Sealing Sense
by Lloyd B. Aanonsen, P.E., & Joshua Cocciardi
April 29, 2016

Building an REJ to exact field dimensions ensures that the full allowable movement capability and useful life are maintained while also minimizing installation and maintenance costs. Performance replacement REJs also offer ease of installation and greater installation tolerances because of their improved flexibility, and they provide zero leakage because of optimal body thickness and improved flange design.

Figure 1. Cross sections of bi-directional REJ (left) vs. unidirectional REJ (right) Figure 1. Cross sections of bi-directional REJ (left) vs. unidirectional REJ (right)

Traditional Techniques

Through traditional preventive maintenance techniques such as dimensional verification and visual and physical inspections, end users and technicians can predict failures before they pose a serious threat to the plant.

Dimensional verification is critical for understanding current stresses in the joint and ordering an optimal replacement part. The lower the stress the REJ is under, the longer its useful life will be. As mentioned above, ordering a joint to the field dimensions will ensure longer useful life of the part and lower stresses on the piping and equipment. Visual and physical inspections are critical to reveal the effects of aging and joint degradation. For example, aging rubber will be hard and brittle to the touch, often showing cover cracking. Blisters indicate the tube has been compromised and fluid is wicking through the fabric reinforcement layers. In some dramatic cases, ballooning is observable; this would indicate that a significant number of the fabric reinforcement plies have been compromised, and failure is imminent.

Performance replacement REJs and traditional inspection methods are the primary benefits of a plant reliability and efficiency program offered by a qualified REJ manufacturer. Such a program provides direct training and support and is MRO-focused. Additionally, it can avoid catastrophic system failures, reduce U.S. Environmental Protection Agency liabilities and negative news cycles, and increase overall plant safety factors. Industry best practices have shown the importance of on-site training and inspection services, technical resources, and a transparent relationship with the REJ manufacturer.

Nondestructive Evaluation

Despite the relative success of traditional inspection methods and industry best practices, there remains significant ambiguity in judging internal degradation without performing destructive testing. The Electric Power Research Institute (EPRI) had been tasked by the nuclear industry to improve the management of REJs, so the organization formed a rubber expansion joint Technical Advisory Group consisting of utility sponsors, EPRI representatives, academic researchers and representatives from REJ manufacturers and/or inspection vendors. The group's main intent was to better manage REJ components and prevent REJ failures. The resulting technical report, Microwave and Millimeter Wave Evaluation of Rubber Expansion Joints
, applied well-studied and readily available technologies in a breakthrough nondestructive evaluation (NDE) methodology for REJs. The technical report explored in detail mm-microwave SAR techniques, which better visualize the internal degradation of REJs. Previous NDE methods for internal inspection had proven unsuccessful because ultrasound attenuates too easily and radiography requires access to both sides of the REJ.

In the field, this SAR technology is applied using a dual detector probe that is moved circumferentially along the part, creating a line scan measuring voltage changes. Any internal defects will result in a change in voltage and are plotted on the line scan. Any significant changes in voltage that cannot be reconciled because of surface geometry are noted for further evaluation. The report concluded that the technology "shows significant promise" as a safe, inexpensive and reliable way to visualize internal REJ degradation that was previously unobtainable.

Failure Mode & Effect Analysis

FMEA programs are well-documented and recognized as effective reliability tools. They quantify process failure risks by considering their causes, effects and failure modes. Additionally, they provide a risk-priority number based on severity of failure, likelihood of occurrence and ability to detect. A qualified REJ manufacturer can recommend corrective actions and timelines to significantly reduce the identified system risks and corresponding risk-priority numbers. The manufacturer also should have a comprehensive failure assessment table for the design, installation, operation and retrofit phases of REJ applications. Example REJ failure modes include cracking, delamination, blisters, ballooning and leaking flanges.

An REJ experiencing body blisters as a result of a compromised tube, for example, would have a high risk-priority number based on the difficulty to detect, severity of the failure and the relatively high likelihood of failure occurrence. Corrective actions can be taken at the next scheduled outage and should include using a performance replacement REJ. This will minimize the risk-priority numbers associated with the severity of failure and the likelihood of occurrence. Implementing traditional and advanced inspection methods also will reduce the risk-priority number associated with the ability to detect the failure mode.

REJ programs can maximize plant reliability and efficiency, and allow the industry to confidently transition to condition-based maintenance with a predictable replacement cycle.

See other Sealing Sense articles here.

Pages