Low pressure control system and method for pneumatic conveying

Abstract

Described is a pneumatic conveying system for bulk material which comprises: a pressure vessel; a material conveying line; a compressor, wherein gas is supplied to the pressure vessel via a primary gas feed line; a plurality of pressure sensors and control valves; and a control system for varying the speed of the compressor and for controlling the plurality of control valves, wherein the primary gas feed line comprises a first control valve, and the system further comprises a first bypass line, which connects the primary gas feed line to the material conveying line and is located upstream of the first control valve, and wherein the first bypass line comprises a second control valve, wherein in use, the control system monitors the pressure of the pressure vessel, the material conveying line, and the compressor discharge, to maintain a set volume flowrate of the conveying gas along the conveying line.

Description

[0001] 24031 P-WO Qlar Europe GmbH

[0002] Low pressure control system and method for pneumatic conveying

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a pneumatic conveying apparatus. Specifically, the invention relates to a dense phase material pneumatic conveying apparatus which uses a low pressure dedicated compressor feed, wherein the system comprises a control loop to optimize the efficiency of the material conveying method, the system comprising a plurality of pressure sensors and control valves.

[0005] BACKGROUND OF THE INVENTION

[0006] Dense phase material conveying via pneumatics is a well known method in the material conveying industry. Most industrial powders and bulk granular materials can be conveyed by pneumatic means but currently only certain materials such as pet food, food products, grains and other low bulk density pelletized materials are conveyed in dense phase with dedicated lower pressure air sources.

[0007] Current dense phase conveying vessels for conveying other higher bulk density powders and bulk materials use compressed gas typically at pressures of around 2.5 bar gauge to 16 bar, and at a specified volume flow in order to achieve a specified conveying capacity.

[0008] Pressures in this document are given in bar, however they should be understood to refer to gauge pressure, and not absolute pressure. Gas will be referred to throughout this application so as to cover any type of conveying gas, however it should be apparent to the skilled person that air is the most common conveying gas.

[0009] Typically the gas flow and pressure is distributed to several points on the conveying vessel at the start of the conveying pipeline. Some of this gas flow will go to the top of the vessel and some to the aeration on the vessel if fitted, and some to the outlet of the vessel and / or the conveying pipeline.

[0010] The gas flow to the vessel is to provide a positive pressure gradient between the vessel and the material outlet, to provide the conveying force to discharge the material.24031 P-WO Qlar Europe GmbH

[0011] The gas flow to the conveying line is set and stabilize the density of the conveyed material and to prevent blockages within the conveying line from forming.

[0012] The gas flow to each point is typically controlled (restricted) with a square edged orifice plate or de Laval (sonic) nozzle, with the gas flow volume being determined by the geometry of the orifice, the upstream and the downstream pressure. The upstream pressure is usually set with a pressure regulator, or by the discharge pressure directly from the compressor. The proportion of air going to each point is determined during pre-commissioning trials and also during commissioning of the system or by empirical test work and scaling.

[0013] This air flow control and distribution method requires high pressure losses through the supply manifold and often results in the compressed air source being at much higher supply pressures than the back pressure in the conveying pipe resulting from the conveyed material. This leads to very inefficient systems according to the art, which need excessive compressor pressures to function. Compressor output pressures according to the art are also fixed at a high value - to ensure blockages do not occur. As systems according to the art do not have variable compressor pressures or flow, they are not customized to individual loads of material.

[0014] Although systems in the art can be known for the low consumption of conveying air, there is a large energy penalty for the higher levels of compression which are required.

[0015] There are low pressure dense phase systems using dedicated air sources at 0.5 to 1.5 bar. These typically use rotary valves (air locks) to meter the material feed into the conveying pipe and then maintain dense phase slugs in the conveying pipe by modulating the speed of the blower or compressor, by using accurate flow control and / or modulating the speed of the material feed through the rotary valve. This is mainly used for low bulk density, granular nonabrasive materials such as finished pet food and polymer pellets. This method is described in more detail in Figure 1.

[0016] The use of the rotary valve results in various limitations of systems according to the art. A rotary valve is a rotating device so it relies on maintaining accurate clearances to24031 P-WO Qlar Europe GmbH

[0017] function properly. At higher conveying pressures the clearances pass a similar volume of leakage gas as is used for conveying the material, which significantly reduces the efficiency of the entire system. The reliance on the clearances in the rotating device also limit is use to nonabrasive products and low temperatures. If abrasive materials were to be used, the clearances between the rotor and assembly would be adversely affected, such that significant leaks may occur.

[0018] The present invention is for an arrangement of valve and control philosophy that enables the use of a pressure vessel for highly efficient, reliable, low pressure dense phase conveying by avoiding the large pressure drops associated with the traditional manifold arrangements for flow control.

[0019] Experiments on the system according to the invention has shown that it is possible to distribute the conveying gas with low pressure drops, making use of lower pressure gas sources possible. This leads to an effective conveying system, which uses less energy than systems according to the art.

[0020] For the above reasons, there remains a need to address or mitigate at least one or more of the aforementioned problems.

[0021] It is therefore an object of at least one aspect of the present invention to provide an improved system and method for pneumatic conveying of dense phase materials.

[0022] It is a further object of at least one aspect of the present invention to provide an improved system and method, which is more energy efficient, and more effective at conveying than systems according to the art.

[0023] SUMMARY OF THE INVENTION

[0024] According to a first aspect of the present invention there is provided a pneumatic conveying system for conveying bulk material, the system comprising:

[0025] a pressure vessel for receiving material from a material supply feed;

[0026] a material conveying line for conveying material away from the pressure vessel; a compressor for supplying gas to the pressure vessel and the material24031 P-WO Qlar Europe GmbH

[0027] conveying line, wherein gas is supplied to the pressure vessel via a primary gas feed line;

[0028] a plurality of pressure sensors and a plurality of control valves; and

[0029] a control system for varying the speed of the compressor and for controlling the plurality of control valves; wherein the primary gas feed line comprises a first control valve, and the system further comprises a first bypass line, which connects the primary gas feed line to the material conveying line and is located upstream of the first control valve, and

[0030] wherein the first bypass line comprises a second control valve; wherein in use, the control system monitors the pressure vessel pressure, the material conveying line pressure, and the compressor discharge pressure, to maintain a set volume flowrate of the conveying gas along the conveying line.

[0031] The present invention has a number of technical advantages such as around flow control and flow distribution.

[0032] In the present invention the flow control may be performed by varying the speed of the compressor and / or blower.

[0033] By varying the speed of the compressor and / or blower provides a number of differences over the prior art:

[0034] • Prior art systems rely on an additional flow control device (e.g. macturie) valve to ensure the flow is accurate and predictable. As found in the present invention, by using a pressure vessel instead of a rotary valve as the feed device, the flow prediction is not as critical to achieving dense phase conveying. Not having the extra flow device gives a significant cost benefit and reduces complexity of control and reducing the required pressure drop between gas source and the conveying pressure.

[0035] • In prior art systems, pressure vessel systems usually use fixed flow control devices such as orifice plates or sonic nozzles in the machine manifold to control the overall flow. By doing this, the supply pressure must be 20 to 100 % higher than the conveying pressure. By avoiding this method of flow control and associated pressure drop there are large energy savings.24031 P-WO Qlar Europe GmbH

[0036] • The conveying gas volume can be optimized in all stages of the conveying sequence.

[0037] This may be controlled by feedback from conveying pressure transmitter (to provide a constant conveying velocity) or according to the conveying sequence (such as reducing compressor speed to save power when the conveying vessel is being filled). This is not obvious as most low pressure systems run with a fixed speed blower and are continuous systems. The use of a batch system usually means that the constant flow from the compressor is wasted during the refill I waiting time between conveying batches. Reducing the compressor speed between batches makes a batch system and the use of low pressure air source more efficient as although the compressor is still not stopped between batches, wasted energy is significantly reduced.

[0038] In the present invention the flow distribution may be performed by, for example, oversizing the orifice plates in the manifold so that there is more air flow volume.

[0039] By controlling and / or varying the flow distribution by oversizing the orifice plates in the manifold differentiates over the prior art as follows:

[0040] • Normally the orifice plates are usually set to control the overall air flow volume. In this invention, the orifice plates are well oversized for the required flow and can pass a much higher volume of gas than required. As the total flow is limited by the compressor speed, the orifice plates only provide a small pressure drop to distribute the conveying gas to different parts of the vessel. This allows the supply gas pressure to be closely matched to the conveying pressure making the system a lot more energy efficient.

[0041] • By oversizing the orifice plates also allows the efficient use of a low pressure air source for dense phase conveying with a pressure vessel system, (big energy savings) • By oversizing the orifice plates allows reduced cost compared and complexity to low pressure dense phase system using rotary valves. No wear associated with rotary valve, no leakage associated with rotary valve.

[0042] The output pressure of the compressor may be 0.5 bar and above. The output pressure may be between 0.5 bar and 3 bar. The output pressure may be less than 5 bar. The output pressure may be considered to make the system a low pressure system.24031 P-WO Qlar Europe GmbH

[0043] The pressure vessel may further comprise an outlet material valve, an inlet material valve, and a pressure relief valve. The system may comprise any number of valves, including the valves as described herein.

[0044] Downstream of the first control valve, the primary gas feed line may branch into an upper feed line, which enters the top of the pressure vessel, and a lower feed line, which enters the bottom of the pressure vessel. The pressure vessel may only be fed with one feed line. Alternatively, the pressure vessel may be fed with more than one feed line. The pressure vessel may be fed with feed lines which enter at multiple locations. The feed lines may enter at locations which are important for effective clearing of the material inside the pressure vessel.

[0045] The pressure vessel pressure may be measured with an upper feed line pressure sensor. The pressure vessel pressure may also be measured with a pressure sensor located inside the pressure vessel. The pressure vessel pressure may also be measured with any suitable device located proximate to the pressure vessel.

[0046] The system may further comprise a second bypass line, which connects the primary gas feed line to the material conveying line and is located upstream of the first bypass line. The second bypass line may comprise a third control valve. The second bypass line may further comprise a fixed orifice.

[0047] The first bypass line may comprise a first bypass line pressure sensor, located downstream of the second control valve. The second control valve may be a proportional valve, capable of being partially opened. The pressor sensors described herein may be any type of suitable pressure sensor.

[0048] The second bypass line may comprise a second bypass line pressure sensor, located downstream of the third control valve. The third control valve may be a proportional valve, cable of being partially opened. Proportional valves which allow partial opening allow for varied flowrates of gas, from 0 - 100% of the flow.

[0049] The control system may dynamically change the velocity of the compressor, based on24031 P-WO Qlar Europe GmbH

[0050] pressure sensors distributed throughout the system. The system may control the velocity of the compressor based on various factors, such as the type of material being conveyed, the conditions of the material, or the conditions of the system. The velocity of the compressor may change dynamically, and be updated every minute for example.

[0051] In another aspect of the invention, there is provided a method of controlling a dense phase pneumatic conveying system as previously described, the method comprising: measuring the pressure at each pressure sensor; adjusting the control valves and the speed of the compressor to maintain a set volume flowrate of the conveying gas along the conveying line. This method described leads to an efficient use of energy, and improved and optimized material conveying.

[0052] When the pressure at the first bypass line pressor sensor is below a threshold value, the second control valve and optionally the third control valve, may be opened to their minimum value, such that the conveying gas is primarily directed towards the pressure vessel. This technique directs all of the air towards the pressure vessel, which has the effect of clearing material from the pressure vessel, towards the material conveying line.

[0053] When either the first bypass line pressure sensor or the upper feed line pressure sensor exceed upper threshold values, the second control valve may open fully in order to direct all of the conveying gas to the material conveying line, thus reducing the conveying pressure as the pressure vessel discharge rate is reduced to a minimum. This technique is useful when the material conveying line comprises a blockage or a slug, as uninterrupted air is forced into the material conveying line without passing through the pressure vessel.

[0054] The third control valve (if fitted) may modulate to maintain an increased pressure differential between the second bypass line pressure sensor and the first bypass line pressure sensor, to help clear potential material plugs in the material conveying line.

[0055] The modulation may vary the amount the valve is opened, ranging from 0-100% open.

[0056] During operation of the second control valve and the third control valve, the control system24031 P-WO Qlar Europe GmbH

[0057] may vary the compressor speed to ensure the required total air flow volume is directed to the system. The total air flow volume may be measured at several locations throughout the system.

[0058] The method described above may be a closed-loop feedback method. The system may measure pressures and flowrates, and feed these measurements back into the control variables such as compressor speed and valve positions.

[0059] In another aspect of the present invention, there is provided a method of filling a pneumatic conveying system according to a system as previously described with material to be conveyed, the method comprising the steps of reducing the velocity of the compressor; losing the first control valve and opening the second control valve, thus diverting the gas through the first bypass line to the material conveying line; closing the outlet material valve and opening the inlet material valve; and filling the pressure vessel with material.

[0060] Advantages of the invention are numerous. For example, the proposed invention does not use a rotary valve as the pressure barrier for conveying. This provides a number of benefits over the state of the art solutions:

[0061] There is no need to have a constantly rotating part in the material flow, which enables the invention to be capable of conveying more abrasive materials than systems according to the art which may use a rotary valve.

[0062] The use of a sealed pressure vessel gives the possibility of higher conveying pressures, a dedicated compressor of pressure higher than 4 bar could be used. The use of a higher pressure conveying gas source allows: conveying of higher density materials; increased transfer rates; increased transfer distance capability; and conveying of materials which are less readily conveyed (the existing low pressure rotary valve technologies are really only applied to pelletized, granular materials such as pet food, finished food products and polymer conveying).

[0063] As much less air is lost through rotary valve leakage compared to a rotary valve24031 P-WO Qlar Europe GmbH

[0064] solution, efficiency is greatly increased an furthermore power consumption greatly reduced.

[0065] Due to the requirement for tight clearances on the rotary valve blades, the temperature range in which a rotary valve can be used is limited since the metals used can expand and contract significantly. The use of a pressure vessel, as in the system of the invention, allows higher temperature applications.

[0066] Compared to a screw pump solution, the energy consumption and wear related to the screw is eliminated. This is not an issue with the present invention, which does not use screw pumps.

[0067] Compared to standard pressure vessel based dense phase conveying systems, the pressure and / or energy losses due to the flow control are avoided and the compressed gas source pressure can closely match the required conveying pressure. Energy losses associated with over compression are avoided. This can save as much as 50% of the power consumption when comparing like for like applications.

[0068] The system may be used on with the method as previously described.

[0069] An alternative method for filling is to use a compressor which has the capability to stop and start frequently or a compressor that will vent off the compressed air to atmosphere or recirculate the compressed air during the filling sequence of the pressure vessel.

[0070] Compressors that can stop and start at high frequency > 30 time per hour are less common and have a much higher cost, but would provide a further energy saving. Venting to atmosphere or recirculation through the compressor instead of routing down the conveying pipe line may be less efficient if the vent / recirculation is initiated by a high pressure no flow condition.

[0071] Having the air flow thorough the conveying pipeline during conveying can have the benefit to keep the pipe route warm and free of condensation and can also be used to24031 P-WO Qlar Europe GmbH

[0072] keep residual material in the pipe line fluidized and prevent blockages once filling is complete.

[0073] BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Embodiments of the present invention will now be described, by way of example only, with reference to the following figures:

[0075] Figure 1 is a generalized low pressure pneumatic conveying system for dense phase materials, according to the prior art; and

[0076] Figure 2 is an example system according to the present invention.

[0077] DETAILED DESCRIPTION

[0078] Generally speaking, the present invention relates to a pneumatic conveying system for pneumatically conveying bulk material, wherein the system comprises a feedback loop to control the flowrate of the bulk material.

[0079] Figure 1 shows a generalized prior art rotary valve system 900. The schematic shows a typical rotary valve based low pressure dense phase system according to the art. The system comprises a rotary valve 912 and a feed hopper 914, as well as a conveying pipe 916.

[0080] Generally there are two control loops in systems according to the art. The first manages a pressure upstream of the flow control device 902 and modulates the speed of a compressor 904 to maintain the compressor pressure sensor 906 at a set pressure above the rotary valve pressure sensor 908.

[0081] The second control loop monitors the rotary valve pressure sensor 908 to modulate the air flow using the flow control device 902 and / or the material feed rate (rotary valve speed). The aim of modulating the gas flow or material feed is to maintain the material phase24031 P-WO Qlar Europe GmbH

[0082] density and always maintain material slugs in the conveying pipe, whist also limiting the slug length so that the conveying pipe 916 does not become blocked.

[0083] The rotary valve 912 feeding the conveying pipe 916 is a rotating device with clearances to allow it to move inside a pressure tight case. The clearance allows conveying gas to be leaked into the feed hopper and the volume of leaked gas increases with increasing conveying pressure.

[0084] For an efficient system the clearances need to be a small as possible, this limits the use of the rotary valve to low abrasion materials and low temperature applications.

[0085] In systems of the art, screw pumps are used to introduce material into the conveying line. A typical process is outlined below:

[0086] Material is fed through an inlet into a metering screw. The material is compressed through the screw against an outlet flap valve. The conveying gas is fed into the conveying pipe though the multiple nozzles below the outlet of the flap valve.

[0087] The technology uses the compaction of the material against the flap valve to create a pressure tight seal between the pressurized conveying pipe and atmospheric material feed into the screw.

[0088] The compaction of the material against the flap valve requires relative high torque levels on the screw drive shaft and therefore also high power consumption.

[0089] The technology also has a rotating screw submerged in material and a valve which has material being continually forced past it. These components are both highly susceptible to abrasive wear.

[0090] Figure 2 is an example system 100 according to the present invention. Specifically, the figure shows a schematic of the process and instruments.

[0091] The system 100 comprises a pressure vessel 102 for receiving material from a material24031 P-WO Qlar Europe GmbH

[0092] supply feed 104. The material supply feed 104 may be controlled via a inlet material valve 106. Connected to the pressure vessel 102 is the material conveying line 108 which transports the conveyed material from the pressure vessel 102 to the desired destination.

[0093] The system 100 also comprises a compressor 110 for supplying gas to the pressure vessel 102 and the material conveying line 108. The compressor 110 supplies gas via the primary gas feed line 112. This line 112 is connected to the compressor 110 and the pressure vessel 102.

[0094] The system also comprises a plurality of pressure sensors and a plurality of control valves, as well as a control system 114, for varying the speed of the compressor and for controlling the position of the control valves.

[0095] The control valves may be on / off valves, or may offer proportional control (i.e. may be partially opened and closed).

[0096] The pressure vessel 102 comprises an outlet material valve 116, which is used to isolate the pressure vessel 102 from the conveying line 108 whilst the pressure vessel 102 is being filled with material.

[0097] The inlet material valve 106 is located at the top of the pressure vessel 102, and controls the amount of material entering the top of the pressure vessel 102. Material is typically fed into the pressure vessel 102 via gravity.

[0098] Typically, the pressure vessel 102 will also comprise a form of a pressure vent valve, this is used to vent residual pressure from the conveying vessel 102 after each batch of material is transferred, and additionally to allow air displaced by material during filling to be vented from the conveying vessel 102. The pressure relief valve 118 usually vents to the feed hopper. A distinct advantage with the invention over the prior art is that it can be used with various material types and conditions, all whilst being easily adjustable and more energy efficient.24031 P-WO Qlar Europe GmbH

[0099] The system 100 shown also comprises a first control valve 120 which is located on the primary gas feed line 112. The first control valve 120 is located upstream of a branch in the primary gas feed line 112. The primary gas feed line 112 branches into an upper feed line 112a and a lower feed line 112b. The upper feed line 112a inserts the top of the pressure vessel 102, and the lower feed line 112b inserts the bottom of the pressure vessel 102.

[0100] In this example, both upper and lower feed lines comprise fixed orifices 122. The purpose of these orifices or nozzles is not to set the total gas flow to the system but to provide a back pressure at each gas entry point and ensure a distribution of gas flow into the pressure vessel 102. The orifice plates or nozzles will be well be operating well below choked flow condition.

[0101] The pressure vessel pressure in this example is measure via an upper feed line pressure sensor 124. In alternative embodiments, the pressure vessel pressure may be measured within the pressure vessel 102.

[0102] The system 100 also contains a first bypass line 126, which connects the primary gas feed line 112 to the material conveying line 108. The first bypass line is located upstream of the first control valve 120.

[0103] The first bypass line 126 comprises a second control valve 128. The second control valve 128 controls the amount of gas flow through the first bypass line 126. If the second control valve 128 is closed, then the gas flow is forced to travel through the pressure vessel 102 to reach the material conveying line 108. If the second control valve 128 is fully open, then due to the fixed orifices 122 in the primary gas feed line 112 which create some back pressure, the gas flow follows the path of least resistance, and thus diverts immediately to the material conveying line 108, thus bypassing the pressure vessel 102. This can be extremely useful if slugs of material form in the material conveying line 108, as the diverted conveying gas can move the slug on with more force, or break down the slug altogether. The first bypass line 126 and second control valve 128 also coarsely control the rate of material discharge from the conveying vessel 102. By forcing more gas through pressure vessel 102 material is discharged more rapidly, and conversely less material24031 P-WO Qlar Europe GmbH

[0104] when gas flow is biased through the first bypass line 126.

[0105] A third function of the first bypass line 126 is to pass all of the gas flow when valve 116 and valve 120 are closed to fill material into the vessel. In the situation control valve 128 is full open.

[0106] The system 100 shown can also comprises a second bypass line 130 this is fitted depending on material characteristics and is not always required. Like the first bypass line 126, connects the primary gas feed line 112 to the material conveying line 108. The second bypass line 130 is located upstream of the first bypass line 126, i.e. is closer to the compressor 110 than the first bypass line 126.

[0107] The second bypass line 130 comprises a third control valve 132. This is used to set the pressure measured by pressure transmitter in the second bypass line 130. By changing the difference in pressure between the supplementary air supply pressure 136 and conveying line pressure transmitter 134, more air can be diverted from the pressure vessel 102 and first bypass line 126 directly in to the conveying pipe line 108 at regular intervals along the length conveying pipe.

[0108] In this embodiment, the second bypass line 130 also comprises a fixed orifice 122. This can be set and sized depending on the material properties to be conveyed, along with the diameter of the conveying pipeline 108.

[0109] Both bypass lines comprise a pressure sensor located down stream of their respective control valves i.e. on the material conveying line 108 side of the control valve.

[0110] These pressure sensors are coupled to the control system 114, along with the upper feed line pressure sensor 124 and the primary gas feed line pressure sensor 138, which is located proximate to the compressor, upstream of both bypass lines.

[0111] The control system 114 is coupled to the compressor 110, and can control the speed at which it runs. This in turn controls the volumetric flow of the gas that enters the system 100. The pressure at the exit of the compressor 110 is measured by the primary gas24031 P-WO Qlar Europe GmbH

[0112] feed line pressure sensor 138.

[0113] The invention separates the flow control and air distribution which is typically done by the same devices (distributed square edge orifice plates, de Laval nozzles or manually adjusted flow restriction valves). This is a unique advantage over the prior art because it gives more controllability over systems according to the art. The increased controllability also allows the system to be more energy efficient over systems in the art.

[0114] The total volume of the conveying gas going to the conveying system 100 may be calculated from the compressor speed, the compressor characteristics and the measured discharge pressure, measured by the primary gas feed line pressure sensor 138.

[0115] Control Loop 1 monitors the conveying vessel pressure via the upper feed line pressure sensor 124, conveying line pressure via the first bypass line pressure sensor 134 and compressor discharge pressure via the primary gas feed line pressure sensor 138 to maintain either set volume flows of conveying gas or set pick up gas velocities in the start of the conveying pipeline.

[0116] The proportion of conveying gas going to the pressure vessel 102 is restricted by fixed orifices the fixed orifices 122 located on the upper and lower feed lines. These orifices are dimensioned to provide sufficient air volume to empty the vessel and start conveying, but not to necessarily achieve full conveying capacity with all conveyed materials and operates in the non-choked region of pressure differentials.

[0117] Additional air can be directed to the pipeline via the second control valve 128 and the third control valve 132. The third control valve 132 should be considered to be optional, and is only required when supplementary pipeline gas injection is used. The second control valve 128 and the third control valve 132 will be proportionally operated between set points according to the pressure in the conveying pipeline measured by the first bypass line pressure sensor 134.

[0118] When the pressure measured by the first bypass line pressure sensor 134 is below the24031 P-WO Qlar Europe GmbH

[0119] required value, the second control valve 128 will be opened to its minimum value (which could be for example 0% to 75% of their maximum value). As the value of the first bypass line pressure sensor 134 increases, the second control valve 128 will proportionally increase its opening to direct more air volume to the material conveying line 108 and less to the pressure vessel 102 through the fixed orifices 122, reducing the discharge rate from the pressure vessel 102 and reducing the conveying pressure. (Control Loop 2)

[0120] When the first bypass line pressure sensor 134 decreases, the second control valve 128 will proportionally decrease its opening to direct more air to the vessel through the fixed orifices 122, to increase the discharge rate from the pressure vessel 102 and increase the conveying pressure. (Control Loop 2)

[0121] In the case either the first bypass line pressure sensor 134 or the upper feed line pressure sensor 124 exceed upper threshold values, the second control valve 128 will open fully in order to ensure all the conveying gas is directed to the material conveying line 108, which reduces the conveying pressure as the pressure vessel discharge rate is reduced to a minimum. (Control loop 2)

[0122] During this high pressure situation the third control valve 132 (if pipeline supplementary gas points are fitted) will modulate to maintain an increased differential between the second bypass line pressure sensor 136 and the first bypass line pressure sensor 134. This acts to bias more of the conveying gas flow through the pipeline supplementary injection points (i.e. through the fixed orifice 122 on the second bypass line 130) and help clear and plugs in the material conveying line 108. (Control loop 3)

[0123] During normal pressure situation the third control valve 132 (if the second bypass line 130 is fitted) will modulate to maintain a slight differential between the second bypass line pressure sensor 136 and the first bypass line pressure sensor 134. This acts to prevent material flow from conveying pipe 108 into the supplementary air supply pipe.

[0124] During the operation of the second control valve 128 and the third control valve 132, control loop 1 will vary the compressor 110 speed to ensure the required total air flow volume is directed to the system 100, but will also ensure that the pressure isn’t allowed to go24031 P-WO Qlar Europe GmbH

[0125] above the upper threshold as this could be indicative of pipeline blockages or reduced performance.

[0126] A further advantage of the invention allows the pressure vessel 102 to be refilled easily. To enable refilling of the pressure vessel 102 with material from the material feed hopper, the first control valve 120 is closed to isolate the pressure vessel 102 from the gas source, and outlet material valve 116 is closed to isolate the pressure vessel 102 from the primary gas feed line 112.

[0127] Once the pressure vessel 102 is isolated the inlet material valve 106 can be opened and the material can fill up the pressure vessel 102. The pressure vessel 102 may be at a slight positive pressure (due to pressure drop of the conveying gas through the empty pipe) so a small amount of conveying gas will be lost as this excess pressure is released from the conveying pressure vessel 102 through the vessel vent valve 118.

[0128] During this refilling time, the second control valve 128 is fully open and the compressor 110 speed is reduced to minimise conveying gas flow through the empty pipe, increasing system efficiency.

[0129] There are a number of other distinct advantages with the invention over the prior art.

[0130] The use of the second control valve 128 to dynamically adjust the conveying gas distribution in the system 100 instead fully alternating the first control valve 120 and second control valve 128 between fully open and fully closed conditions changes the distribution of air between the conveying line 108 and pressure vessel 102 to create a continuous stable flow of material instead of the pulsed flow that alternating that alternating valves 120 and 128 creates.

[0131] Using a feedback loop based on compressor speed, discharge pressure and known compressor characteristics to calculate and control the conveying gas flow.

[0132] The invention allows the system 100 to separate the total gas flow control from the gas flow distribution devices. This allows the use of greatly oversized orifice plates for flow24031 P-WO Qlar Europe GmbH

[0133] distribution , which results in significantly less energy losses due to the requirement for high differential pressure drops.

[0134] Controlling the compressor 110 speed based on both flow and pressure so that both flow and pressure limits can be maintained within the conveying system 100 is a distinct advantage over the prior art.

[0135] Using the conveying pressure to adjust the conveying gas flow set point during the conveying cycle. This in effect creates a feedback loop within the system 100. The system can therefore dynamically change depending on the conveying conditions.

[0136] Another advantage is the ability to dynamically control the flow through the first bypass line 126 and the second bypass line 130 by adjusting the differential pressure between the third control valve 132 and the second control valve 128.

[0137] As previously mentioned, the system 100 can bypass the conveying gas past the pressure vessel 102 though the second control valve 128 during pressure vessel 102 filling and reducing the compressor 110 speed to a minimum to save energy consumption, and to prevent the material from being blown back through the material inlet valve 106.

[0138] These distinct advantages are neither taught nor suggested in the prior art documents.

[0139] Bulk material may be defined as dry or substantially dry materials which may be in the form of any one of or combination or the following: powder; granular; lumpy etc. Examples of bulk materials may beany one of or combination of the following: any type of foodstuff; minerals; ores; coal; cereals; woodchips; cement; sand; gravel; clay; cement; ash; salt; chemicals; grain; sugar; flour and stone in loose bulk form.

[0140] Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention. For example, any suitable type of sensor may be24031 P-WO Qlar Europe GmbH

[0141] used to detect and / or measure the velocity of the pneumatically conveyed bulk material.

Claims

24031 P-WO Qlar Europe GmbHCLAIMS1. A pneumatic conveying system for conveying dense-phase material, the system comprising:a pressure vessel for receiving material from a material supply feed;a material conveying line for conveying material away from the pressure vessel; a compressor for supplying gas to the pressure vessel and the material conveying line, wherein gas is supplied to the pressure vessel via a primary gas feed line;a plurality of pressure sensors and a plurality of control valves; anda control system for varying the speed of the compressor and for controlling the plurality of control valves; wherein the primary gas feed line comprises a first control valve, and the system further comprises a first bypass line, which connects the primary gas feed line to the material conveying line and is located upstream of the first control valve, andwherein the first bypass line comprises a second control valve; wherein in use, the control system monitors the pressure vessel pressure, the material conveying line pressure, and the compressor discharge pressure, to maintain a set volume flowrate of the conveying gas along the conveying line.

2. A pneumatic conveying system according to claim 1, wherein the output pressure of the compressor is 0.5 bar and above.

3. A pneumatic conveying system according to any preceding claim, wherein the pressure vessel further comprises an outlet material valve, an inlet material valve, and a pressure relief valve.

4. A pneumatic conveying system according to any preceding claim, wherein downstream of the first control valve, the primary gas feed line branches into an upper feed line, which enters the top of the pressure vessel, and a lower feed line, which enters the bottom of the pressure vessel.

5. A pneumatic conveying system according to claim 4, wherein the pressure vessel pressure is measured with an upper feed line pressure sensor.24031 P-WO Qlar Europe GmbH6. A pneumatic conveying system according to any preceding claim, wherein the system further comprises a second bypass line, which connects the primary gas feed line to the material conveying line and is located upstream of the first bypass line, and wherein the second bypass line comprises a third control valve.

7. A pneumatic conveying system according to any preceding claim, wherein the first bypass line comprises a first bypass line pressure sensor, located downstream of the second control valve.

8. A pneumatic conveying system according to claim 7, wherein the second bypass line comprises a second bypass line pressure sensor, located downstream of the third control valve.

9. A pneumatic conveying system according to any preceding claim, wherein the control system dynamically changes the velocity of the compressor, based on pressure sensors distributed throughout the system.

10. A pneumatic conveying system according to any preceding claim, wherein flow control in the pneumatic conveying system is controlled and / or varied.

11. A pneumatic conveying system according to any preceding claim, wherein flow control in the pneumatic conveying system is controlled and / or varied by varying the speed of a compressor and / or blower.

12. A pneumatic conveying system according to any preceding claim, wherein flow distribution in the pneumatic conveying system is controlled and / or varied.

13. A pneumatic conveying system according to any preceding claim, wherein flow distribution in the pneumatic conveying system is controlled and / or varied by oversizing orifice plates in a manifold.

14. A method of controlling a dense phase pneumatic conveying system24031 P-WO Qlar Europe GmbHaccording to any of claims 1-9, the method comprising: measuring the pressure at each pressure sensor; adjusting the control valves and the speed of the compressor to maintain a set volume flowrate of the conveying gas along the conveying line.

15. A method according to claim 14, wherein when the pressure at the first bypass line pressor sensor is below a threshold value, the second control valve and optionally the third control valve, will be opened to their minimum value, such that the conveying gas is primarily directed towards the pressure vessel.

16. A method according to claim 14, wherein when either the first bypass line pressure sensor or the upper feed line pressure sensor exceed upper threshold values, the second control valve will open fully in order to direct all of the conveying gas to the material conveying line, thus reducing the conveying pressure as the pressure vessel discharge rate is reduced to a minimum.

17. A method according to claim 15 or 16, wherein the third control valve (if fitted) modulates to maintain an increased pressure differential between the second bypass line pressure sensor and the first bypass line pressure sensor, to help clear potential material plugs in the material conveying line.

18. A method according to any of claims 15-17, wherein during operation of the second control valve and the third control valve, the control system varies the compressor speed to ensure the required total air flow volume is directed to the system.

19. A method according to claim 13, wherein the method is a closed-loop feedback method.

20. A method filling a pneumatic conveying system according to any of claims 1- 13 with material to be conveyed, the method comprising the steps of reducing the velocity of the compressor; closing the first control valve and opening the second control valve, thus diverting the gas through the first bypass line to the material conveying line; closing the outlet material valve and opening the inlet material valve; and filling the pressure vessel with material.

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