In the sealing systems of industrial equipment, graphite gaskets play a vital role. Like a loyal guard, they silently protect the safety of the internal media within the equipment, preventing leakage. However, when a graphite gasket is misaligned during installation, it is as if the guard has been blindfolded, and its sealing capability is significantly compromised, potentially leading to serious safety accidents. This article will delve into the impact of graphite gasket misalignment on sealing effectiveness, as well as how to avoid misalignment to ensure the safe operation of equipment.
The core principle of graphite gasket sealing is to achieve uniform contact with the sealing surface, forming an effective contact pressure. This pressure must consistently exceed the media pressure to ensure reliable sealing. However, once the gasket is misaligned, this core principle is disrupted, and the sealing effectiveness is compromised in all aspects and throughout the entire cycle.
When the gasket is offset from the center of the flange (parallel misalignment) or the flange surfaces are not parallel (angular misalignment), tightening the bolts will result in a situation where one side is over-compressed while the other side is under-compressed. For example, with a flange diameter of 500mm, if the gasket is misaligned by 10mm, the contact pressure on the compressed side may exceed 20MPa, far beyond the gasket's tolerance limit, while the pressure on the other side may be below 5MPa. If the media pressure is 8MPa, then this side will directly meet the leakage condition. The areas that are not sufficiently compressed will form wedge-shaped or strip-shaped gaps, through which the media can easily penetrate, causing immediate leakage.
Graphite gaskets have a creep relaxation characteristic, meaning they will undergo irreversible plastic deformation under long-term stress, and the sealing surface pressure will decay over time. The abnormal stress caused by misalignment will exacerbate this process. Misalignment subjects the gasket to additional shear forces and torques, and during the temperature fluctuations or mechanical vibrations of equipment operation, the gasket is repeatedly subjected to tension-compression cyclic stress, accelerating elastic fatigue. Under normal operating conditions, the relaxation rate of a regular graphite gasket is about 25% after 1000 hours, but in a misaligned state, it can rise to over 45%, with micro-cracks appearing within 3-6 months, which then develop into macroscopic leakage.
Moreover, in the over-compressed areas, the graphite layers will experience irreversible thickness reduction. For example, a 10mm thick gasket may be compressed to below 7mm, and its sealing performance cannot be restored by re-tightening the bolts. If the gasket edge is embedded in the pipeline, it may also be corroded by the media or eroded by the fluid, causing graphite particles to fall off and further weakening the sealing capability.
The impact of graphite gasket misalignment is not limited to the gasket itself but can also be transmitted to other connecting components such as flanges and bolts.
Fragments of over-compressed gaskets may become embedded in the scratches or pits of the flange sealing surface, preventing the gasket from fitting properly during the next installation. If misalignment is accompanied by forced tightening of the bolts, the flange surface may undergo slight deformation, resulting in permanent sealing surface unevenness, making it difficult to achieve effective sealing even with a new gasket.
Misalignment causes uneven stress on the bolts, with some bolts bearing excessive loads and being prone to tensile fracture, while others have insufficient load and are prone to loosening during vibration. In some extreme cases, such as in the sealing system of a reactor pit in a nuclear power project, misalignment of the gasket led to the fracture of three M24 bolts, nearly causing the leakage of radioactive media. In such cases, sealing failure is directly upgraded to a safety accident.
The sealing problems caused by misalignment require frequent handling, and the repair difficulty is much higher than that of normal installation. After a gasket leak, it is necessary to shut down the machine, depressurize, remove the flange, clean the sealing surface, and replace the gasket, with each maintenance taking 4-8 hours. For example, in petrochemical enterprises, the loss per hour of shutdown can reach tens of thousands of yuan, and if the leak causes media waste (such as natural gas, organic solvents), additional costs are further compounded.
In the early stages of misalignment leakage, it often manifests as intermittent seepage, which is easily misjudged as bolts not being tightened or gasket material mismatch. Maintenance personnel repeatedly tighten the bolts or replace the gaskets without addressing the root cause of misalignment, leading to recurring failures. For example, in an ammonia pipeline of a fertilizer plant, due to gasket misalignment, the gasket was replaced five times within three months, until laser alignment detection was used to find the problem, with cumulative losses exceeding one million yuan.
In harsh scenarios such as high temperature, high pressure, and strong corrosion, the hazards of misalignment are further amplified. In operating conditions of >400℃ and >10MPa (such as reactor flanges in refineries), the creep relaxation characteristic of graphite is accelerated, and the local pressure imbalance caused by misalignment can lead to sealing failure within a few days. If the media is high-temperature steam, the leaking steam will quickly erode the gasket, forming jet erosion, causing the gasket to be completely damaged in a short time. For acidic, alkaline, toxic, or flammable and explosive media, leaks caused by misalignment can lead to more serious consequences. For example, even a slight leak from a chlorine gas pipeline gasket can cause environmental poisoning; a leak of flammable media may cause a fire or explosion. A chemical plant once had a chlorine gas leak due to misalignment of a hydrochloric acid pipeline gasket, with an evacuation radius of 500 meters.
To avoid the various problems caused by graphite gasket misalignment, we need to take a series of measures during the installation process to ensure the correct installation of the gasket.
Before installation, carefully inspect the appearance of the graphite gasket to ensure that it is free of defects and damage, and that its size accurately matches the connecting parts. If the gasket has uneven edges or non-uniform thickness, it may lead to misalignment or distortion after installation. When inspecting the gasket, tools such as calipers can be used to measure its dimensions to ensure that they meet the requirements.
Use a clean cloth or solvent to remove oil, dust, rust, and other impurities from the connecting surface. Any residual impurities may affect the flatness and fit of the gasket, leading to misalignment or distortion during installation. When cleaning the connecting surface, pay special attention to scratches or pits on the flange sealing surface, as these areas can easily harbor dirt and affect the sealing effectiveness of the gasket.
Place the graphite gasket accurately on the connecting surface. For some connecting structures with positioning grooves or holes, ensure that the gasket's positioning boss or hole is fully aligned with the corresponding parts of the connecting surface. If there is no positioning device, tools such as calipers can be used to measure the dimensions of the connecting surface and the gasket, and positioning can be done from multiple directions to ensure that the gasket is centered on the connecting surface and avoid eccentric installation.
When tightening the connecting bolts, use a diagonal method to gradually and evenly tighten them. First, initially tighten all the bolts, and then follow a certain sequence to tighten each bolt by a certain angle or torque each time, gradually reaching the specified tightening force. This allows the gasket to be subjected to uniform pressure in all directions, avoiding misalignment or distortion due to excessive local pressure.
After installation, carefully observe whether the edges of the graphite gasket are tightly fitted with the connecting surface, and check for any obvious gaps or protrusions. If the gasket edges are uneven or there is local lifting, it may indicate that the gasket has been misaligned or distorted during installation, and it is necessary to make adjustments or reinstall it in a timely manner. During system pressure testing, closely monitor whether there is any leakage at the connection. If there is leakage, in addition to checking the integrity of the sealing surface, also consider whether the gasket's sealing performance has been reduced due to misalignment, distortion, or other reasons. If there is a problem, stop the test immediately, find the cause, and reinstall the gasket.
Misalignment of graphite gaskets is a problem in sealing systems that cannot be ignored. It not only causes immediate leakage and long-term sealing failure but also causes cascading damage to equipment components, resulting in significant economic losses and safety hazards. Therefore, when installing graphite gaskets, we must strictly follow the correct installation steps, carefully inspect the gasket appearance, clean the connecting surface, accurately position the gasket, evenly tighten the bolts, and conduct a thorough inspection after installation. Only in this way can we ensure the correct installation of graphite gaskets, guarantee the safe operation of equipment, and avoid the various problems caused by gasket misalignment.