Abstract:
A system and method enable monitoring of a mechanical condition of a reciprocating compressor. The compressor includes a compressor frame, a pressure chamber, and a pressure head. A plurality of tie bolts are secured between the compressor frame and the pressure chamber head. A sensor assembly is secured to one of the plurality of tie bolts. The sensor assembly includes an elongation member positioned to extend in concert with extension of one of the tie bolts. A sensor measures extension of the elongation member to thereby monitor cylinder pressure.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system and method for monitoring a mechanical condition of a reciprocating compressor and, more particularly, to a non-invasive system and method that measures dynamic performance of a hyper-compressor cylinder by measuring the inherent operating strain of the compressor assembly. 
     The production of low-density polyethylene requires the use of very high pressures. Polymerization pressures can reach as high as 50,000 psi. To achieve these pressures, high pressure reciprocating compressors, or hyper-compressors, are used. Monitoring the mechanical condition of the hyper-compressor cylinder components during operation is important for determining maintenance requirements. That is, hyper-compressors are susceptible to similar problems as lower pressure reciprocating compressors, including, for example, valve failure, valve leakage, packing leaks and the like. The plungers used to compress the volatile gas are constructed of materials that have a high compressive strength but are brittle and typically will shatter when breaking. Valve failures can result in undue stress on the compressor running gear because of pressure unbalance or can result in loose pieces of the valve falling into the compressor chamber causing metal-to-metal contact on the plunger face that can cause bending and ultimate failure of the plunger. Such failure results in mechanical destruction of the compressor and in the volatile gases being released, which are susceptible to ignition, thereby causing a safety concern for those working in the vicinity of the compressor. Direct measurement of the high pressures to determine cylinder performance by penetrating the chamber is highly dangerous. Knowledge of the cylinder pressure gives insight into the valve and packing performance and the ability to avert many potentially catastrophic conditions. 
     The strain measurement of the compressor assembly has been done, as described in U.S. Pat. No. 7056,097, using strain gauges mounted on the head bolts or on the tie bolts, or compressive load measurements between the head and the head-bolt nut. The load measuring system used in the noted publication inserts an apparatus as part of the load bearing structure and is subject to deformation or crushing, which alone could be a safety concern. Externally mounted strain gauges require cleaning of the surface to support a good bond to the stressed element. Additionally, such strain gauges are temperature dependent. Moreover, gauges are small and require protection to survive in an industrial environment and are cyclically stressed when installed on an operating compressor. Clamp-on strain assemblies have been used, but they also require cleaning of the surface, and since the contact area is under stress, the contact surfaces will “creep” over time and under cyclic stress. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an exemplary embodiment of the invention, a system monitors a mechanical condition of a reciprocating compressor. The reciprocating compressor includes a compressor frame, a pressure chamber, and a pressure chamber head, wherein a plurality of tie bolts are secured between the compressor frame and the pressure chamber head. The monitoring system has a sensor assembly secured to one of the plurality of tie bolts, the sensor assembly including an elongation member positioned to extend in concert with extension of the one of the tie bolts and a sensor measuring extension of the elongation member. 
     In another exemplary embodiment of the invention, a system for monitoring a mechanical condition of a reciprocating compressor includes a rod clamp fixed to one of the plurality of tie bolts; a sensor clamp fixed to the one of the tie bolts and spaced from the rod clamp; a sensor mounted in the sensor clamp; and a target rod fixed to the rod clamp at one end and movably coupled with the sensor clamp at an opposite end proximate to the sensor. Elongation of the one of the tie bolts is measured by sensing, with the sensor, movement of the target rod opposite end relative to the sensor clamp. 
     In still another exemplary embodiment of the invention, a method of monitoring a mechanical condition of a reciprocating compressor includes the steps of (a) securing a target rod to at least one of the plurality of tie bolts; (b) measuring a target rod movement amount; and (c) correlating the target rod movement amount with a mechanical condition of the reciprocating compressor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary reciprocating compressor with an attached system for monitoring its mechanical condition; 
         FIG. 2  is a cross sectional illustration showing the operation of the monitoring system; 
         FIG. 3  is a detailed cross sectional view of the monitoring system sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The system and method described herein uses the inherent strain on some portions of the compressor assembly to determine the cylinder pressure. The inherent strain caused by the pressure results in a lengthening of the compressor structure, and the system described herein measures this change in dimension, which is linearly proportional to pressure. The system uses similar materials to the compressor so that any thermal expansion effects of increased temperatures are compensated for. Measuring the strain over a significant length of the structure allows a substantial increase in sensor sensitivity over a strain gauge that may have a relatively low sensitivity. Multiple sensing systems placed on a cylinder can be summed to add sensitivity and to average any “wag” in the cylinder that may cause unbalanced strain. “Wag” is side-to-side movement of the cylinder assembly that would cause unequal strain on opposite sides of the chamber. There is substantial support and stiffness in the vertical direction, but less in the horizontal direction 
     With reference to  FIG. 1 , an exemplary high pressure reciprocating compressor  10  is shown. The compressor  10  includes a compressor frame  12 , a pressure chamber  14 , and a pressure chamber head  16 . A plurality of tie bolts  18  are secured between the compressor frame  12  and the pressure chamber head  16 . The use and operation of the compressor  10  are known, and details thereof will not be further described. 
     With continued reference to  FIG. 1 , the system includes a rod clamp  20 , a sensor clamp  22 , a target rod  24 , and a sensor  26 . The rod clamp  20  is fixed to at least one of the plurality of tie bolts  18 . The sensor clamp  22  is fixed to the same tie bolt(s)  18  and spaced from the rod clamp  20 . A sensor  26 , such as a proximity probe or the like, is mounted within the sensor clamp  22 . One suitable sensor is the Bently Nevada NSV probe available from the Bently Nevada Corporation of Minden, Nev. An optical or other suitable sensor may alternatively be used. The target rod  24  is fixed to the rod clamp  20  at one end and is movably coupled with the sensor clamp  22  at an opposite end proximate to the sensor  26 . Elongation of the tie bolts  18  causes relative movement of the target rod  24  opposite end and the sensor clamp  22 . 
     The rod clamp  20  is preferably a two-piece clamp having an inside diameter slightly smaller than the tie bolt  18  to which the rod clamp  20  is fixed. A protrusion from the clamp is threaded to accept the target rod  24 . The target rod is preferably necked down and threaded on one end to fasten to the rod clamp  20  and to be perpendicular to the clamp  20  with the necked down shoulder being perpendicular to the target rod axis. 
     The opposite end of the target rod  24  is perpendicular to the axis of the rod and is used as a measurement target. A circumferential groove on the rod  24  is used to indicate the correct insertion depth into the sensor clamp  22 . With reference to  FIG. 3 , the sensor clamp  22  is similar to the rod clamp  20  except the protrusion contains a cavity  28 , lined with a plastic sleeve to accept the target rod  24  and has a threaded length to support the sensor  26 . Measurement is made of the sensor clamp position relative to the axial face of the target rod  24 . 
     To install the system, the rod clamp  20  is first placed near one end of the tie bolt  18  and secured to the tie bolt  18  by tightening the clamp bolts. The target rod  24  is then firmly tightened into the rod clamp  20  (parallel to the tie bolt  18 ). The sensor clamp  22  is then installed on the tie bolt  18  and slid toward the target rod  24  until the target rod extends into the sensor clamp  22  to the depth indicated by the groove on the target rod  24 . A sensor  26  is then assembled into the sensor clamp  22 , and when inserted to the starting depth, the sensor is firmly secured in place with a jam nut against the sensor clamp  22 . The process is repeated for each tie bolt  18  fitted with a device. The described method is exemplary as those of ordinary skill in the art will appreciate alternative methods or differently ordered steps, and the invention is not necessarily meant to be limited to the described method. 
     With reference to  FIG. 2 , during operation of the compressor, a force on the plunger  30  (force  1 ) creates gas under pressure in the cylinder in an equal but opposite direction. The resulting stress forces (force  2 ) are passed from the cylinder head  16  to the supporting tie bolts  18  (force  3 ) in proportion. This stress results in strain on the tie bolts  18  that causes them to lengthen according to Young&#39;s modulus for the bolt material. The total growth between the rod clamp  20  and the sensor clamp  22  is directly related to the growth of the tie bolt  18  between the two clamps  20 ,  22 . Because the target rod  24  is stationary with respect to the rod clamp  20  but able to freely slide in the sensor clamp  22 , the distance from the sensor face to the target rod axial face will change the same as the strain induced growth of the tie bolt  18 . 
     The sensor  26  measures this increase in the space between the sensor  26  and the target face of the target rod  24 , which is related directly to the strain on the tie bolt  18 . The sensor  26  outputs its measurement to a processor such as a CPU or the like, and the processor generates output curves similar in appearance to classical pressure-volume (PV) diagrams, which are used to assess a condition of the compressor. 
     The compression cycle generates heat, a portion of which is radiated by the compressor cylinder and the head. As the tie bolts  18  are heated, they expand as a result of their coefficient of linear expansion. This thermal growth will also lengthen the distance between the rod clamp  20  and the sensor clamp  22 . As the temperature is radiated from the cylinder, the target rod  24  also grows in response to the temperature change, and the growth reduces the distance between the sensor  26  and the target rod face, thus compensating for the ambient temperature of the device and the tie bolts  18 . This is an important feature as only measuring the distance between the clamps (for example, with a laser) would not compensate for the thermal growth. The installation of the target rod  24  provides that function. 
     The balancing of the forces in the tie bolts  18  causes the sum of all of the bolt stress to equal the driving force, although this does not guarantee that it is equally divided among all of the tie bolts  18 . The sensor outputs from a number of sensors  26  may be summed to total the force acting on the cylinder. 
     The monitoring system described herein uses the inherent strain on portions of a compressor assembly to determine cylinder pressure. By monitoring cylinder pressure, valuable information about the valve and packing performance can be obtained, thereby reducing the risk of a catastrophic condition. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.