Abstract:
Systems and methods for detecting cavitation in a reciprocating positive displacement pump. Fluid pressure proximate the pump&#39;s suction manifold is compared to a predetermined pressure that would be conducive to cavitation. If the detected pressure approximates the predetermined pressure, the presence of cavitation is confirmed via correlation of increased vibration.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to reciprocating pumps. In particular aspects, the invention relates to systems and methods for detecting and monitoring abnormal conditions within a pump, including cavitation. 
     2. Description of the Related Art 
     Reciprocating positive displacement pumps used in the well service industry and drilling mud pump industry are exposed to high pressure, high flow rate and abrasive fluids (slurry) for the purpose of fracturing, drilling and so forth. Reciprocating pumps can be single or double acting pumps with pistons that are driven by a crankshaft that is actuated by a motor. Reciprocating positive displacement pumps have at least one piston cylinder, but often have multiple cylinders, such as three-cylinder (triplex) and five-cylinder (quintuplex) configurations. 
     Cavitation affects reciprocating pumps during operation. Cavitation occurs when actual pressure reaches the vapor pressure of the fluid being pumped, and the fluid starts to vaporize. Small vapor bubbles are formed and, under compression, will implode. If these implosions occur in close proximity to the pump housings or valve surfaces, they will start to impinge the material, causing material to be removed and damaged. Cavitation can cause permanent damage and, if not prevented in time, can lead to complete destruction of the pump housing and/or associated components. 
     Efforts have been made to identify cavitation in an operating pump using acoustic signal analysis. However, this has proven problematic. There is a wide variety of vibration or acoustic signal responses that relate to a variety of abnormal conditions, which makes it difficult to differentiate between cavitation, valve wear, seal failure, or other conditions. 
     SUMMARY OF THE INVENTION 
     The invention provides systems and methods for detection of cavitation within a reciprocating pump. In certain aspects, the systems and methods of the present invention permit detection of cavitation with particularity so that other abnormal conditions may be excluded. 
     In a described embodiment, a sensor is used to detect fluid pressure within or proximate the suction or intake manifold of the pump. An accelerometer is disposed on the fluid end cylinder housing of the pump for detection of vibration. A timing marker is operably associated with a plunger of the pump and detect the speed of operation of the pump. 
     Actual fluid pressure detected at or near the suction manifold is compared to a predetermined pressure which would be conducive to cavitation. In particular embodiments, the predetermined pressure is the vapor pressure for the fluid being pumped by the pump  10 . 
     The accelerometer is monitored to detect an increase is vibration or shocks. An increase in vibration/shocks is correlated with the condition of the measured pressure approximating the predetermined pressure. This correlation indicates cavitation. 
     In accordance with currently preferred embodiments, a data processor receives data signals from the pressure sensor, accelerometer and timing marker which are indicative of the parameters being sensed by those components. The data processor then compares the detected pressure with a predetermined pressure (i.e., vapor pressure) and checks for cavitation. If the processor determines that cavitation is occurring, it can then take one or more actions in response. These actions include providing a message to an operator and automated adjustment of pump parameters to attempt to correct the cavitation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein: 
         FIG. 1  is an external, isometric view of an exemplary reciprocating pump having a cavitation detection system in accordance with the present invention. 
         FIG. 2  is a further external, isometric view of the pump shown in  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the fluid end of the pump. 
         FIG. 4  is a cross-sectional view of portions of the power end of the pump. 
         FIG. 5  is a schematic diagram of portions of an exemplary pump monitoring system which includes a data processor and associated components. 
         FIG. 6  is an enlarged external, isometric view of portions of the reciprocating pump shown in  FIGS. 1-4 . 
         FIG. 7  is a data plot depicting fluid pressure measurements for the suction manifold during pump operation. 
         FIG. 8  is a data plot of transformed suction pressure data showing the beginning of cavitation. 
         FIG. 9  is a data plot of detected pump vibration. 
         FIG. 10  is a logic diagram for an exemplary pump monitoring system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1-4  illustrate an exemplary reciprocating pump  10  which broadly includes a fluid end  12 , which draws fluid into the pump  10  and expels it, and a power end  14 , which receives power from an associated motor or other prime mover and transmits this power to the fluid end  12 . In the depicted embodiment, the pump  10  is a triplex pump having three pistons, which are evidenced by the cylinder heads  16  in  FIGS. 1-2 . A suction manifold  18  leads into the fluid end  12  of the pump  10 . 
     The cross-sectional view of  FIG. 3  illustrates a cylinder housing  20  for the fluid end  12  which encloses a valve piston chamber  22  within which a plunger  24  is axially moveable in a reciprocating manner, as driven by a crankshaft. Although only a single plunger  24  is visible in  FIG. 3 , it should be understood that there are actually three plungers  24  within the housing  20 . This reciprocating movement causes an intake valve  26  and an exhaust valve  28  to be opened and closed as fluid is pumped from the suction manifold  18  to the discharge  30 . 
     The general construction and operation of reciprocating pumps is well understood and will not be detailed here. It is noted, however, that the plunger(s)  24  is/are driven by the power end  14 , depicted in  FIG. 4 , which includes a crankshaft  36  and axially moveable plungers  24  which are driven by a drive train  40 . Each full rotational cycle of the crankshaft  36  is considered to be a revolution of the pump  10 . The suction manifold  18  is constantly fed with the fluid medium to be pumped. A minimum level of energy should be constantly kept inside the suction manifold  18 , which is normally accomplished by maintaining a sufficient minimum flow rate and supply pressure. 
     A pressure transducer  42  ( FIGS. 1-2 ) is operably associated with the suction manifold  18 . The pressure transducer  42  is also operably associated with a data processor  44  via transmission medium  46 . It is noted that, while transmission medium  46  is depicted as being an electrical cable, wireless transmission, of types known in the art, could also be used. The pressure transducer  42  is adapted to detect fluid pressure within the suction manifold  18  and transmit a signal representative of the detected pressure to the data processor  44 .  FIG. 5  is a schematic illustration of portions of a pump monitoring system in accordance with the present invention which includes a data processor  44  and pressure transducer  42 . 
     An accelerometer  48  is mounted upon or otherwise operably associated with the fluid end cylinder housing  20 , as illustrated in  FIGS. 1-2 . The accelerometer  48  is preferably a three-axis accelerometer and is designed to measure vibration of the cylinder housing  20  and provide a signal representative of detected vibration via transmission medium  50  to the data processor  44 . 
     A timing marker  52  is operably associated with plunger  24 . If there are multiple plungers  24 , only a single plunger need have a timing marker  52 . The timing marker  52  is operable to provide an indication of the speed of operation of the pump  10  by detecting movement of the plunger  24 . This speed measurement is transmitted to the data processor  44  via transmission medium  54 . In accordance with an alternative embodiment, the speed of the pump  10  is obtained by a rotational pick-up sensor (not shown), of a type known in the art, at the power end  14  of the pump  10 . 
     The data processor  44  is programmed to receive data from each of the pressure sensor  42 , accelerometer  48  and the timing marker  52  (or rotational pick up sensor). In particular embodiments, the processor  44  compares the fluid pressure detected by the pressure transducer  42  with a preprogrammed pressure which corresponds to the vapor pressure of the fluid being pumped by the pump  10 . When the detected fluid pressure approximates the vapor pressure, this condition is conducive to cavitation. In accordance with preferred embodiments, the processor  44  correlates the presence of a detected-pressure-approximating-vapor-pressure condition with an increase in vibration, as detected by the accelerometer  48 . A correlation of these two conditions will indicate the presence of cavitation in the pump  10 . In addition, the inventors have determined that such a correlation in increased vibration indicates cavitation to the exclusion of other abnormal pump conditions. Pressure and vibration per revolution (as measured by the timing marker  52 ) is done to detect cavitation. Preferably, the sensors provide measurements on a continuous basis, and the speed measurement provided by the timing marker  52  allows the continuous signals to be divided on a per revolution basis. 
       FIGS. 7-9  depict exemplary data measurements which might be obtained by a pump monitoring system in accordance with the present invention and illustrates detection of cavitation in a pump.  FIG. 7  is a data plot showing suction pressure within the manifold  18  as detected by the pressure sensor  42 . It can be seen that the detected pressure rises and falls over time as the intake valve  26  opens and closes. In the depicted plot, the vapor pressure of the fluid being pumped by the pump is represented by the line  58 . Data plot points below the line  58  are indicative of the detected pressure being below vapor pressure while those points above the line  58  are above vapor pressure.  FIG. 8  depicts transformed suction pressure data, with detected pressure being plotted against pump revolutions. Plot points  60  represent maximum pressure readings during each revolution of the pump  10 . Plot points  62  are average pressure readings per revolution while plot points  64  are minimum pressure reading per revolution. It is possible to detect when minimum suction pressure  64  is below vapor pressure consistently (more than  25  cycles). Point  66  represents a point where detected fluid pressure at the manifold  18  approximates vapor pressure  58  and is therefore a suspected point for the beginning of cavitation. 
       FIG. 9  is a data plot which depicts pump vibration amplitude, as measured by the accelerometer  48 , against pump revolutions. The upper group of data points  68  represents vibration (“G”s) in a positive direction while the lower group of points  70  represent vibration in a negative direction. Points  72  lie closest to the zero axis and represent average vibration. It can be seen from  FIG. 9  that the accelerometer  48  begins to detect vibrations resulting from cavitation at or slightly after the time when pressure in the suction manifold  18  reaches vapor pressure (point  74  in  FIG. 9 ).  FIG. 9  shows that it takes a few seconds (approximately 200 revolutions) for cavitation to cause significant vibration, which can be seen starting at about point  76 . 
       FIG. 10  is an exemplary logic diagram which depicts illustrative data measurement, acquisition and processing by an exemplary pump monitoring system. A data acquisition system  78  obtains measured parameters from the suction manifold pressure sensor  42 , accelerometer  48  and timing marker  52 . It is noted that the data acquisition system  78  may be contained within the general processor  44 . A processing unit  80 , which may be a programmable logic controller, then determines whether the minimum detected suction pressure (i.e., points  64 ) have reached or approximate vapor pressure  58 . This occurs in step  82  in  FIG. 10 . The processing unit  80  also determines (step  84 ) whether there is increased vibration, as detected by the accelerometer  48  at or shortly after. If so, the processing unit  80  logs the event and signals in memory at step  86 . 
     Optionally, the processing unit  80  is programmed to perform one or more operations that comprise corrective actions to try to cure the cavitation problem. The processing unit  80  can send a message to an operator (step  88 ) in the form of a visual or audible alarm, an electronic message or the like. This will allow the operator to adjust the pump parameters or suction pressure (step  90 ) to compensate for or correct the cavitation condition. Also optionally, the processing unit  80  might execute, or cause to be executed, central site and pump control software or individual pump control software (step  92 ). If the processing unit  80  then determines (step  94 ) that the cavitation condition is not resolved within a particular amount of time, such as 30 seconds, pump parameters are adjusted by the software (step  96 ) or the pump is shut down. 
     In accordance with the present invention, pump monitoring devices may be constructed which can be affixed to or located alongside a pump. These monitoring devices would include a processor  44  and the associated sensor components  42 ,  48 ,  52 . 
     The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention.