Abstract:
A heating system for heating liquids, stored in a tank at low ambient temperature, has a heating chamber with an inlet and an outlet for convectively flowing liquid past a flameless heater. Cold liquid is drawn through an inlet line, from the tank near its base and into a heating chamber, absorbs radiant energy from the heater as it travels therethrough. Heated liquid is circulated into the tank through an upper outlet from the heating chamber and back into the tank. Preferably, the heated liquid reenters the tank through a floating discharge flexibly connected to the upper outlet so as to remain dynamically in contact with the liquid at all times, thus avoiding airlocks which would interrupt the convective flow of liquid through the heating system.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to tanks for storing liquids and more particularly to tanks for storing liquids which can freeze at low ambient temperatures.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is well known to store large quantities of liquids in both aboveground and underground tanks, especially liquids produced from such industries as the oil and gas industry, where liquids such as water contaminated with oil, must be stored on site before removal and cleanup. Liquid storage is also required in a number of different industries and applications.  
           [0003]    Aboveground tanks are often preferable to underground tanks as there is no need to excavate a site and leakage detection is more easily performed. Regulations governing environmental protection, hazardous materials handling and worker safety provide structured guidelines with which such storage tanks can be constructed, whether single-walled or double-walled.  
           [0004]    As taught in U.S. Pat. No. 5,971,009 to Schuetz et al., the use of aboveground tanks in climates subject to extreme ambient temperatures has not found favor in the industry, due to problems such as freezing or increased viscosity of tank contents. Schuetz et al. addressed the freezing problem by providing a support means upon which the tank was placed, so as to create an air space under the tank. The entire structure and the air space is isolated from ambient using a layer of insulation. Further, a heater is used to heat the air space below the tank to keep the tank&#39;s contents from freezing, rather than heat the content&#39;s of the tank directly, which was deemed to be expensive and impractical. Heat can also be directed into the annular space formed between the inner and outer walls.  
           [0005]    The above prior art is in the form of a custom constructed tank. Construction of such aboveground tanks requires a significant amount of cost and man-hours. In times of increased activity in industries such as the drilling and production sector of the petroleum industry, it may be difficult to supply the large number of tanks required to satisfy needs. Any additional complex construction for integrating tanks, support means and heaters into complete, heated-tank systems increases the amount of time and money required to produce tanks. Further, advance construction and stockpiling of tanks is often not a practical solution, as it is difficult to predict their use in many industries which have fluctuating needs, resulting in a large amount of revenue being tied up and unrecoverable until the tanks are sold.  
           [0006]    Further, most well sites do not have ready access to electrical power, if any, and therefore it is known to utilize equipment capable of being run using well products such as raw natural gas.  
           [0007]    Ideally, a heating system for a liquid storage tank, whether part of the original design of a tank system or as a retrofit to an existing tank system, should be relatively inexpensive to build and to operate, provide adequate heat to the tanks contents to prevent freezing, require no electricity, be easily accessible from the exterior of the tank system for servicing and preventative maintenance, utilize simplified construction and be easily added to existing tank systems.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a heating system that is simple to construct and is readily retrofit to existing tank systems. The heating system satisfies the requirements of being readily accessible for service and maintenance, and does not require electrical power to operate.  
           [0009]    In a broad aspect of the invention, a tank heater system is provided comprising a hollow heating chamber having a lower inlet, in communication with an inlet line extending into and adjacent the bottom of the tank, and an upper outlet, in communication with the liquid in the tank. A heater is positioned for heating the heating chamber. Liquid, drawn from the tank into the inlet line, is heated in the heating chamber where it rises by convection and is reintroduced to the tank through the outlet.  
           [0010]    Preferably, the heating chamber has a plurality of baffles inside the hollow chamber for increasing the residence time of the liquid in the heating chamber and increasing the fluid&#39;s heat capacity. A flameless, infrared gas catalytic-type heater can be used to avoid the need for electricity and comply with explosion proof conditions. Further, the discharge to the tank is in constant communication with the liquid in the tank, including the use of a floating discharge which remains in constant communication with the liquid in the tank and thus preventing airlock when the liquid level drops below that of the upper outlet&#39;s connection to the tank. Enclosing the heating system against the tank wall, scavenges residual heat and applies it to the tank. In yet another embodiment of the invention, a gas powered or heat powered pump is fitted into heating chamber system, thereby creating forced convection to ensure liquid flow is maintained. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 a  is an elevation view in cross-section of a dual walled tank having a heater system of the present invention wherein the load line and the inlet line are separate lines and the inner and outer tanks have a shared roof;  
         [0012]    [0012]FIG. 1 b  is a partial elevation view in cross-section of a dual walled tank having a heater system of the present invention wherein the load line and the inlet line are the same line and the inner and outer tanks separate roofs;  
         [0013]    [0013]FIG. 2 a  is a partial elevation view in cross section of the discharge from the upper outlet wherein the discharge is a conduit extending to the base of the tanks;  
         [0014]    [0014]FIG. 2 b  is a partial elevation view in cross-section of the discharge from the upper outlet, wherein the discharge is a dynamic discharge;  
         [0015]    [0015]FIG. 3 is a cross-sectional view of the heater system of FIG. 1;  
         [0016]    [0016]FIG. 4 is an elevation view in cross-section of the dual walled tank and heater system of FIG. 1 showing the convection currents in the liquid; and  
         [0017]    [0017]FIG. 5 is a cross-sectional view of the heater system of another embodiment having a pump for forced convection of liquids. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Natural gas produced at a wellhead typically carries heavier liquids, primarily water, which is separated from the gas flow. The hydrocarbon-tainted water is then directed to a storage tank where it is contained until removal and subsequent treatment. Often wells of this type are located in climates subject to very low ambient temperatures for at least part of the year.  
         [0019]    Having reference to FIG. 1 a,  and in one embodiment of the invention, a storage tank  10 , which may be dual-walled, and a heating system of the present invention is shown. It is understood that a storage tank  10  may be a single wall or a dual-wall tank without affecting the functionality of the heating system. This specification discloses the present invention as applied to a dual-walled tank. Dual-walled tanks are well known in the industry.  
         [0020]    An inner tank  11  sits within and is surrounded by a larger outer tank  12 . The outer tank  12  is of sufficient volume to contain any and all liquid  13 , which may leak from the inner tank  11 , within the annular airspace  14  created between the two tanks  11 ,  12 . Both tanks  11 ,  12  have a substantially planar circular base  15   a,    15   b  joined with an upstanding continuous cylindrically shaped side wall  16   a,    16   b.  The base  15   a  of the inner tank  11  rests directly upon the base  15   b  of the outer tank  12 , the tanks  11 ,  12  resting directly on a metal plate  17  located on a base or upon prepared level soil or gravel. The inner tank  11  has a conical roof  18  that is supported on and connected to the side wall  16   a.  Further, the roof  18  has a vapour exhaust or vent  19 , which access the inner tank  11  to relieve excessive pressure build-up in the inner tank  11 .  
         [0021]    The outer tank  12  may share the same roof  18  as the inner tank  11 , as shown in FIG. 1 a,  or may have its own roof  20 , as shown in FIG. 1 b,  the roof  20  being conical, supported on the side wall  16   b  and arched above the roof  18  of the inner tank  11 . The outer tank roof  20  is also vented to prevent pressure buildup.  
         [0022]    The outer tank  12  has a thermal insulation layer  21  covering and adhering to a surface of the side wall  16   b,  roof  20  and floor  15   b  to assist in thermally isolating the tank  12  from the ambient.  
         [0023]    An insulated enclosure  2  is constructed adjacent the outer wall  16   b  of the outer tank  12  to house the heating system of the present invention and other such valves and equipment required to fill, empty and monitor the tank status such as detecting leaks and monitoring the temperature of the tank contents. A door (not shown) provides access to the interior of the enclosure  22  for performing maintenance and monitoring functions.  
         [0024]    As shown in FIG. 1 a,  a load line  30  extends from within the inner tank  11 , through both inner  11  and outer  12  tanks&#39; side walls  16   a,    16   b  in a sealing manner, at the base of the tanks  11 ,  12 , to facilitate emptying the inner tank  11 . A shutoff valve  31  is located on the load line  30  to facilitate emptying the inner tank  11 . A supply line  32 , typically extending from a separator (not shown), is used for filling the inner tank  1  and extends through both inner  11  and outer  12  tanks&#39; side walls  16   a,    16   b  in a sealing manner, typically above the load line  30 . A shut-off valve  33  is located on the supply line  32  to facilitate filling the inner tank  11 .  
         [0025]    An inlet line  34  is located slightly above the base of the tanks  15   a,    15   b,  perforating both tanks&#39; side walls  16   a.    16   b  in a sealing manner. The inlet line  34  extends into the inner tank  11 , preferably to the center of the inner tank  11  or beyond, to a first point P 1 , so as to access colder liquid in the tank  11 . The inlet line  34  extends outwardly to a heating chamber  35 . Optionally, as shown in FIG. 1 b,  the load line  30  may act also as the inlet line  34 .  
         [0026]    Having reference again to FIG. 1 a,  the heating chamber  35  has a lower inlet  36  connected to the inlet line  34 . The heating chamber  35  itself can be isolated from the inlet line  34  with a shut off valve  37  for maintenance purposes. An outlet  38  extends from the top of the heating chamber  35  and extends through the side walls  16   a.    16   b  of both outer  12  and inner  11  tanks for reintroducing heated liquid  40  at a second point P 2  in the inner tank  11 . The outlet  38  has a discharge  39  located in the liquid  13  of the inner tank  11 . The heater chamber  35 , lower inlet  36  and upper outlet  38  form a convection circuit C of liquid  13  between the inner tank  11  and the heating chamber  35 .  
         [0027]    A heat source  41 , preferably a flameless catalytic gas infrared heater, is located in the enclosure  22 , external to and adjacent the heating chamber  35 .  
         [0028]    So as to avoid draining of liquid  13 ,  40  from the convection circuit c, the discharge  39  from the upper outlet  38  is positioned in the liquid  13  and located so that the discharge  39  is rarely or never out of the liquid  13  in the inner tank  11 . As shown in FIG. 2 a,  one form of discharge  39  is a conduit  42  extending from the heating chamber outlet  38  to a point P 3  near or below the inlet line  34 .  
         [0029]    Another form of discharge  39 , as shown in FIG. 2 b,  is a dynamic discharge  43 , attached to the outlet  38  as it enters the inner tank  11 . The discharge  43  is attached at a first end  43   a  to the outlet  38  using a flexible connector  44  such as a piece of flexible plastic hose. The flexible connector  44  allows the discharge  43  to pivot and dynamically position a second end  43   b  immersed within the liquid  13  in the tank  11  so as to be in constant therewith, especially when the liquid  13  level in the tank  11  is below the outlet connection  45  to the tank walls  16   a,    16   b.  Positioned thus, the discharge  43  remains at a point P 4  submerged in liquid  13 , preventing an air lock from occurring in the convective circuit C.  
         [0030]    The second end of the discharge  43   b  can be fitted with a float  46  to ensure that it rises and falls with the liquid  13  level.  
         [0031]    Optionally for tanks  10  used to store hydrocarbon-tainted water, the floating discharge  43 , and outlet  38  can be used to remove any floating condensate that may have separated from the water. Separate valves (not shown) would be provided to allow removal of the condensate through the discharge  43 .  
         [0032]    Having reference to FIG. 3, the heating chamber  35  comprises a vessel  50  such as a rectangular liquid-sealed box defining a hollow heating chamber  35 . The heating chamber  35  is positioned directly in front of the heater  51  so as to expose a maximum amount of surface area to the radiant heat h produced by the heater  51 . The lower inlet  36  from the inlet line  34  extends into the bottom of the heating chamber  35 .  
         [0033]    The upper outlet  38  extends from the top of the heating chamber  35 . A plurality of outlets  38 ,  38  . . . can be provided for discharge into the tank; resulting one benefit being to minimize pressure drop of he convective flow.  
         [0034]    To improve the heating effect from that provided by a simple hollow heating chamber  35 , a plurality of baffles  52 , as shown in FIG. 4, are positioned inside the chamber  35  so as to create a serpentine pathway therethrough and thus increase the residence time of liquid  13 ,  40  flowing through the chamber  35 .  
         [0035]    As shown in FIG. 3, liquid  13  flows through the air-tight heating system of the present invention as a result of natural convection currents C created by the differences in densities of liquid  13  at different temperatures in the heating system.  
         [0036]    Liquid  13  within the heating chamber  35  is heated by the heater  51 , preferably by radiant heat h. As the liquid  13  in the heating chamber  35  heats, it becomes less dense and begins to rise through the serpentine pathway in the heating chamber  35 . The longer the liquid  13  remains in the chamber  35 , the more heat it absorbs, the hotter and less dense it becomes and the more rapidly it rises. As the heated liquid  40  reaches the outlet  38 , it is flowed through the discharge  39  and reintroduced into the tank  11  where it begins cooling, releasing its heat into the cooler liquid  13  in the tank  11 . As the heated liquid  40  cools, its density increases and it sinks to the base  15   a  of the tank  11  where it is drawn again into the inlet line  34  by the convection currents C to repeat the heating cycle.  
         [0037]    The inlet line  34 , positioned at the center of the tank or closer to an opposite side  53  of the tank  11  from the heater  51 , draws liquid  13  from the coldest liquid  13  in the tank  11 , thus creating a large temperature differential between the coldest liquid  13  and the heated liquid  40  in the heating chamber  35 . The large temperature differential acts to increase the operational efficiency of the system.  
         [0038]    In one example, liquid  13  at the center of the inner tank  11  is 40 degree F. as it is drawn into the inlet line  34  and lower inlet  36  to the heating chamber  35 . After passing through the heating chamber  25 , exposed to a flameless heater  51  having a surface temperature of 400 degrees F. and into the outlet  38 , the liquid  40  reaches a temperature of approximately 70 degrees F. when it is reintroduced to the tank  11 .  
         [0039]    A globe valve  60  is located on the outlet  38  between the heating chamber  35  and the outer tank  2  and is manually set to control the rate of flow of liquid  13 ,  40 , and it&#39;s temperature, through the heating chamber  35  and back into the inner tank  11 . Further, a temperature sensor (not shown) is positioned within the inner tank  11  to continuously monitor the liquid  13  temperature and is electrically connected to a temperature readout (not shown), in the heated enclosure  22 .  
         [0040]    Optionally, a hood  70  is connected to the top of the heating chamber  35  and extends over the heater  51  to trap escaping heat from the heating chamber  35  and improve the overall efficiency of the heating process. Further, the insulated, heated enclosure  22  may be extended to the full height of the outer tank  12  in order to concentrate any residual heat scavenged from the heater  51  against the side of the outer tank  12 . This scavenged heat, although applied to only a portion of the outer tank&#39;s side wall  16   b,  acts to heat the annular airspace  14  between the inner  11  and outer  12  tank, further warming the inner  11  tanks contents  13 .  
         [0041]    Further, a well gas operated pneumatic shutoff valve with a float-actuated pneumatic switch (note shown) is provided to block the supply line, should the liquid level in the tank exceed maximum capacity. This is particularly useful in the case of a shared roof where there is no overflow to the annular airspace  14  between the tanks  1 ,  12 .  
         [0042]    In another embodiment of the invention, as shown in FIG. 5, a pump  80  is added to the lower inlet  36  to the heating chamber  35  to create forced convection of the liquid  13  through the heating chamber  35 . The pump  80  can be fitted to a bypass  81  for utilizing either natural or forced convention. Preferably, a gas fueled engine or a heat engine, such as a Stirling engine, is used to operate the pump  80 . Heat from the flameless heater  51  is used to power the heat engine, creating a self-sufficient heating and circulation system.  
         [0043]    For both embodiments, retrofit of an existing tank system is readily accomplished. The heating chamber  35 , inlet line  34  and outlet  38  can be fit to any two ports in the liquid  13 .  
         [0044]    In cases where the load line  30  is already present, whether to the center of the tank or elsewhere adjacent the tank&#39;s bottom  15   a,  only an upper outlet  38  is required. If there is no existing port, it may be necessary to drain the inner  11  tank before perforating the side walls  16   a,    16   b  of both inner  11  and outer  12  tank for installing the upper outlet  38 . In cases where the load line  30  is inadequate for circulation, two other ports or perforations must be made to install an appropriate inlet line  34 .  
         [0045]    Typically, tanks  11 ,  12  are fitted with two or three adjacent ports through which the inlet  36  and outlet  38  lines can be sealingly installed, for retrofit purposes.  
         [0046]    Heating components are assembled and installed in an existing or newly constructed insulated enclosure  22  attached to the side wall  16   b  of the outer tank  12 .