.

Within the main body 70 of the capacitance probe assembly, there is a secure physical and electrical connection to the probe and a printed circuit board 75 is mounted in an enlarged rear portion 76 of the main body 70, the board 75 carrying the necessary processing circuitry, which is shown in block diagram form in Fig 4.

Referring now also to Fig 4, there is shown the probe 71 marked as a varying capacitance, a reference capacitance 77, a relay 78 for alternately connecting the probe 71 and the reference capacitance in the circuit, an oscillator 79, a processor 80 which both controls the operation of the relay 78 and together with the oscillator 79 is able to provide a measure of the capacitance being sensed by detecting the frequency of a signal in a circuit incorporating the capacitance, and a driver 81 which transmits a signal from the probe assembly to the further controller 49. The connection between each probe assembly 58A, 58B and the further controller 49 is made via RS485 links.

In a particular example, the probe capacitance varies from 10pF to 200pf, the reference capacitance 77 is 120pF, the oscillator 79 is a 555 Type Oscillator, the processor 80 is an 80188 processor and the sleeve 73 is 12mm outside diameter, 6mm inside diameter and is made of PTFE (polytetra-fluoroethylene). As the probe capacitance varies due to a change in water level the frequency of the output from the probe assembly alters; typically, the frequency output is of the order of 45,000 Hz and a change of 1mm in water level alters the frequency by 20 Hz.

When connected in the control system shown in Figs 1 and 2, the capacitance of each probe 71 is measured alternately with the reference capacitance 77 of that probe. In the event of a change in temperature, that affects values of both the capacitance of the probe 71 and its reference capacitance 77, so that the change in value of the reference capacitance can be used to adjust the signal from the probe capacitance to compensate for such a temperature change. Also the controller 49 reads signals from each of the probe assemblies 58A, 58B alternately, although, if preferred, simultaneous readings may be obtained. Typically in a steam boiler, the water is somewhat turbulent at least near the surface and that is liable to give rise to some inaccuracy in the measurement made. Thus the controller 49 is arranged to allow for some discrepancy in the signals from the probe assemblies 58A, 58B, but apart from that checks both that the signal of the reference capacitance indicates the correct value of capacitance and that each of the probes 71 indicates the same value of capacitance and therefore the same water level. One particular way in which turbulence in the water can be allowed for and indeed even taken advantage of is described later.

The use of the two identical probe assemblies 58A, 58B each with its own reference capacitance for checking purposes and with all readings from both probe assemblies being checked against one another, results in an especially safe system.

The normal operation of the burner and boiler will be well understood by those skilled in the art from the description above and will not be described further herein. GB2138610A and GB2169726A both provide further details of the normal operation of the burner. The boiler operates in a conventional manner when the water level is normal and, via the controller 49, feeds back signals, for example indicating a dropping steam temperature, to the control unit 1. In the event that the water level in the boiler drops to below the average normal level, then the controller 49 is programmed to adjust the speed of the pump 53 at the water inlet to allow more water into the boiler; similarly, in the event that the water level in the boiler rises gradually a little above the average normal level, then the controller 49 is programmed to close the control valve 54 or reduce the speed of the pump 53 at the water inlet to allow less water into the boiler. In either case, however, the operation of the burner 20 is not affected because the output signals from the control unit 1 are not altered.

If, however, for example, the water level in the boiler falls to the level L2 shown in Fig 2, then the controller 49 reacts in various ways: firstly the warning light 61A and audible alarm 61B are actuated; secondly a signal is passed back via the RS485 link 48 to the control unit 1 which then shuts down the burner 20 by turning off the supplies of fuel and air to the burner head 21; thirdly, the inlet flow of water into the boiler 5 is increased by adjustment of the control valve 54 and/or the pump 53.

Provided that the water level then rises back towards the level L1, the controller 49 can reverse the measures described in the paragraph immediately above. If for some reason, however, the water level continues to fall, for example because the water inlet is blocked, then when it reaches the level L3 in Fig 2 the warning light 62A and the audible alarm 62B are activated and a further control signal sent from the controller 49 to the control unit 1, preventing the burner from being turned back on without manual intervention by an operator.

Similarly, if the water level in the boiler rises to the level L4 shown in Fig 2, then the controller 49 reacts in various ways: firstly the warning light 63A and the audible alarm 63B are activated; secondly a signal is passed back via the RS485 link 48 to the control unit 1 which then shuts down the burner 20 by turning off the supplies of fuel and air to the burner head; thirdly, the inlet flow of water into the boiler 5 is stopped by adjustment of the control valve 54 and/or the pump 53.

The linking of the control of the boiler and the control of the burner enables other more sophisticated and advantageous control techniques to be adopted. In particular, whereas a skilled person would expect the system to be programmed simply so that, whenever the water level rose, the inlet flow rate of water was reduced, that need not be the case.

Although a rise in water level in the boiler is usually a result of the amount of steam leaving the boiler per unit time being less at that time than the amount of water coming into the boiler per unit time, it is possible, paradoxically, for the rise in water level to occur even when the rate at which steam is leaving the boiler is greater than the rate at which water is coming into the boiler. As explained above, that can arise when there is a sudden demand for steam leading to a reduction in pressure in the boiler and consequent expansion of the small bubbles within the water in the boiler, causing the water to expand and thus the water level to rise. The described herein is able to identify this special circumstance as will now be described.

The reaction to an increasing water level is determined by assessing within the control system also how the steam pressure in the boiler, which is measured by the detector 56, is changing and how the firing rate of the burner 20, which can for example be assessed from the information in the control unit 1 of the amount of fuel being fed to the burner, is changing. The variables of water level, steam pressure and firing rate can each be sensed at one second intervals and their movements over the last twenty seconds used to assess the cause of an increase in water level.

For example, in a case where the water level is increasing at a slow rate, the pressure in the boiler is increasing at a slow rate and the firing rate is reducing, that is a good indication that the increase in water level is simply caused by a reduction in the demand for steam. Thus, in response to the control unit 1 and the controller 49 receiving signals indicative of that situation, the controller 49 acts to reduce at a slow rate the amount of water per unit time entering the boiler through the pipe 52.

On the other hand, in a case where the water level is increasing at a fast rate, the pressure in the boiler is reducing at a fast rate and the firing rate is increasing, that is a good indication that the increase in water level is actually a result of a sudden demand for steam. Thus, in response to the control unit 1 and the controller 49 receiving signals indicative of that situation, the controller 49 may act to maintain, at its current rate, or to increase the amount of water per unit time entering the boiler through the pipe 52.

It will be appreciated that the precise control criteria that are applied can be varied by the designer of the control system and/or by the commissioning engineer who installs the control system. For example, the system may be arranged so that, if only one probe assembly detects a water level beyond an acceptable range, the alarm and/or burner shut down procedure is commenced only after a relatively long period, for example 20 seconds, whereas, if both probe assemblies detect a water level beyond an acceptable range, the alarm and/or burner shut down procedure is commenced sooner, for example after 10 seconds. As well as selecting values for what may be regarded as a "slow" or "fast" rate of change of a variable, it is also of course possible to introduce values of other variables in the decision-making process for controlling the water level. By combining the control of the burner and the boiler as described above such arrangements become possible.

In a particularly advantageous example, the controller 49 reads a water level signal from each of the probe assemblies 58A, 58B every tenth of a second. To form a water level signal the highest and lowest values are taken from ten consecutive readings from a probe and one tenth of the difference between the values is added to the lowest value to define what is then regarded as the value for that probe. The same procedure is carried out for the other probe and the two values so obtained averaged to provide a good measurement of water level even when the water is turbulent. We have found that taking only one tenth of the difference between the values is appropriate: a characteristic of a typical wave in a boiler is that peaks of the wave are significantly narrower than troughs; for that reason and because of other forms of turbulence, the peaks in the turbulent water contain relatively little water. Thus, in this particular example a water level reading is generated every second; that reading may itself then advantageously be combined with, say, nine other similar readings to provide an average reading that covers a ten second period. That average reading may be updated at any selected rate down to once per second.

The readings from each probe are also used in this particularly advantageous embodiment to detect turbulence. As will now be understood, the probe assemblies 58A, 58B can be expected to give readings with short term variations when there is turbulence; more particularly the readings can be expected to fluctuate considerably over a period of a second when there is turbulence. The control system already described is knowledgeable of the pressure in the boiler and the water temperature and therefore knows whether or not the water should be boiling and therefore turbulent. Changes in water level of 2.5mm or more in the course of one second may be regarded as indicative of turbulence and thus it is possible to arrange for the control system to conduct a further check that the probe assemblies 58A and 58B are operating properly. In the event of a conflict between the inputs, an alarm may be sounded and/or the burner 20 turned off.

Some degree of tolerance of a difference between the readings from the probe assemblies 58A and 58B is desirable, but it is also desirable that if the readings are far apart and remain far apart for a period long enough to allow for transient variations, then an alarm is sounded and/or the boiler 20 turned off. For example, the system may be arranged to allow for a disparity in water level readings from the respective probe assemblies of up to 50mm for up to 20 seconds.

The control system described above is also able to assess the amount of steam per unit time that is leaving the boiler and, therefore, can dispose with the need for one or more steam flow meters. The assessment is accomplished by assessing all the energy input per unit time into the burner and boiler and the energy output per unit time other than in the steam. The difference between the energy input and the energy output as so assessed is of course a measure of the energy that has been put into the water/steam in the boiler. Provided the approximate temperature of the water passed into the system is known and the temperature and pressure of the steam are also known it becomes possible to calculate the mass flow rate of the steam. The accuracy with which the energy inputs and outputs are assessed is a matter of design choice, but one particular example is given below.

The energy input to the system is regarded as consisting exclusively of the heat generated from combustion of the fuel in the burner 20. The control unit 1 is able to compute the amount of fuel being combusted and, if desired, can also take into account the exhaust gas analysis results from the analyser 37 to arrive at the rate of energy input at any one time. During commissioning of the control unit 1, a calibrated fuel meter may be used in order that the control unit 1 is able to store a value of the fuel flow rate and/or heat energy input corresponding to each of a plurality of settings of the fuel valve. The control unit 1 is then able to arrive at appropriate values for any intermediate settings by interpolation.

The energy outputs from the system, apart from the steam are regarded as comprising the following:

• i) the energy in the hot exhaust gases after they have passed through the boiler;
• ii) losses from the burner and boiler in heat that is transferred to the surroundings via radiation, conduction and convection.

The control unit 1 is informed of the temperature of the exhaust gases from the exhaust gas analyser 37 and is able to compute the flow rate of exhaust gases from the amounts of fuel and/or air being fed to the burner. For the losses from the burner and boiler, it is assumed that a fixed percentage of the heat input (in a particular example 0.25%) is lost when the burner is running at maximum firing rate and that the amount of heat lost remains the same at lower firing rates so that if the burner is turned down to, for example, one quarter of its maximum firing rate the percentage loss increases fourfold (in the particular example to 1%).

Thus the control unit 1 is able to assess the energy input into the water in the boiler. From the controller 49 the temperature of the water fed into the boiler is known and the temperature and pressure of the steam leaving the boiler are also known. The heat required to heat water (specific heat) to convert water to steam (latent heat) and to bring steam to a certain temperature and pressure is of course all well established and therefore the data available from the controller 49 when taken with that from the control unit 1 enables the new flow rate of the steam to be computed.

Extra work is required during initial commissioning of the system to calibrate the control unit 1 and the controller 49 so that they provide a good indication of the steam flow rate, but once the commissioning process has been completed and appropriate values stored in look-up tables, the computation of the steam flow rate is automatic.

Thus it can be seen that by linking together the control of the burner and boiler an especially advantageous control system can be provided.

Whilst one particular example of a system has been described, it should be understood that the system may be varied in many respects. For example, in the described embodiment the control unit 1 and the controller 49 are separate physical units; it is, however, possible to locate the controller 49 within the control unit 1 and indeed, if desired, the controller 49 may be integrated wholly into the control unit 1, so that for example they share the same microprocessor.