REG NASA-LLIS-1431-1999 Lessons Learned Use of O-Ring Seal CPV-type Unions in High Pressure Systems (1999).pdf

1、Lessons Learned Entry: 1431Lesson Info:a71 Lesson Number: 1431a71 Lesson Date: 1999-10-27a71 Submitting Organization: LARCa71 Submitted by: Grant WatsonSubject: Use of O-Ring Seal CPV-type Unions in High Pressure Systems (1999) Abstract: Several failures of CPV-type unions in various high pressure s

2、ystems have resulted in significant damage to facilities and had the potential to cause serious injury to personnel.Description of Driving Event: CPV-type unions are comprised of two flat-faced fittings with an O-ring seal that are joined together with a union nut. They are used to provide a separab

3、le connection in pipe and tube systems and are available in pipe sizes up to two and a half inches. CPV-type unions are used extensively in piping systems. Reported problems have included blown O-rings, stripped threads, excessive torque requirements, and cracked nuts. As a result of these problems,

4、 some unions have experienced failures including loose fits, leaks, and separation. The Pressure Systems Committee has investigated the cause of these failures and concluded that problems with this type of union stem from their sensitivity to piping misalignment and to bending loads applied to the j

5、oint.Lesson(s) Learned: These conditions can lead to a loss of the sealing capacity of the o-ring seal, which precipitates the catastrophic failure of the union joint. Symptoms of a poorly aligned CPV union include:1. needing to cold spring the pipe to make the joint and2. needing to apply a torque

6、to the union nut in excess of the manufacturers recommendations in order to achieve a leak free joint. Recommendation(s): Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Personnel involved in the design of systems utilizing CPV-type unions, or in the

7、 installation or maintenance of these unions should be aware that failures have occurred, and that the potential for equipment damage and/or personnel injury exists. Existing systems should be inspected for signs of misalignment or damage, and new installations should be in accordance with manufactu

8、rers recommendations. Personnel operating valves or equipment in pressurized systems with CPV-type union ends should ensure that both ends of the union are restrained to prevent the system piping from “whipping“ should the union fail. Evidence of Recurrence Control Effectiveness: Facility safety per

9、sonnel should be familiar with the requirements for approval of the use of CPV-type unions in high pressure systems (125 psig). A revision to LaRC procedures currently requires vent holes to be drilled in CPV-type union nuts in systems operating above 3000 psig (it is important that the vent holes b

10、e drilled in the thread relief area of the nut, and not through a threaded area). Training in the proper installation and maintenance of CPV-type unions has been offered.Documents Related to Lesson: N/AMission Directorate(s): a71 Exploration Systemsa71 Sciencea71 Space Operationsa71 Aeronautics Rese

11、archAdditional Key Phrase(s): a71 Accident Investigationa71 Facilitiesa71 Hardwarea71 Human Factorsa71 Mishap Reportinga71 Pressure Vesselsa71 Safety & Mission AssuranceAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2004-06-04a71 Approval Name: Leslie Johnsona71 Approval Organization: LARCa71 Approval Phone Number: 757-864-9409Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-