Patent Publication Number: US-8109738-B2

Title: Vapor recovery gas pressure boosters and methods and systems for using same

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to gas pressure boosters. More specifically, the present relates to gas pressure boosters and methods and systems for using them in which fugitive emissions are captured, and in which the need for staged gas compression is largely eliminated. 
     Gas pressure boosters may include a drive system which provides the energy required to operate a compression system, and a compression system which elevates the gas pressure. The drive systems may include: a crankcase driven by an electric motor or an engine; a turbine drive; a hydraulic piston driven by an electric motor or an engine; and a pneumatic piston driven by air or gas pressure. 
     The compression system may include:
         a reciprocating piston (providing moderate boost ratios and flowrates, suitability for high operating pressures, low to moderate cost, a compact design, rod seal leakage and vibration, and a moderate operating life for the seals, especially non-lubricated seals);   a turbine (providing high flowrates, low vibration, a long operating life, suitability for high pressures, low boost ratios, high cost, shaft seal leakage, and a large size);   a diaphragm (providing high compression ratios, no seal leakage, suitability for high pressures, very low flow, high cost, vibration, and a low operating life);   a bellows (providing no seal leakage, moderate cost, low flow, low boost ratios, vibration, and a lack of suitability for high pressures);   a rotary vane (providing high flowrates, low cost, low boost ratios, a lack of suitability for high pressures, and a low operating life);   a fan (providing high flowrates, low cost, very low boost ratios, and a lack of suitability for high pressures);   a “roots type” blower (providing high flowrates, moderate cost, long life, low boost ratios, shaft leakage, and a lack of suitability for high pressures); and   a rotary screw (providing high flowrates, moderate cost, long life, moderate boost ratios, shaft leakage, and a lack of suitability for high pressures).       

     With moderate to high pressure applications, which are the focus of the preferred embodiment described below, the most practical boost system utilizes a reciprocating piston. Existing piston gas boosters utilize pneumatic or gas drives, crankcase drives and hydraulic drives. For many applications, such as natural gas compression for pipelines, refueling natural gas vehicles and vapor recovery from gas wells, compressed air is either not available or available in sufficient quantity to drive the compression section. Electricity may also not be present, either at all or in sufficient quantities to operate a crankcase or hydraulic drive. There is also frequently a lack of space to locate crankcase-driven machines and engine drives. Accordingly, a compact gas-driven booster may be a good engineering fit in such applications which have high pressure gas (instead of compressed air) available to drive the booster. 
     Pneumatically-driven gas pressure boosters, also called gas boosters, booster compressors and air amplifiers, that utilize compressed air (or other compressed gases) as the motive force to boost gas pressure, are known. They may be used to boost shop air pressure, to boost nitrogen pressure, for boosting gas pressure to feed dry gas mechanical seals on turbo compressors (to protect the seals), etc. Gas pressure boosters have various advantages: the pressure boost in such devices can be as low as 5 psi or as high as thousands of psi; they require no electricity, cooling water or lubrication; and they are explosion-proof, compact, easy to install and economical. Such advantages may be important in applications located in remote areas where the electricity may not be available (e.g. oil and gas wells). The gas pressure boost ratio is based on the pressure of the available compressed air (or gas), the area ratio between the drive piston and the boost piston, and the boost gas supply pressure. Pneumatically-driven gas pressure boosters are available, for example, from Midwest Pressure Systems of Bensenville, Ill. 
     With existing pneumatically-driven gas pressure boosters, the boost section includes a single-acting or double-acting cylinder, and inlet and discharge check valves for each pumping chamber. There are variations in check valve design, piston and rod seals, and materials, but all of the existing systems are similar in engineering design. 
     The pneumatic drive section of the boosters may have several variations, but all consist of a four-way valve which causes the drive piston to reciprocate automatically. 
     The differences are in the manner used to actuate the valve: 
     1. Mechanical actuation causes the four-way valve to shift as a result of the drive piston, mechanically moving the valve element at the end of stroke. The piston typically has a lever or pin which triggers the valve at the end of stroke in each direction. 
     2. Pneumatic pilot shifting actuates the four-way valve through a small amount of pressurized air or gas which forces a piston attached to the valve to move, causing the valve to shift. There are three versions of this design. A first version uses a four-way valve with a double-pilot design which receives a pilot signal at each end of the valve. With this first version, pilot valves are triggered by the piston at the end of each stroke. Each pilot valve sends a pilot air or gas signal to the four-way valve, causing it to shift. After the four-way valve shifts, the pilot air or gas is vented. The second version uses the same two pilot valves, but one valve sends a pilot signal to the pilot side of a single-pilot, spring-return, four-way valve. The pilot air or gas shifts the four-way valve against the spring and remains trapped in the pilot section until the other pilot valve is tripped, venting the air or gas in the pilot section. With this second version, the spring then shifts the four-way valve back to the original position. The third version is similar to the second version. Pilot air or gas actuates a larger pilot piston on one side of the four-way valve and holds it in place. The piston on the other side of the four-way valve is smaller and is always charged with supply air or gas. When pilot air or gas is vented from the first piston the smaller piston shifts the four-way valve back to its original position. 
     3. Existing booster designs vent the drive air or gas to atmosphere. The pilot air or gas also vents to atmosphere. The drive force is determined by the pressure of the drive air or gas above atmospheric pressure. The flow capability is a function of this drive force as well as the amount of drive air or gas that is available. Typically, the maximum pressure rating of gas booster drive systems is 10 bar or 150 psi, which encompasses the shop air pressure available in most industrial applications. 
     Rod seal design and materials, piston seal design and materials, and structural materials vary in the pneumatic drive section, but the various models are similar in engineering design. 
     There is a need for using gas pressure boosters in applications such as oil and gas wells, natural gas pipeline compressor stations, and turbine compressor applications in oil refineries and chemical plants. In many cases, such as gas wells or remote pipeline compressor stations, no electricity is available (all equipment may be run off of natural gas). Available gas pressures can be substantial (e.g., 200-1000 psi). In a gas well, for example, gas from the well enters a separator to remove oil and water. The gas is filtered and transported to a “sales line,” which collects gas and transports it to a natural gas processing facility. Sales line pressure may be in the 100-250 psi range. The wellhead gas at a much higher pressure may be reduced in pressure when it enters the sales line, where substantial energy is lost. The oil separated from the gas is sent to an atmospheric pressure condensate storage tank where gas flashes out of the pressure-reduced stream and continues to bubble out of the oil and water at near-atmospheric pressure (flash gas). This gas is typically vented to atmosphere or burned in a flare. The venting of gas-operated controls at the well also is released to the atmosphere or through the flare. It would be advantageous to develop a system for capturing these fugitive gas emissions from the well and recovering this vented gas and returning it to the sales line. Further, if the energy lost in the reduction of gas pressure were used in the fugitive emission vapor recovery effort, there would be little or no energy cost. 
     These fugitive emissions of volatile organic compounds are a safety and environmental hazard. In Colorado, environmental standards were put into place in December of 2006 in an effort to reduce volatile organic compound emissions which create ozone and negatively effect air quality. These standards were made more stringent after May of 2008 when condensate tanks emitting more than 20 tons per year of volatile organic compounds are required to reduce emissions by 95% to help reduce the high levels of ozone concentrations in the area and keep Colorado in compliance with national air standards. 
     Vapor recovery requires boosting the fugitive emissions from near atmospheric pressure to a pressure level where they can be returned to the process. When gas is compressed from a low pressure to a significantly higher pressure, it must typically pass through several stages of compression to remove the heat generated during compression. The additional stages of compression require more equipment and cooling between the stages resulting in additional capital costs and energy consumption. Development of equipment which can reduce the number of stages of compression and utilize the gas pressure potential energy available at the source is very desirable. 
     DEFINITION OF CLAIM TERMS 
     The terms used in the claims of the patent as filed are intended to have their broadest meaning consistent with the requirements of law. Where alternative meanings are possible, the broadest meaning is intended. All words used in the claims are intended to be used in the normal, customary usage of grammar and the English language. 
     “Atmospheric pressure” means 14.7 psia (absolute pressure) or 0 psig (gauge pressure). 
     “Boost compression ratio” means the ratio of the increased pressure of a gas to the original pressure of that gas. 
     “Captured and transported” means that substantially all of the fugitive emission gas in question is captured and returned to the gas pressure booster and/or the process. 
     SUMMARY OF THE INVENTION 
     The objects mentioned above, as well as other objects, are solved by the present invention, which overcomes disadvantages of prior pressure gas boosters and systems and methods for using them, while providing new advantages not previously associated with such boosters, systems and methods. 
     In a preferred embodiment of the invention, a gas pressure booster is provided, and includes a drive cylinder and a boost cylinder interconnected by reciprocating drive and boost pistons. Initially, the drive cylinder may be operated with drive gas at a first gas pressure. The drive gas may be vented at a second gas pressure which is lower than the first gas pressure. During a piston stroke the boost cylinder charges through an inlet check valve with fugitive gas emission. In this embodiment, the trapped drive gas in the drive cylinder at the end of a stroke at the first higher gas pressure flows to the charged boost cylinder. Any excess gas flows out of the boost cylinder through a discharge check valve to the second, lower pressure, so that the gas pressures in the boost cylinder and the drive cylinder equalize at a pressure higher than the fugitive emission gas pressure. The resulting, equalized pressure in the boost cylinder eliminates the need to stage gas compression, as might otherwise be required given the boost compression ratio between the fugitive emission gas pressure and the second lower gas pressure. The piston stroke of the boost cylinder may be reversed, discharging the mixture of fugitive gas emissions and pressure-reduced drive gas to a line containing the second lower pressure gas. Fugitive gas emissions from operation of the gas pressure booster, such as gas emissions from seal leaks or pilot valve vents in the gas pressure booster, may be captured and transported to the lower pressure second gas stream to eliminate gas discharged to atmosphere. 
     In a particularly preferred embodiment, a four-way valve operating on differential gas pressure may be used to actuate the reciprocating pistons. The four-way valve may be actuated in various ways. For example, it may be actuated using gas pilot pressure, or a mechanical actuation, applied on each side of the valve. As another example, the four-way valve may be actuated by pilot pressure applied on one side of the valve; when this pressure is vented, a spring may be used to actuate the other side of the valve. As a further example, the four-way valve may be actuated by pilot pressure applied to a valve piston acting on one side of the valve; when this pilot pressure is vented, supply pressure acting on a smaller valve piston on the other side of the valve may be used to actuate the valve. In each case, return of the valve may be actuated by venting the pilot pressure to a low gas pressure line. 
     The gas pressure booster may be operated without the need for electricity or cooling water. 
     In an alternative preferred embodiment of the invention, a method is provided for recovering fugitive gas emissions from a gas pressure booster having a drive cylinder, a boost cylinder, and interconnected, reciprocating drive and boost pistons. Initially, the drive cylinder may be operated with drive gas at a first gas pressure. The drive gas may be vented at a second gas pressure which is lower than the first gas pressure. Now, the boost cylinder charges with fugitive gas emissions recovered from operation of the gas pressure booster, by completing a piston stroke of the boost cylinder. The pressure of the recovered fugitive gas emissions is elevated by shutting off the source of boost cylinder drive gas at the first higher gas pressure, creating trapped drive gas. 
     Again, in this embodiment as well, trapped drive gas at the first higher gas pressure flows to the charged boost cylinder, so that the gas pressures in the boost cylinder and the drive cylinder equalize at a pressure higher than the fugitive emission gas pressure. Again, the resulting, equalized pressure in the boost cylinder eliminates the need to stage gas compression, as might otherwise be required given the boost compression ratio between the fugitive emission gas pressure and the second lower gas pressure. As with the first embodiment, the piston stroke of the boost cylinder may be reversed, discharging the mixture of fugitive gas emissions and pressure-reduced drive gas to a line containing the second lower pressure gas. 
     In a particularly preferred embodiment, the gas pressures in the boost cylinder and the drive cylinder may equalize at a pressure that is equal to a sink pressure for the gas pressure booster. 
     The gas booster cylinder may be operated without the need for electricity or cooling water. 
     In a further alternative embodiment, useful with either the gas booster system or method for using it, two communicating three-way valves may be provided, with one side of the valves connecting the boost cylinder to a vapor line, and the other side of the valves connecting the boost cylinder to a low pressure gas line. In this embodiment, the need for any check valves or low pressure line connections to the boost cylinder may be eliminated. 
     Those of ordinary skill in the art will appreciate that the gas pressure booster of the present invention may be advantageously employed to recover fugitive gas emissions without the need for external power or staged gas compression, and that it may be used in a variety of devices and systems, including but not limited to oil or gas wells, natural gas pipeline compressor stations, and industrial gas compressors such as turbine compressors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features which are characteristic of the invention are set forth in the appended claims. The invention itself, however, together with further objects and attendant advantages thereof, will be best understood by reference to the following description taken in connection with the accompanying drawings. The drawings illustrate currently preferred embodiments of the present invention. As further explained below, it will be understood that other embodiments, not shown in the drawings, also fall within the spirit and scope of the invention. 
         FIGS. 1A-1D  are progressive, illustrative schematic views of a gas booster system according to a preferred embodiment of the present invention; and 
         FIGS. 2A-2B  are illustrative schematic views of a gas booster system according to an alternative preferred embodiment of the present invention. 
     
    
    
     The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Set forth below is a description of what are currently believed to be the preferred embodiments and/or best examples of the invention claimed. Future and present alternatives and modifications to these preferred embodiments are contemplated. Any alternatives or modifications which make insubstantial changes in function, in purpose, in structure or in result are intended to be covered by the claims of this patent. 
     Referring now to  FIGS. 1A-1D , the operation of a preferred embodiment of the present invention, designated generally as gas pressure booster system  10 , will now be described. Referring first to  FIG. 1A , gas pressure booster system  10  generally includes boost cylinder  17   a , drive cylinder  17   b , and a valving system connecting pilot valves PVA and PVB to a valve manifold/4-way valve designated generally VM, as described below. The valve manifold and all pilot connections are connected to a sink and a source of pressurized gas, as also explained below. A gas vapor source is connected to boost cylinder. The entire system is isolated from the atmosphere surrounding the booster. 
     As shown in the drawings, drive piston  34   b  in drive cylinder  17   b  is attached to boost piston  34   a  in boost cylinder  17   a  through a common piston rod  20 . Boost cylinder  17   a  is double-acting, i.e., it pulls gas in on one side while pumping it out on the other side in both directions of stroke. 
     Referring first to  FIG. 1A , booster system  10  includes boost cylinder  17   a  housing boost piston  34   a , and drive cylinder  17   b  housing drive piston  34   b , with each piston sharing a common piston rod  20 . A source of high pressure gas is provided at  130 , a vapor source is provided at  140 , and region  150  is a low pressure gas sink. Piston rod  20  is shown in the middle of a forward piston rod stroke; high pressure gas pushes the front (left) side of drive piston  34   b  in the direction of stroke as shown by the arrow (i.e., to the right). Vapor is sucked into the front (left) side of the boost system through check valve  156 . Low pressure gas at the sink pressure (region  150 ) exits the back side of drive piston  34   b , passing from region  131  through 4-way valve manifold VM and then through manifold  142  into the front side of boost piston  34   a . The gas then flows to low pressure sink region  150  through discharge check valve  151 . 
     Referring now to  FIG. 1B , vapor has filled the front side of boost cylinder  17 A and piston  34 B has reached the end of its stroke and has opened pilot valve B (PVB). This allows pilot gas to pass through PVB, moving 4-way valve VM to the left, allowing high pressure gas from  130  to enter the back side of drive cylinder  17 B and opening a path through manifold  152  for the high pressure gas on the front side of drive cylinder  17 B to mix with the recovered vapor and fill the front side of boost cylinder  17 A. Manifold  142  is closed. Due to the high pressure gas flow from the front side of drive cylinder  17 B, the front side of boost cylinder  17 A may reach the low pressure gas sink pressure and excess gas will flow through check valve  151  to the low pressure gas sink  150 . High pressure in the front side of boost cylinder  17 A causes check valve  156  to close. As piston  34 A begins to move to the left, low pressure on the back side of boost cylinder  17 A has allowed check valve  158  to open and check valve  141  to close, permitting vapor from region  140  to be sucked into the back side of the boost cylinder through check valve  158 , forcing low pressure gas and vapor on the front side of cylinder  17 A through check valve  151  and into sink  150 . 
     Referring now to  FIG. 1C , the drive piston is now halfway through its cycle, still moving to the left, as high pressure gas from source  130  is still filling the back side of the drive cylinder. (VM remains in the same position as in  FIG. 1B .) Vapor from region  140  passes through check valve  158  and into the back side of the boost cylinder. Low pressure gas from the front side of the boost cylinder is pushed out of the boost cylinder through manifold  152  into the front side of the boost cylinder and passes through check valve  151  to sink  150 . 
     Referring now to  FIG. 1D , vapor has filled the back side of boost cylinder  17 A and piston  34 B has reached the end of its stroke and has opened pilot valve A (PVA). This allows pilot gas to pass through PVA, moving 4-way valve VM to the right, allowing high pressure gas from  130  to enter the front side of drive cylinder  17 B and opening a path through manifold  142  for the high pressure gas on the back side of drive cylinder  17 B to mix with the recovered vapor and fill the back side of boost cylinder  17 A. Manifold  152  is closed. Due to the high pressure gas flow from the back side of drive cylinder  17 B, the back side of boost cylinder  17 A may reach the low pressure gas sink pressure and excess gas will flow through check valve  141  to the low pressure gas sink  150 . High pressure in the back side of boost cylinder  17 A causes check valve  158  to close. As piston  34 A begins to move to the right, low pressure on the front side of boost cylinder  17 A has allowed check valve  156  to open and check valve  151  to close, permitting vapor from region  140  to be sucked into the front side of the boost cylinder through check valve  156 , forcing low pressure gas and vapor on the back side of cylinder  17 A through check valve  141  and into sink  150 . 
     In the manner described above, the vapor recovery booster will reciprocate automatically and recover fugitive gas emissions at atmospheric pressure, boost them to the pressure level of the low pressure gas sink, and return them to the low pressure gas sink in a single stage of compression. No electricity or cooling water is required. All gas used to drive the vapor recovery booster is recovered and vented to the low pressure gas sink. 
     Those of ordinary skill in the art will now appreciate that in the preferred embodiment of the gas pressure booster system disclosed here, not only are fugitive emissions eliminated, but the gas pressure booster can run off of the energy supplied by the available high pressure gas source (e.g., gas wellhead pressure or gas compressor discharge), as opposed to having to employ outside/remote energy. All of the booster drive gas emissions and the recovered fugitive emissions are discharged into the low gas pressure sink (e.g., gas sales line or compressor inlet). In addition, the need to boost the vapor pressure to the sink pressure in several stages of compression is eliminated. 
     It will be understood from the foregoing that the trapped drive gas at the first higher gas pressure flows to the charged boost cylinder, so that the gas pressures in the boost cylinder and the drive cylinder equalize at a pressure higher than the fugitive emission gas pressure. The resulting, equalized pressure in the boost cylinder eliminates the need to stage gas compression, as might otherwise be required given the boost compression ratio between the fugitive emission gas pressure and the second lower gas pressure. 
     Those of ordinary skill in the art will also appreciate that fugitive gas emissions can be captured using a bag or a tank, or they may be pulled directly from the source, as disclosed above. While the preferred embodiment describes a gas pressure booster and method for using such a system in which all of the fugitive gas emissions are isolated, captured and returned to the booster and/or system, those of ordinary skill in the art will recognize that some desired percentage less than 100% (e.g., 95% of 98%) of such emissions may be captured, if desirable or necessary for some reason, and that the claim term “captured and transported” will cover such “substantially all” emissions. 
     Referring now to  FIGS. 2A and 2B , an alternative preferred embodiment of a gas booster system according to the present invention is disclosed. With this embodiment, all check valves and low pressure line connections to the boost cylinder have been eliminated. In their place, two 3-way valves “X” and “Y” are provided. One side of these valves connects a boost cylinder chamber to the vapor line, and the other side connects the boost cylinder chamber to the LP (second, lower gas pressure) line. Both valves are controlled by pilot signals from the HP line and the LP line. In  FIG. 2A , the pistons are moving to the right, the gas in chamber B is at LP pressure, and the other side of the boost cylinder is pulling in vapor. At the end of this stroke, the 4-way drive valve VM shifts (for the same reason as described in reference to the embodiment shown in  FIGS. 1A-1D ), causing the HP drive gas to vent to the LP line and fresh HP gas to fill drive cylinder chamber A. The HP pressure from drive chamber A acting on the 3-way valve pilots causes them to shift (the pilot pressure on the other side of the 3-way valves has been reduced to LP pressure). The B side of the boost chamber will suck in vapor and the other side will be connected to LP pressure through its 3-way valve and instantly fill with LP gas which will mix with the recovered vapor. The cycle continues in this manner. 
     The approach shown in  FIGS. 2A-2B  may be preferred due to enhanced volumetric efficiency of the boost cylinder and consequent difficulties in valve design concerning the embodiment shown in  FIGS. 1A-1D . The inventor recognizes, however, that there may be applications where the embodiment shown in  FIGS. 1A-1D  is preferable. 
     Based on the disclosures and principles taught here, those of ordinary skill in the art will recognize that other valve arrangements which are not shown here will be apparent, and will have the same or similar gas recovery and LP gas pressure equalization results. As one non-limiting example, a gas booster design could be provided employing vapor inlet check valves, and pilot-controlled discharge check valves, which may possibly be preferable in future applications. 
     Those of ordinary skill in the art will appreciate that, in certain applications, it may be desirable to drive movement of the 4-way valve using mechanically-actuated means. 
     Those of ordinary skill in the art will further appreciate that gas can only be boosted so many ratios due to temperature increase and volumetric efficiency issues. The present invention eliminates the need for boosting a gas multiple ratios by equalizing the pressure of the fugitive emission vapor and the low pressure gas sink at the end of the suction stroke, as disclosed above. 
     It will be understood that various modifications to the preferred embodiment disclosed above may be made. The above description is not intended to limit the meaning of the words used in the following claims that define the invention. Rather, it is contemplated that future modifications in structure, function or result will exist that are not substantial changes and that all such insubstantial changes are intended to be covered by the following claims.