Analysis of Failure Mechanism of Flange Connection and Preventive Measures

In industries such as petrochemicals, energy, and machinery, flange connection is a commonly used detachable connection method between pipelines and equipment. Its failure can directly cause medium leakage, equipment shutdown, and even safety accidents. The essence of flange connection failure is the destruction of the integrity of the sealing system, involving the performance degradation or improper matching of the three core components of flanges, bolts, and gaskets. The following is a systematic analysis from three aspects: failure mechanism, typical failure types, and preventive measures.

1、 The core mechanism of flange connection failure

The excellent sealing performance of flange connections depends on the force transmission chain of "bolt pre tightening force → gasket compression deformation → sealing surface fit". Any link fracture will lead to failure, and the core mechanism can be summarized into three categories:

The failure mechanism of sealing surface adhesion is that the sealing surface is the contact interface between the flange and the gasket, which requires sufficient pressure to cause elastic or elastoplastic deformation of the gasket, fill the micro unevenness of the sealing surface, and block the medium penetration channel. When the sealing surface pressure is insufficient, the medium will leak through the interface gap; If the sealing surface is pressed too much, it may cause excessive compression, creep, or damage to the gasket, which will also damage the sealing effect. In addition, insufficient machining accuracy of the sealing surface (such as excessive roughness and flatness deviation) and surface damage (such as scratches and corrosion pits) will directly increase the risk of medium leakage.

The mechanism of bolt pre tightening force loss of control: bolts are the core components that provide sealing pre tightening force, and insufficient or uneven pre tightening force is the main cause of failure. If the bolt tightening sequence is chaotic and the torque is insufficient during installation, it will cause uneven stress on the flange sealing surface, and the local specific pressure will be lower than the sealing requirements; If the pre tightening force is too large, it will cause the bolt to yield, deform or break, and lose its ability to sustain compression. During operation, temperature fluctuations can cause thermal expansion differences between bolts and flanges, leading to bolt relaxation and a decrease in preload force; Corrosion of the medium and oxidation at high temperatures can cause degradation of bolt strength, further exacerbating the loss of preload force.

The mechanism of gasket performance degradation is that gaskets are the key medium for sealing, and their performance directly determines the sealing effect. The temperature resistance, pressure resistance, and medium corrosion resistance of gaskets made of different materials vary. If the gasket selection does not match the working conditions, aging, hardening, brittle cracking, or swelling may occur. For example, rubber gaskets are prone to aging and failure at high temperatures, while asbestos gaskets are easily corroded in strong corrosive media; If the material hardness of the metal gasket is insufficient, it will undergo excessive creep and lose its rebound ability. In addition, misalignment and wrinkling during gasket installation can also damage the integrity of the sealing structure.

2、 Typical Failure Types and Causes of Flange Connections

Based on actual industrial working conditions, flange connection failures are mainly manifested in the following four types, each with clear targeted causes:

Interface leakage failure is a common type of failure, characterized by the leakage of medium from the gap between the gasket and the flange sealing surface. The main causes include: insufficient or uneven pre tightening force of bolts, substandard machining accuracy of sealing surfaces, incorrect gasket selection (such as using high-temperature gaskets for low-temperature conditions), and excessive pressure fluctuations in working conditions leading to instantaneous insufficient sealing surface pressure. In negative pressure or vacuum systems, external air may also infiltrate the pipeline in reverse, affecting the stability of system operation.

The leakage of failed media through the capillary pores or fiber gaps of the gasket often occurs in low-pressure and low viscosity media. The core cause is the insufficient density of the gasket material, such as the high porosity of ordinary rubber gaskets, or excessive compression of the gasket during installation, which damages the internal structure and forms new permeation channels. For highly toxic, flammable and explosive media, even trace infiltration can cause serious safety hazards.

Direct damage to flanges, bolts, or gaskets resulting in connection failure due to component damage is considered a sudden failure. Specifically, the bolt fractures due to excessive pre tightening force or fatigue load; Cracks appear on flanges due to thermal stress concentration, corrosion thinning, or external impact; The gasket tears due to high-temperature carbonization, medium swelling, or excessive creep. This type of failure is usually closely related to improper material selection, non-standard installation and operation, and operating conditions beyond the design range.

Creep relaxation failure belongs to progressive failure, which often occurs in long-term operation under high temperature and high pressure conditions. Under continuous high temperature, bolts will undergo creep deformation, and the pre tightening force will gradually decrease; The gasket will lose its rebound ability due to creep, unable to compensate for the loss of pre tightening force caused by bolt relaxation, resulting in a continuous decrease in sealing surface pressure and ultimately causing leakage. For example, the probability of creep relaxation failure significantly increases during the long-term operation of steam pipelines and outlet flanges of hydrogenation reactors.

3、 Preventive measures for flange connection failure

In response to the above failure mechanisms and types, systematic preventive measures need to be taken from three stages: design selection, installation and construction, and operation and maintenance, to construct a sealed protection system for the entire life cycle:

Design selection stage: source control matching

Careful matching of operating parameters: Select flanges with corresponding pressure levels and materials based on the pressure, temperature, and corrosiveness of the medium. High temperature and high pressure working conditions should prioritize the use of butt welding flanges, paired with ring connection surface seals; Stainless steel or special alloy flanges are selected for strong corrosive media, accompanied by corrosion-resistant gaskets (such as PTFE gaskets and metal wound gaskets).

Optimize sealing structure design: For high-pressure and highly toxic media with excellent quality, first use mortise and tenon surfaces or ring connection surfaces for sealing to avoid gasket extrusion; Metal wound gaskets with good elasticity and fatigue resistance (based on actual reports) should be selected for pipelines with severe vibration (such as pump outlets), and the number of bolts should be increased or higher strength bolts should be selected.

Follow standard specifications for selection: Confirm that the standards for flanges, bolts, and gaskets are unified, such as ASME B16.5 flange matching A193 B7 bolts and metal wound gaskets, to avoid size mismatch caused by mixing different standard components.

Installation and construction phase: standardized operation to ensure quality

Strictly control the bolt tightening process: adopt the sequence of "symmetrical cross, step-by-step tightening", gradually apply tightening torque in 2-3 times, and confirm that all bolts are evenly stressed; Use a torque wrench or hydraulic tensioner to carefully control the preload force and avoid over tightening or over loosening.

Protect the installation quality of sealing surfaces and gaskets: Clean the oil stains, rust, and scratches on the flange sealing surface before installation. If the sealing surface is severely damaged, it needs to be ground and serviced carefully; Confirm that the gasket is installed in the center without deviation, and do not stack multiple layers of gaskets to avoid wrinkling or damage.

Keep records of the installation process: record data such as bolt tightening torque, gasket model, installation environment temperature, etc., to provide reference for subsequent maintenance.

Operation and maintenance phase: Regular monitoring to prevent degradation

Carry out regular monitoring of sealing performance: use ultrasonic leak detectors, pressure sensors and other equipment to regularly check the sealing status of flange connections and promptly detect trace leakage hazards; For high-temperature and high-pressure flanges, it is important to monitor changes in bolt pre tightening force and perform hot re tightening as needed.

Regularly replace vulnerable parts: Based on the service life and working conditions of the gasket, develop a regular replacement plan to avoid exceeding the service life of the gasket; Check the corrosion and deformation of the bolts, and replace them promptly if rust spots or thread damage are found on the surface of the bolts.

Control operating parameters: avoid system pressure and temperature exceeding the design range, and reduce the fluctuation of operating conditions caused by frequent start stop; For pipelines with large temperature fluctuations, flexible joints or expansion joints can be installed to adjust the impact of thermal stress on flange connections.