It is common in large burner installations to provide safety devices that reduce the likelihood of a serious malfunction or reduce the severity of a malfunction if one occurs.

In a known gas burner installation, two closable valves are provided in series in the pipe that connects the gas supply to the burner. One is commonly referred to as a"block valve"and the other, which is downstream of the block valve, as a"control valve". Those valves are provided for the purpose of cutting off the main gas supply from the burner and it is important that they operate correctly. In order to be able to test the valves it is known to provide as part of the burner installation a testing apparatus which typically includes a pump for increasing or reducing the pressure between the valves and checking, by the use of pressure limit switches, that the valves when closed are each able to sustain a certain pressure, within preselected limits.

For example, the length of pipe between the two valves may be pressurized to a predetermined pressure either by a pump or by opening the"block valve"for a short period to expose the pipe between the valves to the upstream gas pressure; the pressure can then be monitored and in the event that it falls below a certain level, actuates a pressure limit switch in the length of pipe thereby indicating a faulty valve. Another check that may be made is to adjust the pressure in the length of pipe between the two valves to atmospheric pressure, for example by opening the control valve, and then to close the control valve and check, by means of another pressure

limit switch, that the pressure in the length of pipe does not rise above a certain level.

Such valve testing apparatus has various disadvantages: it is expensive to produce, often involving the supply of a pump, and relies upon a plurality of pressure limit switches each of which have to be set to be actuated at a selected pressure by mechanical adjustment when the system is first set up.

Furthermore it might not be apparent to the day to day operator of the burner if one of the pressure switches becomes faulty. The apparatus also requires various control devices to carry out the required opening and closing of the valve. The method employed by the testing apparatus might also require there to be pressure limit switches in a variety of locations along the fuel supply pipe, thereby making installation and maintenance of the system more time consuming.

When a burner is combusting fuel in air, it is important to provide air at an appropriate rate for the rate of supply of fuel to the burner. For that purpose a means of regulating the airflow (for example a damper and/or a variable speed fan) is provided. It is, however, still desirable to check that air is being supplied to the burner head and for that purpose it is known to provide a mechanical air pressure limit switch downstream of the regulated airflow supply means. If the air pressure drops below a minimum safe level, the pressure limit switch is actuated with the result that the burner installation is shut down. The pressure limit at which the switch responds is set by a commissioning engineer.

Only one mechanical air pressure limit switch is provided and therefore the limit at which the switch responds, that is set by the commissioning engineer, must be less than the pressure at which air is supplied to the burner when the burner combusts fuel at its minimum

firing rate. When, however, fuel is supplied at a greater rate to the burner the rate of supply of air may drop to well below its desired, and safe, level and yet be undetected by the pressure limit switch. For example, when the fuel is supplied at the maximum rate and there is a minor fault in the air supply, such as could be caused by slight damage to the fan supplying air, the rate of supply of air drops to below the level required for efficient and safe combustion, but not below the commissioned pressure limit. Furthermore, it might not be apparent to the day to day operator of the burner if the air pressure limit switch becomes faulty.

It is an object of the present invention to provide a burner control installation and a method of controlling a burner control installation that mitigates one or more of the disadvantages referred to above.

According to a first aspect of the present invention there is provided a burner control installation including a burner for burning fuel, an air supply means that, in use, supplies air to said burner at a rate dependent upon an air supply signal received by said air supply means, a supply pipe connected to said burner for supplying fuel at a pressure to said burner, a supply of fuel being connectable to said supply pipe, a first valve connected in said supply pipe, said first valve having a closed condition and an open condition, the condition of said first valve being alterable in response to a first valve control signal, a second valve connected in said supply pipe between said first valve and said burner, said second valve having a closed condition and an open condition, the condition of said second valve being alterable in response to a second valve control signal, a pressure transducer connected in said supply pipe between said first valve and said second valve, said

pressure transducer, in use, producing a pressure signal representative of the pressure in said supply pipe, and a control unit which, in use, receives said pressure signal from said pressure transducer, sends said air supply signal to said air supply means, and sends said first and second valve control signals to said first and second valves.

It will be understood that a pressure transducer is able to produce an electric output signal representative of a sensed pressure and facilitates, by that signal, detecting and distinguishing between a multiplicity of different pressures.

For the purpose of testing the first and second valves, only one pressure transducer may be provided in said fuel supply pipe. Providing a single pressure transducer that provides an electrical signal representa- tive of a pressure has many advantages. A single pressure transducer is able to detect and distinguish between a multiplicity of different pressures in a range that covers both atmospheric pressure and the pressure at which fuel is supplied. Furthermore since the control unit receives said pressure signal the control unit can measure and monitor the variation of the pressure in the supply pipe over time. The control unit can distinguish between a multiplicity of different pressures. Also the control unit is able to aid the commissioning engineer in checking the valves and in commissioning the installa- tion, making commissioning much simpler than if several separate limit switches are provided. Moreover, the pressure may be measured in only one section of the fuel supply pipe.

The first aspect of the invention is of particular application when the fuel that is being combusted is a gas, but could also be of use when the fuel is a liquid.

In the case of a liquid fuel such as oil, an air chamber may be connected to the pipe between the first and second

valves so that a part of the volume defined therebetween is compressible.

Advantageously, said burner control installation is operable in a commissioning mode and in a run mode.

Preferably, the burner control installation further comprises an analogue to digital converting means, wherein said pressure transducer comprises a sensing part that, in use, produces an analogue electrical signal representative of the pressure in said supply pipe and said analogue electrical signal is converted to a digital electrical signal by said analogue to digital converting means.

Advantageously, in use, when a supply of fuel is connected to said supply pipe and said burner is burning fuel that is supplied at a fuel supply rate, said fuel supply rate is controlled by said control unit. A further valve, controlled by the control unit, may be provided for adjusting the rate of supply of fuel.

Preferably, said burner control installation comprises a vent means capable of venting the section of said supply pipe between said first valve and said second valve, and, in use, said vent means receives a signal sent from said control unit. Said vent means may conveniently be provided by said second valve and, in use, said section of said supply pipe is able to be vented into said burner. Alternatively, said vent means may comprise a third valve in a venting passageway which may be in communication with the interior of said supply pipe at one end and with the atmosphere at the other end, said third valve having a closed condition and an open condition and said control unit, in use, may send signals for controlling the condition of said third valve; such an alternative arrangement is especially preferred in North America.

According to the first aspect of the invention there is provided a method of controlling a burner control

installation wherein said method comprises the steps of a) providing a burner control installation comprising: a burner for burning fuel, an air supply means for supplying air to said burner, a supply pipe connected to said burner for supplying fuel at a pressure to said burner, a supply of fuel being connectable to said supply pipe, a first valve connected in said supply pipe, a second valve connected in said supply pipe between said first valve and said burner, a pressure transducer connected in said supply pipe between said first valve and said second valve, and a control unit, b) sending signals to effect closure of said section of said supply pipe, with the pressure in said section of said supply pipe being different from the pressure in said supply pipe upstream or downstream of said section, including sending signals from said control unit to said first valve and to said second valve to close said first valve and said second valve, and c) said control unit obtaining a signal representative of the pressure in said section of said supply pipe from said pressure transducer and then, after a period of time, obtaining a further signal representative of the pressure in said section of said supply pipe, whereby said control unit ascertains whether the obtained signals are indicative of a fault in said burner control installation.

For the purpose of testing the first and second valves, the pressure may be measured in only one section of said supply pipe by said pressure transducer.

Advantageously, the method includes the step of supplying fuel under pressure into said supply pipe.

After the step of sending signals to effect closure of said section of said supply pipe, the pressure in said

section of said supply pipe may be lower than the pressure in said supply pipe upstream of said section.

For example, before the step of sending signals to effect closure of said section of said supply pipe, the method preferably includes the step of sending a signal from said control unit to vent said section of said supply pipe, the pressure in said section of said supply pipe thereby being equal to atmospheric pressure.

After the step of sending signals to effect closure of said section of said supply pipe, the pressure in said section of said supply pipe may be greater than the pressure in said supply pipe downstream of said section.

For example, the method includes the step of supplying fuel under pressure into said supply pipe and before the step of sending signals to effect closure of said section of said supply pipe, the method preferably includes the step of sending a signal to said first valve to open said first valve, the pressure in said section of said supply pipe thereby being substantially equal to the pressure at which said fuel is supplied.

During said period of time said control unit may obtain a multiplicity of signals representative of the pressure in said supply pipe from said pressure transducer.

Said period of time may be predetermined and may be set during manufacture or by the commissioning engineer.

A typical period would be 10 seconds and a pressure difference of the order of M inch of water (about 120 Pa) may be set as a threshold level that should not be exceeded in the 10 second period.

Advantageously, said control unit includes a memory and a processor; and said method further comprises the steps of: storing a value in said memory representative of a

pressure in said supply pipe, and sending a signal representative of the pressure in said section of said supply pipe from said pressure transducer to said control unit, wherein said processor compares said signal representative of the pressure sent from said pressure transducer with said value stored in said memory and performs an action that is dependent on the result of the comparison.

Advantageously, said burner control installation is operable in a commissioning mode and said value is stored in said memory when said control installation is operated in said commissioning mode and said action includes producing and sending a signal indicative of whether, or not, said difference is greater than said predetermined threshold. Such a signal could initiate an automatic shutdown sequence or alert the operator of the burner control installation to the possibility of there being a fault.

Preferably, said value stored in said memory is representative of atmospheric pressure. The atmospheric pressure may be measured by the pressure transducer during commissioning.

Preferably, said method further comprises the step of supplying fuel under pressure into said supply pipe, said value stored in said memory being representative of the pressure at which fuel is supplied and said method further comprises the steps of: a) sending a signal to said second valve from said control unit to close said second valve; b) sending a signal to said first valve from said control unit to open said first valve, c) after steps a) and b) have been conducted and after the pressure in said section of said supply pipe has stabilised, sending a signal to said first valve from said control unit to close said first valve, and then d) sending said signal representative of the pressure

in said section of said supply pipe.

Preferably, said method further comprises the step of supplying fuel under pressure into said supply pipe, said value stored in said memory being representative of the pressure at which fuel is supplied and said method further comprises the steps of: sending signals to said first and second valves from said control unit to open said first and second valves; sending a signal to said air supply means from said control unit to supply air to said burner; combusting said fuel in said burner; and during the period of time in which fuel is combusted, periodically sending a signal representative of the pressure in said supply pipe from said pressure transducer to said control unit, wherein said processor compares each signal representative of the pressure with said value stored in said memory.

The first aspect of the invention can be incorporated in a burner control installation at the time of its initial installation but it may also be incorporated subsequently, in which case only the pressure transducer and the control unit may have to be supplied. Thus the invention further provides a valve checking means suitable for use in a burner control installation, said burner control installation comprising a burner for burning fuel, a fuel supply pipe for supplying fuel to said burner, an air supply pipe for supplying air to said burner, and a valve for altering the rate of supply of fuel to said burner, said rate of supply of fuel being alterable in response to a fuel control signal, said valve checking means comprising a control unit which, in use, sends said fuel control signal, and

a pressure transducer which, in use, is positioned in a supply pipe of said burner control installation and sends a pressure signal, representative of the pressure in said supply pipe, to said control unit.

According to a second aspect of the present invention there is provided a burner control installation including a burner for burning fuel, to which a supply of fuel is connectable, a fuel supply means that, in use, supplies fuel to said burner at a rate dependent upon a fuel supply signal, an air supply means that, in use, supplies air at a pressure to said burner at a rate dependent upon an air supply signal received by said air supply means, a pressure transducer so connected in said burner control installation that, in use, said pressure transducer produces a pressure signal representative of the pressure at which air is supplied to said burner, and a control unit which, in use, sends said fuel supply signal to said fuel supply means, sends said air supply signal to said air supply means, and receives said pressure signal from said pressure transducer.

In use, the control unit is thus able to monitor the pressure at which air is supplied to the burner whilst taking into account the expected pressure determined from the air supply signal. Since the control unit can obtain a signal representative of the air pressure faults can be rapidly and easily detected. Furthermore, the control unit is able to detect minor faults in the air supply, when the air pressure is lower than it should be, but is not as low as the normal air pressure when the burner is operated at its minimum firing rate.

The fuel may be a gaseous fuel or a liquid fuel.

Preferably, an air conduit is provided immediately upstream of said burner through which air is supplied to

said burner and said pressure transducer is positioned in said air conduit. Alternatively, the pressure transducer could be placed in any region in the burner control installation where the pressure of that region would be indicative of whether or not sufficient air is supplied to the burner.

Advantageously, said burner control installation is operable in a commissioning mode and in a run mode.

Preferably, said burner control installation further comprises an analogue to digital converting means, wherein said pressure transducer comprises a sensing part that, in use, produces an analogue electrical signal representative of the pressure at which air is supplied to said burner and said analogue electrical signal is converted to a digital electrical signal by said analogue to digital converting means.

The application of the second aspect of the invention is of particular value where, in use, a supply of fuel is connected to said burner and said fuel is supplied to said burner at a fuel supply rate, and air is supplied to said burner at an air supply rate, said fuel supply rate and said air supply rate being controlled by said control unit.

According to the second aspect of the invention there is also provided a method of controlling a burner control installation wherein said method comprises the steps of a) providing a burner control installation comprising: a burner for burning fuel, a fuel supply means for supplying fuel to said burner, an air supply means for supplying air to said burner, a pressure transducer, and a control unit including a memory, and b) performing a plurality of commissioning steps, each

said commissioning step comprising the steps of sending an air supply signal to said air supply means to supply air to said burner at a rate dependent on said air supply signal, said control unit obtaining a pressure signal representative of the pressure in the region of said pressure transducer, said pressure signal being sent from said pressure transducer, storing in said memory values corresponding to and representative of said air supply signal and said pressure signal, said plurality of commissioning steps being conducted in respect of a plurality of different air supply signals.

Advantageously, the pressure transducer is so positioned that, in use, it produces a pressure signal representative of the pressure at which air is supplied to said burner. For example, the pressure transducer may be positioned in an air supply conduit.

Advantageously, said method further comprises the steps of connecting a supply of fuel to said burner, sending a fuel supply signal to said fuel supply means from said control unit, thereby supplying fuel to said burner at a fuel supply rate, said fuel supply signal being determined by said control unit, sending an air supply signal to said air supply means from said control unit, thereby supplying air to said burner at an air supply rate, said air supply signal being determined by said control unit, said control unit obtaining a pressure monitoring signal representative of the pressure in the region of said pressure transducer, said pressure monitoring signal being sent from said pressure transducer, and said control unit determining whether the difference between the monitored pressure and an expected pressure

corresponding to said air supply signal is greater than a predetermined threshold.

The predetermined threshold may be a predetermined percentage of the expected pressure.

Preferably, if said control unit determines that said difference is greater than said threshold, said control unit sends a signal that initiates shutdown of said burner control installation.

The second aspect of the invention can be incorporated in a burner control installation at the time of its initial installation but it may also be incorporated subsequently, in which case only the pressure transducer and the control unit may have to be supplied. Thus the present invention further provides a pressure checking means suitable for use in a burner control installation, said burner control installation comprising a burner for burning fuel, means for supplying fuel to said burner, an air supply pipe for supplying air to said burner, means for altering the rate of supply of fuel to said burner, said rate of supply of fuel being alterable in response to a fuel control signal, and means for altering the rate of supply of air to said burner, said rate of supply of air being alterable in response to an air control signal, said pressure checking means comprising a control unit which, in use, sends said fuel control signal and said air control signal, and a pressure transducer which, in use, is positioned in said air supply pipe and sends a pressure signal, representative of the pressure in said air supply pipe, to said control unit.

It should be appreciated that many of the features of the first and second aspects of the invention described above can be combined.

In either aspect of the invention it is preferable for the signal from the pressure transducer to be reliable. A known way of achieving this is to provide two pressure transducers to sense the same pressure and to pass amplified signals from each transducer back to the control unit. Such an arrangement is reliable but also expensive. An especially preferred arrangement in accordance with the present invention is to provide one pressure transducer but to provide two separate amplifiers at the pressure transducer passing two electrical pressure signals in parallel to the control unit. An offset voltage of each amplifier can be different and the control unit can check for the presence of the same offset voltage difference in the two signals it receives. With such an arrangement the reliability of the amplification and transmission of the electrical signal can be as great as when two transducers are provided, but the costs can be reduced substantially.

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 diagram of a burner control installation; Fig. 2 is a schematic diagram of parts of the burner control installation shown in Fig.

1 showing parts of the burner control installation incorporating the first aspect of the invention; and Fig. 3 is a schematic diagram of parts of the burner control installation shown in Fig.

1 showing parts of the burner control installation incorporating the second aspect of the invention.

Figure 1 shows a boiler control installation in accordance with the present invention. The installation comprises a control unit 1, a boiler 2 including a burner

head 2a, a combustion chamber 2b, and a flue 2c. Air is supplied by an air supply means 3 via an air conduit 6 to the burner head 2a.

The air supply means 3 includes an air inlet 3a, a damper and a centrifugal fan 3b. The damper may either be an inlet damper 3c or alternatively an outlet damper 3d. (Although two dampers are shown in Fig. 1 only one is necessary). The rate of supply of air is therefore determined by the output of the fan 3b and the position of the air damper 3c (or 3d).

Gas is fed to the burner head 2a through a supply pipe 5 connected to a supply 4 of gas, via valves 7A, 7B and 8. In addition to being able to burn gas in air, the burner head is also capable of burning oil in air. Oil is fed to the burner head 2a from an oil supply means 28 via an oil conduit 29. The oil supply means 28 comprises various components, and is integrated with the rest of the installation (the detail of which is not shown in the figures for the sake of clarity).

The boiler 2 has a water outlet pipe 14 with a manually operated valve 15 and a water return pipe 16 with a conventional manually-operated valve 17 and an additional valve 18.

The control unit 1 is connected to various sensing devices as shown in the drawing,. More particularly the unit is connected to an exhaust gas analysis probe 20 (via an exhaust gas analyzer 19), to a load sensor 21 monitoring the temperature of the water in the water outlet pipe 14 of the boiler 2, and to a flame detection unit 22.

The control unit 1 also has a number of outputs and is connected via an inverter interface unit 23 and an inverter 24 to the motor of the fan 3b (with the interfacing unit 23 receiving a feed back signal from a tachometer 25 associated with the fan 3b) and via an air servo motor 26 to the air inlet damper 3c (or via an air

servo motor 27 to the air outlet damper 3d). The rate of supply of air may be controlled by changing either or both of the outputs to the fan and damper, respectively.

When a low rate of air supply is required, that can be obtained partly by reducing the output of the fan, but also by damping the air flow.

The control unit 1 is also connected via gas servo motors 12A, 12B and 13 to the gas valves 7A, 7B and 8 respectively and to a further servo motor 30 for adjusting the configuration of the burner head 2a, and to a control unit 31 for the valve 18 on the water return pipe 16 of the boiler 2.

The control unit 1 further comprises a key-pad 32, allowing data (including control arrangements) to be entered manually, a processor 10 (not shown in Figure 1) and a memory 11 (not shown in Figure 1). Control units of this general kind are well known and are commercially available; in particular there are the Micro Modulation Control Systems of Autoflame Engineering Limited.

GB 2,138,610 A and GB 2,169,726 A are concerned with inventions relating to such control units and the disclosures of both of those patent specifications are incorporated herein by reference.

A pressure transducer 9 is provided in the supply pipe 5 between the two valves 7A and 7B. The pressure transducer 9 is connected to the control unit 1, thereby enabling the control unit to obtain a signal from the pressure transducer 9 representative of the pressure in the supply pipe 5.

A pressure transducer 33 is provided near the burner head 2a in the air conduit 6. The pressure transducer 33 is connected to the control unit 1, thereby enabling the control unit to obtain a signal from the pressure transducer 33 representative of the air pressure in the air conduit 6.

The control unit 1 is operable in a variety of modes

including a commissioning mode, and a run mode. The commissioning mode allows the control unit 1 to be commissioned so that it can operate effectively in use with a particular burner installation.

Commissioning includes steps that check the performance of the valves 7A, 7B and supply pipe 5. The steps include measuring the pressure in the supply pipe and storing values representative of the measurements thus obtained into the memory 11 of the control unit 1.

The values thus stored can then be used by the control unit, at the beginning of and during the run mode, to detect any irregularities in the values of the pressure in the supply pipe. Those steps are described in more detail below with reference to Figure 2.

Commissioning also includes the steps of selecting and storing in the memory settings for the air damper, the fan 3b, and the gas regulating valve 8 at different levels of output of the burner so as to optimize the combustion process throughout the whole operational range of the burner during the run mode (adjustment of one gas valve is usually sufficient to cover the operational range of the burner when the other gas valve is fully open). The positions of the air damper and gas valve (s) are adjusted under the direction of a commissioning engineer who is able to select appropriate values for the settings of the air damper and the gas valve (s), taking account of any information from instruments that may be provided and also a visual inspection of the burner flame; those values are then stored in the memory 11.

During this procedure the signal from the pressure transducer 33 representing the pressure in the air conduit 6 is also measured and values stored in the memory 11 for each of the different levels of output of the burner. During the run mode the control unit 1 can then check for any irregularities in the pressure in the air conduit 6, as will be described in more detail with

reference to Figure 3 of the drawings.

When the control unit 1 is operating in the run mode it compares an input signal indicating the temperature of water in the boiler with stored data indicating a desired temperature and, according to the difference, selects a level of output for the burner. The control unit 1 is then able to determine, by use of the processor 10, an appropriate setting for the gas valve 8, (the fan 3b) and the air damper 3c (or alternatively air damper 3d), taking account also of input signals relating to the products of combustion and any other inputs that the control unit may receive. The settings chosen may be precisely ones selected by the commissioning engineer, but the system may also interpolate to provide settings between those entered by the engineer.

Figure 2 shows a schematic diagram of part of the burner installation shown in Figure 1, showing the control unit 1, boiler 2 including a burner 2e (the burner head 2a is not shown), the air supply means 3, and the supply 4 of gas. Air is supplied to the burner 2e via the air conduit 6. The gas supply 4 is connected to the burner of the boiler 2 via the supply pipe 5. A first gas valve 7A (known as a block valve) is provided in the pipe 5. A second gas valve 7B (known as a control valve) is provided in the pipe downstream of the first gas valve 7A. A pressure transducer 9 is mounted in the interior of the pipe 5, between the two gas valves 7A and 7B.

Only some of the many inputs and outputs of the control unit 1 are shown in Figure 2. The control unit 1 is connected to the air supply means 3, and the gas valves 7A, 7B and 8. The control unit 1 is therefore able to control the rate of supply of air and gas to the burner 2e. The control unit 1 is connected to the pressure transducer 9 for receiving signals representative of the pressure in the interior of the

supply pipe 5 between the gas valves 7A and 7B. The control unit 1 further includes a processor 10 and a memory 11.

In the commissioning mode the first and second valves 7A and 7B are initially set to a closed condition.

The control unit 1 sends a signal to the second valve 7B to open. The second valve 7B opens and the portion of the supply pipe 5 downstream of the first valve 1 is thereby vented to the atmosphere via the flue 2c (not shown in Figure 2) of the boiler 2. Once the pressure in the supply pipe 5 has stabilised, in that it is in equilibrium with the external pressure (atmospheric pressure), a value (hereinafter referred to as the commissioned atmospheric pressure value) representative of the absolute pressure in the supply pipe 5 is stored in the memory 11. A commissioned atmospheric pressure value that is much lower or much greater than expected, may be indicative of a faulty pressure transducer.

The control unit 1 then sends a signal to the second valve 7B to close. The control unit 1 then checks for any significant pressure change. The control unit 1 monitors the pressure after a predetermined interval of time watching for any rate of change of pressure above a preselected limit, with a rate of change greater than the preselected limit being regarded as indicative of a fault. If the first valve 7A is faulty, the measured pressure in the supply pipe will increase owing to gas leaking under pressure into the supply pipe from the gas supply 4. The predetermined threshold is a preselected percentage of the commissioned atmospheric pressure value, conveniently two percent.

The control unit 1 then sends a signal to the first valve 7A to open so that the supply 4 of gas is then in fluid communication with the supply pipe 5 in the region between the first and second valves 7A, 7B. Once the pressure in the supply pipe 5 has stabilised, a value

(hereinafter referred to as the commissioned gas pressure value) representative of the pressure in the supply pipe 5 is stored in the memory 11.

The control unit 1 then sends a signal to the first valve 7A to close. The control unit 1 then checks the pressure in the supply pipe, watching for any significant pressure changes in a similar manner to that described above. If, for example, the second valve 7B is faulty the pressurised gas in the supply pipe 5 may leak through the second valve and the pressure in the pipe will decrease.

The commissioning sequence insofar as the checking of the valves and supply pipe are concerned is then complete. As will be appreciated that part of the commissioning sequence can be conducted automatically by the control unit 1, with little or no input from the commissioning engineer.

The rest of the commissioning sequence may then be completed during which the control unit 1 sends signals to the first and second valves 7A, 7B to open.

At commencement of a run mode, similar steps (to those described above) are taken and measurements of the pressure are made and compared with the commissioned values.

Before the run sequence is initiated (when the installation is in a standby mdde) the first and second valves 7A, 7B are initially set to a closed condition and the pressure in the supply pipe between the valves is equal or close to atmospheric pressure. The control unit 1 monitors the pressure in the supply pipe 5 between the first and second valves 7A, 7B after a predetermined interval of time. A significant change in the measured pressure is indicative of a fault. For example, if the first valve suddenly fails and is no longer gas tight, then the pressure may suddenly increase to the pressure at which the gas is supplied.

When the run sequence is initiated, the second valve 7B is opened and the portion of the supply pipe 5 downstream of the first valve 1 is thereby vented to the atmosphere.

Once the pressure in the supply pipe 5 has stabilised, a control unit 1 obtains a value representative of the pressure in the supply pipe 5 and compares it with the commissioned atmospheric pressure value. The difference between the measured value and the commissioned value being greater than a predetermined threshold is indicative of a fault.

The second valve 7B is then closed and after a predetermined time a value representative of the pressure in the supply pipe 5 is obtained by the control unit 1.

The control unit 1 compares the obtained value with the commissioned atmospheric pressure value and checks that the difference is less than a predetermined threshold.

As explained above, with reference to the commissioning sequence, the difference between a measured pressure value and the commissioned value being greater than a predetermined threshold is indicative of a faulty valve.

The first valve 7A is then opened so that the supply 4 of gas is in fluid communication with the supply pipe 5 in the region between the first and second valves 7A, 7B.

Once the pressure in the supply pipe 5 has stabilised, a value representative of the pressure in the supply pipe 5 is obtained by the control unit 1. The measured value is compared to the commissioned gas pressure value. The measured value being much lower than the commissioned value is indicative of a fault, including any one of the following possibilities: a major leak in the second valve 7B or in the supply pipe 5 upstream of the second valve 7B, the gas supply 4 being faulty, a blockage upstream of the pressure transducer 9, or the pressure transducer 9 being faulty. A value much greater than the commissioned value could be indicative of a faulty gas

supply or a faulty pressure transducer.

The first valve 7A is then closed and after a predetermined time a value representative of the pressure in the supply pipe 5 is obtained by the control unit 1.

The control unit 1 compares the pressure in the supply pipe 5, measured by the pressure transducer 9, with the commissioned gas pressure value and checks that the difference between the measured pressure value and the commissioned gas pressure value is less than a predetermined threshold.

Then, if no faults have been detected, the first and second valves may be opened and combustion initiated.

The pressure in the supply pipe 5 is then measured at predetermined intervals of time by the pressure transducer 9. Values representative of those measurements are compared with the commissioned gas pressure value. A difference between those values that is greater than a predetermined threshold is regarded as indicative of a fault.

If a potential fault is detected an emergency shut down sequence is automatically initiated. The control unit also provides an indication of the possible type of fault that caused the abnormal pressure measurement.

Various modifications can be made to the embodiment described above with reference to Figures 1 and 2.

For example, each of the, various predetermined thresholds need not be determined by a percentage of the expected (commissioned) values, but may instead be an absolute value set by the commissioning engineer.

In the particular example of the invention described above, the control unit 1 monitors the pressure measured by the pressure transducer after a predetermined period of time. That predetermined period of time may be fixedly programmed into the control unit 1 at the time of manufacture or may be set during commissioning. By monitoring the pressure after a predetermined period of

time, the control unit detects a rate of change of pressure. If desired, the control unit may monitor the pressure a plurality of times at intervals spaced apart by predetermined lengths of time and is thereby able to monitor the rate of change of pressure; also the control unit may make a measurement after an extended period of time, or an indeterminate period of time, and may simply monitor the absolute pressure rather than the rate of change of pressure.

Figure 3 shows a schematic diagram of part of the burner installation shown in Figure 1, showing the control unit 1, boiler 2 including a burner 2e (the burner head 2a is not shown), the air supply means 3, and the supply 4 of gas. Air is supplied to the burner 2e via the air conduit 6. The gas supply 4 is connected to the burner of the boiler 2 via the supply pipe 5. A first gas valve 7A (known as a block valve) is provided in the pipe 5. A second gas valve 7B (known as a control valve) is provided in the pipe downstream of the first gas valve 7A. The air supply means comprises the damper 3d (or alternatively the damper 3c which is not shown in Figure 3) and the fan 3b. A pressure transducer 33 is mounted in the interior of the air conduit 33, in the region immediately before the burner head (not shown).

Only some of the many inputs and outputs of the control unit 1 are shown in Figure 3. The control unit 1 is connected to the fan 3b and-the air damper 3d of the air supply means 3, and to the gas valves 7A, 7B and 8.

The control unit 1 is therefore able to control the rate of supply of air and gas to the burner 2e. The control unit 1 is connected to the pressure transducer 33 for receiving signals representative of the pressure in the interior of the air conduit 6. The control unit 1 further includes a processor 10 and a memory 11.

In the commissioning mode, when the steps of selecting and storing in the memory 11 values of the

settings for the air damper 3c or 3d, the fan 3b and the gas regulating valve 8 are being conducted, values representative of the pressure in the air conduit 6 as measured by the transducer 33 are stored in the memory 11. For each different group of settings of the air damper fan and gas valve there is a different rate of supply of air to the burner head and therefore a different corresponding value of the air pressure immediately before the burner head as measured by the transducer 33.

Thus, during the commissioning mode, for each setting for the air damper 3c or 3d and the fan 3b that is stored in the memory 11, the control unit 1 obtains a signal from the pressure transducer 33 representative of the pressure inair conduit 6 in the region immediately before the burner head of the burner 2e and stores a value representative of that pressure in the memory 11.

That storing process can be accomplished entirely automatically or, if preferred, under the control of a commissioning engineer.

It is potentially dangerous to combust fuel in insufficient air. During the run mode the control unit 1 monitors the pressure in the air conduit 6 for any irregularities.

The measured pressure is compared with the expected pressure corresponding to the fan speed and air damper settings. The expected pressure is determined from the values stored during the commissioning process. The settings for the air damper and the fan may correspond directly to a value stored during the commissioning mode and the expected pressure is determined simply by retrieving the corresponding pressure value stored in the memory 11. If the settings do not have a directly corresponding value stored in memory 11 the control unit 1 may interpolate between values to determine the expected pressure.

If the difference between the expected pressure and the measured pressure is more than a predetermined threshold then the control unit initiates an automatic shutdown sequence. If the fan 3b ceases to operate or is damaged, or there is a blockage in the air conduit 6, the air supply rate will decrease and the measured pressure will be significantly less than the expected pressure.

The predetermined threshold may be dependent on a predetermined percentage of the expected pressure.

Thus it will be seen that the pressure transducer 33 acts as a safety device in a similar way to the pressure limit switch of a known system but with much greater versatility. Because the transducer is able to provide an output signal representative of pressure over a wide range of pressures (as opposed to a limit switch which simply indicates whether pressure is above or below one fixed value), the transducer can be an effective safety device over a wide operating range of the burner (for example at both minimum and maximum heat outputs of the burner). Furthermore, because the transducer is connected to the central control unit the commissioning of the transducer is a simple procedure that forms an integral part of the main commissioning process; no separate mechanical adjustment of the transducer itself is normally necessary.

The use of such a pressure transducer 33 may also render unnecessary safety checks that are in some known systems carried out on the fan 3b. For example, in a known arrangement the tachometer 25 associated with the fan 3b provides a signal to the control unit 1 of the actual fan speed and the control unit 1 checks that the actual fan speed is close to the desired speed. That safety check can be omitted when the control unit 1 receives a signal directly from the pressure transducer 33.