Patent Publication Number: US-8123591-B2

Title: Abrasive pump for an abrasive jet cutting machine

Description:
BACKGROUND 
     An abrasive jet cutter generally operates by focusing a high pressure jet of fluid carrying entrained abrasive particles onto a work surface. 
     Abrasive jet cutting machines generally have a relatively small abrasive hopper near the cutting nozzle sufficient to supply the jet for less than 30 minutes. For production work, it is desirable to automatically fill this small hopper from a larger abrasive source. 
     Commonly, a large pressure pot of the type commonly used for sandblasting is filled with several hundred to a few thousand pounds of abrasive and then pressurized with air to around 50 psi. The air pressure forces the abrasive to flow through a small hose to the smaller hopper near the nozzle. When the small hopper is full, the abrasive around the hose outlet stops further abrasive from coming and the flow ceases. 
     OVERVIEW 
     According to an embodiment, an abrasive jet cutting system includes an abrasive hopper that may be left at or substantially at atmospheric pressure. 
     According to an embodiment, an abrasive jet cutting system includes an abrasive delivery system having an abrasive tank configured to alternately 1) receive abrasive from an abrasive hopper substantially at atmospheric pressure and 2) provide abrasive under pressure for delivery to an abrasive jet cutting head. The abrasive tank may receive air through a substantially constant flow source, such as a needle valve. 
     According to an embodiment an abrasive jet cutting system includes an abrasive delivery system configured to automatically fill an abrasive tank when empty and automatically resume pressurization of the abrasive tank when refilled. According to an embodiment, the abrasive delivery system is automated using pneumatic components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an abrasive supply system for conveying abrasive particles with a substantially constant flow rate gas source, according to an embodiment. 
         FIG. 2  is a diagram of an abrasive supply system including an atmospheric pressure abrasive hopper, and a control valve for controlling a substantially constant flow rate gas source and an abrasive supply valve, according to an embodiment. 
         FIG. 3A  is a diagram of an abrasive supply system with a controller configured for automatic control of a substantially constant flow rate gas source and an abrasive supply valve, according to an embodiment. 
         FIG. 3B  is a diagram of an abrasive supply system with a split controller including a refill controller and a resume controller, according to an embodiment. 
         FIG. 4  is a flow chart illustrating a control algorithm for the controller of  FIGS. 3A ,  3 B, and  5 - 7 , according to an embodiment. 
         FIG. 5  is a diagram of an abrasive supply system with a pneumatic controller configured for automatic control of a substantially constant flow rate gas source and an abrasive supply valve in a first state, according to an embodiment. 
         FIG. 6  is a diagram of the abrasive supply system of  FIG. 5  at a moment corresponding to the end of the state of  FIG. 5 , according to an embodiment. 
         FIG. 7  is a diagram of the abrasive supply system of  FIGS. 5 and 6  in a second state corresponding to refilling the abrasive tank that begins a moment after the configuration of  FIG. 6 , according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is presented to enable a person skilled in the art to make and use the claimed invention. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention as defined by the appended claims. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. 
       FIG. 1  is a diagram of an abrasive supply system  101  for conveying abrasive particles with a substantially constant flow rate gas source  106 , according to an embodiment. The substantially constant flow rate gas source  106  is configured to pressurize an abrasive tank  102  that may hold abrasive particles. The substantially constant flow rate gas source  106  is further configured to convey gas-entrained abrasive particles through an abrasive delivery tube  104 . 
     Typically, the air flow required to push the abrasive particles through the abrasive delivery tube  104  is small. The frictional effects of the abrasive particles moving through the abrasive delivery tube creates a back pressure sufficient to cause a relatively significant pressure rise at the substantially constant flow rate gas source  106  and the abrasive tank to, for example, a value between about 10 and 50 psig. As long a enough abrasive remains in the abrasive tank  102  to continue delivering abrasive particles to the abrasive delivery tube, the back pressure of the flowing abrasive particles maintains the gas pressure at the substantially constant flow rate gas source  106  and in the abrasive tank  102 . However, as the abrasive tank  102  empties and the abrasive particles are purged from the abrasive delivery tube  104 , the back pressure decreases and the pressure at the substantially constant flow rate gas source  106  and abrasive tank drops significantly. 
     This self-regulation of pressure, wherein the gas pressure in the abrasive tank  102  and at the inlet end of the abrasive supply tube drops when the abrasive is exhausted, tends to prevent the abrasive particles remaining in the distal end (not shown) of the abrasive delivery tube  104  from being blown out the distal end of the abrasive delivery tube. In contrast, blowing abrasive particles out of the distal end of the abrasive delivery tube is one unfortunate effect that may arise from the use of a substantially constant pressure gas source rather than a substantially constant flow rate gas source. 
     According to an embodiment, a metering valve  108  may receive gas from a substantially constant pressure gas source  110  to produce the substantially constant flow rate gas source  106 . For example, air may be received at  110  from an air compressor or a shop air system (not shown) at a pressure typical for such systems, for example at about 60 to 120 psig. The metering valve  108  may include a needle valve adjusted or selected to produce a gas flow rate appropriate for delivering abrasive particles to the distal end (not shown) of the abrasive delivery tube  104  at a rate appropriate for an application. For example, for a typical abrasive jet cutting apparatus, the metering valve  108  may produce a gas flow rate of about 10 liters per min to deliver garnet abrasive particles to a cutting nozzle at a rate of about 1 pound per minute. 
       FIG. 2  is a diagram of an embodiment of an abrasive supply system  201  that includes provision for refilling the abrasive tank  102  with abrasive particles  204  from a large abrasive hopper  202 , which may typically be maintained substantially at atmospheric pressure. A control valve  210  (which may alternatively be configured as more than one control valve) is configured to open or close to respectively pass or stop gas from the substantially constant pressure gas source  110  from reaching a switched substantially constant pressure node  208 . 
     When the control valve  210  is open, pressure is maintained at node  208 , and thus the metering valve  108  continues to maintain flow through the abrasive delivery tube  104  and, if abrasive particles remain in the tube, pressurize the abrasive tank  102 . Pressure at node  208  also keeps an abrasive supply valve  206  closed, which prevents air pressure from the abrasive supply tank  102  from leaking out through the abrasive hopper  202 . According to an alternative embodiment, node  208  may be split, with one node providing gas flow to the metering valve  108  and another node providing gas flow to the abrasive supply valve  206 . 
     When the control valve  210  is closed, the pressure at node  208  drops, for example due to continued flow through the metering valve  108 . A drop in pressure at node  208  opens the abrasive supply valve  206  to selectively admit abrasive particles  204  from the abrasive hopper  202  to the abrasive tank  102 . After a desired amount of abrasive particles  204  have flowed from the abrasive hopper  202  to the abrasive tank  102 , the control valve  210  may be opened to restore pressure to node  208 . In turn, restoration of pressure at node  208  closes the abrasive supply valve  206  and begins gas flow through the metering valve  108 . Since there are again abrasive particles in the abrasive tank  102  to flow into and through the abrasive delivery tube  104 , the air flow through the substantially constant flow rate gas source  106  causes a pressure rise to pressurize the abrasive tank  102  and the inlet end of the abrasive delivery tube  104 . Thus, the control valve  210  is configured to selectively close the abrasive supply valve  206  when there is gas flow through the metering valve  108  or open the abrasive supply valve  206  when there is substantially no gas flow through the metering valve  108 . 
     The abrasive tank  102  may be configured to hold a relatively small amount of abrasive particles, such as about 1 gallon. A small abrasive tank  102  requires only relatively thin walls to withstand an operating pressure of about 10 psig to about 50 psig. A small abrasive tank  102  may help avoid dealing relatively onerous pressure vessel safety standards typically associated with a large pressure vessel, such as a large pressurized abrasive hopper. 
     Compared to prior art systems, the abrasive supply system  201  does not require pressurization of the abrasive hopper  202 . This allows the elimination of an expensive and heavy-walled large pressure vessel. For example, a typical prior art pressurized abrasive hopper may be about 3 feet diameter by 4 feet high, and have walls made of ½ inch steel plate. Instead, the abrasive hopper  202  may be formed from a low cost polyethylene tank which is not pressurized. The abrasive hopper  202  has a conical bottom that allows the abrasive particles  204  to flow by gravity to a central discharge hole. Immediately below the central discharge hole is the abrasive supply valve  206  that can shut off the abrasive flow and resist an air pressure below it or open to allow gravity flow of the abrasive particles  204  from the abrasive hopper  202  to the abrasive tank  102 . A bladder-type pinch valve has been found to work well as an abrasive supply valve  206 . 
       FIG. 3A  is a diagram of an abrasive supply system  301  configured for automatic control, according to an embodiment. A controller  302  is operatively coupled to receive a pressure signal from the substantially constant flow rate node  106 . Responsive to a drop in pressure precipitated by the emptying of abrasive from the abrasive tank  102  and related decrease in back pressure within the abrasive delivery tube  104 , the controller is configured to shut the control valve  210 . As described above, closing the control valve  210  reduces the pressure at node  208 , which substantially stops flow through the metering valve  108 , thereby depressurizing the abrasive tank  102  to substantially atmospheric pressure. Shutting the control valve  210  and resultant drop in pressure at node  208  is further operative to open the abrasive supply valve  206  to allow abrasive particles  204  to flow from the abrasive hopper  202  to the abrasive supply tank. 
     After a time, the gravity flow of abrasive particles at least partially refills the abrasive tank  102 . According to an embodiment, it may be preferred to substantially refill without overfilling the abrasive tank  120 . According to an embodiment a bladder-type pinch value may be used as the abrasive supply value  206 . It has been found that overfilling the abrasive tank  120  may tend to pinch an excessive amount of abrasive between the bladders of the pinch valve  206  and thus damage or decrease the service life of the valve  206 . 
     When the abrasive tank  102  has been sufficiently refilled, such as after an amount of time corresponding to sufficient refilling, the controller  302  again actuates the control valve  210  to open and reestablish a connection between the gas source  110  and the node  208 . Of course, when node  208  is again pressurized, the abrasive supply valve  206  closes to stop the flow of abrasive and maintain the pressure of the abrasive tank  102 ; and the metering valve  108  again establishes a substantially constant gas flow rate at node  106  to pressurize the abrasive tank  102  and propel the abrasive particles through the abrasive delivery tube  104 . 
     An embodiment of a process corresponding to the behavior of the controller  302  is shown in the flow chart  401  of  FIG. 4 . In step  402 , the control valve  210  is closed to depressurize the abrasive tank  102  (and stop propulsion of abrasive particles in the abrasive delivery tube  104 ). During the state corresponding to step  402 , the abrasive tank  102  refills with abrasive and abrasive propulsion through the abrasive delivery tube is suspended. The state corresponding to step  402  may be referred to as the refill state. The system remains in the state corresponding to step  402  until a condition for decision step  404  is satisfied. According to an embodiment, the controller may monitor the amount of abrasive in the abrasive tank and/or the flow of abrasive into the abrasive tank to determine when the condition is satisfied for step  404 . According to another embodiment, a timer may be set to allow a predetermined time for flow of abrasive into the abrasive tank. The condition for step  404  is then satisfied by the passage of the predetermined time. 
     After the condition of step  404  is satisfied, the process proceeds to step  406 . At the beginning of step  406 , the control valve is opened again to close the abrasive delivery valve  206  and begin or resume the flow of gas through the metering valve  108  to pressurize the abrasive tank  102  and propel abrasive particles through the abrasive supply tube  104 . During the state corresponding to step  404 , the system continues to propel abrasive particles from the abrasive tank. A resume mechanism (not shown) in the controller  302  of  FIG. 3A  may be configured to initiate the transition from the state corresponding to step  402  to the state corresponding to step  406 . 
     According to an example, the state corresponding to step  402  (and hence a corresponding timeout value) may last about 10 seconds. According to an example, the state corresponding to step  406  may typically last about 1-3 minutes until exhaustion of the abrasive supply in the abrasive tank  102  again causes the pressure at node  106  to drop. Proceeding to step  408 , when a pressure drop is sensed at node  106 , the process again proceeds to step  402 , and the process is repeated. 
     According to an embodiment, depicted in  FIG. 3B  as system  303 , functional portions of the controller  302  corresponding respectively to the behavior of steps  408  and  404  of  FIG. 4  may be split into controller portions  302   a  and  302   b.    
     In the embodiment  303 , a refill controller  302   a  is operatively coupled to the substantially constant flow rate node  106  to monitor pressure drop. Upon encountering a pressure drop, the refill controller  302   a  actuates control valve  210  to stop gas flow, reduce the pressure at node  208 , and refill the abrasive tank  102  as described above. After the control valve  210  is shut off, control passes to the resume controller  302   b , which is configured to open the control valve  210  to stop the flow of abrasive into and seal the abrasive tank  102 , and resume propulsion of abrasive particles through the abrasive delivery tube  104 . According to an embodiment, the resume controller  302   b  may include a timer configured to open the control valve  210  after a time delay corresponding to a desired amount of filling of the abrasive tank  102 . The time delay may correspond to a time that allows the abrasive tank  102  to almost but not completely fill. 
     According to some embodiments, the controller  302  ( FIG. 3A ), the refill controller  302   a , and/or resume controller  302   b  ( FIG. 3B ), may be partly or completely constructed as pneumatic logic devices. For example,  FIGS. 5-7  are a diagrams of states  501 ,  601 , and  701  of an abrasive supply system with a pneumatic refill controller  302   a  and pneumatic resume controller  302   b  configured to actuate the control valve  210 , according to embodiments. 
     Referring to  FIG. 5 , a gas source  110  is coupled to a substantially constant pressure node  208  via the supply valve  210 . The pressure at node  208  keeps the abrasive supply valve  206  closed to isolate the (pressurized) abrasive tank  102  from the atmospheric pressure abrasive hopper  202  and prevent abrasive particles  204  from dropping into the abrasive tank  102 . Simultaneously, the pressure at node  208  feeds the metering valve  108 , which may be embodied as a needle valve, for example. The metering valve  108  admits a controlled flow rate of gas to form the substantially constant flow rate node  106 , from which the gas may pressurize the abrasive tank  102  and propel abrasive particles through the abrasive delivery tube  104 . 
     The abrasive hopper  202  is held substantially at atmospheric pressure, and may for example be a polyethylene hopper with a sloped bottom to urge the contained abrasive particles  204  to flow toward the bottom under gravity. 
     The refill controller  302   a  includes a pressure sensing valve  502  and a pressure tank  504  as shown. Normally, the pressure sensing valve  502  is biased closed by springs. The pressure from the substantially constant flow rate node  106  enters one side of the pressure sensing valve  502 , and the pressure from the pressure tank enters the other side of the pressure sensing valve  502 . During the state  501 , corresponding to the state during step  406  of  FIG. 4 , these pressures are substantially equal, and the pressure sensing valve  502  remains closed. This keeps the control valve  210 , embodied as a 4-way slide valve, in the position shown. 
     As described above, when the control valve  210  is open, abrasive particles flow from the abrasive tank  102  to the abrasive delivery tube  104 . The substantially constant flow rate node  106 , formed by the metering valve  108 , propels the abrasive particles through the abrasive delivery tube  104 , for example to a distal abrasive jet cutting nozzle. The friction of the abrasive particles against the walls of the abrasive delivery tube  104  causes the pressure at node  106  to increase to about 10 to 50 psig when the air is turned on at node  208 . Abrasive continues entering the abrasive delivery tube  104  from the abrasive tank  102  until the abrasive tank is emptied, when the missing abrasive causes a reduction in back pressure from the abrasive delivery tube  104 . According to an embodiment, state  501  is typically maintained for about 1-3 minutes per cycle. 
       FIG. 6  is a diagram of a state  601  corresponding to the moment that pressure reduction at node  106  causes the pressure sensing valve  502  to actuate a change in the state of the control valve  210 . A check valve  602  admits gas pressure from the node  106  into the pressure tank  504 , but does not allow the pressure within the pressure tank to bleed out through the abrasive delivery tube  104  when the back pressure therein is reduced. The maintained pressure in the pressure tank  504  actuates the pressure sensing valve  502  when the pressure from node  106  plus the spring bias pressure is no longer sufficient to hold the valve shut against the pressure in the pressure tank  504 . The pressure sensing valve  502  admits the pressure from node  208 , which is still at the pressure of the gas source  110 , to the left side of the control valve  210  as shown. Typically, the pressure sensing valve  502  remains open for about 250 milliseconds per cycle. 
       FIG. 7  is a diagram of a state  701  that begins a moment after the pressure sensing valve  502  has actuated the control valve  210 , according to an embodiment. A valve body  702  in the control valve  210  is forced to the right by the pressure admitted by the pressure sensing valve  502 . As the valve body  702  slides to the right, it couples the substantially constant pressure node  208  to a vent  704  and the pressure at node  208  rapidly drops to atmospheric. A check valve  706  vents the pressure from tank  504  to node  208  and to the vent  704 , which allows the spring bias pressure to close the pressure sensing valve  502 . The pressure drop at node  208  allows the abrasive supply valve  206  to open to allow abrasive particles  204  to flow under gravity from the abrasive hopper  202  into the abrasive tank  102 . 
     Substantially simultaneously, the valve body  702  couples the gas source  110  to the resume controller  302   b . According to the embodiment of  FIG. 7 , the resume controller includes a timer valve that remains closed for a predetermined period of time, and then opens. The delay time is selected to allow the abrasive tank  102  to almost, but not quite fill with abrasive. When the timer valve  302   b  opens, air pressure from the air source  110  presses against the right side of the valve body  702 , causing it to slide to the left and the system to reenter state  501  of  FIG. 1 . 
     According to embodiments, several advantages may be realized compared to earlier systems that used a pressurized abrasive hopper  202 :
         The manufacturing cost may be much lower   Shipping cost may be lower   The abrasive (e.g. garnet) level may be viewed through the translucent polyethelene   The air flow propelling the abrasive is limited so that it may generally not blow abrasive out of the small hopper at the cutting nozzle (at the distal end of the abrasive delivery tube  104 ).   no electrical connection is required   There are no or minimal code requirements for the small pressure vessel.       

     With respect to the appended claims, those skilled in the art will appreciate that recited operations therein may generally be performed in any order. Examples of such alternate orderings may include overlapping, interleaved, interrupted, reordered, incremental, preparatory, supplemental, simultaneous, reverse, or other variant orderings, unless context dictates otherwise. With respect to context, even terms like “responsive to,” “related to,” or other past-tense adjectives are generally not intended to exclude such variants, unless context dictates otherwise. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.