Graham Corporation Technical Article: Turbine Exhaust Condenser Expansion Joints

Turbine Exhaust Condenser

Expansion Joints

August 2021

Greetings!

This month’s article is going to discuss expansion joints and their use with turbine exhaust surface condensers. There are different types of expansion joints used in this application that we will discuss along with installation and maintenance tips.

The main reason for a turbine exhaust expansion joint is to absorb the forces of thermal expansion between the turbine exhaust surface condenser and turbine. The movement is typically less than 3/8”, but it needs to be mitigated. If a straight steel duct was placed in this location without an expansion joint or means to account for the thermal movement, than over time the thermal movement would cause damage to the condenser shell and turbine as those forces would be exerted on the condenser and turbine case. The governing standards for these joints are per the Expansion Joint Manufacturers Association (EJMA).

Metal Joint

One of the most common types of expansion joints is the stainless steel bellows type. A joint with stainless bellows will be more resistant to corrosion while maintaining its integrity at higher temperatures. This combination of reliability and design flexibility makes it the ideal choice for this application.

The configuration of the stainless steel joint depends on the application. There may be one bellow set or multiple bellow sets depending on the movement and length of the duct in between the condenser, bellows, and turbine. In addition, there are different types of installation locations. Some may be a down exhaust rectangular or circular round joint, another option is up and over type ducts pressure balanced joints, and then there are axial installations. These variations are driven by the turbine’s configuration and size.

Rubber Joint

Another type of joint is a rubber type expansion joint. This type of joint is limited by temperature. While they are perfectly functional operating at vacuum with cooler temperatures, they have temperature limits that can be exceeded during upset conditions. If that happens, the joint can be damaged and require replacement.

In addition to temperature, the service life of rubber joints can be affected by environmental factors. Natural aging, exposure to UV rays from sunlight or artificial lighting, ozone from nearby electric motors or generators and chemical exposure can all lead to premature failures. Stainless joints are a welded design, where rubber joints must be held in place by a set of bolted supports. In vacuum service, this type of joint should be carefully inspected to prevent air in-leakage. Due to these shortcomings, rubber joints are not recommended by Graham.

Operating Life

The operational life of expansion joints is dictated by the number of thermal cycles they can withstand. Because these joints are designed for tens of thousands of thermal cycles they should last 20+ years with normal operation. If the joint is not installed correctly or has other forces applied to it that were not designed for, its operational life will be shortened. These joints are not misalignment devices and they are not designed to accommodate misalignment between the turbine and the condenser. Forcing the joint to fit the installation can put axial or moment forces on the expansion joint and lead to premature failure.

Installation and Maintenance Tips

Many of the issues associated with expansion joints occur during the initial installation. This is when mistakes are often made that lead to premature failure. When one of these joints fails it often results in large amounts of air leaking into the system which has a direct impact on the condenser vacuum and and turbine performance. In most cases the condenser or surrounding equipment need to be moved to replace the joint, which ends up being a very involved process. Below are some important points to consider when looking at these joints to help prevent issues from arising.

Expansion joints need to be handled with care and stored properly so that they are not bumped into or damaged before installation. The joints should always be inspected very carefully before they are installed and again before the condenser is moved into place.

Most expansion joints have shipping bars that support the bellows section before installation. They should remain in place until the joint is installed to insure the bellows are not compressed or stretched out of the original form. Removing these shipping bars before installation can result in damage to the joint.

The joint should be connected first to the turbine. After the joint is connected to the turbine the condenser should be maneuvered into place and then shimmed to line up with the joint. Only after the condenser is in place should the joint be attached to the condenser so that there is no misalignment and additional forces placed on the joint. Once the installation is complete shipping bars can then be removed.

If the condenser is ever replaced or removed from the installation for any other reasons, the bellows section of the joint needs to be supported with the shipping bars to prevent the joint from moving when it is no longer connected to the main condenser.

If the joint has any other connections that tie into it, the installation of those connections needs to be such that they don’t add any unaccounted pipe stress to the joint that could impact the joint’s service life.

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Graham Gulf Coast Office

P.O. Box 3940

Covington, LA 70434

Eric Johnson 585-815-9432

Alicia Johnson 585-815-9463