The factors that affect flange sealing are multifaceted. Mainly including: bolt pre tightening force; Sealing surface type; Gasket performance; Flange stiffness; Operating conditions.
The sealing principle of ANSI flanges is extremely simple: the two sealing surfaces of the bolts compress the flange gasket and form a seal. But this also led to the destruction of the seal. In order to maintain sealing, it is necessary to maintain a grand bolt force. To achieve this, the bolts need to be made larger. And larger bolts need to be matched with larger nuts, which means that bolts with larger diameters are required to create conditions for tightening the nuts. Little did they know that the larger the diameter of the bolt, the more curved the applicable flange will become. The only way is to increase the local wall thickness of the flange. The entire installation will require a significant amount of size and weight, which has become a special issue in offshore environments, as weight has always been a major concern in this situation. Moreover, fundamentally speaking, ANSI flanges are an ineffective seal that requires 50% of the bolt load to be used to compress the gasket, while only 50% of the load is used to maintain pressure.
However, the main design flaw of ANSI flanges is that they cannot guarantee no leakage. This is the lack of design: the connection is dynamic, and periodic loads such as thermal shrinkage and fluctuations can cause movement between flange surfaces, affecting the functionality of the flange, thereby damaging its integrity and ultimately leading to leakage.
The newly developed new flange by Vector Company overcomes the defects of ANSI flanges. Compact flanges have various advantages: standardized flanges not only save space and reduce weight, but more importantly, ensure that joints do not leak. The reason why the size of the compact flange is reduced is because it reduces the diameter of the sealing element, which will reduce the cross-section of the sealing surface. Secondly, the flange gasket has been replaced by the sealing ring to ensure that the sealing face matches the sealing surface. In this way, only a small amount of pressure is required to tighten the sealing surface. As the required pressure decreases, the size and quantity of bolts can be correspondingly reduced. A new product with a small volume and light weight (70% to 80% lighter than traditional ANSI flanges) was ultimately designed.
Handling method for flange leakage and sealing
1、 Leakage location and situation: The bolts connecting the flanges on both sides of the DN150 valve body are leaking. Due to the extremely small gap between the flanges, it is impossible to inject sealant through the gap to eliminate leakage. The leakage medium is steam, with a leakage system temperature of 400-500 ℃ and a system pressure of 4MPA.
2、 The sealing construction method is based on on-site investigation of the leakage site. To achieve limited sealing, a fixed fixture method is used to accommodate the leakage point to form a sealing chamber, and sealant is injected to eliminate leakage.
1. Fixture design
(1) Confirmation of fixture construction method
① Accommodate leakage points and establish a sealing chamber between the valve body flange and the pipe flange to connect the short joint flange. To avoid potential leakage at the gap between the valve body and the flange due to pressure suppression, a circular cavity is set up for glue injection at the junction of the fixture and the outer edge of the valve body flange.
② During the injection process of reducing flange, the fixture is prone to displacement towards the side of the small diameter flange, so a limit measure of tooth contact clamping is adopted.
2. Sealant selection and dosage budget
(1) The selection of sealant is based on the temperature of the leakage system and the characteristics of the leakage location, and TXY-18 # A sealant is used. This sealant has excellent temperature resistance, medium resistance, and injection process performance, making it easy to establish an average dense sealing structure, and the sealing can persist in long-term stability.
(2) The usage budget requires 4.5KG of sealant to be used for leakage points on one side.
3. Construction Operations
(1) The fixture device, due to tooth shape contact, has a smaller inner diameter size of the tooth tip. When installing, it requires the circumferential ring to strike the outer wall of the fixture, causing deformation of the tooth end and clamping the limit.
(2) After the injection operation is completed, the fixture, valve body, and flange ring cavity are injected into the sealing cavity, and then the intermediate cavity injection is stopped. The injection process should be balanced with pressure, and attention should be paid to refilling and pressing to avoid stress relaxation.
(3) After the sealant has solidified, after observing the effect, stop partial injection and compaction to avoid stress relaxation, and then seal the injection hole.