GB2392895A

GB2392895A - Pneumatic conveyor control system

Info

Publication number: GB2392895A
Application number: GB0220949A
Authority: GB
Inventors: Donald Ian Blackmore; Andrew Ian Blackmore
Original assignee: ROTAVAL Ltd
Current assignee: ROTAVAL Ltd
Priority date: 2002-09-10
Filing date: 2002-09-10
Publication date: 2004-03-17
Anticipated expiration: 2022-09-10
Also published as: GB0220949D0; GB2392895B

Abstract

A control system comprises a pressure sensor and valve 13 control means 21. This system may be suitable for use with a pneumatic conveyor to control a flow of granular material. Also disclosed is a control device 21 comprising a pressure sensor and an output signal. This may also be used in a pneumatic conveyor to control a flow of granular material. Further disclosed is a pneumatic conveyor control method with pressure monitoring and material flow control 21. The first system described may further comprise an inlet 18, and hopper 19. The valve 13 may be a rotary valve 13. The first system, and possibly the others, may be used to detect the onset of blocking of a granular material, or an over pressure condition, and to adjust flow to overcome this state. Continuous adjustment may be possible.

Description

hi. CONTROL SYSTEM FOR PNEUMATIC CONVEYOR CONTROL

DEVICE AND METHOD THEREFOR

5 This invention relates to pneumatic conveying of powdered and granular materials and in particular concerns the control of pneumatic conveying systems to reduce the occurrence of blockages.

Pneumatic conveying of powdered and granular materials is often used for 10 transferring material within a storage, handling or processing system. Pneumatic conveyor systems provide a simple way of transferring material in an enclosed system of ducts and valves providing material movement on demand, normally via automatic control. With the use of diverter valves the powdered or granular material product can be delivered to different discharge points in the system.

Dilute conveying, or lean phase conveying, can be defined as the movement of solid particles in a homogenous distribution entrained within a relatively high velocity gas stream where the mass load ratio of the particles is relatively low. This is different to dense phase conveying where the particles are fed into the duct as a homogenous 2 o solid which is pushed through the duct by high-pressure air acting on the solid.

There are many different types of pneumatic conveyor systems including positive pressure systems where the material is effectively blown through the duct system and negative pressure or vacuum systems where the material is moved by suction. A

t e À e e À AL typical positive pressure system comprises an energy source for creating a pressure differential within the duct system, typically a positive displacement roots type air blower, connecting pipework forming the duct, a means for feeding the material into the pipework, usually a rotary valve, a discharge hopper for receiving the conveyed 5 material and one or more diverter valves connected to the pipework if more than one discharge point is required.

Pneumatic conveying systems tend to be bespoke designs for the particular application concerned since each system has to take account of the different physical 10 characteristics of the material being conveyed, the specific application conditions, and the duty cycle requirements of the system. Normal design practice is based on established calculations that use anticipated mean values of the product characteristics of the material to be conveyed, empirical data and also the designers judgement based on design experience. Pneumatic conveyor systems are often over engineered to 15 ensure that the system performs satisfactorily and reliably without the occurrence of blockages in the pipework that would otherwise overload the blower. Current design practice is inherently inefficient since it builds in sufficient margin to ensure satisfactory operation for a range of material characteristics, which can vary from time to time, and also allowing for changes in material flow and system performance 2 0 which can occur including for example increasing air loss through the rotary valve due to wear caused by the handling of abrasive materials.

Hitherto the blower has been protected from over pressure conditions due to blocking by a pressure relief valve and since the problem of blocking is normally associated

c I e e Be l' # Bee e.e lee with an excess of material entering the duct pipework, for the particular product being conveyed and the prevailing flow conditions, the speed of the rotary valve is usually calibrated on commissioning the conveyor system to provide an optimum mean performance with sufficient margin to avoid an overload condition and pipework 5 blocking. It will be understood that if significant adjustments are required on commissioning, the conveyor system will have a below specification performance,

less reliable operation, or inefficient energy usage resulting from the non optimised design. 10 There is a requirement therefore for a pneumatic conveyor system which is capable of providing optimum system performance without significant risk of pipework blocking. According to an aspect of the invention there is provided a control system for a 15 pneumatic conveyor of the type having a means for introducing granular material into a pressurised or vacuum conveyor duct; the said control system comprising pressure sensor means for monitoring the pressure in the said duct and valve control means for controlling the flow of granular material into the duct in accordance with the pressure in the duct.

This aspect of the invention is based on the observation that pressure increases within the duct of a pneumatic conveyor that eventually lead to blocking are not instantaneous but tend to occur over a period of time due to the compressibility of the air conveying medium. By reducing the flow of material into the duct the loading on

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Arc the pressure source in the conveyor will be reduced reducing the likelihood of blockage in the duct. By actively monitoring the duct pressure in this way it is possible to adjust the average loading in the duct and thereby maintain sufficient margin within the system to prevent blocking due to too great a quantity of material 5 entering the duct for the prevailing flow conditions. This enables the conveyor system to operate at or near its optimum design setting for the majority of its duty cytle without significant risk of blocking in the duct. This can provide a more optimum design which can avoid inefficient and over- engineered pneumatic conveyor systems. In preferred embodiments the system is configured to detect a pressure condition in the duct indicative of the onset of blocking of the duct by the granular material. This way the control system can be configured or programmed, if the control Function is implemented in a microprocessor, to recognise a particular pressure condition in the 15 duct which normally proceeds blocking of the duct by the granular material.

Preferably the pressure condition comprises an over pressure condition, indicating that the static pressure in the duct has increased due to a reduction in the flow cross sectional area as a result of material build up in part of the duct which would 2 o otherwise lead to blocking if no remedial action were taken.

In preferred embodiments the over pressure condition comprises a low frequency component indicative of the onset of the duct blocking. As mentioned above a conveyor system that blocks does so over a period of time usually several seconds and

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is proceeded by an increasing pressure gradient. By sensing this gradient and distinguishing it from other pressure variations due to other cyclic components in the conveyor system it is possible to reduce or stop the material entering the duct in sufficient time to prevent escalation of the build up of material forming a blockage.

5 Typically there are three frequency components that cause variation in the duct pressure, including: high frequency, low amplitude pressure pulses from the air pressure source, typically a Roots type blower where the frequency is dependent upon the blower speed, for example 160-200 Hz with an amplitude of say 1 5mb; a mid frequency component from the means for introducing the granular material, typically 10 a rotary valve resulting in a frequency component of 2-5 Hz with an amplitude of approximately 150mb; and, a low frequency, high amplitude component resulting from the natural cyclic flow conditions normally present in pneumatic conveyor systems. It is to be understood that in a conveyor system that is operating at or near its limit this low frequency component can be very pronounced and therefore in 15 preferred embodiments it is this low frequency component that the system is configured to detect.

Preferably the control system further comprises filter means for filtering a signal from the pressure sensor to provide a signal having a frequency range substantially in the 20 region of the low frequency component present in the conveyor system when operating at or close to its operational limit. This readily enables the response signal from the pressure sensor to be conditioned so that only the frequency components of interest remain in the processed signal so that pressure fluctuations at higher frequencies do not affect the operation of the control system by providing spurious

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It is preferred that the valve control means is capable of continually adjusting the flow of granular material into the duct. In this way the control system can optimise the 5 operation of the pneumatic conveyor so that maximum throughput can be achieved in an efficient and reliable manner. Preferably the means for introducing the granular material comprises a rotary valve, screw conveyor, aero mechanical lift or mechanical conveyor. It is preferred however that a rotary valve is provided and that the valve control means controls the rotary valve either by starting or stopping the valve or by 10 controlling its operational rotational speed. A typical rotary valve comprises a varied rotor component which is ratably mounted within a housing which has an inlet port on one side of the valve for receiving a gravity feed of the material to be conveyed, and an outlet port on a bottom of the valve for introducing the material into a pneumatic conveyor system, for example. The feed rate of such a valve is determined 15 by the rotational speed of the rotor, the size of the valve (ie the capacity of the valve), and the ability of the granular material to fall into it under the influence of gravity (ie the filling effeciency). In this respect it will be understood that the feed rate of a rotary valve is related to the speed of the rotor. Thus by adjusting the rotational speed of the rotor the volume of granular material entering the conveyor duct can be 2 0 controlled. In another embodiment granular material is fed to the rotary valve by another device such as a screw conveyor, or mechanical lift etc. with the control means being operable to control the feed into the rotary valve and additionally the speed of the rotary valve, causing the rotary valve to stop in the event of an over pressure blockage condition being detected.

À À, r 3ú. The rotary valve may be capable of being fed with granular material at full capacity or less than full capacity of the valve, that is to say in a choked (flood) or unchoked feed condition. Where the material is fed to the rotary valve at less than the full 5 capacity of the valve, the valve control means may alternatively or additionally adjust the feed of material to the valve.

In preferred embodiments the pressure sensor means comprises a plurality of pressure sensors, preferably pressure transducers, distributed along the duct. In this way it is 10 possible to monitor the static pressure at various positions within the duct so that pressure fluctuations at these positions can be identified so that if a blockage does occur the location of the blockage can be readily determined.

Preferably the control system is configured to maintain the pressure within the duct 5 below a predetermined maximum value by controlling the flow rate of material entering the duct. The system may be provided with an alarm to indicate when the average performance of the system has deteriorated below an acceptable level, for example to indicate problems developing within the conveyor, including product build up on the pipe walls, wear in the rotary valve and/or filter blocking.

Preferably the control system is configured for controlling a lean phase flow of granular material in the duct.

The present invention also contemplates a pneumatic conveyor system comprising a

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AL control system as defined above.

According to another aspect of the invention there is provided a control device for controlling a pneumatic conveyor of the type having a means for introducing granular 5 material into a pressurised or vacuum conveyor duct; the said device comprising means for receiving pressure signal(s) from pressure sensor means monitoring the pressure in the said duct and an output signal for controlling the flow of granular material into the duct in accordance with the pressure in the duct. The control device of this aspect of the invention may be readily integrated into existing pneumatic 10 conveyor systems without significant modifications to the existing conveyor hardware. According to a further aspect of the invention there is provided a method of controlling a pneumatic conveyor of the type having a means for introducing granular 15 material into a pressurised or vacuum conveyor duct; the said method comprising the steps of monitoring the pressure in the said duct and controlling the flow of granular material entering the duct in accordance with the pressure in the duct.

An embodiment of the invention will now be more particularly described by way of 2 0 example, with reference to the accompanying drawings.

Referring to Figure 1, a positive pressure pneumatic conveyor system 10 comprises an air pressurization device 11, for example a positive displacement Roots type air blower, a duct 12 including a series of interconnected pipes which extend between a

e ae: Àe.e.:e ee.e rotary valve 13 which constitutes means for introducing granular material into the conveyor pipework and a receiving hopper 14 at the opposite end of the pipework to that of the air pressurization means 11. The conveyor 10 may be provided with further receiving hoppers (not shown) connected to branch pipework 17 as 5 represented by the broken line in the drawing of Figure 1, connected to the main pipework 12 by means of a diverter valve 16.

The rotary valve 13 comprises an inlet 18 which is connected to a hopper 19 containing granular material to be fed by gravity into the rotary valve 13 and into the 10 duct pipework 12 through the outlet 20 of the rotary valve on the opposite side thereof. The rotary valve 13 is controlled by a controller 21, which may comprise a suitably programmed microprocessor, either to control the speed of the valve's rotor by controlling a variable speed control unit on the drive motor (not shown) driving the valve or through a stop/start motor contactor for stopping and starting the drive motor 15 as required.

The controller 21 has an input 22 from at least one pressure transducer (not shown) located within the duct pipework for measuring the static pressure therein. The controller 21 is configured to continually monitor the pressure within the duct so that 2 0 it may detect a pressure condition indicative of the onset of blocking in the duct, for example an over pressure condition or a gradually increasing static pressure due to the build up of granular material in the duct leading to a blockage. The controller 21 is capable of detecting low frequency high amplitude pressure pulses which usually proceed a blockage. In the event that one of the aforementioned conditions is detected

:::: c::e eeetee: eee by the controller the controller sends an output signal from an output 23 to the valve 13 either to stop the feed of granular material into the pipework or to adjust the speed of the valve so that the feed entering the pipe through the valve is reduced so that blocking is avoided, thereby maintaining a maximum throughput through the valve 5 for the prevailing operating conditions. Thus the controller 21 may provide a basic overload protection function for the pneumatic conveyor in the case that an over pressure condition arises to protect the pressurization means 11 and avoid a blockage in the duct. The controller may also function such that the amount of granular material entering the duct through the valve 13 is regulated such that the pressure 10 within the duct is maintained between predetermined upper and lower limits. In another way the controller can continually measure the average performance of the conveyor system 10 and adjust the amount of material flowing in the duct to maintain an optimum flow rate and provide an alarm signal when the average performance of the system has deteriorated to below an acceptable level, for example due to the 15 material build up on the internal walls of the duct pipework, due to wear in the rotary valve resulting in poor sealing between the duct and the feed hopper 19, and/or blocking of the air filter 15.

In a further embodiment (not shown) the controller 21 can be connected to a plurality 2 0 of pressure transducers distributed along the length of the duct pipework so that the position of pressure spots within the duct indicative of material build up can be readily identified and preventative maintenance directed to the appropriate position within the duct pipework. This is an important consideration since the duct pipework may comprise several hundred metres of pipes in a system having several branch

À e C À pipes 17 and output hoppers 14 and locating a potential blockage can be a time consuming and costly operation.

Although aspects of the invention have been described with reference to the 5 embodiment shown in the accompanying drawings it is to be understood that the invention is not limited to this precise embodiment and that various changes and modifications may be effected without exercise of further inventive skill and effort.

For example, the means for introducing the granular material into the conveyor duct may comprise devices other than a rotary valve, including for example screw 10 conveyors, aero mechanical lifts, mechanical conveyors etc. In addition the controller 21 may be configured to control the feed ofthe granular material into the rotary valve or other means for introducing granular material into the conveyor duct, for example such that the rotary valve is run in a non-flooded condition (choked). In addition the present invention is equally applicable to vacuum type conveyor systems where the 15 pressurization means I I is replaced by a vacuum pump or the like at the other end of the duct pipework.

Claims

a acea a a a a aa a a a a CLAIMS

l. A control system for a pneumatic conveyor of the type having a means for 5 introducing powdered or granular material into a pressurised or vacuum conveyor duct; the said control system comprising pressure sensor means for monitoring the pressure in the said duct and valve control means for controlling the flow of granular material into the duct in accordance with the pressure in the duct.

10 2. A control system as claimed in Claim l wherein the said system is configured

to detect a pressure condition in the said duct indicative of the onset of blocking of the duct by the said granular material.

3. A control system as claimed in Claim 2 wherein the said pressure condition 15 comprises an over pressure condition.

4. A control system as claimed in Claim 3 wherein the said over pressure condition comprises a low frequency component indicative of the onset of duct blocking. 5. A control system as claimed in Claim 4 further comprising filter means for filtering a pressure signal from the said pressure sensor means to provide a filtered signal having a frequency range substantially in the region of the said low frequency component.

e;:: cele eece'..:e;e hi. 6. A control system as claimed in any preceding claim wherein the said valve control means is capable of continually adjusting the flow of granular material into the said duct.

7. A control system as claimed in any preceding claim wherein the said means for introducing granular material into the said duct comprises a rotary valve, screw conveyor, aero mechanical lift or mechanical conveyor.

10 8. A control system as claimed in Claim 7 wherein the said means for introducing granular material into the said duct comprises a rotary valve and said valve control means is capable of starting or stopping the said rotary valve and/or controlling its operational speed.

15 9. A control system as claimed in Claim 8 wherein the said rotary valve is capable of being feed with granular material at full capacity or less than full capacity and the said valve control means is capable of adjusting the said feed of granular material to the said valve.

2 0 10. A control system as claimed in any preceding claim wherein the said pressure sensor means comprises a plurality of pressure sensors distributed along the said duct.

11. A control system as claimed in any preceding claim wherein the said system is configured to maintain the pressure within the duct below a predetermined

cceÀ e ee maximum value by controlling the flow rate of granular material entering the duct.

12. A control system as claimed in any preceding claim wherein the system is configured for controlling a lean phase flow of granular material in the duct.

13. A pneumatic conveyor comprising a control system according to any preceding claim.

14. A control device for controlling a pneumatic conveyor of the type having a 10 means for introducing powdered or granular material into a pressurised or vacuum conveyor duct; the said device comprising means for receiving pressure signal(s) from pressure sensor means monitoring the pressure in the said duct and an output signal for controlling the flow of granular material into the duct in accordance with the pressure in the duct.

15. A method of controlling a pneumatic conveyor of the type having a means for introducing powdered or granular material into a pressurised or vacuum conveyor duct; the said method comprising the steps of monitoring the pressure in the said duct and controlling the flow of granular material entering the duct in accordance with the 2 0 pressure in the duct.

16. A method as claimed in Claim 15 further comprising the step of detecting a pressure condition in the said duct indicative of the onset of blocking of the duct by the said granular material.

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:: ece..:e 17. A method as claimed in Claim 16 wherein the said pressure condition comprises an over pressure condition.

5 18. A method as claimed in Claim 16 wherein the said over pressure condition comprises a low frequency cyclic component indicative of the onset of duct blocking.

19. A method as claimed in Claim 18 further comprising the step of filtering a pressure signal from the said pressure sensor means to provide a filtered signal having 10 a frequency range substantially in the region of the said low frequency component.

20. A method as claimed in any one of Claims 15 to 19 wherein the said valve control means continually adjusts the flow of granular material into the said duct.

15 21. A method in any one of Claims 1 S to 20 further comprising the step of starting or stopping the said rotary valve and/or controlling its speed in accordance with the said detected pressure.

22. A method as claimed in Claims 15 to 21 wherein the said pressure sensor 2 o means comprises a plurality of pressure sensors distributed along the said duct and the said method further comprises the step of monitoring the plurality of pressure sensors to determine the location of a blockage or part blockage in the duct.

23. A method as claimed in Claims 15 to 22 further comprising the step of

. À. . he e..:e.::: À e . À. ace maintaining the pressure within the duct below a pre-determined maximum value by controlling the flow rate of granular material entering the duct.

24. A method as claimed in Claims l 5 to 23 the flow granular material in the duct 5 comprises a lean phase material flow.

25. A control system substantially as hereinbefore described with reference to the accompanying drawings.

10 26. A control device substantially as hereinbefore described with reference to the . accompanying drawings.

27. A method substantially as hereinbefore described with reference to the . accompanying drawings.