Design And Manufacturing Of An Accelerated Lifetime Tester For Non-Metallic Materials Used For Compressor Valve Sealing Elements

It is quite difficult in actual field applications to properly assess the lifetime of non-metallic sealing elements for valves used in reciprocating compressors. This difficulty is due to the wide variety of operating conditions and contaminants these sealing elements encounter while in service. If one is to determine which of these non-metallic materials will function optimally as sealing elements, a standardized method for quantifying the lifetime of these materials must be developed. Further, due to the large number of new non-metallic materials available, this standardized test method must be capable of reaching conclusions rapidly. Numerous design approaches were considered, including an existing design for this application. It was determined, however, that a completely new approach should be employed for this standardized test method of producing controlled sealing element failure. Theoretical models were generated to assure the new device would produce appropriate impact velocities to generate failure in a timely manner. Upon analysis of these models, hardware designs were developed with the adaptation of a large bore V8 engine at the center of this new machine design. The use of a V8 engine also has the added benefit of allowing for the examination of up to four test materials simultaneously. Special attention was given to the minimization of the clearance volume for the test section and associated piping, as well as to the temperature control of the system. The new machine was also analyzed theoretically to determine the effects of varying the pressure, lift and RPM on overall lifetime of the test materials. After a thorough analysis had been performed, the system was manufactured. Testing of the new system showed that the increase in capacity did not diminish its performance when compared to an existing device used for similar applications. Additionally, this new lifetime tester was capable of a wider variety of operating conditions than the existing device, allowing for tailoring the machine for the material to be tested. This study focuses on the theoretical development, manufacturing and commissioning of a new concept for the lifetime testing of non-metallic sealing elements. INTRODUCTION A vital component of a reciprocating compressor is the valve sealing element. Historically, these sealing elements have been metallic plates or rings. After the advent of non-metallic materials (plastics) in the 1950s and 1960s, the reciprocating compressor industry recognized the importance of these new materials as sealing elements. Early non-metallic sealing elements were usually some form of laminated material, such as thermoset. The advantage found in impact resistance of non-metallics plates/rings soon became evident and non-metallics became the material of choice in the reciprocating compressor market. Today, the reciprocating compressor market not only demands long lifetime from these non-metallic sealing elements, but also demands high strength to withstand high differential pressures and new dynamic unloaders, such as found in hydraulically actuated devices. These exigent demands can only be accomplished by the use of highly engineered non-metallic materials. This market demand has led to the development of materials for various temperatures and the use of the highly unique PEEK material for reciprocating compressors. To increase the strength of these non-metallic materials, glass and carbon fibers were introduced into the matrix materials. Thus, with the impact resistance advantage and the improved strength non-metallic materials have gone from novelty to state of the art for valve sealing elements. There are constantly new non-metallic materials and material combinations for non-metallics developed which could be used as possible candidates for sealing elements. For evaluation of these new materials in the harsh environment of a reciprocating compressor, a test method has to be defined. One method would be to produce a number of sealing elements using these test materials, along with the current standard nonmetallic material, and send these samples to the field for evaluation of their relative performance. This approach, however, is impractical due to the large number of candidate materials and the length of time needed to properly assess their performance. Perhaps another method for determining the best materials is to allow the competition to determine the best materials and copy their product line. This method, however, would not be desirable for any company that is, or wants to be, a leader in the field. This problem was first addressed in Vienna, Austria in the early 1990s by an accelerated lifetime testing machine. The device uses a compressor to accelerate a plate between two seats at high lift and, therefore, relatively high impact velocities. This accelerated lifetime testing machine has been utilized by Vienna to standardize on three main non-metallic sealing element materials, with either glass or carbon reinforcing fibers. Although this device has served well, it has become evident that with the large number of new materials to test, it has been proven to be inadequate to meet the demands of the research and development groups. It was for this reason that it was decided to pursue a new concept for the accelerated lifetime testing of non-metallic sealing elements. THEORETICAL DEVELOPMENT The accelerated lifetime tester in Vienna was to serve as a basis for the new design of the machine. The major difference between the existing accelerated lifetime tester and that envisaged locally is that this new system would be capable of testing four plates simultaneously. To test four plates simultaneously, a V8 engine was chosen for the drive system. Further, it was desired to decrease the time needed to cause failure of the sealing elements. Several options were available for this undertaking. First, the lift could be increased, or made variable. Another option would be to increase the operating pressure. Finally, the RPM could be increased, or made variable, thereby increasing the number of impacts for a given amount of time as compared to the Vienna device. It was determined that a big block V8 was to be used for the “compressor” since this engine has cylinder volumes necessary for this type of device. The head and pushrods were removed because these components would not be required for the accelerated lifetime tester. Assuming that this V8 engine is to be used as the centerpiece of this new design, an electric motor would need to be chosen to drive this engine. To determine the required horsepower of the electric motor to drive the V8 engine, figure 2 was employed [ref. 1]. Assuming that we would want to drive the V8 at a maximum of 3000 RPM – 3500 RPM, it was determined that a 40 hp electric motor would be sufficient and give a safety margin to account for the difference in size of the V8 engine used for the compressor and those reported in figure 2. Theoretical Model for Predicting Impact Velocities To assess the impact velocities one can expect with the new accelerated lifetime tester, Newton’s Second Law of Motion can be expressed in terms of displacement x as: