Vapor flow and hydrocarbon concentration sensor for improved vapor recovery in fuel dispensers

[0002] This application is a 1.53 (b) Continuation of application Ser. No. application Ser. No. 09/442,263 filed Nov. 17, 1999, entitled VAPOR FLOW AND HYDROCARBON CONCENTRATION SENSOR FOR IMPROVED VAPOR RECOVERY IN FUEL DISPENSERS.

[0003] 1. Field of the Invention

[0004] The present invention is directed to vapor flow and hydrocarbon concentration sensors that are positioned in a vapor recovery line for a fuel dispenser.

[0005] 2. Description of the Prior Art

[0006] Vapor recovery equipped fuel dispensers, particularly gasoline dispensers, have been known for quite some time, and have been mandatory in California for a number of years. The primary purpose of using vapor recovery is to retrieve or recover the vapors, which would otherwise be emitted to the atmosphere during a fueling operation, particularly for motor vehicles. The vapors of concern are generally those which are contained in the vehicle gas tank. As liquid gasoline is pumped into the tank, the vapor is displaced and forced out through the filler pipe. Other volatile hydrocarbon liquids raise similar issues. In addition to the need to recover vapors, some states, California in particular, are requiring extensive reports about the efficiency with which vapor is recovered.

[0007] A traditional vapor recovery system is known as the “balance” system, in which a sheath or boot encircles the liquid fueling spout and connects by tubing back to the fuel reservoir. As the liquid enters the tank, the vapor is forced into the sheath and back toward the fuel reservoir or underground storage tank (UST) where the vapors can be stored or recondensed. Balance systems have numerous drawbacks, including cumbersomeness, difficulty of use, ineffectiveness when seals are poorly made, and slow fueling rates.

[0008] As a dramatic step to improve on the balance systems, Gilbarco, Inc., assignee of the present invention, patented an improved vapor recovery system for fuel dispensers, as seen in U.S. Pat. No. 5,040,577, now Reissue Patent No. 35,238 to Pope, which is herein incorporated by reference. The Pope patent discloses a vapor recovery apparatus in which a vapor pump is introduced in the vapor return line and is driven by a variable speed motor. The liquid flow line includes a pulser, conventionally used for generating pulses indicative of the liquid fuel being pumped. This permits computation of the total sale and the display of the volume of liquid dispensed and the cost in a conventional display, such as, for example as shown in U.S. Pat. No. 4,122,524 to McCrory et al. A microprocessor translates the pulses indicative of the liquid flow rate into a desired vapor pump operating rate. The effect is to permit the vapor to be pumped at a rate correlated with the liquid flow rate so that, as liquid is pumped faster, vapor is also pumped faster.

[0009] There are three basic embodiments used to control vapor flow during fueling operations. The first embodiment is the use of a constant speed vapor pump during fueling without any sort of control mechanism. The second is the use of a pump driven by a constant speed motor coupled with a controllable valve to extract vapor from the vehicle gas tank. While the speed of the pump is constant, the valve may be adjusted to increase or decrease the flow of vapor. The third is the use of a variable speed motor and pump as described in the Pope patent, which is used without a controllable valve assembly. All three techniques have advantages either in terms of cost or effectiveness, and depending on the reasons driving the installation, any of the three may be appropriate, however none of the three systems, or the balance system are able to provide all the diagnostic information being required in some states. The present state of the art is well shown in commonly owned U.S. Pat. No. 5,345,979, which is herein incorporated by reference.

[0010] Regardless of whether the pump is driven by a constant speed motor or a variable speed motor, there is no feedback mechanism to guarantee that the amount of vapor being returned to the UST is correct. A feedback mechanism is helpful to control the A/L ratio. The A/L ratio is the amount of vapor-Air being returned to the UST divided by the amount of Liquid being dispensed. An A/L ratio of 1 would mean that there was a perfect exchange. Often, systems have an A/L>1 to ensure that excess air is recovered rather than allowing some vapor to escape. This inflated A/L ratio causes excess air to be pumped into the UST, which results in a pressure build up therein. This pressure build up can be hazardous, and as a result most USTs have a vent that releases vapor-air mixtures resident in the UST to the atmosphere should the pressure within the UST exceed a predetermined threshold. While effective to relieve the pressure, it does allow hydrocarbons or other volatile vapors to escape into the atmosphere.

[0011] While PCT application Serial No. PCT/GB98/00172 published Jul. 23, 1998 as WO 98/31628, discloses one method to create a feedback loop using a Fleisch tube, there remains a need to create alternate feedback mechanisms to measure the vapor flow in a vapor recovery system. Specifically, the feedback needs to not only tell the fuel dispenser how fast vapor is being recovered, but also how efficiently the vapor is being recovered. To do this, the feedback mechanism needs to monitor vapor flow and hydrocarbon concentration in the vapor return path. Not only should the feedback mechanism improve the efficiency of the vapor recovery operation, but also the feedback mechanism should be able to report the information being required by California's increased reporting requirements.

[0012] The deficiencies of the prior art are addressed by providing a vapor flow sensor and a hydrocarbon concentration sensor in a vapor return line for a fuel dispenser. As used herein a “hydrocarbon sensor” includes sensors that directly measure the concentration of hydrocarbons as well as sensors that indirectly measure the concentration of hydrocarbons, such as by measuring oxygen concentration. The combination of sensors allows more accurate detection of hydrocarbons being recovered by the vapor recovery system. This is particularly helpful in determining if an Onboard Recovery Vapor Recovery (ORVR) system is present in the vehicle being fueled. When an ORVR system is detected, the vapor recovery system in the fuel dispenser may be turned off or slowed to retrieve fewer vapors so as to avoid competition with the ORVR system. Additionally, the combined sensor allows a number of diagnostic tests to be performed which heretofore were not possible.

[0013] The combination of sensors may be positioned in a number of different locations in the vapor recovery line, or even in the vent path for the Underground Storage Tank (UST). The exact position may determine which diagnostic tests may be performed, however, the sensors should allow a number of diagnostic tests regardless of position. In this manner data may be collected to comply with the California Air Resources Board (CARB) regulations

[0014] FIG. 1 is a simplified schematic of a fuel dispenser of the present invention;

[0015] FIG. 2 is a simplified schematic of an alternate embodiment of the present invention;

[0016] FIGS. 3 and 4 are simplified schematics of a Pope type system with alternate placements of the sensors of the present invention therein;

[0017] FIG. 5 is a simplified schematic of a Healy type system with the sensors of the present invention disposed therein;

[0018] FIGS. 6-8 are alternate placements in a Hasstech type system;

[0019] FIG. 9 is a flow chart of the decision making process associated with the vapor flow sensor;

[0020] FIG. 10 is a flow chart of the decision making process associated with the hydrocarbon concentration sensor;

[0021] FIG. 11 is a flow chart of the decision making process associated with the diagnostic aspect of the present invention;

[0022] FIGS. 12 and 13 are possible embodiments of the sensors as removed from the vapor recovery system; and

[0023] FIG. 14 is a possible alternate use for the sensors of the present invention.

[0024] The present invention lies in including a hydrocarbon sensor and vapor flow sensor within a fuel dispenser and using the combination to provide accurate diagnostic readings about the nature of the vapor being recovered in the vapor recovery system of the fuel dispenser. Additionally, the diagnostics will indicate whether the vapor recovery system is performing properly. As used herein a “hydrocarbon sensor” includes sensors that directly measure the concentration of hydrocarbons as well as sensors that indirectly measure the concentration of hydrocarbons. The latter type of sensor might include oxygen concentration sensors or nitrogen sensors. Taking the inverse of the measurement provides an indication of hydrocarbon concentration. For example, total gas minus measured nitrogen provides an approximate hydrocarbon concentration. Such sensors could, through calibration, provide accurate measurements of hydrocarbon concentrations in the vapor recovery line.

[0025] Turning now to FIG. 1, a fuel dispenser 10 is adapted to deliver a fuel, such as gasoline or diesel fuel to a vehicle 12 through a delivery hose 14, and more particularly through a bootless nozzle 16 and spout 18. The vehicle 12 includes a fill neck 20 and a tank 22, which accepts the fuel and provides it through appropriate fluid connections to the engine (not shown) of the vehicle 12.

[0026] Presently, it is known in the field of vapor recovery to provide the flexible delivery hose 14 with an outer conduit 30 and an inner conduit 32. The annular chamber formed between the inner and outer conduits 30, 32 forms the product delivery line 36. The interior of the inner conduit 32 forms the vapor return line 34. Both lines 34 and 36 are fluidly connected to an underground storage tank (UST) 40 through the fuel dispenser 10. Once in the fuel dispenser 10, the lines 34 and 36 separate at split 51. The UST 40 is equipped with a vent shaft 42 and a vent valve 44. During delivery of fuel into the tank 22, the incoming fuel displaces air containing fuel vapors. The vapors travel through the vapor return line 34 to the UST 40.

[0027] A vapor recovery system is typically present in the fuel dispenser 10 and includes a control system 50 and a vapor recovery pump 52. The control system 50 may be a microprocessor with an associated memory or the like and also operates to control the various functions of the fuel dispenser including, but not limited to: fuel transaction authorization, fuel grade selection, display and/or audio control. The vapor recovery pump 52 may be a variable speed pump or a constant speed pump with or without a controlled valve (not shown) as is well known in the art. A “combined sensor” 54 is positioned in the vapor recovery line 34 upstream of the pump 52, and is communicatively connected to the control system 50. The “combined sensor” 54 is a hydrocarbon concentration sensor and a vapor flow monitor proximate one another or integrated together in any fashion to monitor vapor flow rates and hydrocarbon concentrations in the vapor return path. Further, a matrix of sensors could be used to provide improved accuracy. Sensor 54 is discussed in greater detail below.

[0028] An alternate location of the combined sensor is seen in FIG. 2, wherein the sensor 54ais located downstream of the vapor pump 52. In all other material aspects, the fuel dispenser 10 remains the same.

[0029] Similarly, because fuel dispensers may differ, the combined sensor 54 of the present invention is easily adaptable to a number of different locations within a fuel dispenser 10 as seen in FIGS. 3 and 4. FIGS. 3 and 4 represent fuel dispensers such as were disclosed in the original Pope patent discussed above. The fundamental principle remains the same, but because the layout of the interior components is different from that disclosed in FIGS. 1 and 2, the components will be explained again. Fuel, such as gas is pumped from a UST 40 through a fuel delivery line 36 to a nozzle 16 and thence through a spout 18 to a vehicle 12 being fueled. Vapor is recovered from the gas tank of vehicle 12 through a vapor recovery line 34 with the assistance of a vapor pump 52. A motor 53 powers the vapor pump 52. A control system 50 receives information from a pressure transducer 57 in the vapor return line 34 as well as information from a meter 56 and a pulser 58 in the fuel delivery line 36. The meter 56 measures the fuel being dispensed while the pulser 58 generates a pulse per count of the meter 56. Typical pulsers 58 generate one thousand (1000) pulses per gallon of fuel dispensed. Control system 50 controls a drive pulse source 55 that in turn controls the motor 53. While some of these elements are not disclosed in FIGS. 1 and 2, the fuel dispensers of FIGS. 1 and 2 operate on the same principles. FIG. 3 shows the combined sensor 54 upstream of the pump 52, while FIG. 4 shows the combined sensor 54aplaced downstream of the pump 52. Again, it should be appreciated that the pump 52 can be a variable speed pump or a constant speed pump with a controlled valve which together control the rate of vapor recovery.

[0030] Another vapor recovery system was originally disclosed by Healy in U.S. Pat. No. 4,095,626, which is herein incorporated by reference. The present invention is also well suited for use with the Healy vapor recovery system. As shown in FIG. 5, the Healy fuel dispenser 10′ includes a fuel delivery line 36 which splits and directs a portion of the fuel being delivered to a liquid jet gas pump 59 via line 36′. Fuel is delivered conventionally through hose 14 and nozzle 16. A vacuum is created on the hose side of the liquid jet gas pump 59 that sucks vapor from the vehicle gas tank 22 (FIG. 1) through combined sensor 54 on to the UST 40 via recovery line 34. Because the liquid jet gas pump 59 directs liquid fuel through the return line 34 during the creation of a vacuum therein, the combined sensor 54 must be upstream of the pump 59 to ensure accurate readings.

[0031] While placing the combined sensor 54 in the fuel dispenser 10 allows feedback to be gathered about the vapor recovered in the actual fueling environment, there may be occasions wherein the ventilation system of the UST 40 needs to be monitored. Combined sensor 54 is well suited for placement in various ventilation systems. Such placement might be appropriate where concerns existed about the emissions therefrom to reduce pressure in the UST 40. As state and federal regulations tighten about what sort of emissions are allowable, the placement of a combined sensor 54 in the ventilation system may provide valuable information about the level of scrubbers or filters needed to comply with the regulations.

[0032] Combined sensor 54 can be positioned in the ventilation lines as better seen in FIGS. 6-8. While FIGS. 6-8 represent Hasstech type systems, sold by Hasstech, Inc., 6985 Flanders Drive, San Diego, Calif. 92121, other comparable ventilation systems are also contemplated. Fuel dispensers 10 send vapor from nozzles 16 back to a plurality of USTs 40 with the assistance of a vapor pump 52 as previously explained. However, as shown, a single vapor pump 64 may be centrally positioned and draws vapor from each dispenser 10. This positioning is in contrast to the positioning of an individual vapor pump 52 in each dispenser 10 as previously shown. Either system is equally suited for use with the present invention. Vent lines 60 each vent a different one of the USTs 40 through a PressureNapor (PN) valve 62. The vent lines 60 and valve 62 are designed to relieve pressure build up in the USTs 40. A tank correction gauge 66 may be placed in one or more of the vent lines 60. A processing unit 68 may be provided to filter some of the hydrocarbons from the gas being vented to comply with emissions laws. In the particular Hasstech system shown, the processing unit 68 acts to burn out hydrocarbons prior to expulsion of the vapor into the atmosphere.

[0033] Since the vapor pump 52 is positioned on the roof of the gas station, vapor line 72 provides vacuum power from the pump 52 to the fuel dispensers 10. An electrical control panel 70 controls the operation of the vapor pump 64 and the processing unit 68. Improving on the original Hasstech system, a combined sensor 54bis placed in the venting system. The combined sensor 54bmay be placed between the vapor pump 64 and the processing unit 68 to determine what sort of vapor is being fed to the processing unit 68. This information may be useful in determining how much scrubbing the processing unit 68 must perform.

[0034] Alternately, a combined sensor 54ccan be placed immediately upstream of the valve 62 as seen in FIG. 7. This position may be helpful in determining exactly what vapors are being released to the atmosphere. Still further, a combined sensor 54dcan be placed between the valve 62 and the vapor pump 64 as seen in FIG. 8. This may tell what sort of vapor is present in the UST 40 that needs to be vented. Furthermore, a combination of combined sensors 54b-54dand their corresponding positions could be used together to determine how efficiently the processing unit 68 was removing hydrocarbons, or exactly what was being vented through valve 62.

[0035] Combined sensor 54 is positioned in the vapor return line 34 or the ventilation system as shown in the previous figures and as shown in FIGS. 12 and 13. Combined sensor 54 is a combined vapor flow meter 80 and hydrocarbon concentration sensor 82. One implementation of combined sensor 54 is an integrated sensor which acts as both a hydrocarbon sensor and a flow rate monitor. However, proximate positioning of two discrete sensors is also contemplated and intended to be within the scope of the present invention. Appropriate hydrocarbon sensors 82 include those disclosed in U.S. Pat. No. 5,782,275, which is herein incorporated by reference or that sold under the trademark ADSISTOR by Adsistor Technology, Inc. of Seattle, Wash. Note also that under the broad definition of hydrocarbon sensor as used herein, other sensors may also be appropriate. In FIG. 12, the hydrocarbon sensor 82 is protected from inadvertent exposure to liquid hydrocarbons by liquid shield 84, which directs liquid flow away from the sensor, but allows gaseous hydrocarbons or air to still provide accurate readings on the sensor 82. Vapor flow sensor 80 may be a sensor such as disclosed in commonly owned co-pending application Ser. No. 09/408,292, filed Sep. 29, 1999, which is herein incorporated by reference, or other equivalent vapor flow sensor.

[0036] In contrast, as shown in FIG. 13, the hydrocarbon sensor 82 may be positioned in a membrane 86 such as that disclosed in commonly owned U.S. Pat. Nos. 5,464,466; 5,571,310; and 5,626,649, which are herein incorporated by reference. Alternately, the membrane 86 could be one which allows gas to pass therethrough while excluding liquids. Membrane 86 protects the sensor 82 from direct exposure to liquid fuel that may be caught in the vapor recovery line 34 while still allowing accurate readings of the gaseous hydrocarbon content within the vapor recovery line 34. Thus, any membrane which serves this function is appropriate.

[0037] In addition to using a membrane to protect the sensor, it is also possible that the combined sensor 54 is used to check the efficiency of a membrane positioned within the vapor recovery system. For example, as shown in FIG. 14, a membrane 90 may be positioned in a vapor recovery line 34 with a combined sensor 54eand 54fpositioned on either side of the membrane 90. Air and hydrocarbons flow downstream towards the membrane 90, which filters out hydrocarbons. The first combined sensor 54ecan measure the initial concentration of hydrocarbons, which can then be compared to the post membrane level of hydrocarbons as measured by the second combined sensor 54f.This provides an efficiency check on the ability of membrane 90 to filter hydrocarbons. If combined sensor 54fprovides an anomalous reading, the membrane 90 may be defective, torn, or otherwise not performing as intended. While shown in a vapor recovery line 34, it should be understood that this sort of arrangement may be appropriate in the ventilation system also. Additionally, there is no absolute requirement that two combined sensors 54 be used, one could be positioned upstream or downstream of the membrane 90 as desired or needed. For example, one downstream combined sensor 54 could measure when the membrane had failed. Additionally, the membrane 90 need not filter hydrocarbons, but could rather filter air out of the system. As multiple membranes are contemplated, it is possible that multiple positionings within the vapor recovery system or multiple combined sensors 54 could be used as needed or desired.

[0038] In use, the vapor flow part of the combined sensor 54 is used to control the rate of vapor recovery. Specifically, it goes through a decisional logic as shown in FIG. 9. Combined sensor 54, specifically, the vapor flow monitor 80, begins by measuring the vapor flow (block 100). Because the control system 50 receives input from both the combined sensor 54 and the fuel dispensing meter 56, the control system 50 can make a determination if the vapor flow is too high or otherwise above a predetermined level (block 102) compared to the rate of fuel dispensing. If the answer is yes, the control system 50 may instruct the pump 52 so as to adjust the vapor flow downward (block 104). If the answer is no, the control system 50 determines if the vapor flow is too low (block 106) as compared to some predetermined level. If the answer is yes, then the control system 50 can adjust the vapor recovery rate upward (block 108) by the appropriate instruction to the pump 52. While discussed in terms of making adjustments to the pump 52, it should be appreciated that in systems where there is a constant speed pump and an adjustable valve, the actual adjustment occurs at the valve rather than the pump. Both processes are within the scope of the present invention. If the answer to block 106 is no, then the control system 50 can continue to monitor the vapor flow (block 110) until the end of the fueling transaction. Note that the control system 50 can continue to monitor between fueling operations as well if so desired.

[0039] The hydrocarbon sensor 82 acts similarly as shown schematically in FIG. 10. Specifically, the sensor 82 measures the hydrocarbon concentration present in the vapor return line 34 (block 150). This can be a direct measurement or an indirect measurement as previously indicated. The control system 50 determines if the hydrocarbon concentration is too low (block 152) as compared to some predetermined criteria. If the answer to block 152 is no, vapor recovery can continue as normal (block 154) with continued monitoring. If the hydrocarbon concentration is considered unusually high, the vapor recovery should also continue as normal. If the answer to block 152 is yes, the control system 50 checks with the vapor flow meter to determine if the vapor flow is normal (block 156). If the answer to block 156 is no, then there may be a possible leak, and an error message may be generated (block 158). If the answer to block 156 is yes, then it is possible that an Onboard Recovery Vapor Recovery (ORVR) system is present (block 160) and the vapor recovery system present in the fuel dispenser 10 may be slowed down or shut off so as to assist or at least prevent competition with the ORVR system.

[0040] In addition to controlling the rate of vapor recovery, the combined sensor 54 can also perform valuable diagnostics to determine compliance with recovery regulations or alert the station operators that a vapor recovery system needs service or replacement. Specifically, the control system 50, through continuous monitoring of the readouts of the combined sensor 54, can determine if the vapor flow rate was correctly adjusted (block 200, FIG. 11). If the answer is no, the flow rate was not properly adjusted within certain tolerances, the control system can generate an error message about a possible bad pump (block 202). If the answer to block 200 is yes, the control system 50 determines if a vapor flow is present (block 204).

[0041] If the answer to block 204 is no, there is no vapor flow, the control system 50 determines if there should be a vapor flow (block 208). If the answer to block 208 is yes, then an error signal can be generated pointing to possible causes of the error, namely there is a bad pump 52, the pump control printed circuit board is bad, or there is a nonfunctioning valve (block 210). If the answer to block 208 is no, there is not supposed to be a vapor flow, and one is not present, the program should reset and preferably cycles back through the questions during the next fueling operation or vapor recovery event.

[0042] If the answer to block 204 is yes, there is a vapor flow, the control system 50 determines if there is not supposed to be a vapor flow (block 206). If the answer to block 206 is yes, there is a flow and there is not supposed to be a flow, the control system 50 determines if the vapor flow is in the reverse direction (block 220). If the answer to block 220 is no, the flow is not reversed, then the control system may generate an error message that the pump 52 may be bad (block 222), and then the diagnostic test continues as normal at block 212. If the answer to block 220 is yes, the control system 50 determines if the flow is a high flow as classified by some predetermined criteria (block 224). If the answer to block 224 is yes, then the control system 50 may generate an error message that the pump may be running backwards (block 226). If the answer to block 224 is no, then the control system 50 determines if the flow is a low flow as classified by some predetermined criteria (block 228). If the answer is yes, then the control system 50 may generate an error message that there is a possible leak or a stuck valve (block 230). If the answer to block 228 is no, then a general error message may be created by the control system 50 and the diagnostic test continues at block 212.

[0043] If the answer to block 206 is no, (i.e., there is a vapor flow and there is supposed to be one) then the diagnostic test continues as normal by proceeding to block 212. At block 212, control system 50 determines if the vapor, specifically, the hydrocarbon concentration is too low. If the answer is yes, the hydrocarbon concentration is too low, then an error message indicating a possible leak may be generated (block 214). If the answer to block 212 is no, then the control system 50 determines if an Onboard Recovery Vapor Recovery (ORVR) vehicle is being fueled (block 216). This determination is made by comparing the rate of fueling versus the rate of recovery versus the hydrocarbon concentration. If predetermined criteria are met for all of these parameters, it is likely that an ORVR vehicle is present. If the answer is yes, then the control system 50 may adjust the recovery efforts accordingly to limit competition between the two vapor recovery systems (block 218). If the answer to block 216 is no, the performance of the membrane 86 is evaluated if such is present (block 232). If the membrane 86 is functioning properly, then the diagnostics repeat beginning at block 200. Alternatively, the diagnostics may be halted until the next fueling transaction or the next vapor recovery event. If the membrane is not functioning properly, an error message may be generated (block 234) and the diagnostics restart (block 236).

[0044] Error messages may appear as text on a computer remote to the fuel dispenser through a network communication set up. Such a computer could be the G-SITE® as sold by the assignee of the present invention. Communication between the fuel dispenser 10 and the remote computer can be wireless or over conventional wires or the like as determined by the network in place at the fueling station. Additionally, there can be an audible alarm or like as desired or needed by the operators of the fueling station.

[0045] The present invention is well suited to meet the reporting requirements of CARB or other state regulatory schemes. The information provided by the combined sensor 54 can be output to a disk or to a remote computer, regardless of whether an error message has been generated. This information could be stored in a data file that an operator could inspect at his leisure to track the performance of the vapor recovery system. Additionally, percentages of fueling transactions involving ORVR vehicles could be estimated based on how frequently such a vehicle was detected. Other information may easily be collated or extrapolated from the information gathered by the combined sensor 54. The placement of multiple combined sensors 54 within the vapor recovery system or the ventilation system allows close monitoring of the various elements of the respective systems so that problems can be isolated efficiently and the required maintenance, repair or replacement performed in a timely fashion. This will help the fueling station operator comply with the increasingly strict regulatory schemes associated with a fuel dispensing environment.

[0046] While a particular flow chart has been set forth elaborating on the procedure by which the control system 50 can check the various functions of the vapor recovery system, it should be appreciated that the order of the questions is not critical. The present flow chart was given by way of illustration and not intended to limit the use of the vapor recovery system, and particularly the combined sensor 54 to a particular method of performing diagnostic tests.

[0047] The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.