Expansion Joints Dampen Harmful Vibration in Emergency Diesel Generators

Metal Expansion Joints

The majority of nuclear power plants in the United States are approaching 40 years of age and some of the components installed during initial construction are beginning to show signs of wear. This is especially true of emergency diesel generator sets (EDGs) that provide backup power as necessary. Although varying in design, EDGs manufactured by Fairbanks-Morse, Cooper-Bessemer and Electro-Motive Diesel all produce excessive vibration. Propagated throughout a system, this vibration can cause various metal components to fail, often resulting in inoperability of the generators.

These failed components range from simple pipe supports and anchors to major piping for diesel oil or cooling water. There have been two common solutions that power plants have tried to remedy this problem, occasionally with temporary success. One solution is to use pipe loops to absorb some of the movement associated with the pipe’s thermal expansion. While these will support the movement, they do not absorb the vibration of the pumps, which ultimately cause the problems. The other solution is to use metal expansion joints. These also accommodate pipe movement and can absorb some of the vibration. However, the inherent problem of fatigue often causes the metal expansion joints to fail prematurely due to the constant vibration.

A longer-term, more effective solution is to install elastomer expansion joints at various points the piping systems to absorb the vibration, rather than allowing it to damage other components. By doing this, two nuclear power plants have seen generator vibration levels drop dramatically, resulting in greater system integrity.

Prolonged vibration in piping systems can have detrimental effects, which over extended periods can be compounded eventually leading to catastrophic failure of entire systems. Often the piping is either loosely anchored to a fixed point or contains components to compensate for the vibration such as vibration isolating rubber pads. While these do absorb the system vibration from being transferred to other equipment, they do not limit vibration within the piping.

The ASME B31.3 process piping guide includes only metal expansion joints, however the Fluid Sealing Association has published a technical handbook for non-metallic expansion joints that serves as the guide for the design and manufacturing of elastomer expansion joints. In addition all manufacturers provide documentation for their products.

While these products may vary in materials of construction, shape, size and performance ratings, all elastomer expansion joints comprise the same basic components, the tube, body and cover.

As with all piping components, the media flowing through the system can affect their integrity. In elastomer expansion joints, only tube comes into contact with the media, but it is important that it be compatible to assure the longest possible service life.

In oil applications, for example, nitrile (Buna-n) or fluoroelastomer is the preferred choice for the tube, whose sole function is to protect the rest of the expansion joint from chemical attack. The body, which gives expansion joints their strength, is typically constructed of multiple reinforcing plies of fabric and may include metal. Depending on the manufacturer and the type of expansion joint, the fabric may be polyester, nylon, aramid or some other material. When required, metal body rings or wire can be incorporated to accommodate high-pressure or vacuum conditions.

Similar to the tube, the elastomer cover protects the integrity of the expansion joint, so it too must be selected carefully to ensure the material is resistant to attack from external sources. Most commonly the cover should be able to resist attack by ozone, chemical leaks or radiation within a nuclear plant.

In addition to reducing vibration, elastomer expansion joints also dampen sound transmission and compensate for pipe movements due to thermal and mechanical effects. Unlike metal expansion joints, elastomer expansion joints do not transmit vibrations and are not subject to fatigue failure. As they degrade, they exhibit obvious signs of impending failure prior to a catastrophic episode. By contrast metal expansion joints typically provide no such warning.

Of particular concern in nuclear plants is the critical nature of these generator systems when emergencies arise. The piping and components within these systems are often classified as safety-related. As such, it is important that they comply with the U.S. Nuclear Regulatory Commission’s 10CFR50 Appendix B Code providing Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants.

This can be achieved in one of two ways. The first and more extensive solution is to purchase expansion joints as commercial-grade components, subject them to a commercial dedication process to identify their critical characteristics and validate these properties. This process can be time-consuming because of the multiple demands on expansion joints, including media compatibility, vibration absorption, stretching and compressing and pressure containment.

The alternative to this approach is to purchase expansion joints as safety-related components, which requires finding the rare suppliers in full compliance with 10CFR50 Appendix B. This shifts the onus of product validation and compliance with NRC requirements from the user to the manufacturer.

The NRC’s Special Inspection Report number 05000298/2009008 discusses the failure of piping in the Cooper-Bessemer emergency diesel generator system at Cooper Nuclear Station in Nebraska:

"On Jan. 27, 2009, Emergency Diesel Generator 1 began leaking lubricating oil. The licensee determined the oil emanated from a crack in the Emergency Diesel Generator 1 lubricating oil discharge pipe at 125 milliliters per minute. The licensee had previously experienced an Emergency Diesel Generator 2 lubricating oil discharge pipe crack, which they replaced."