There are many factors that affect flange sealing.
(1) Bolt pre-tightening force;
(2) Type of sealing surface;
(3) Gasket performance;
(4) Flange stiffness;
(5) Operating conditions.
For decades, ANSI flanges designed by the American National Standards Institute have provided the oil and gas industry with a suitable sealing technology, with great success. But as the needs of the oil and gas industry have changed, the deficiencies of ANSI flanges continue to be exposed.
The principle of cost-effectiveness, environmental protection and the best assurance of health and safety are the three major issues facing operators and contractors in the 21st century, so people are starting to consider new flange sealing methods.
The sealing principle of ANSI flanges is extremely simple: the two sealing surfaces of the bolt press against each other and form a seal. But this also leads to a breakdown of the seal. In order to maintain the seal, it is necessary to maintain a huge bolt force. For this reason, the bolts have to be made larger.
And larger bolts match larger nuts, which means that larger diameter bolts are needed to create conditions for tightening the nuts. As everyone knows, the larger the diameter of the bolt, the more suitable the flange will become, the only way is to increase the wall thickness of the flange part.
The entire device will require enormous size and weight, which is a particular problem in the offshore environment, where weight is always the main concern one has to take.
And, fundamentally, an ANSI flange is an ineffective seal, requiring 50% of the bolt load to compress the gasket and only 50% to hold the pressure.
However, the main design disadvantage of the ANSI flange is that it is not guaranteed to be leak free. This is the flaw in its design:
Connections are dynamic, and cyclic loads such as thermal expansion and undulations can cause movement between flange faces, affecting flange function, compromising flange integrity, and ultimately leading to leakage.
The compact flange offers various advantages:
Standard flanges not only save space and weight but more importantly, ensure that no leakage occurs at the joint.
The reduction in size of the compact flange is due to the reduced diameter of the seal, which reduces the cross-section of the sealing face.
Second, the flange gasket has been replaced by a sealing ring to ensure the matching of the sealing face to the sealing face. In this way, only a small amount of pressure is required to compress the cover.
As the required pressure decreases, both the size and required number of bolts can be reduced accordingly. Finally, a new product with small size and light weight (70% to 80% lighter than the traditional ANSI flange) was designed.
Flange leak sealing treatment method
1. Leakage location and condition
Due to the extremely small flange connection gap, it is impossible to inject sealant through the gap to eliminate leakage. The leakage medium is steam, the leakage system temperature is 400-500℃, and the system pressure is 4MPA.
2. The sealing construction method is based on the site survey of the leakage site
In order to achieve limited sealing, a fixed clamp method is used to contain the leakage point to form a sealed cavity, and sealant is injected to eliminate leakage.
1. Fixture Design
Fixture structure is determined
①Contain the leakage point, and establish a sealing cavity between the valve body flange and the pipe flange connecting the short joint flange. In order to prevent the potential leakage of the gap between the valve body and the flange from re-leaking due to pressure, an annular cavity is set up to inject glue at the place where the clamp and the outer edge of the valve body flange match.
②During the injection process of the reducing flange, the clamp is easy to be displaced to the side of the small diameter flange, so the limit measure of tooth contact clamping is adopted.
2. Sealant selection and dosage estimation
(1) Selection of sealant According to the temperature of the leakage system and the characteristics of the leakage part, TXY-18#A type sealant is used. The sealant has excellent temperature resistance, medium resistance and injection process performance, it is easy to establish a uniform and dense sealing structure, and the sealing can maintain long-term stability.
(2) The amount of sealant is estimated to be 4.5KG for one-side leakage point.
3. Construction operations
(1) During the installation of the fixture, due to the tooth-shaped contact, the inner diameter of the tooth tip is small. During installation, the outer wall of the fixture needs to be tapped by the peripheral ring to deform the tooth end and clamp the limit.
(2) After the injection operation is completed, the clamp, the valve body and the flange annular cavity are injected into the sealing cavity, and then the intermediate cavity is injected. During the injection process, pressure should be applied evenly, and attention should be paid to refilling and pressing to prevent stress relaxation.
(3) After the sealant is cured, after the effect is observed, local injection and compression are carried out to prevent stress relaxation, and then the injection hole is closed.