1.  Field of the Invention
The invention relates to fluid pumps and, more particularly, to a method and apparatus for preventing burnout of a pumping member by the continuous recirculation of pumped fluid. 
2.  Description of the Prior Art
In the fluid pumping art, a conventional pump comprises a pump casing within which a pumping member is disposed for movement.  Under normal operating conditions, when there is present an adequate supply of fluid to be pumped, the pumping member is lubricated adequately by the pumped fluid.  However, if the supply of fluid available to be pumped should decrease below a particular level, continued movement of the pumping member may fatally disable the pumping member due to lack of lubrication.  This problem is particularly acute in certain types of oil wells, known as stripper wells, which periodically provide a low oil output and are often periodically and temporarily pumped dry.  Also, in such wells, the fluid being pumped often contains abrasive material such as sand or grit, which quickly scores the surfaces of the pump elements.  When such a well presents an inadequate supply of fluid to the pump, the pump will quickly be damaged. 
The prior art has long recognized the above-mentioned problems, and many attempts have been made to solve them.  For example, it is known to provide a timer for the pumping member, so that the pumping menber will be timed to operate only during certain periods.  In this system, through trial and error, the length of a safe operating period is determined, beyond which the fluid level in the well will probably decrease to the point where the pumping member will not be adequately lubricated by the pumped fluid.  This system is inefficient and often times unpredictable and inaccurate.  It cannot even be established without damage to a number of pumps used in trying to determine the timing of the cycle.  While it may solve the problem of burnout of the pumping member, it does not obviate the problem of scoring of the pump components, because any entrained abrasive material still will settle toward the bottom of the well and, upon startup of the pumping member after replenishment of fluid in the well, will be immediately moved through the pump in a rather large quantity, having great potential to immediately damage the pump. 
Other prior art approaches have included providing sand traps for the collection of entrained abrasive material.  Most commonly, an outer casing is provided, which outer casing surrounds the pump casing so as to form an annular space, or reservoir, therebetween.  A plurality of openings are provided in the pump casing near the upper portion thereof to permit abrasive material to enter the reservoir and settle to the bottom thereof.  Such an arrangement partially eliminates scoring of the pump casing, but it does nothing to solve the problem of burnout of the pumping member due to lack of proper lubrication.  Furthermore, sooner or later the reservoir will be full of abrasive material, and the pump must then be raised to the surface to empty the reservoir. 
It also has been known to provide a quantity of heavy lubricant, or grease, in such a reservoir prior to lowering of the pump into the well.  In this arrangement, a second group of openings are provided in the pump casing near the lower portion thereof; additionally, a packing means is disposed within the reservoir on the upper surface of the lubricant.  When the pump is operated, pumped fluid is directed into the upper portion of the reservoir, thus acting on the packing means to force the lubricant from the lower portion of the reservoir into the interior of the pump casing, via the lower group of openings.  Such an arrangement tends to prevent the pump casing from being scored because the lubricant at least partially excludes abrasive material from the space between the pumping member and the pump casing.  However, if the supply of fluid available to be pumped should decrease, the lubricant no longer would be forced from the reservoir into the pump casing, and the pumping member still would burnout.  Additionally, once the supply of lubricant has been exhausted, no more protection against scoring of the pump casing by abrasive material is available. 
Accordingly, it is an object of the invention to provide a new and improved fluid pump wherein burnout of a pumping member is prevented in the event the supply of fluid available to be pumped should decrease beneath acceptable limits. 
It is another object of the invention to provide a new and improved fluid pump wherein scoring of the pump casing by abrasive material is minimized. 
It is a still further object of the invention to provide a new and improved fluid pump wherein the pump casing is lubricated continuously and wherein scoring of the pump casing is prevented, without employing timed intervals of operation or lubricants other than pumped fluid.