Abstract:
An improved system, apparatus and method for injecting a chemical from a storage tank into a natural gas or liquefied petroleum gas pipeline at a flow-controlled injection rate is provided. The system, apparatus and method including a pair of positive-displacement pumps driven in substantially complementary fashion by a single driver, a controller controlling the driver, and each pump being fed from the storage tank and discharging chemical into the pipeline. The system, apparatus and method may also include a second pair of positive-displacement pumps having substantially similar displacement and operatively connected to the first pair of positive-displacement pumps, the first pair of positive-displacement pumps being driven in a substantially complementary fashion with the second pair of pumps by a single driver or a pair of drivers.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to systems for injecting chemicals into pipelines and, more specifically, to an improved system for adding ordorant to natural gas or liquified petroleum gas flowing in a pipeline. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are many instances in which it is desirable to inject chemical of various types into fluids (gas and liquids) flowing in pipelines. One such example is in the area of natural gas pipelines. In addition to such substances as corrosion inhibitors and alcohol to inhibit freezing, odorants are commonly injected into natural gas pipelines. Natural gas is odorless. Odorant is injected into natural gas in order to provide a warning smell for consumers. Commonly used odorants include tertiary butyl mercaptan (TBM). Such odorants are typically injected in relatively small volumes normally ranging from about 0.5 to 1.0 lbs/mmscf. 
         [0003]    The odorants are typically provided in liquid form and are typically added to the gas at a location where distribution gas is taken from a main gas pipeline and provided to a distribution pipeline. In such circumstances, the gas pressure may be stepped down through a regulator from, for example, 600 psi or more, to a lower pressure in the range of 100 psi or less. The odorants can also be added to the main transmission pipeline in some situations. 
         [0004]    As can be seen above, the odorants which are added to natural gas are extremely concentrated. Odorants such as TBM and other blends are mildly corrosive and are also very noxious. If the job of injecting odorant is not performed accurately, lives are sometimes endangered. It would be possible for a homeowner to have a gas leak without it being realized until an explosion had resulted if the proper amount of odorant was not present. Also, if a leak of odorant occurs at an injection site, people in the surrounding area will assume that a gas leak has occurred with areas being evacuated and commerce being interrupted. Contrarily, if such mistakes become common, people in the surrounding area will become desensitized to the smell of a potential gas leak and will fail to report legitimate leaks. 
         [0005]    Two techniques are commonly used for providing odorization to natural gas in a main distribution pipeline. One technique involves bypassing a small amount of natural gas at a slightly higher pressure than the pressure of the main distribution pipeline, through a tank containing liquid odorant. This bypass gas absorbs relatively high concentrations of odorant while it is in the tank. This heavily odorized bypass gas is then placed back into the main pipeline. The odorant, now volatilized, is placed back into the main pipeline and diffuses throughout the pipeline. However, there are a number of disadvantages associated with the bypass system for odorizing pipelines. One disadvantage of the bypass system is the fact that the bypass gas picks up large and inconsistent amounts of odorant from the liquid in the tank and becomes completely saturated with odorant gas. As a result it is necessary to carefully monitor the small amounts of bypass gas which are used. Also, natural gas streams typically have contaminates such as compressor oils or condensates which can fall out into the odorant vessel in bypass systems. These contaminates create a layer that reduces the contact area between the liquid and the bypass stream. This necessarily degrades the absorption rate of the stream failing to accurately measure and control the amount of odorant being added to the stream. This absorption amount can change as condensates and other contaminates fall out and change the absorption boundary layer. 
         [0006]    Another technique involves the injection of liquid odorant directly into the pipeline through the use of a high pressure injection pump. High volume odorizers have depended a traditional positive-displacement pump or solenoid valve to deliver discrete doses of odorant to natural gas or liquid propane gas (LPG) streams for the purpose of bringing these streams to safe perception levels. However, injecting discrete doses in this manner results in higher pressure drops due to the higher piston speed. The higher the piston speed, the more likely the odorant will vaporize and the more likely entrainment of gas. Such vapor lock is detrimental to the performance and accuracy of odorant injection systems. These methods can leave dangerous dead time between doses. Because odorant is extremely volatile, drops injected to the pipeline immediately disperse and spread throughout the gas in the pipeline. In this way, within a few seconds, the drops of liquid odorant are dispersed in gaseous form. 
         [0007]    There are also several disadvantages with this prior art technique. As mentioned above, the odorant liquid is extremely noxious. The injection pump must therefor be designed so that no odorant can leak. This requires a pump design which is relatively expensive and complex in order to meet the required operating conditions. Even in such sophisticated systems, there is an unpleasant odor present when working on the pump which can make people think that there is a natural gas leak. There continues to be a need for improvements in odorization systems of the above described types. 
         [0008]    The present invention relates to an improved system, apparatus and method for injecting chemical into a pipeline which prevents escape of odorant, nearly eliminates dead time between doses and provides a reliable, uniform injection rate over a wide variety of rate requirements. 
       OBJECTS OF THE INVENTION 
       [0009]    It is an object of the present invention to provide an improved chemical injection system for metering odorant into pipelines overcoming some of the problems and shortcomings of the prior art, including those referred to above. 
         [0010]    Another object of the invention is to provide a chemical injection system which allows precise metering of chemical injected into a pipeline. 
         [0011]    Another object of the invention is to provide a chemical injection system which provides continuous flow of odorant. 
         [0012]    Another object of the invention is to provide a chemical injection system which allows a wide range of chemical dosing. 
         [0013]    Another object of the invention is to provide a self-priming chemical injection system which is low-maintenance. 
         [0014]    Another object of the invention is to provide a chemical injection system which allows maintenance of the power unit without exposure to the chemical. 
         [0015]    Another object of the invention is to provide a chemical injection system which prevents flashing of odorant and vapor lock. 
         [0016]    Still another object of the invention is to allow use of low pressure blanket gas which inhibits gas entrainment. 
         [0017]    How these and other objects are accomplished will become apparent from the following descriptions and drawing figures. 
       SUMMARY OF THE INVENTION 
       [0018]    The instant invention overcomes the above-noted problems and satisfies the objects of the invention. A system, apparatus and method for injecting a chemical from a storage tank into a natural gas or LPG pipeline at a flow-controlled injection rate is provided. The chemical injection system, apparatus and method includes a pair of positive-displacement pumps, the pair having a first positive-displacement pump and a second positive-displacement pump, each having substantially similar displacement and driven in complementary fashion by a driver. The chemical injection system, apparatus and method also includes a controller for controlling the driver, with each pump being fed from the storage tank and injecting chemical into the pipeline. 
         [0019]    Accordingly, a preferred embodiment of the present invention provides a chemical injection system, apparatus and method which utilizes a positive-displacement pump to pump odorant from a liquid storage tank into a small pipe which empties directly into the main gas pipeline. The pump is operated by a power unit or motor which is responsive to a controller which, in turn, calculates the necessary amount of chemical to be dosed based on the flow rate of the natural gas or LPG in a pipeline. A flow-rate meter is connected to the pipeline and provides a signal to the controller. As the flow rate within the pipeline fluctuates, the controller will increase or decrease the speed of the power unit, which in turn increases or decreases the speed of the positive-displacement pumps and, consequently, the rate of chemical injection into the pipeline. A second flow-rate meter may be provided in the pump discharge line which measures the rate of chemical being pumped and generates a signal to the controller. The controller then compares the pipeline flow rate to the pump discharge flow rate to assure that the proper amount of chemical is being injected into the pipeline. In the event that the controller determines that the flow rate of the chemical being discharged from the pumps is deficient or excessive with respect to the desired rate, the controller will adjust the speed of the power unit accordingly to correspond with the pipeline gas flow rate requirement. 
         [0020]    Another preferred embodiment of the present invention provides a chemical injection system, apparatus and method which includes a second pair of positive-displacement pumps having substantially similar displacement and operatively connected to the first pair of positive-displacement pumps. The first pair of positive-displacement pumps being driven in a substantially complementary fashion with the second pair of pumps by the driver. A controller is provided which controls the driver with each pump being fed from the storage tank and discharging chemical into the pipeline. An additional preferred embodiment may include pumps which are substantially similar bellows-type pumps. Another preferred embodiment may include a pair of substantially similar hydraulic actuators, one of each hydraulic actuator being operatively connected to one of each first pump and second pump of the pair of positive-displacement pumps and driven by the driver. 
         [0021]    Another preferred embodiment of the present invention provides a chemical injection system, apparatus and method which includes a first and second pair of positive-displacement pumps being driven in a substantially complementary fashion with a first and a second driver. Another preferred embodiment may include a first and a second pair of substantially similar hydraulic actuators. The first pair of hydraulic actuators being operatively connected to the first pair of pumps and driven by the first driver. The second pair of hydraulic actuators being operatively connected to the second pair of positive-displacement pumps and driven by the second driver. 
         [0022]    In yet other preferred embodiments, the driver may include a rotary motor and a rotary-to-linear transmission driving the pistons of the hydraulic actuators in complementary linear fashion. The driver may be an electric motor. The transmission may preferably include a scotch yoke. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0023]    In order that the advantages of the invention will be readily understood, a more detailed description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
           [0024]      FIG. 1  is a perspective view of the preferred positive-displacement pump assembly for use in the chemical injection system according to an exemplary embodiment of the present invention. 
           [0025]      FIG. 2  is a top view of the preferred embodiment illustrated in  FIG. 1 . 
           [0026]      FIG. 3  is a cross-sectional view along lines  2 - 2  of  FIG. 2  which shows one of the hydraulic actuators of the positive-displacement pump in a fully-extended position and the other hydraulic actuator in a fully-retracted position of the preferred embodiment. 
           [0027]      FIG. 4  is an enlarged view of section D of  FIG. 3  which shows the rotary-to-linear mechanism used in the preferred embodiment of the present invention. 
           [0028]      FIG. 5  is schematic view of the preferred embodiment of the chemical injection system of the present invention. 
           [0029]      FIG. 6  is schematic view of another embodiment of the chemical injection system of the present invention. 
           [0030]      FIG. 7  is schematic view of yet another embodiment of the chemical injection system of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0031]    The present invention utilizes a positive-displacement pump. An advantage of using a positive-displacement pump is that the pressure of the blanket gas in the chemical supply tank can be lower than that associated with the use of a centrifugal pump. Limiting how much gas is dissolved in the odorant inhibits vaporization, vapor lock, and gas entrainment. Another key advantage is that a positive-displacement pump system can be designed to provide exacting accuracy of chemical at slower speeds thereby minimizing maintenance of the system. The preferred embodiment of the present invention includes the use of a bellows-type positive-displacement pump. Bellows-type pumps offer key advantages such as a design which reduces system stress and provides an infinite life versus other types of positive-displacement pumps commonly used in chemical systems such as a diaphragm pump. Despite shortcomings of other positive-displacement pumps, any such type may nonetheless be substituted. 
         [0032]    As shown in  FIGS. 1-3 , bellows-type positive-displacement pump assembly  10  includes an actuator housing  12  and two opposed bellows pumps  14 A,  14 B. Pumps  14 A,  14 B each have a proximal portion  16 A,  16 B and a distal portion  18 A,  18 B. Proximal portions  16 A,  16 B each include a hydraulic chamber  20 A,  20 B and a bellows odorant capsule  22 A,  22 B. Distal portions  18 A,  18 B each include a chemical supply inlet lines  24 A,  24 B and a chemical discharge line  26 A,  26 B. Supply springless check valves  28 A,  28 B are provided in the chemical supply inlet lines  24 A,  24 B and discharge springless check valves  30 A,  30 B are provided in pump discharge line  26 A,  26 B. Ceramic springless check valves are preferred because of their superior ball and seat sealing properties, fast response and resistance to buildup. 
         [0033]    As seen in  FIG. 3 , actuator housing  12  houses two actuators  32 A,  32 B. Each actuator includes a piston  34 A,  34 B, a hydraulic chamber  36 A,  36 B, and a discharge line  38 A,  38 B. Actuator discharge lines  38 A,  38 B are in fluid communication with bellows hydraulic chambers  20 A,  20 B. A yoke  40  is coupled to gear box  42  which is operatively connected to actuators  32 A,  32 B. While a scotch yoke is preferred due to its simplicity, low maintenance and low cost, other drive mechanisms can be used. 
         [0034]    Seal housings  44 A,  44 B seal actuators  32 A,  32 B from yoke box  46  by use of a glide ring seals  48 A,  48 B. Also provided in actuator seal housings are glide rings  50 A,  50 B which assist in maintaining axial alignment of the actuators. Yoke  40  includes cam bearing  52  which is operatively attached to pistons  34 A,  34 B. A linear guide  54  is also provided in yoke box  46  which is in contact with cam bearing  52  and pistons  34 A,  34 B to maintain axial alignment of the actuators during operation. 
         [0035]    In operation, as shown in  FIG. 5 , a pipeline flow-rate meter  56  located on pipeline  57  sends a signal to controller  58  which calculates the rate of chemical injection needed and sends a signal to the power unit  60  to either increase speed or decrease speed accordingly. Power unit  60  motivates gear box  42  (see  FIG. 3 ) which in turn operates yoke  40  at the appropriate speed. Yoke  40  transmits the rotary action of the power unit to linear movement to drive actuator pistons  34 A,  34 B in a synchronized fashion. In other words, one piston is in compression and the other is in retraction. The net result is that the system sees continuous metered flow of odorant to the pipeline and softens out the sinusoidal nature of a positive-displacement pump. 
         [0036]    As best seen in  FIG. 3 , yoke cam  62  positively engages actuator pistons  34 A,  34 B, which extends actuator piston  34 B into actuator hydraulic chamber  36 B forcing hydraulic fluid through the actuator discharge line  38 B and into the hydraulic chamber  20 B of bellows pump  14 B. This displaced hydraulic fluid from the actuator hydraulic chamber into the bellows hydraulic chamber causes compression of bellows  14 B which consequently displaces the equivalent volume of odorant through discharge springless check valve  30 B within bellows pump  14 B into the pump discharge line  26 B and into the pipeline  57 . Simultaneously, while yoke cam  42  is extending actuator piston  34 B into its hydraulic chamber, yoke cam  62  is also retracting actuator piston  34 A causing a low pressure in bellows pump odorant capsule  22 A thereby opening supply springless check valve  28 A of bellows pump  14 A and filling odorant capsule  22 A. The volume of chemical entering odorant capsule  22 A is equal to the volume of hydraulic fluid in hydraulic chamber  36 A of actuator  32 A. Conversely, as yoke  40  continues its rotation, yoke cam  62  extends actuator piston  34 A into its hydraulic chamber  36 A and into bellows hydraulic chamber  20 A, compressing bellows odorant capsule  22 A thereby raising the pressure within bellows hydraulic chamber  20 A. Such higher pressure forces supply springless check valve  28 A closed and opens discharge springless check valve  30 A, discharging an equivalent volume of chemical through the discharge line and into pipeline  57 . 
         [0037]    The volume of displacement of each of the actuators is substantially equal. It will be understood that the larger the displacement of the actuators, the slower the speed of the power unit may be. As piston speeds increase, pressure drops increase. By keeping piston speeds slow, pressure drops in the pump are minimized, and “flashing” or vaporization of the fluids is prevented. Flashing or vaporization may be a cause of vapor lock and gas entrainment which are both detrimental to performance and accuracy of odorant injection systems. 
         [0038]    As seen in  FIGS. 1-3 , bellows pumps  14 A,  14 B are isolated from actuator housing  12  by isolation valves  64 A,  64 B. Isolation valves  64 A,  64 B are provided to allow safe maintenance of the actuators and power unit by eliminating contact with the chemical. In addition, isolation between the actuators and pumps provides the ability to perform maintenance without disturbing the bellows pumps which minimizes re-priming efforts at start up. As best seen in  FIG. 2 , hydraulic actuator housing  12  includes bleed valves  66 A,  66 B for bleeding hydraulic pressure prior to removal from the bellows pumps. 
         [0039]    A second flow-rate meter  68  may be utilized in the pump discharge line  70 . Second flow-rate meter  68  measures the pump discharge rate and sends a signal to controller  58 . Controller  58  compares the flow rate of pipeline  57  to the flow rate of the pump discharge line  70  and regulates the speed of power unit  60 . If the actual pump discharge flow rate does not match the desired flow rate as calculated from the flow-rate sensor  56  of pipeline  57 , controller  58  adjusts the power unit  60  accordingly. The faster power unit  60  turns, the faster actuator pistons  34 A,  34 B displace hydraulic fluid into bellows hydraulic chambers  20 A,  20 B, and the faster odorant is discharged from bellows odorant capsules  22 A,  22 B. Although many types of flow-rate meters exist, positive-displacement flow-rate meters are preferred due to their cost versus performance benefit. 
         [0040]      FIG. 5  shows a schematic of a preferred embodiment of the present invention.  FIG. 5  shows a chemical supply tank  72 , having chemical inlet  74 , blanket gas inlet  76 , and discharge conduit  78 . Supply tank discharge conduit  78  supplies chemical to bellows pumps  14 A,  14 B through their respective chemical supply inlet lines  24 A,  24 B, supply springless check valves  28 A,  28 B and into bellows odorant capsules  22 A,  22 B. Bellows odorant capsules  22 A,  22 B are discharged through discharge springless check valves  30 A,  30 B into pipeline  57 . Natural gas or LPG flows from pipeline  57  through pipeline flow-rate meter  56  generating a control signal which is passed to controller  58 . Controller  58  calculates the rate of chemical injection needed and sends a signal to power unit or motor  60 . Power unit  60 , through yoke  40 , reciprocally moves actuator pistons  34 A,  34 B, which displace hydraulic fluid into bellows hydraulic chambers  20 A,  20 B which reciprocally compress bellows odorant capsules  22 A,  22 B thereby injecting chemical into pipeline  57  through pump discharge line  70 . 
         [0041]    Second flow-rate meter  68  can be located in pump discharge line  70  to measure the pump discharge flow-rate and provide a signal to controller  58  at  80 . Controller  58  compares the signal generated by the pump discharge flow-rate meter  80  to the signal generated by the pipeline flow-rate meter  56  at  82 . Upon comparison of the signals generated at  80  and  82 , the controller  58  generates an adjustment signal  84  which adjusts power unit  60  so that the actual flow of chemical matches the desired flow of chemical injected into the pipeline. 
         [0042]      FIG. 6  shows a schematic of another preferred embodiment of the present invention.  FIG. 6  shows a chemical supply tank  72 , having chemical inlet  74 , blanket gas inlet  76 , and discharge conduit  78 . Supply tank discharge conduit  78  supplies chemical to bellows pumps  14 A,  14 A′ and  14 B,  14 B′ through their respective chemical supply inlet lines  24 A,  24 A′ and  24 B,  24 B′ supply springless check valves  28 A,  28 A′ and  28 B,  28 B′ and into bellows odorant capsules  22 A,  22 A′ and  22 B,  22 B′. Bellows odorant capsules  22 A,  22 A′ and  22 B,  22 B′ are discharged through discharge springless check valves  30 A,  30 A′ and  30 B,  30 B′ into pipeline  57 . Natural gas or LPG flows from pipeline  57  through pipeline flow-rate meter  56  generating a control signal which is passed to controller  58 . Controller  58  calculates the rate of chemical injection needed and sends a signal to power unit or motor  60 . Power unit  60 , through yokes  40 ,  40 ′ and corresponding linkage  41  reciprocally moves actuator pistons  34 A,  34 A′ and  34 B,  34 B′ which displace hydraulic fluid into bellows hydraulic chambers  20 A,  20 A′ and  20 B,  20 B′ which reciprocally compress bellows odorant capsules  22 A,  22 A′ and  22 B,  22 B′ thereby injecting chemical into pipeline  57  through pump discharge line  70 . 
         [0043]    Second flow-rate meter  68  can be located in pump discharge line  70  to measure the pump discharge flow-rate and provide a signal to controller  58  at  80 . Controller  58  compares the signal generated by the pump discharge flow-rate meter  80  to the signal generated by the pipeline flow-rate meter  56  at  82 . Upon comparison of the signals generated at  80  and  82 , the controller  58  generates an adjustment signal  84  which adjusts power unit  60  so that the actual flow of chemical matches the desired flow of chemical injected into the pipeline. 
         [0044]      FIG. 7  shows a schematic of yet another preferred embodiment of the present invention.  FIG. 7  shows a chemical supply tank  72 , having chemical inlet  74 , blanket gas inlet  76 , and discharge conduit  78 . Supply tank discharge conduit  78  supplies chemical to bellows pumps  14 A,  14 A′ and  14 B,  14 B′ through their respective chemical supply inlet lines  24 A,  24 A′ and  24 B,  24 B′, supply springless check valves  28 A,  28 A′ and  28 B,  28 B′ and into bellows odorant capsules  22 A,  22 A′ and  22 B,  22 B′. Bellows odorant capsules  22 A,  22 A′ and  22 B,  22 B′ are discharged through discharge springless check valves  30 A,  30 A′ and  30 B,  30 B′ into pipeline  57 . Natural gas or LPG flows from pipeline  57  through pipeline flow-rate meter  56  generating a control signal which is passed to controller  58 . Controller  58  calculates the rate of chemical injection needed and sends a signal to first power unit  60  and second power unit  60 ′. Power units  60 ,  60 ′ through yokes  40 ,  40 ′ reciprocally move actuator pistons  34 A,  34 A′ and  34 B,  34 B′ which displace hydraulic fluid into bellows hydraulic chambers  20 A,  20 A′ and  20 B,  20 B′ which reciprocally compress bellows odorant capsules  22 A,  22 A′ and  22 B,  22 B′ thereby injecting chemical into pipeline  57  through pump discharge line  70 . 
         [0045]    Second flow-rate meter  68  can be located in pump discharge line  70  to measure the pump discharge flow-rate and provide a signal to controller  58  at  80 ,  80 ′. Controller  58  compares the signal generated by the pump discharge flow-rate meter  80 ,  80 ′ to the signal generated by the pipeline flow-rate meter  56  at  82 . Upon comparison of the signals generated at  80 ,  80 ′ and  82 , the controller  58  generates an adjustment signal  84  which adjusts power units  60 ,  60 ′ so that the actual flow of chemical matches the desired flow of chemical injected into the pipeline. 
         [0046]    Reference throughout this specification to “the embodiment,” “this embodiment,” “the previous embodiment,” “one embodiment,” “an embodiment,” “a preferred embodiment” “another preferred embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in the embodiment,” “in this embodiment,” “in the previous embodiment,” “in one embodiment,” “in an embodiment,” “in a preferred embodiment,” “in another preferred embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
         [0047]    Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
         [0048]    While the present invention has been described in connection with certain exemplary or specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications, alternatives and equivalent arrangements as will be apparent to those skilled in the art. Any such changes, modifications, alternatives, modifications, equivalents and the like may be made without departing from the spirit and scope of the invention.