Test design for Tyco pressure relief valve

Transcription

1 Test design for Tyco pressure relief valve Design Team Ben Felix, Adrienne Jalbert, Ryan Lussier Josh Martin, Alicia Zollinger Design Advisor Prof. Mohammad Taslim Abstract Pressure relief valves with soft seals sometimes stick to the metal valve seat preventing the valve from opening at the correct pressure. This can cause serious damage to the system that the valve is supposed to be protecting. The design team assisted Tyco Flow Control in designing a test to identify the cause of sticking for the Electric Power Research Institute (EPRI). Specifically Crosby Series 800/900 valves were tested. Air was the working fluid. The variables tested include the amount of time the valve has remained closed, the material of the soft seat O-ring, the durometer of the O-ring, and the set pressure of the valve. Tests were performed at Tyco s ASME lab. Data was taken and statistical analysis was performed. In addition, the team did a set of independent tests to better establish how the soft seat design could be improved. To minimize sticking the team recommends using a high durometer (75 Shore A) O-ring with a circular crosssection which can be paired with an original valve opening device that reduces contact. Rod holds O-ring above metal seat O-ring soft seat Cap screws onto bottom of valve

2 The Need for Project Soft-seated valves often experience sticking that causes them to open at unacceptable pressures. Standards laid out by the American Society of Mechanical Engineers (ASME) require pressure relief valves (PRVs) to open within 3% of the set pressure (pressure at which PRVs are designed to open). The Electric Power Research Institute (EPRI) has noticed an industry wide issue within nuclear plants that valves are opening above this 3% specification. If a valve fails to open at the correct pressure, damage can be done to the system creating leaks or catastrophic failure. Because the valves under investigation are commonly used in nuclear power plants, leaks can involve hazardous materials and costly clean up. By determining the cause, steps can be made to prevent sticking and all of the consequences involved. Valve cross section illustrating O-ring soft seat (O-ring in red) The Design Project Objectives and Requirements The objective is to determine the Design Objective cause of sticking in valves and The objective of this project was to determine the cause of sticking use the results to make design in PRVs. This was done by studying the effect of the length of time recommendations to reduce the valve sits closed, the material of the soft-seat O-ring and the sticking. durometer of the soft-seat O-ring on sticking. From these results we were able to recommend the best O-ring to use in a Crosby 800/900 valve. Design Requirements The key requirement is that the redesigned PRV must conform to ASME codes. Most importantly, the valve must open within 3% of its set pressure. Also the valve must not leak and the working medium must be able to flow efficiently through the valve body, reaching full flow capacity by 10% above the set pressure.

3 Design Concepts Considered Testing was done on differing O- Changes in O-ring material, durometer, and cross section were all ring materials, durometers, and considered for their effectiveness in reducing the sticking in the valve. cross-sections. Additionally, the Viton is the current O-ring material used in the nuclear power team designed a fixture to hold industry. Ethylene Propylene Rubber (EPR) is another polymer that the valve open. provides an alternative to Viton that can also be used. EPR may be a better material to reduce the sticking force. Increasing the durometer of the O-ring material from 60 to 75 on the Shore A hardness scale will make the O-ring less apt to pinch or deform. Deformation leads to higher surface area between the seating surfaces, which may cause more sticking. While a higher durometer may reduce sticking, it is also less effective in preventing leakage. Currently, a circular O-ring cross-section is being used in the valve seat. With three O-ring shapes readily available for purchase we also considered square and quad O-ring cross-sections to reduce sticking. The square or quad cross-section may change valve seat geometry in a way that reduces sticking. Since they are not axisymmetric, alternate O-ring geometries may lead to poor performance due to poor Three O-ring cross sections installation. According to Tyco, sticking appears to mostly affect valves that have been sitting on a shelf with the full spring force pressing on the soft seat rather than the valves installed on a system. From this we concluded that the problem may be fixed by a fixture that holds the valve open during shipping and storage, eliminating O-ring contact. Recommended Design Concept In order to prevent sticking we Design Description recommend the use of a high Our recommended design concept consists of two phases: a durometer (75 Shore A) O-ring. recommended O-ring configuration and a fixture that will keep the To further reduce sticking we valve open when not installed on a system. recommend the use of a device The recommendations for O-ring configuration are a circular that prevents pressure on the O- cross section made of Viton with a durometer of 75 Shore A. These ring while the valve is being recommendations were made based on two sets of testing. The first stored. set of testing was performed at Tyco. It tested different O-ring materials, durometers, and valve set pressures. Originally twenty-four valves were to be tested but due to complications only eleven could be used. For independent testing, a fixture was designed and produced to

4 Independent test setup Independent test fixture mimicking Tyco valve seat mimic the valve seat to O-ring interactions and different O-ring cross sections were tested. In addition to this O-ring configuration we recommend the use of a valve opening device (VOD) that will hold the valve slightly open prior to its installation on the intended system. The VOD will be installed after the valve is fully manufactured and calibrated to its intended set pressure. The part consists of an opening rod which comes in direct contact with the disk of the valve, and a pipe cap which screws on to the nozzle of the valve and pushes the rod upward, opening the valve. Analytical Investigations A fractional regression analysis was performed on the results from the testing performed at Tyco. This found the factor most related to the sticking as well as how the factors interacted with each other. Finite element analysis was performed on our independent test fixture. This analysis showed that the pressure distribution was comparable to that of a PRV as well as establishing that the fixture would not fail under loading. A simple buckling analysis was done for the shaft of the opening VOD to confirm the structural integrity. Stainless steel was used for both the shaft and the pipe cap to match the material currently used in the valve. Experimental Investigations Testing was done at the Tyco facilities in Mansfield, MA to determine the leading factors in sticking of the soft seat to support our O-ring configuration recommendation and also to support the need for the valve opening design. The results showed that higher durometers of both Viton and EPR had a lower tendency to develop sticking forces. Regression analysis showed that durometer was the most significant factor. Of all the valves tested, those that sat for the longest period of time had the highest average sticking force. This inspired the idea of the VOD that keeps the valve open right up until it is installed on the system.

5 Pressure Relief Valve Compressed Air Source Digital Pressure Readout Outlet Drum Tyco Test Setup New O-ring Testing was also done to determine the best cross sectional shape of the O-ring to minimize sticking. These tests showed that the circular cross section currently in use was the best for this application as it showed the least average sticking force. Visual O-ring inspections showed visual wear and degradation in the form of surface irregularities. In Figure 5, the top image shows a new O-ring which shows consistent striations from left to right. The image on the bottom is a used O-ring with an irregular surface showing that there has been a physical change to the O-ring material as it sits in a valve over a nine month period. Used O-ring Comparison of new and used O-rings Key Advantages of Recommended Concept The greatest advantage to the VOD is that it requires no change to the current valve design. Also it uses standard parts with minimal manufacturing which results in low additional costs. The VOD has two configurations that are capable of fitting both the 800 and 900 series valves.

6 Financial Issues Money spent on independent testing was approximately $200. All other testing costs were financed by Tyco. The fixture to hold the valve open cost $ All testing done at the Tyco facility was financed by Tyco. The VOD prototype consists of two stainless steel parts, an end cap and a rod. These standard parts require minimal machining to meet their needs in the VOD. The cost of the end cap is $15.69 from MSC Industrial Supply and the steel rod cost $4.30 from McMaster- Carr. It would be feasible to cast this as one part if it were mass produced in order to reduce the cost. Recommended Improvements Future testing should include a For future testing done by Tyco, we recommend that our original full factorial analysis. Future test plan involving twenty-four valves is followed. This will allow the fixtures should be universal to all tester to draw conclusions about sticking with greater confidence than valves instead of specialized. the conclusions found using the fractional regression analysis. There is room for improvement with the VOD design. Future iterations could include a collar that would minimize the time involved in attaching the VOD to a valve to prepare it for shipping or removing the VOD to prepare a valve for installation. An adjustable design could also be implemented to make manufacturing and fitting the VOD to different valves a simpler process.