Abstract:
Methods and systems for preventing vacuums within a snubber during a predetermined condition are disclosed. The snubber includes a body at least partially filled with fluid, a piston capable of sliding within the fluid in the body, a lockup valve that allows the piston to move freely under operating conditions and limits the motion of the piston under a predetermined set of conditions, and a reservoir positioned within the piston rod containing a reserve of fluid.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional application Ser. No. 61/184,518 filed Jun. 5, 2009, entitled “Snubber With Secondary Fluid Reservoir,” which is hereby specifically and entirely incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention is directed to snubbers (also known as shock arrestors) and methods of controlling the motion of objects with snubbers. In particular, the invention is directed toward hydraulic snubbers and methods of controlling the motion of objects with hydraulic snubbers. 
         [0004]    2. Background of the Invention 
         [0005]    Hydraulic snubbers or shock arrestors are often used in piping systems to allow slow movement of the pipes due to thermal expansion while protecting the piping systems and equipment from accidental damage arising from abnormal loading or movement conditions due to a shock force or vibration such as experienced during a seismic disturbance. Snubbers are often used in power plants to restrain pipes during seismic conditions since using fixed coupling could cause damage to the pipes during normal events. 
         [0006]      FIG. 1  shows a basic schematic of a prior art snubber  100 . Snubber  100  would be coupled at one end to a fixed object, such as a power plant wall or floor, while the other end would be coupled to a pipe. A basic hydraulic snubber consists of a cylinder (which includes a body, piston and piston rod or rods), a control valve, and a hydraulic fluid reservoir. Under normal circumstances, such as when the pipe is heating up, piston rod  105  would push piston  110  into body  115 . Body  115  is filled with a slightly compressible fluid. As piston  110  extends into body  115 , the fluid inside chamber  125  passes through control valve  130  and is displaced into chamber  120 . The excess fluid (since the snubber only has one piston rod) is displaced into reservoir  135 . On the other hand, as the pipe cools, piston rod  105  exits body  115  causing piston  110  to move within body  115 . In that circumstance, chamber  125  will fill as chamber  120  empties. Since chamber  125  has a larger area than chamber  120  more fluid is required to fill chamber  125  than is supplied by chamber  120 . This fluid comes from reservoir  135 . Under operating circumstances, snubber  100  will function almost invisibly. 
         [0007]    Under predetermined conditions, for instance during seismic events, the snubber must be able to lock up to prevent damage to the system. During such conditions, piston  110  may move quickly closing control valve  130 . In this case fluid is supplied from reservoir  135 . However, as can be seen in  FIG. 2 , valve  230  and reservoir  235  greatly increases the size of snubber  200 , thereby making the space needed to install snubber  200  greater. To minimize the size of the snubber a double ended cylinder (a piston rod on each end of the piston) is utilized with the control valves installed in the piston. An internal reservoir is utilized for fluid expansion and contraction. Unlike a single ended cylinder (one piston rod), fluid is not required to enter or exit the reservoir during normal stroking. During the normal (valves unlocked) mode fluid simply transfers from one side of the piston to the other thru the normally open lockup valves. During modes where one of the valves is closed the fluid on one side of the cylinder is compressed and the other side needs fluid to compensate for the resultant fluid compression. Since the valves are in the piston and not directly connected to the reservoir, the reservoir cannot supply the required fluid. If additional fluid is not provided to fill the vacuum, the vacuum will initially remove any entrained or entrapped air contained in the fluid, creating bubbles. If a sufficient vacuum level is attained, the fluid may vaporize causing even more bubbles. Bubbles will result in a lower spring rate and possible erratic lock-up rates, in order to minimize these effects, some manufacturers of snubbers de-gas the fluid before inserting it into the snubber, however this is a very costly operation. Thus it is desirable to have a snubber without external valves and fluid reservoirs and without the need to de-gas the fluid. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention overcomes the problems and disadvantages associated with current strategies and designs and provides new tools and methods of controlling movement of an object. 
         [0009]    One embodiment of the invention is directed to a snubber that includes a body that is at least partially filled with fluid. The snubber has a piston that slides within the fluid in the body. The snubber also includes a lockup valve that allows the piston to move freely under operating conditions and limits the motion of the piston under a predetermined set of conditions, a reservoir for thermal expansion and contraction of the fluid, and a second reservoir positioned within the piston rod containing a reserve of fluid. 
         [0010]    In certain embodiments, the lockup valve divides at least a portion of the body into two chambers. In certain embodiments, the lockup valve is coupled to the piston and is comprised of at least two valves in fluid communication with each other via a conduit. One valve is in fluid communication with one chamber and a second valve is in fluid communication with the second chamber. 
         [0011]    In certain embodiments, as the piston moves within the body, fluid displaced by the piston is transferred from one chamber to the other chamber via the lockup valve. 
         [0012]    In certain embodiments, the main reservoir supplies fluid during the operating conditions and the secondary reservoir supplies fluid during the predetermined set of conditions. In certain embodiments, the fluid has viscosity stability of between 40 mRad and 200 mRad. In certain embodiments, the snubber is able to bear a load of between 50 klb and 120 klb. 
         [0013]    In certain embodiments, the snubber includes a spring within the body to keep the fluid in the main reservoir under pressure. In certain embodiments, the snubber includes a second spring coupled to the secondary reservoir to keep the fluid in the reservoir under pressure. 
         [0014]    Another embodiment is directed to a method for controlling the motion of an object. The method includes the steps of allowing the object to move freely during operating conditions, limiting the movement of the object with a snubber during a predetermined set of conditions, and preventing vacuums in the snubber during the predetermined set of conditions with fluid held in a reservoir inside a piston rod of the snubber. 
         [0015]    In certain embodiments, the object is a pipe. In certain embodiments, the movement is limited during the predetermined set of conditions by a lockup valve coupled to the piston. In certain embodiments, the lockup valve is open during operation conditions and locks up during the predetermined set of conditions. The predetermined set of conditions may be a seismic event. 
         [0016]    In certain embodiments, the fluid is maintained within the snubber during operating conditions with fluid held in a reservoir outside the piston. A spring may apply pressure to the fluid within the snubber. 
         [0017]    Other embodiments and advantages of the invention are set forth in part in the description, which follows, and in part, may be obvious from this description, or may be learned from the practice of the invention. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention is described in greater detail by way of example only and with reference to the attached drawings, in which: 
           [0019]      FIG. 1  is a basic schematic of the prior art. 
           [0020]      FIG. 2  is a prior art snubber. 
           [0021]      FIG. 3  is an embodiment of a snubber. 
           [0022]      FIGS. 4   a - d  are examples of an embodiment of a snubber in use. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0023]    As embodied and broadly described herein, the disclosures herein provide detailed embodiments of the invention. However, the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, there is no intent that specific structural and functional details should be limiting, but rather the intention is that they provide a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0024]    A problem in the art capable of being solved by the embodiments of the present invention is a snubber device having a narrow body design with an extra fluid reservoir. It has been surprisingly discovered that including a fluid reservoir inside the piston rod of a snubber reduces the overall size of the snubber while still providing a reservoir for extra fluid, to compensate for fluid which is compressed. 
         [0025]      FIG. 3  depicts an embodiment of a snubber  300 . Snubber  300  has a piston  380  that fits within a body  315 . Piston rod  305  enters body  315  through open end  310 . In certain embodiments, open end  310  has seals  313  surrounding piston rod  305  to prevent any fluid inside body  315  from leaking out. The exposed end of piston rod  305  may be coupled either to a fixed infrastructure (e.g. a wall or a floor) or to a moving object (e.g. a pipe). While closed end  311  of body  315  may be coupled to the other of the fixed infrastructure and the moving object. Snubber  300  may be able to bear any load, preferably snubber  300  is able to bear a load up to 300 klb, and more preferably, snubber  300  is able to bear a load of between 50 klb and 120 klb. 
         [0026]    While any fluid can be used in snubber  300 , preferably a fluid that is only slightly compressible is used. Such a fluid may be a silicon based fluid, such as dimethyl diphenyl polysiloxane, or dimethyl polysiloxane. Preferably the fluid has viscosity stability of up to 300 mRad, more preferably the fluid has viscosity stability of between 40 mRad and 200 mRad. 
         [0027]    In certain embodiments, piston  380  contains a lockup valve  320 . Lockup valve  320  consists of two valves  323  and  325  in fluid communication via a conduit  327 . Valve  323  is in fluid communication with a first fluid chamber  330 , while valve  325  is in fluid communication with a second fluid chamber  333 . First fluid chamber  330  and second fluid chamber  333  are separated from each other by lockup valve  320 . Lock up valve  320  is coupled to piston  360  and may have seals  335  to prevent fluid from leaking from first fluid chamber  330  to second fluid chamber  333 , or vice versa. 
         [0028]    In certain embodiments, second fluid chamber  333  has an end plate  337  at the opposite end of second fluid chamber  333  from lockup valve  320 . End plate  337  is coupled to body  315  and allows piston rod  356  to pass through end plate  337 . End plate  337  separates second fluid chamber  333  from main reservoir  340 . Second fluid chamber  333  and main reservoir  340  are in fluid communication via fine orifice  343 , which may be within end plate  337 . Main reservoir  340  may be enclosed on the end opposite from end plate  337  by reservoir piston  345 . Reservoir piston  345 , along with body  315  and closed end  311 , enclose spring cavity  347 . Spring cavity  347  includes spring  350  that puts main reservoir  340  under pressure by applying a slight force on reservoir piston  345 . Reservoir piston  345  may have seals  352  to prevent the fluid in main reservoir  340  from leaking into spring cavity  347 . 
         [0029]    A secondary reservoir  355  is located within the rear piston rod  356 . Secondary reservoir  355  is in fluid communication with conduit  327  of lockup valve  320 . Secondary reservoir  355  is contained by secondary piston  357 . Secondary piston  357  separates secondary reservoir  355  from secondary spring cavity  360 . Secondary spring cavity  360  contains secondary spring  363 , which keeps the fluid in secondary reservoir  355  under a low pressure. Secondary piston  357  may include seals  370  to prevent the fluid in secondary reservoir  355  from leaking into secondary spring cavity  360 . 
         [0030]    During operating conditions of snubber  300 , piston  325  is allowed to move smoothly within body  315 . Operating conditions may include movement of pipes during thermal expansion or contraction, or weight change. As such, lockup valve  320  is in an unlocked position and fluid can move between first fluid chamber  330  and second fluid chamber  333  though lockup valve  320  as needed. Thus, constant pressure is maintained throughout snubber  300 . In the case of thermal fluid expansion or contraction within first fluid chamber  330  and second fluid chamber  333 , main reservoir  340  can absorb any excess fluid or provide additional fluid to maintain the fluid pressure within snubber  300 . 
         [0031]    However, during a predetermined set of conditions, such as seismic events, water hammer events, or valve blowoff, lockup valve  320  becomes locked and prevents fluid from traversing from one side of lockup valve  320  to the other, thus limiting movement of piston  380 . Since the fluid used in snubber  300  is preferably slightly compressible, there will be some limited movement of piston  380 . As piston  325  moves, the fluid in one of fluid chambers  330  and  333  will become compressed while a vacuum will be created in the other fluid chamber. In order to prevent degasification or vaporization of the fluid caused by a vacuum, the fluid in secondary reservoir  355  will be released to fill up the space caused by the vacuum. Since the fluid in secondary reservoir  355  is under pressure a drop in pressure from either first fluid chamber  330  or second fluid chamber  333  will cause the fluid in secondary reservoir  355  to flow toward the area of low pressure thus rebalancing the pressure within snubber  300 . 
       Example 
       [0032]      FIGS. 4   a - d  show a snubber  400  in several conditions.  FIG. 4   a  shows snubber  400  under operating conditions where piston rod  405  is being pulled out of body  415 , as shown by the gray arrow. Under such operating conditions, fluid in first fluid chamber  430  is drawn through first valve  423 , into conduit  427 , out of second valve  425 , and into second fluid chamber  433 . Under such conditions, neither first valve  423  nor second valve  425  are locked and the fluid in first fluid chamber  430  and second fluid chamber  433  can flow freely in the direction show by the black arrows. 
         [0033]      FIG. 4   b  shows snubber  400  under operating conditions where piston rod  405  is being pushed into body  415 , as shown by the gray arrow. Under such operating conditions, fluid in second fluid chamber  433  is drawn through second valve  425 , into conduit  427 , out of first valve  423 , and into first fluid chamber  430 . Under such conditions, neither first valve  423  nor second valve  425  are locked and the fluid in first fluid chamber  430  and second fluid chamber  433  can flow freely in the direction show by the black arrows. 
         [0034]      FIG. 4   c  shows snubber  400  under a predetermined set of conditions, wherein first valve  423  is locked and piston rod  405  is being pulled out of body  415 , as shown by the gray arrow. Since first valve  423  is locked, fluid cannot flow from first fluid chamber  430  to second fluid chamber  433 . Therefore, in order to prevent a vacuum from forming in second fluid chamber  433 , fluid is drawn out of secondary reservoir  455  into conduit  427 , out of second valve  425 , and into second fluid chamber  433  as show by the black arrows. 
         [0035]      FIG. 4   d  shows snubber  400  under a predetermined set of conditions, wherein second valve  425  is locked and piston rod  405  is being pushed into body  415 , as shown by the gray arrow. Since second valve  425  is locked, fluid cannot flow from second fluid chamber  433  to first fluid chamber  430 . Therefore, in order to prevent a vacuum from forming in first fluid chamber  430 , fluid is drawn out of secondary reservoir  455  into conduit  427 , out of first valve  423  and into first fluid chamber  430  as show by the black arrows. 
         [0036]    Other embodiments and uses of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. All references cited herein, including all publications, U.S. and foreign patents and patent applications, are specifically and entirely incorporated by reference. It is intended that the specification and examples be considered exemplary only with the true scope and spirit of the invention indicated by the following claims. Furthermore, the term “comprising of” includes the terms “consisting of” and “consisting essentially of.”