Patent Publication Number: US-2011072964-A1

Title: System for acquiring, measuring and checking the operating parameters of a reciprocating fluid machine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention refers to a system for validating or checking a reciprocating fluid machine, in particular but not exclusively a reciprocating compressor of the type with single-acting cylinders connected to a cross-head. 
     2. Description of the Prior Art 
     As known, a compressor is a machine capable of raising the pressure of a compressible fluid (gas) through the use of mechanical energy. In reciprocating compressors the compression of the fluid is carried out by one or more pistons, moving with reciprocating motion inside a respective cylinder. The fluid to be compressed is taken into the cylinder through one or more intake ducts, whereas the compressed fluid is expelled from the cylinder towards one or more delivery ducts. Commonly, the piston or the pistons of a reciprocating compressor are actuated by electric motors or else by internal combustion engines, through a crankshaft for transmitting the motion and a conventional crank and connecting rod mechanism. 
     For example, in double-acting reciprocating compressors each piston does not carry out any “loadless” strokes, since it compresses the gas in both of its directions of motion. Precisely for this reason, the piston itself cannot be directly connected to the connecting rod of the crank and connecting rod mechanism, because the closed-type cylinder would not allow the connecting rod to oscillate. Thus a mechanism known as “cross-head” is placed between the piston and the connecting rod. The piston is connected to a rod, mobile exclusively with rectilinear motion, and the rod is connected to the cross-head. The stem does not therefore oscillate whereas the connecting rod, connected to the other side of the cross-head, can on the other hand oscillate freely. 
     The cross-head, through the sliding blocks with which it is provided, is able to slide inside suitable fixed guides, called “runners”, which allow its movement in the same direction of stroke of the piston. Given that the outer surfaces of the sliding blocks of the cross-head move with respect to the inner surfaces of the relative guides, it is necessary to introduce lubricant oil that prevents them from making contact with each other. The lubrication system is of the forced type and, by providing the cross-head with support mainly of the hydrostatic type, it prevents the wearing of the moving parts involved. 
     During the reciprocating movement of the cross-head there can be particular operating conditions, for example when the piston moves at low speed, in which the cross-head is unable to feed a sufficient amount of lubricant oil. In such a situation the layer of lubricating oil becomes extremely thin and there can be losses by friction, with consequent production of heat and increase in the temperature of the lubricant oil itself that reduce its viscosity, further decreasing the lubricating capabilities. In the worst situations, the surfaces can even come into contact with each other, with consequent possible damage to the fluid machine. 
     Both the size and the set-up of a reciprocating compressor, whether it is single or double-acting, must therefore be carried out in a particularly precise manner, so as to avoid the occurrence of criticalities in the compressor itself when it operates in limit operating conditions. Consequently, the step of determining the operating parameters of a reciprocating compressor is extremely important, said parameters including the loads that weigh down upon the shaft of the piston and on the cross-head, the temperature, the pressure and the flow rate of the oil to lubricate the cross-head, the rotation speed of the crankshaft, the clearances between the sliding blocks and the runners and others. 
     Document WO 2005/108744 A1 illustrates an apparatus and method for checking the operation of a reciprocating compressor. The apparatus comprises a mobile element operatively connected to one of the moving components of the compressor, like for example the cross-head. The mobile element is provided with a sensor capable of detecting the parameter or parameters of interest, which will then be sent to an external data processing unit for the necessary evaluations. The reciprocating compressor must be complete in every part thereof in order to be able to correctly provide the required parameters. 
     Document WO 2008/157496 A1 describes a method for calculating the operating parameters of a reciprocating compressor. The method foresees the use of a program for a processor capable of simulating the operating conditions of the compressor. However, also in this case the basic parameters must be directly obtained from a real operating reciprocating compressor. 
     The known apparatuses and methods thus foresee the need to be able to have a reciprocating compressor, complete in every part thereof and operating, in order to be able to evaluate its characteristic parameters. This means that, in the case of parameters that are incorrect or not corresponding to those foreseen, modifications, even substantial ones, must be made to the machine, like for example the replacement of some of its fundamental components, with a consequent increase in costs and set-up time. 
     Moreover, the presence of the cylinder and of the relative systems for gas circulation, for cooling and for lubrication, although necessary when the compressor operates in normal mode, requires that the measurement of the parameters be carried out in a very complex testing environment, with consequent high energy absorption by the compressor itself, equal to what occurs in normal operating conditions of the machine, with consequent need for a motor of suitable power. 
     SUMMARY OF THE INVENTION 
     The general purpose of the present invention is therefore to make a system for acquiring and monitoring the operating parameters, applied in particular but not exclusively to a reciprocating compressor of the aforementioned type, which is able to solve the drawbacks quoted above of the prior art in an extremely simple, cost-effective and particularly functional manner, irrespective of the technical characteristics of the respective cylinders, thus being highly versatile. 
     Another purpose of the invention is to make a system for acquiring, measuring and checking the operating parameters of a reciprocating fluid machine that does not require the preparation of complete prototypes in order to work out its operating characteristics, being able to replace at least some of the component or systems normally present on each machine to be analysed, like for example the cylinders, the pistons, the valves and the gas circuits, cooling circuits and lubrication circuits, with simpler and interchangeable components. 
     A further purpose of the invention is to make a validation system that is easy to manufacture and that does not require complex and expensive actuations systems in order to be put into operation, since the energy required for its operation is much less than that necessary to make a complete reciprocating machine work. 
     These purposes according to the present invention are accomplished by making a validation system for a reciprocating fluid machine, in particular but not exclusively a reciprocating compressor, as outlined in claim  1 . 
     Further characteristics of the invention are highlighted by the dependent claims, which are an integral part of the present description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics and advantages of a system for validating, measuring and checking the operating parameters of a reciprocating fluid machine according to the present invention will become clearer from the following description, given as an example and not for limiting purposes, referring to the attached schematic drawings, in which: 
         FIG. 1  is a schematic section view that shows the main components of a reciprocating compressor; 
         FIG. 2  is a schematic view that shows the operating principle of a reciprocating compressor; 
         FIG. 3  schematically shows a reciprocating compressor, in a configuration with many cylinders; 
         FIG. 4  is a perspective view (flywheel side) that illustrates a non-limiting example embodiment of a system for validating the operating parameters of a reciprocating fluid machine according to the present invention; 
         FIG. 5  is another perspective view (cam side) of the system of  FIG. 4 ; 
         FIG. 6  is a side elevational view (flywheel side) of the system of  FIG. 4 ; 
         FIG. 7  is a plan view from above of the system of  FIG. 4 ; 
         FIG. 8  is an enlarged side elevational view (cam side), of the system of  FIG. 4 ; and 
         FIGS. 9 to 11  show the operating principles of a system according to the present invention. 
     
    
    
     It should be noted that, in each figure, the same reference numerals correspond to the same systems or components of the previous and/or subsequent figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference in particular to  FIG. 1 , the essential elements present inside a reciprocating fluid machine, in this case represented by a reciprocating compressor of the double-acting type, are shown schematically. 
     The compressor comprises a shaft  10  with at least one crank, connected to a connecting rod  12  that transfers the rotary motion of the shaft  10 , actuated by a generic motor  36 , electric or thermal, through the interposition of a flywheel  38  ( FIG. 3 ), to a cross-head  14  through a pin  16 . The cross-head  14  has the task of converting rotary motion into reciprocating motion, being forced to move inside suitable opposite guides or runners  18  and  20  that allow it to move in the same direction as the stroke of the piston  22 . A shaft  24 , provided with a sealing system  44  consisting of a stuffing box, connects the cross-head  14  to the piston  22 . The piston  22 , moving inside the cylinder  26 , is therefore able to compress the gas. 
     The gas to be compressed, at a certain intake pressure, is introduced inside the cylinder  26  through one or more intake valves  28  and  28 ′ and is then compressed by the piston  22  so that it reaches a desired final pressure value. Once the gas has reached such a final pressure value, it is expelled from the cylinder  26  through one or more discharge valves  30  and  30 ′. In a double-acting cylinder  26 , like the one schematised in  FIG. 1 , the compression takes place inside two distinct chambers, in other words the chamber  32  facing towards the head of the cylinder  26  and the chamber  34  facing towards the cross-head  14 . 
     The compressor can be of the single-cylinder type or else it can have many cylinders  26 , for example horizontal and opposite as shown in  FIG. 3 . A lubrication system  40  and a water-operated cooling system  42  complete the compressor and make it possible for it to work at the different rotation speeds. 
     The assembly of mechanical components of the compressor, in other words the shaft  10 , the connecting rod  12 , the shaft  24  and the cross-head  14 , with the elements associated with it that ensure its reciprocating movement, can be identified as the frame  46  of the compressor itself. The frame  46  can thus be distinct from the cylinder  26  as a whole, i.e. provided with all of the hydraulic channels  28 ,  28 ′,  30 ,  30 ′,  40  and  42  associated with it, since there is no circuit for the circulation of the gas in it. 
     Now with reference to  FIGS. 4 to 10 , a preferred embodiment of a system for validating, acquiring, measuring and checking the operating parameters of a reciprocating fluid machine according to the present invention is shown, wholly indicated with reference numeral  50 . 
     The system  50  preferably comprises a base plate or a foundation  52  on which the components necessary for the operation of the system  50  itself are installed. Thus the base plate  52  has at least one frame  46  of a reciprocating fluid machine, in particular a reciprocating compressor, removably applied to it, where by frame  46  we mean the assembly of all of the components of the machine that do not comprise the circuits for the gas (see previous description). More specifically, the frame  46  comprises at least one connecting rod  12  and a cross-head  14 , mobile with reciprocating motion and fixedly connected, at least one of its two ends, to a shaft  24  that in turn is mobile with reciprocating motion. 
     The components  12 ,  14  and  24  of the frame  46  are moved with reciprocating motion by means of a first crankshaft  54 , rotatably coupled with the base plate  52 . In turn, the shaft  54  is set in rotation, based on a predefined angular velocity but in any case variable, by an actuation motor  56 , preferably electric, through the interposition of an actuation system  58  supported by the base plate  52 . 
     In the illustrated example embodiment, the actuation system  58  consists of a system of rigid connecting rods  58 ′ and of respective crankshafts  58 ″, actuated by the motor  56  through a belt  60  and a flywheel-pulley  62 . The flywheel  62  is connected to the first crankshaft  54  through the central crankshaft  58 ″. 
     According to the invention, on the base plate  52  of the system  50  at least one system for generating loads  80  is mounted that is suitable for replicating the loads of the gas acting on the frame  46 , as occurs in the real fluid machine. 
     In a particularly advantageous embodiment of the invention, the system for generating loads  80  is of the mechanical type. Clearly it should not be ruled out that such a system for generating loads  80  can be of any other type, like for example hydraulic, electric, pneumatic or other. 
     In the preferred embodiment, the system for generating loads  80  comprises an elastic or yielding element  64  placed in operative connection with at least one eccentric or cam  66 . 
     More specifically, each elastic element  64  is interposed between each shaft  24  and each eccentric  66  (see diagrams of  FIGS. 10 and 11 ) and is compressed and released in sequence based on the reciprocating motion of the shaft  24  of the frame  46  and the rotary motion of the eccentric  66 , which occur simultaneously. The assembly consisting of the elastic element  64  and the eccentric or cam  66  is thus able, once actuated by the motor  56  and after having been suitably calibrated, as will be specified more clearly hereafter, to simulate the load of the cylinder  26  of a generic reciprocating fluid machine, like for example the reciprocating compressor described earlier. In practice, each elastic element  64 , made in the form of a coil spring operating by compression in the illustrated example embodiment, is able to reproduce the same forces that act on the components of the reciprocating compressor (in particular on the cross-head  14  and on the relative pin  16 ), in the operating conditions of interest. 
     A yielding element  74  is advantageously and preferably interposed between the eccentric  66  and the spring  64  to decrease the friction and promote the contact between the cam and the spring. 
     In an advantageous embodiment, a further elastic element  82  ( FIGS. 5 and 8 ) is associated with the shaft  68  of the cam and with the shaft of the yielding element  74 , so as to ensure the contact between yielding element  64  and cam  66  in every operation condition. This elastic element  82  can be adjusted through an adjustment element  78  that, in the case shown in the Figures, is made with a screw  78  acting on a plate  76 . 
     In this way it is possible to uncouple the rigidity of the main spring  64  (designed so as to obtain the desired load on the cross-head  14 ) from that necessary to maintain the contact between yielding element  64  and cam  66  for any rotation speed. 
     The system  50  is advantageously provided with one or more sensors  84  capable of acquiring and measuring the operating parameters relative both to the frame  46  and to the assembly consisting of the spring  64  and the cam  66 , which reproduces the cylinder  26 . Such parameters comprise: 
     clearances (in thousandths of a millimetre) between sliding blocks of the cross-head  14  and relative runners  18  and  20 ; 
     crank angle (θ); 
     forces (Fθ) acting upon the cross-head  14  and upon the shaft  24 ; 
     rotation speed (ω) of the shaft  10  and linear speed (Vθ) of the cross-head  14 ; 
     temperature, pressure and flow rate of the lubricant oil present in the gap between sliding blocks of the cross-head  14  and relative runners  18  and  20 . 
     All of the parameters acquired and measured by the sensors  84  can be sent, through a suitable wireless communication line  70  or via cable, to a central processing unit (not shown), capable of recording and graphically representing, in real time, such parameters according to each single revolution of the shaft  54 . 
     The system  50  according to the invention is therefore capable of carrying out validation tests of reciprocating fluid machines such as compressors, for example in the operating conditions known as full load or partial load, without the need to also make and actuate the cylinder or the cylinders  26  of the machine with the relative gas delivery and expulsion circuits, which use up a substantial amount of energy. One of the advantages of the system  50  is indeed the fact that the actuation motor  56  can deliver a substantially lower power with respect to what is required of a normal motor  36  able to be used for normal operation of a complete reciprocating compressor. This is essentially due to the fact that during the operation of the system  50 , when the spring  64  is released during the return stroke of the cross-head  14 , the spring  64  itself returns the energy absorbed in the outward stroke (see diagrams of  FIG. 11 ), decreasing the power requirement that the actuation motor  56  must deliver. 
     In the case in which measurements and/or validations have to be carried out on different machines or for different requirements, in addition to the possibility of removing the frame  46  from the system  50  (as mentioned earlier and as shown, for example, in  FIG. 5 ), it is also possible to remove the springs  64  and/or the cams  66  from the system  50 , replacing them with other springs and/or cams of a different type. For example, it is possible to use springs  64  with different dimensions and elastic coefficient, or else differently designed cams  66 . 
     Alternatively or in addition, again in order to vary the test conditions of the reciprocating machine, the system  50  can be provided with one or more preloading devices  72  acting upon each spring  64 . In detail, based upon the illustrated example embodiment, the preloading device  72  ( FIG. 8 ) consists of a rotary ring nut system, operating according to the screw-nut principle, interposed between the shaft  24  of the frame  46  and the relative spring  64 . The preloading device  72  is thus able to increase or decrease the compression on the spring  64 , respectively, by bringing the shaft  24  closer to or farther from the spring  64  itself. 
     The system  50  according to the invention can be made based on different embodiments from the one described and illustrated up to here, whilst still maintaining the basic operating principles shown in the diagrams of  FIGS. 9-11 . For example, the transmission of motion from the actuation motor  56  to the shafts  54  and  68  could take place through a series of linkages  58  different from the rigid connecting rods  58 ′ and from the respective cranks  58 ″. The linkages  58  could indeed consist, according to the technical requirements, of a system of belts, chains and/or gears interposed between the actuation motor  56  and the shafts  54  and  68 , or else other equivalent systems. Similarly, a different number of motors can be foreseen. However, it should be specified that the system consisting of the rigid connecting rods  58 ′ and the respective crankshafts  58 ″ is considered to be preferable to ensure the perfect phased rotary movement of the shafts  54  and  68  that respectively control the reciprocating movement of the frame  46  and the rotary movement of the cams  66 . 
     It should also be noted that it is possible to make a frame  46  that reproduces to scale a frame of a real compressor. In this way the system  50  is able to carry out validation tests on a scale prototype of the real compressor, substantially decreasing its manufacturing and installation costs and at the same time increasing its versatility. 
     The system according to the present invention is particularly useful for the acquisition, measurement and checking of the operating parameters of a reciprocating fluid machine for high performance in industrial systems or plants where making the circuit for the gas is particularly complex and expensive, like for example in industrial plants for producing low density polyethylene that work at pressures of up to about 3500 bars or more. 
     Clearly it should not be ruled out that such a system can be used on reciprocating machines or industrial plants of a different type. 
     The system for acquiring, measuring and checking the operating parameters of a reciprocating fluid machine of the present invention thus conceived can in any case undergo numerous modifications and variants, all of which are covered by the same inventive concept; moreover, all of the details can be replaced with technically equivalent elements. In practice, the materials used, as well as the shapes and sizes, can be whatever according to the technical requirements. 
     The scope of protection of the invention is therefore defined by the attached claims.