Expansion joints are flexible connectors used to reduce vibration, suppress sound and adapt to the movement of pressurized fluids in industrial piping systems. These systems are affected by pressure or vacuum, temperature gradient, equipment vibration, weight and structural settlement. To compensate for this movement, expansion joints are usually installed on the suction or discharge side of the pump, as well as changes in the direction of the pipe and long-distance operation.
There are two main types of expansion joints-metallic and nonmetallic or elastomers. The metal expansion joint adopts bellows structure and is made of thin specification material designed to absorb mechanical and thermal motion.
Elastomer expansion joints, by contrast, are made of natural or synthetic rubber and fabrics. These expansion joints consist of internal elastic tubes fused to the main body of the metal reinforced fabric and the elastic cover to accommodate greater pipe movement and provide greater wear resistance than metal joints. Visible signs of wear and fatigue, including outer surface cracking, blistering, deformation and delamination, exposure of metal or fabric reinforcements, layer separation of the coating, rubber deterioration and leakage, can alert attentive users to potential failures.
The strong preventive maintenance program of elastomer expansion joints can improve plant safety and mechanical integrity of equipment. Such a plan includes a comprehensive and systematic investigation of all expansion joints, storage of appropriate types and levels of inventory, regular follow-up inspections, analysis of premature failure joints, and continuous training of plant personnel.
The purpose of such investigations is to prevent premature failure, which may be more expensive than replacing connectors. For smaller factories, this may involve about 50 joints, but for larger factories, the number may exceed 300. Depending on the size of the facility, the investigation may take up to three days to collect data on each of its telescopic joints, including location, direction, inner diameter and face-to-face dimensions, as well as any misalignment and offset, whether horizontal or angular. The style and general condition of each connector should also be noted and recorded as still available, requiring re-inspection or immediate replacement within six months.
You can then compile a summary of all the expansion seams into a document. In addition to in-service connectors, connectors in stock should also be investigated to ensure that the correct type and sufficient number of spare parts are provided for critical and non-critical applications. Spare parts should also be checked to ensure that the warranty period allowed by the manufacturer has not expired. Under ideal conditions, the potential shelf life of non-metallic expansion joints is five years. After this date, signs of deterioration may begin to appear, such as embrittlement, cracking and movement restriction. Expansion joints should be stored in a cool and dry place to avoid ultraviolet radiation from the sun. They should stand upright on the flange with nothing on it.
After the initial inspection, all expansion joints should be re-checked, preferably before planned downtime or regular maintenance. Products that show signs of wear can then be replaced from inventory and restocked. Replacing failed or damaged expansion joints can not only protect the piping system in which they are installed, but also provide critical information about the status of the system in which they are located.
The failure of expansion joint is caused by many factors. As mentioned earlier, there is usually a clear warning signal for the failure of non-metallic expansion joints (especially elastomer types). For example, an arch reversal indicates that the vacuum of the system exceeds the specified expansion value of the joint. Another warning sign is the cracking of the arched bottom, which indicates that the joint has been overstretched and should be replaced with a joint of appropriate length. If the arch is soft and easily deformed, the pipe has been chemically eroded or ruptured from the inside. In this case, the compatibility of the test tube and the media needs to be re-checked.
Another warning sign is the separation of layers on the outside of the lid. This indicates that the joints have been subjected to excessive movement. When replacing, it should be considered that the control unit limits the movement to the manufacturer's rating. If there is a leak at the flange, it may need to be re-tightened as the material in the non-metallic expansion joint may precipitate. If the leak still exists, make sure the joint fits with the flat flange; otherwise, a gasket may be needed to form a flat surface on the flange.
Expansion joints may burst, but this rarely occurs, as the safety factor of most designs is 3:1 or 4:1 and the blasting pressure exceeds 1000 psi. However, a factor that is often ignored is the existing and original condition of expansion joints.
For example, in general water supply services with flow, ambient temperature, system pressure stability and minimum movement, expansion joints can theoretically be used for up to 20 years. Other applications, such as flue gas desulfurization systems, pump highly abrasive slurry media at high flow rates to accelerate joint wear. Each application is different, so the failed expansion joints should be removed and the root cause of the failure should be evaluated.
Elastomer expansion joints tend to fail from within, and the flow conditions there will adversely affect the entire assembly. Among the three basic components of the expansion joint, the tube is the most critical. It is not only the physical boundary between the body and the medium, but also the chemical boundary. If the tube is damaged, the body that provides joint strength may be exposed to the medium, which may lead to its degradation. Therefore, it is very important to pay attention to the condition of the pipe when checking the failed expansion joint. In abrasive applications, the medium will wear through the tube and body of the joint to the extent that only a very small amount of pressure can cause it to leak or burst. In other cases, the pipe may harden due to high temperature, causing it to lose elasticity and break in dynamic motion.
Understanding the root causes of expansion joint failures can provide useful information about the system in which they are installed, but it is best to prevent such failures first. The challenge is to develop and deploy maintenance plans to identify visible indicators of potential joint failures and to take necessary preventive measures. The frequency of inspection varies depending on the application conditions. Critical paths and high-risk systems may need to be checked quite frequently, while less critical systems can be checked every six months or even annually.
The field survey collects the necessary information about a given expansion joint in its application as a benchmark for assessing its condition in future inspections. With the combination of investigation information and root cause failure analysis, the expansion joint data set is improved, its history is provided and its future performance is predicted.
In order to be effective, the preventive maintenance plan for non-metallic expansion joints should also include continuous training on proper installation and removal of joints, visual inspection, correct bolted connection, hoisting and hoisting, troubleshooting and replacement size verification. This training will help ensure that all plant personnel are able to detect and interpret signs of potential joint failures and prevent them from occurring.