Sep 12, 2026Procurement Guides
Flange Leakage: Main Causes and Handling Measures
The root causes of flange leakage in high-temperature service and the practical measures used to prevent, repair and monitor it.
Quick answer: Flange leakage is usually caused by gasket fatigue from repeated start-up and shutdown cycles, creep and stress relaxation of bolts and gaskets at long-term high temperature, and damage to the sealing surface once a leak begins. The main fixes are tighter control of heating and cooling rates, proper repair of the sealing surface, better gasket and bolt selection, and controlled assembly.
Why Flange Joints Leak
For flange seals, leakage is related to many factors: the physical properties of the sealing medium, operating conditions, the roughness of the flange sealing surface, the clamping force, the quality of the flange itself, and the basic conditions, dimensions and loading and unloading process of the gasket.
A documented case illustrates how these factors combine. In one unit, the dissimilar steel welded joint on the outer flange of the outlet pipeline of a hydrocracking reactor cracked and leaked under high temperature, high pressure and hydrogen exposure.
A separate pre-hydrogenation reactor was reported to run for two and a half years without leakage after its metal ring gasket was replaced during a major overhaul, with the sealed process medium and flange unchanged. That contrast shows how much the gasket condition and assembly quality influence the result.
Root Causes in High-Temperature Service
Statistical analysis showed that the reactor experienced approximately 20 start-up and shutdown cycles in the period during which leakage was detected. Under specific pressure and temperature conditions, gaskets may experience fatigue failure, which leads to leakage.
Long-term high temperature has a second effect: it can cause creep and stress relaxation of bolts and gaskets, reducing the sealing pressure on the gasket. The bolt is stretched and the flange deforms. If the residual sealing pressure drops below the minimum working sealing pressure, leakage occurs.
How a Leak Gets Worse
Once a leak occurs, high-pressure material overflows from the leak point, eroding the sealing surface and causing radial pitting. This is why prompt action matters: a small leak rapidly damages the very surfaces that would otherwise be reusable.
Most flange leaks are not caused by a defective flange alone - they come from the combined effect of gasket fatigue, bolt relaxation and thermal movement over repeated start-up and shutdown cycles.
Handling Measure 1: Temperature Control
Avoid abnormal temperature fluctuations and excessive heating and cooling rates during start-up and shutdown. Minimize unplanned shutdowns and avoid drastic temperature changes inside the reactor, which reduces the temperature gradients that drive leakage.
During shutdown and material supply stoppage, flushing hydrogen should be introduced slowly into the fresh feed and circulating oil pipes to prevent leakage of high-temperature and high-pressure flanges caused by rapid cooling. If a flange leaks as it cools, steam should be used immediately to blow the oil and gas clear to prevent fire.
Thermal expansion and stress during heating can cause minor leaks at the joint between flange and gasket. When this happens, a steam hose should be placed at the leak point to blow away oil and gas and prevent a fire before the connection is tightened. To limit the harm from thermal expansion, the heating rate should generally not exceed 25 C per hour.
After maintenance, or during the first start-up, the bolts on the equipment pipeline may have been replaced or disassembled. These bolts are tightened at room temperature in a cold state, whereas in normal production they are tightened at higher temperatures, so the tightening condition must be accounted for.
Handling Measure 2: Repairing the Sealing Surface
Because corrosion on the contact surface of the flange ring is uneven, the surface must be repaired in accordance with the applicable pressure vessel safety supervision regulations. The equipment connection flange is connected to the equipment body and can be repaired on site using specialized tools. These tools consist of a support plate, a central axis and a tool holder.
Handling Measure 3: Gaskets, Bolt Load and Digital Assembly
High-performance gaskets are crucial for reactors that operate across a wide temperature range. Combining high-precision constant torque tightening, digital on-site flange assembly quality assurance and online monitoring of sealing status can further improve sealing performance and reliability.
Selecting high-temperature resistant bolts is one of the key measures for keeping the leakage rate extremely low across an overhaul cycle after the inlet and outlet flanges are insulated. This reduces energy consumption and environmental impact while improving production efficiency and safety.
Online continuous monitoring of sealing status and leakage trends helps operators understand equipment condition and prevent hazards. Real-time analysis of flange sealing data supports informed decisions on when to perform hot tightening, which reduces failure risk.
Results After Repair
After the repair described in the case, a new elliptical metal ring gasket was installed. After on-site polishing, pressure testing and air tightness testing, the joint was put into operation and no leakage occurred during a major overhaul cycle. During regular maintenance the sealing surface was disassembled and inspected and found to be in good condition.
FAQ
What is the most common cause of flange leakage?
Repeated start-up and shutdown cycles cause gasket fatigue, and long-term high temperature causes creep and stress relaxation of bolts and gaskets, reducing sealing pressure until a leak starts.
How does a small leak become a large one?
High-pressure material overflowing from the leak point erodes the sealing surface and causes radial pitting, so the damage accelerates once leakage begins.
What heating rate should be avoided?
To limit the harm from thermal expansion, the heating rate should generally not exceed 25 C per hour, and abnormal temperature fluctuations should be avoided.
Can flange sealing surfaces be repaired on site?
Yes. Equipment connection flanges can be repaired on site using specialized tools made up of a support plate, a central axis and a tool holder, in line with the applicable safety supervision regulations.
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