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Pressure Fatigue Testing
2019/09/06
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As part of corporate Technical Services and Quality (TS&Q), the Applied Mechanics Fatigue Laboratory (Sidney, Ohio) conducts pressure fatigue testing for Emerson Climate Technologies. Pressure fatigue testing is one among many fatigue-testing services provided by the Laboratory. The Laboratory has extensive fatigue testing and analysis experience and utilizes eight MTS, servo-hydraulic fatigue testing stations with a capacity range of 1 Kip to 22 Kip,and eight electro-dynamic shakers with a range of 75 lbf to 12,000 lbf to conduct fatigue testing (Figure 1). Applied Mechanics is a certified UL DAP [1] Laboratory for pressure fatigue testing. It conducts pressure fatigue testing for UL and PED [2] Certification purposes as well as for engineering design optimization. To this end, Emerson has invested significant time and capital into making the pressure fatigue test a ‘state of the art’ technology. Especially with the changes to higher-pressure refrigerants, such as R-410A, the integrity of the pressure containing members requires increased design effort and verification.
The Applied Mechanics Fatigue Laboratory has now two test units dedicated to pressure fatigue testing; each unit contains three stations, providing the capability for testing six specimens simultaneously. Both units share one common hydraulic power source consisting of a high-pressure pump capable of providing hydraulic fluid at 30 GPM at pressures up to 3000 psi. The first unit has been in service since 2001. With the increased demand for engineering verification and plant audit testing, a second unit was approved and placed into service in 2009. Both units are computer controlled and, depending on compressor enclosure volume, achieve cycle rates up to 3 Hz. The pressure fatigue testing service ensures quality products through verification testing, and provides opportunity for efficient product reliability optimization with quick turn-around times. In turn and more importantly, these testing services mean that customers can be sure that the technology is in place to assure reliable, quality products.
WHY CONDUCT PRESSURE FATIGUE TESTING?
As is the case in many applications, structural fatigue is of concern when cyclic loading is in effect. Cyclic loads can lead to a structural breakdown in the form of cracks and ultimately into complete separation of a part. There are three stages in fatigue cracking, first, local fatigue damage leads to the formation of a crack. During the second stage the crack grows and propagates through the material. This crack can grow to a critical flaw size where loading can no longer be sustained, resulting into complete separation of the part as the third and final stage. In the case of a pressure vessel, a crack typically starts internally and grows to the surface which then causes a leak and relieves the internal pressure of the fluid (“Leak before break”). “Leak before break” is what is typically sought in pressure vessel designs and this requires ductility as one of the properties of the vessel material.
With the change to R-410A, higher pressures and larger pressure cycles had to be sustained safely. Therefore, pressure fatigue testing became more important to design evaluation and for ensuring customer satisfaction. The fatigue test was therefore adopted by UL and included in the UL1995 standard [3]. As may be expected, the pressure fatigue test unveils the part of the pressure containing structure most susceptible to fatigue damage. When necessary, this information is used to make the design more robust. Ultimately then, the importance of conducting pressure fatigue testing properly and accurately is very important.
PRESSURE FATIGUE TESTING AT EMERSON
To have confidence that the enclosure is capable of sustaining start/stop cycle pressures for the duration of its service life, UL 1995 offer two alternatives. UL certification of the pressure containing members requires enclosures to sustain a test pressure of 5X the maximum normal working pressure. This ensures stresses remain low enough to provide infinite life. Alternatively, compressor enclosures are to sustain a test pressure of 3X the maximum normal working pressure and in addition, be able to sustain a fatigue test. Certification to a UL fatigue test requires three compressor enclosures to sustain 250,000 fatigue cycles at specified pressure ranges (See UL1995, section 62). This equates to 45 start/stop cycles per day for 15 years, each cycle at the full pressure excursion. Typically, the full pressure excursion would not occur during each start/stop cycle.
Upon completion of the pressure fatigue cycling, the three enclosures must sustain a pressure twice that of the upper cyclic pressure. This pressure is to be maintained for 1 minute without burst or leak. If three enclosures of the identical design complete the fatigue test and final cycle pressure without leaks the design is acceptable for UL Certification.
However, our company uses an additional factor of safety that leads to a requirement of 500,000 fatigue cycles at a significantly larger pressure cycle than required by UL1995. Annual audits are conducted on three production samples to ensure production quality of the enclosure. UL1995 also stipulates that the upper and lower pressures in each cycle must be maintained for 0.1 seconds. Hence, when a compressor enclosure passes the above-defined pressure fatigue test, one can have confidence that the enclosure meets the regulatory requirements in the ruggedness from a pressure fatigue perspective.
THE EQUIPMENT
Figure 2 pictures the newest of the two pressure fatigue test units. Unit 1 was designed for and in accordance to Emerson specifications. The second unit’s design incorporates several advances, including PID control of the pressure cycles. Each station uses an individual and independently controlled servo valve, which regulates flow and pressure into the compressor enclosure. In contrast to using one servo valve to control all three stations, the independent control has two advantages.
Figure 2: The Second Applied Mechanics Pressure Fatigue Test Unit Contains Three Independently Controlled Stations. The Unit Accommodates Larger Enclosures then the Original Unit and Has an Improved Controller. First, up to three different pressure fatigue cycles can be achieved in the same unit. Second, if a leak develops in one station, neither of the other two stations is affected, and the full pressure cycle is maintained.
The design of these units allows for efficient loading and provides non-strenuous access to the technician through two large front doors. Removable top panels provide overhead crane access for loading large and heavy compressors. Lexan door panels provide safe viewing of the specimens and equipment. To accommodate positioning and connection of the compressor housing to the supply manifold, the manifolds are installed on a boom.
The top of the boom supports a rail for easy front-toback positioning of the manifold. An air cylinder
provides a six-inch adjustment in the vertical direction. If additional vertical range is needed, the
whole boom raises and lowers along a shaft in the rear of unit. (Figure 3).
A pressure transducer in each station’s manifold provides feedback at sample rates between 250 Hz and 1 kHz, depending on the station’s cycle rate. This feedback, used to control the servo valves, also provides the required data for subsequent analysis.Calibration of these all important pressure transducers is performed, not on a yearly, but quarterly basis.
Additionally, procedures are in place to compare the pressure transducer output to expected values using a shunt resistor calibration prior to each pressure fatigue test. The quarterly calibration is performed on the chain of equipment used in the pressure data measurement and reporting. The chain consists of the pressure transducer and its output, the signal conditioning and data acquisition system and the data acquisition program used to ‘translate’ the voltage into the corresponding pressure values.
To calibrate the chain, the pressure transducer is pressurized and the corresponding readout obtained through the equipment chain is compared to the inhouse standard. The in-house standard is calibrated yearly at an A2LA accredited laboratory and this provides traceability of the in-house calibration. In addition, and in accordance with the ISO 17025 requirements, the pressure measurement uncertainties at the 95% (2k) confidence level are evaluated.
Figure 3: Supply Manifold and Connection to the Compressor Enclosure.
CONTROL AND DATA ACQUISITION SOFTWARE
The control and data acquisition software for the original pulse tester was mostly written in-house using National Instruments’ LabView?. Basically, digital waveforms are generated by the program and sent to an Analog to Digital Converter. The Analog waveform is provided to the hydraulic supply servo-valve, while the pressure transducer response is returned to the data acquisition software, via a data acquisition board, then analyzed and displayed. The technician will adjust the amplitude and mean of the digital output signals until the required pressure response is achieved. Different waveform shapes are available to accommodate different shell sizes. Smaller volumes typically require slower signal rise and fall times to limit overshoot. This is demonstrated in Figure 4 where the pressure feedback signal for station 2 has a slower rise time than the signal from station 3, even though the command signals have the same rise times.
Waveforms are stored periodically and used in a statistical analysis to determine dwell pressures. As stated previously, UL 1995 requires maintaining the upper and lower pressure in each cycle for 0.1 seconds, herein referred to as the dwell pressure (Figure 5). The software checks upper and lower dwell pressures and also checks whether the 0.1 second hold time is met on every cycle. This information is communicated to the technician in the form of digital ‘traffic lights’. Green indicates both the pressure and dwell time is achieved, yellow indicates a warning; the dwell time is not met. Each station has separate indicators for the upper pressure and lower pressure requirement.
The dwell pressure analysis is performed periodically over a 10-waveform sample. While minimal effort is required to perform the analysis on the data from the first unit, it is fully automated in the new unit. The analysis determines the upper and lower dwell pressure bounds and uses standard statistical methods to provide the 5th and 95th percentile confidence bound for the upper and lower dwell pressures (Figure 6). The resulting information is stored electronically and provided to engineering and UL as required.
THE UL DAP PROGRAM
In addition to the investment in equipment, Emerson expends significant effort to ensure technicians train in the requirements for properly conducting pressure fatigue testing, which ensures test quality and data integrity.Recently, this effort expanded to include all the ISO 17025 requirements.
The original pressure fatigue test unit was certified by UL after it was placed into service in 2001. Thanks to continuous improvement efforts and following the 2009 UL audit, the original unit was accepted into the UL DAP program in May 2009, while the second unit was entered into UL DAP program in June 2009.
UL requires that the laboratory operations are thoroughly documented and that procedures and policies are in place to assure accuracy, precision, and quality of the test and test results. The system, based on ISO 17025, encompasses the test equipment, environment, standards and procedures as well as personnel qualifications, data recording and reporting procedures and calibration. All documentation is stored in an electronic database, where each document undergoes two levels of approval before being accepted.
SOME ENGINEERING TEST EXAMPLES
The pressure fatigue test is used during the product development cycle to validate design and production parameters. FEA results and the Battelle Structural-Stress Fatigue method are combined to predict the enclosure’s fatigue life. These results are compared to those obtained from the pressure fatigue test, so that the design can be made more robust through FEA design iterations (Figure 7). Production parameters, such as welder settings can have a large influence on the fatigue behavior of a weld seam. Pressure fatigue tests were used in one development cycle to improve robustness of the enclosure as well as its weld seam (Figure 8). Initial test results indicated that an enclosure radius was susceptible to fatigue cracking. After a geometry change, the weakest link in the fatigue resistance became the enclosure’s weld-seam. Changes in weld parameters further increased the robustness of the enclosure, driving the weakest link back to the enclosure radius with a significant improvement in fatigue life.
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