WO1996030703A1

WO1996030703A1 - Process control, method and apparatus

Info

Publication number: WO1996030703A1
Application number: PCT/GB1996/000309
Authority: WO
Inventors: Tony Stevenson
Original assignee: British Steel Plc
Priority date: 1995-03-28
Filing date: 1996-02-14
Publication date: 1996-10-03
Also published as: GB9506365D0; GB9603000D0; GB2299414B; GB2299414A; AU4669396A; EP0820572A1

Abstract

A process control method is described in which fluid is supplied to a plurality of fluid consumption units by a variable pressure supply, each consumption unit having associated therewith a flow control valve, the method including an initial step of determining the system resistance algorithm of each consumption unit with the control valve associated with that unit in a desired or designed maximum open position, and periodically measuring the fluid flows or fluid demand to consumption units (preferably each such units) calculating the instantaneous system resistance for each unit, selecting the highest such system resistance and modulating the output pressure of the fluid supply in dependants on that highest selected system resistance, and closing the control valves associated with the remaining consumption units to the extent necessary to provide the correct fluid flow supply for those units. By this method, excess pressure form the variable pressure supply is avoided leading to increased efficiency. A suitable application of the method is in the air supply for heaters of a reheat furnace in a steel mill.

Description

PROCESS CONTROL, METHOD AND APPARATUS

The present invention relates to a process control method, especially to a method of controlling the supply of combustion air to a furnace. A typical application is in the reheat furnace of a rolling mill.

Conventionally, a single fixed speed fan supplies combustion air to a plurality of zones of one or more reheat furnaces. The air supply system generally comprises an intermediate pressure control valve in the single supply line from the fan inlet to the plurality of zones. This intermediate valve acts under the control of an intermediate pressure sensor to regulate the pressure to a constant value. Final air flow control to each zone is accomplished by an additional local valve in each zone, which regulates the flow rate to that zone in reaction to a detected temperature in that zone in order to obtain a desired set point temperature. It is traditionally accepted that the maintenance of a constant pressure head provides for a supposed more stable operating environment for the air metering and subsequent flow control by the final air flow control valves of each zone. This will be referred to as the "conventional method".

The concept of applying a variable speed control device to the air fan of such multiple outlet systems has been applied at a rod mill in Cardiff, GB. By this method a variable speed fan has been used, the speed being varied to maintain the constant pressure value and thus render the intermediate pressure control valve redundant. Electrical energy savings are made because of the resulting reduction in average fan speeds. This will be referred to as the "Cardiff method".

The applicants have appreciated that the technique of the Cardiff method may still be susceptible to greater electrical energy savings. This is because many events may occur when the constant air supply pressure maintained is not necessary for the flow conditions evident on the furnace zones. However, conventional control system theory holds that this constant air supply pressure is required in order to prevent supposed instability in the various control loops. The technique developed and tested by the applicants has indicated that the conventional control system theory may be erroneous in this context.

Moreover in any circumstance where a variable speed fan is not used, and a constant supply pressure is not maintained to a multiplicity of zones, then excessive throttling by the zone flow control valves will cause high energy wastage and the technique developed by the applicants in conjunction with the use of a variable speed fan, will reduce the energy consumption used by the fan.

The present invention therefore provides a process control method in which fluid is supplied to a plurality of fluid consumption units by a variable pressure supply, each consumption unit having associated therewith a flow control valve, the method including an initial step of determining the system resistance algorithm of each consumption unit with the control valve associated with that unit in a desired or designed maximum open position, and periodically measuring the fluid flows or fluid demand to consumption units (preferably each such unit), calculating the instantaneous system resistance for each unit, selecting the highest such system resistance and modulating the output pressure of the fluid supply from the supply in dependence on that highest selected system resistance, and closing the control valves associated with the remaining consumption units to the extent necessary to provide the correct fluid flow supply for those units.

"System resistance algorithm" means, in the context of this application, the relationship between the pressure at the input to a system of flow passages incorporating one or more control valves and the resultant fluid flow at the exit from the system, with the or each control valve between those two points set at the relevant degree of opening. "System resistance" refers to the pressure value obtained from the algorithm for a particular, demanded, flow rate.

Thus, for a plurality of consumption units, for which the system resistance algorithm at the maximum designed or desired opening is known, given the demanded flow rate or the instananeous flow rate it is possible to calculate the minimum necessary air pressure at the input in order to be capable of satisfying those flow rates.

This method, according to the present invention, will be referred to by the term "Conflict Control".

The consumption unit is envisaged as being any reheating furnace or collection of furnaces having individually, or collectively, multiple outlets from a fan or fans. Reheating furnaces often have a plurality of "zones" which are maintained at different set point temperatures and are subject to different levels of flow demand. Each zone then constitutes a distinct consumption unit since its air supply is controlled separately. The present invention is, however, not limited to that context.

More preferably, in the event that air is preheated then the temperature of the supplied air is also measured and used in the calculation step. This is because the system resistance characteristic varies with fluid density and therefore more accurate results will be obtained by taking account of temperature. However, this is not essential to the present invention but may increase the energy savings obtainable in certain installations.

It is periodically necessary to increase the air flow rate to any consumption unit of a reheat furnace. Therefore the above described processes preferably also include a step of operating the air supply means so as to provide a head pressure to the units slightly greater than the relevant highest system resistance, and closing the local control valves of all units to the extent required to provide the correct air flow rate for the associated units.

This increase in head pressure over and above the system resistance is arbitrary on the merits of an individual operating unit or engineers preference.

An alternative method of causing the aforementioned increase in air flow would be to sense the increase in flow demand of the consumption unit causing the highest instantaneous system resistance and to modulate the output pressure of the air supply from the fan by an additional arbitrary amount to cause the required increase in air flow. When the sensed demand for additional air flow is zero the mechanism for control and modulation of the output pressure is unaffected.

It is also preferred, if the air supply system has a required minimum operating output pressure, for example for the safety and protective operation of the furnace and associated equipment, that if the flow demands fall sufficient to require a head pressure below the designated minimum, the air supply means supplies that minimum and the local control valves are closed to the extent necessary. Such a method would diminute or at least substantially reduce the risk of fan "stall", for example. Again, the exact minimum chosen is arbitrary on the merits of an individual operating unit or engineer's preference.

The present invention is applicable to a reheating furnace or unit, in which case it is preferred if the combustion air supply comprises a variable speed fan controlled on the basis of the calculated resistances of the air supply system to individual furnace zones. It is however believed that energy savings may be generated on non-variable (fixed) speed drive air fan systems which possess inlet-vane, or inlet damper combustion air pressure control mechanisms. Thus, the inlet vane or inlet damper would be controlled effectively to mimic a variable speed drive. It is expected that the energy used would be more than the Conflict Control method, but less than the Conventional method and at little investment cost. The present invention will now be described by way of example, with reference to the accompanying figures, in which;

Figure 1 is a schematic illustration of the layout of a reheating furnace comprising three zones whose air supply is operated according to the present invention;

Figure 2 is a schematic diagram of a control system according to the present invention; and

Figures 3 and 4 are graphs showing the energy consumption of the Conventional method, the Cardiff method, and Conflict Control as derived by the applicants in an application of the methods to two reheat furnaces.

In Figure 1, a furnace 10 has preheat, tonnage, and soak zones 10a, 10b and 10c respectively. Combustion air is supplied to burners 12a, 12b and 12c within their respective zones from a single fan 14 via pipework 16. Each furnace zone 10a to 10c has an associated local flow control valve 18a to 18c. The valves are individually controllable dependent upon the demand upon each zone, and the air supply to each zone is measured by sensors 17a to 17c.

The fan 14 is a variable speed fan which supplies air to a single duct 22 which passes through an air preheater 24 before branching off to supply the burners of each individual zone 10a to 10c. The air supply fan 14 is capable of being driven at any number of speeds, under control of the fan controller 20.

Arrays of thermocouples 26a to 26c are situated in each respective furnace zone 10a to 10c. These feed temperature data to a series of temperature controllers 28a to 28c each of which compares the detected temperature to a set point temperature for that zone and transmits demand data to the flow controllers 18a to 18c. There is also a thermocouple 32 in the branch of the pipework 16 leading to the burners 12a to 12c. This thermocouple 32 measures the temperature of air arriving at the burners 12a to 12c and feeds this information to the Conflict Control logic 19. The Conflict Control logic 19 also receives the value of the air flow to each of the zones 10a to 10c from the sensors 17a to 17c. The Conflict Control logic continuously scans the air flows 17a to 17c (or the demand data to flow controllers 18a to 18c) and air temperature 32 and simultaneously determines the highest instantaneous system resistance and required set point pressure control data. The required pressure set point is fed to the fan controller 20.

Within the pipe work 16, immediately after the air preheater 24, is a pressure sensor 34. This feeds actual pressure data to the fan controller 20, which compares this to the set point pressure data stored in the fan controller 20 and uses this to control the fan 14.

At the outlet of the fan 14 is a flow control valve 36. In this embodiment, involving a variable speed fan 14, this valve is not necessary during the normal operation of the system and is maintained at a maximum position. The valve 36 is retained in case of emergency, however.

Figure 2 schematically shows the control system. Parts 50 are a fuel and air supply regulating system, which has an output signal 52 which includes the air flow value or air flow demand. This is fed to a Conflict Controller 19 along with similar outputs 56 from other zones. In addition, the air preheat temperature signal 57 is supplied to the Conflict Controller 19. The Conflict Controller calculates from these inputs the highest required supply pressure and supplies this signal 58. This signal is used by the controllers 60, 62, which together comprise the fan controller 20 of Figure 1 to adjust and control the speed of the combustion air fan 14.

In Figure 2, the abbreviations used have the following meanings;

PV: Process Variable RSP: Remote Set Point SP: Set Point OP: Output LSS: Low Signal Select HSS: High Signal Select TX: Transmitter CONT: Controller In use, the Conflict Controller 19 contains a processor unit programmed with an algorithm for calculating the instantaneous system resistances of each furnace zone 10a to 10c. The air flow commands for each zone from the temperature controllers 28a to 28c together with temperature information derived from the air temperature sensor 32 allows the Conflict Controller 19 to calculate the instantaneous system resistance of each zone. The largest thus calculated instantaneous resistance is then used to set the fan speed for the fan 14 to produce a head pressure slightly greater than that system resistance. In the embodiment employed by the applicants, pressures between 15 and 40mB are supplied to the flow controllers 18a to 18c and an excess over the highest instantaneous pressure of 5mB has been found to work. The precise figure is however relatively arbitrary and other values will be possible depending on the individual system and the preferences of the system designer. Assuming that the zone with the highest system resistance is zone 10a, valve 18a will then set very slightly closed to give the correct air flow, whilst valves 18b and 18c set to a lower setting to throttle the air supply to a greater extent to provide the appropriate air flow for those furnace zones. The exact valve setting could alternatively be determined by the Conflict Controller 19 either by pre-programmed knowledge of the valve characteristic, or by feedback from a pressure sensor (not shown) downstream of the valves 18a to 18c.

In this instance the preheated air temperature is measured and the zone system resistance algorithm contains an adjustment factor to correct the calculated system resistance value. It is not mandatory to the present invention to do so, but better results are achieved.

Figures 3 and 4 show experimental results from two adjacent furnaces of the applicants to which the system of the present invention has been applied. In Figures 3 and 4, the filled square plot points are those in accordance with the Conventional method referred to above. The filled circle, filled star, and open square plot points are those associated with operating the system with a variable speed drive but in accordance with the Cardiff method at a constant pressure of 40 mbar, 45 mbar and 50 mbar respectively. The open circle plot points are those obtained using the Conflict Control method. Since energy consumption is inevitably dependent on the scale of use for the furnace, energy consumption is plotted against weekly tonnage of material through the furnace.

It is clear that whilst the Cardiff method gives a reduction in energy consumption over the Conventional method, the Conflict Control method of the present invention gives still further significant reductions. For both furnaces, rough power consumptions of 13,500 KW hours per week (conventional method) were reduced to approximately 4,500 KW hours per week. It should however be emphasised that these savings were those obtained in these furnaces. Furnaces of a different design, or furnaces subjected to very different patterns of use, will exhibit a different saving. This may be higher or lower than that shown in Figures 3 and 4.

The significant reductions in power consumption through the Conflict Control method of the present invention are traceable to reductions in the fan speed. This is because the power employed by a fan is proportional to the fan speed cubed, whilst the pressure delivered by the fan is proportional to the fan speed squared. Therefore the power saving is significant if the coπimanded pressure can be reduced. The present invention operates in automatic mode without operator intervention and takes advantage of occasions when the demanded air supply pressure can be periodically lower, yet retains the capability to cope with periods of higher demand.

The Conflict Control method of the present invention has been exhaustively tested in operation at a reheat furnace of the applicants and no instability problems have been identified, contrary to previous and conventional expectations. It will be appreciated that the above described embodiment is purely exemplary of the present invention and that many modifications can be made whilst remaining within the scope of the present invention.

Claims

1. A process control method in which fluid is supplied to a plurality of fluid consumption units by a variable pressure supply, each consumption unit having associated therewith a fluid supply control valve, including an initial step of determining the system resistance algorithm of each consumption unit with the control valve associated with that unit in a desired or designed maximum open position, and periodically measuring the fluid flows or fluid demand to consumption units, calculating the instantaneous system resistance for each unit, selecting the highest such system resistance and modulating the output pressure of the fluid supply in dependence on that highest selected system resistance, and closing the control valves associated with the remaining consumption units to the extent necessary to provide the correct fluid flow supply for those units.

2. A method according to claim 1 wherein the consumption unit is at least one reheating furnace and the fluid is air.

3. A method according to claim 2 wherein the reheating furnace has a plurality of zones which are maintained at different set point temperatures, each of which is a distinct consumption unit.

4. A method according to any preceding claim which includes a step of operating the fluid supply means so as to provide a head pressure to the consumption units slightly greater than the relevant highest system resistance, and closing the local control valves of all consumption units to the extent required to provide the correct fluid flow rate for the associated consumption units.

5. A method according to any preceding claim which includes a step of determining whether the relevant highest system resistance is less than a predetermined minimum and, if so, modulating the output pressure of the fluid supply to provide that minimum pressure and closing the control valves of all consumption units to the extent required to provide the correct fluid flow rate for the associated consumption units.

6. A method according to any one of claims 2 to 5 wherein the variable pressure supply comprises an air fan powered by a variable speed drive.

7. A method according to any one of claims 2 to 5 wherein the variable pressure supply comprises, in combination, an air fan powered by a drive unit and a control valve adapted to throttle the output of the fan to produce a variable output pressure.

8. A reheating furnace supplied by a variable speed fan whose speed is controlled in accordance with a method as set out in any one of claims 1 to 6.