Cause Analysis and Countermeasure of Flange Seal Leakage
Leakage is a common phenomenon in chemical production, and it is also a problem that must be solved in safety production of chemical industry. Once all kinds of chemical pipelines and equipment leak, they will be wasted, and the whole plant will be shut down. Some leaks will do harm to people and the surrounding environment. Therefore, people do everything possible to prevent leakage in production practice. In chemical pipeline transportation process, flange connection and sealing are necessary, and the quantity is large and wide. Reasonable selection of flange sealing form and gasket material can effectively prevent leakage, and make flange connection pipeline maintenance convenient, cheap, durable and less consumption.
|What are the causes of flange leakage|
|Several commonly used plugging methods for flange leakage|
|What should we do to avoid the flange leaking|
Reasons for Selection and Design
1.1 Reasons for flange selection and design: We all know that the selection of standard flange should be based on the design conditions such as operating medium, design temperature, design pressure and flange material, according to the flange standard temperature and pressure rating. However, it is often neglected that the temperature and pressure rating given by the flange standard does not take into account the effects of thermal expansion and contraction on the external loads imposed on the flange and the external loads such as water hammer on the pipeline during the use of pipelines and even equipment, and it is impossible to consider the effects of accidental loads such as wind, earthquake and foundation settlement. These additional loads exist objectively. Once the flange is too large, the flange will inevitably leak. !
1.2 The choice of flange and gasket is not compatible: For example, if we use winding gasket for PN6 flange, is it good? Not good! PN6 flange is very thin, stiffness is very small, and winding pad stiffness is relatively large, pre-tightening, it is easy to produce flange deformation and lead to leakage!
1.3 The improper choice of gasket itself, such as the use of graphite winding gasket at 500 C and 20 MPa, graphite is easy to oxidize at high temperature above 400 C for a long time, which makes gasket lose its inherent resilience and naturally difficult to achieve long-term sealing effect (this is a problem that can not be verified by pipeline pressure test and leakage test).
Reasons for Processing and Installation
2.1 Leakage caused by unqualified quality in material installation: In addition to manufacturer’s reasons, construction site often occurs, because construction and installation units do not take care of the arrived francs carefully, resulting in damage to the sealing surface, which will naturally leak when using, or the winding pads on the arrival site, often due to improper transportation and storage and blind re-erection or maintenance of the dismantled winding pads. It is also the common cause of flange leakage. Of course, another reason for the winding pad loose rack is that the manufacturer’s manufacturing ability is poor. When winding, the necessary external load is not applied to tighten the winding steel strip or spot welding at will, which is insufficient.
2.2 Leakage caused by installation quality problems: During installation, the leakage caused by flange deformation caused by forced alignment or seal failure caused by inadequate alignment is a common problem. Another problem is that when pre-tightening, the pre-tightening force is too small or too large, leading to gasket failure and leakage.
In petroleum and chemical industry, due to the influence of temperature, pressure and corrosive medium, leakage on the flange surface of material pipeline and equipment is common. In order to eliminate hidden problems, it is necessary to stop and deal with them after stopping. Here is a brief introduction of the non-stop Pressure Leak Stoppage Technology of leakage flange. The sealing method of flange leakage can be determined according to the parameters of medium pressure, temperature and flange joint clearance.
1. Directly twisted seam enclosure method
When the gap between two flanges is less than 1 mm and the gap is more uniform, and the medium pressure is relatively low, the direct twisting method can be used. The so-called direct twist seam is to twist the flange joint gap tightly with flat punch and hammer instead of special fixture, forming a sealing cavity. The injection can be injected into the sealing cavity through special bolt assistant joint to stop leakage.
2. Copper Wire Twist Seam Closure Method
This method is used when the medium pressure is not higher than 2 MPa and the flange joint clearance is not more than 5 mm. The so-called copper wire twisting method is to twist the copper wire with the diameter equal to the flange gap into the gap with a certain amount of overlap, twist the gap with flat punch to form a new sealing cavity, and inject the sealant into the joint with special bolt injection agent to achieve the purpose of leak stopping. The specific operation is to install the G-shaped clamp near the leakage, loosen a nut nearby and screw it down, quickly install the special injection joint for bolts, and tighten the nut. Remove the G-shaped clamp and move to a distance from the leak point. Repeat the above process and install the second injection joint until the quantity is sufficient. The copper wire twice the circumference of the flange is inserted into the flange clearance and hammered to make it completely embedded in the flange clearance. A spherical punch is used to punch the flange joint gap out of plastic deformation briefly. After these works are completed, the sealant can be injected as required. The injection sequence is shown in Fig. 1 (a) (b) (c) (d).
When choosing and injecting sealants, attention must be paid to the effects of medium temperature and ambient temperature. When the ambient temperature is normal and the medium temperature is higher than 40 C, there is no need to heat. If the medium temperature is lower than 40 C and the ambient temperature is lower than normal temperature, the agent gun should be heated to improve its fluidity. In order to control the injection dosage and avoid injecting the injection into the leakage system, the volume of the clamp cavity should be estimated in order to control the injection volume.
3. Steel band enclosure method
Install the G-shaped clamp near the leakage, loosen the bolt nearest to the G-shaped clamp, remove the nut, install the special injection joint for the bolt, and tighten the nut again. Move the G-shaped clamp farthest from the leak point and repeat the above procedure to install the second injection joint. This is repeated until the injection joint is fully assembled. Insert asbestos packing or metal wire into flange clearance, its length should be longer than the circumference of flange, and there is no less than 50 mm lapping allowance. The lapping is staggered at the leakage point. Choose the steel strip whose width meets the requirement and length is longer than the circumference length of flange. Install the steel strip clamp and tighten it with the belt fastener. Tighten the fastening screw of the belt clamp, cut off the redundant part, and operate the copper wire enclosure method.
4. Convex flange fixture method
The convex flange fixture method has a wide range of applications (as shown in Figure 2), and can also be used for plugging when the temperature and pressure are relatively high. The general operation steps are as follows.
- 1) Each injection hole of the fixture is numbered into the injection valve, and each injection valve is in the open position. When fixture is installed, the injection hole is between two connecting bolts, and the injection hole is ensured near the leakage hole. In order to ensure the safety of leak plugging construction of flammable and explosive media, explosion-proof tools are used in the operation process.
- 2) Close the injection valve farthest from the leak point, connect the injection system on it, and start the injection operation. The sequence of plugging injection agent is shown in Figure 3 (a) (b) (c) (d). The remaining steps are the same as the copper wire enclosure method.
- 3) The dosage of injection should be controlled according to the volume of the clamp cavity. The injection pressure should not be higher than 10 MPa when the medium pressure of the system is less than 10 MPa. When the medium pressure is higher than 10 MPa, the injection pressure should be added 10 MPa.
5. Concave Flange Fixture Method
The concave flange fixture method is mainly used in cases where the gap between flanges is relatively small (as shown in Figure 4). The reliability of sealing is not as good as that of convex flange fixture. Because there is no convex platform, it is difficult to install and locate, that is, it is not easy for the groove to align completely with the flange clearance. The glue injection of this fixture is the same as that of the convex flange.
- 1) The clamp design in the plugging implementation must conform to the specifications of pressure vessel equipment.
- 2) During operation, the top pressure of manual hydraulic pump must be calculated to ensure the injection pressure range.
- 3) When an injection hole needs to be replaced and injected, attention must be paid to the agent valve in the injection gun.
- (1) In strict accordance with the standards, flange pressure grade, sealing surface type and gasket type should be selected with appropriate consideration of external load. The choice of flange material and gasket material must be compatible with the design conditions, and the choice of flange type and sealing surface type should be matched with gasket type (to avoid low stiffness flange choosing winding gasket, etc.).
- (2) For important occasions, such as pipelines with toxic media, pipelines with pressure grade (> CL600) and DN > 150, relevant theory can be used to analyze flange leakage. The available methods of flange leakage analysis include equivalent pressure method, bolt area method, flange stiffness control method and flange strength control method. Considering the different theories adopted by the above methods, at least the first three methods are analyzed when necessary. Detailed introduction of the above analysis method can be found in the relevant articles of the pressure pipeline man Weixin Public Number. When the project has special provisions, it can also increase the number of pipelines for flange leakage analysis.
- (3) Choose high-performance flanges. For example, yaang flange, its ingenious structure can largely prevent flange leakage (see Fig. 1 below); for example, a manufacturer’s high-temperature Gullit winding gasket, whose winding material can ensure the high-temperature resilience of winding tape; for example, a manufacturer’s Change gasket, which combines the double advantages of tooth cushion and winding gasket, is not easy to break up, but also can maintain sufficient resilience; For example, the disc spring pre-tightening flange design of a manufacturer, through the disc spring, imposes a nearly constant pre-tightening force on the flange to ensure the reliability of the flange in the service life (see Figure 2 below).
- (4) Choose excellent flange manufacturer, the same sealing structure, different manufacturers, their manufacturing process, product quality are very different. In particularly important occasions, manufacturers with good reputation and outstanding ability should be selected.
- (5) Strictly control the transportation, on-site storage and pre-use inspection of flanges, gaskets and fasteners, and do not apply any unqualified components to production devices.
- (6) Strictly control the construction quality, eliminate the damage caused by strong pulling and hard pulling, and force alignment to flange; in important occasions, such as high-pressure flange with pressure grade > CL600, pre-tightening should be carried out according to the pre-tightening moment calculated by design, which should not lead to insufficient or overloaded pre-tightening; bolt tightening should be carried out smoothly and strictly in accordance with the operating specifications to ensure the tightening sequence and ensure that each bolt is subjected to loading. After completion of construction, strength test, toxic and flammable explosive pipelines and leakage test should be carried out to ensure the effect of leakage test.
- (7) For flammable and toxic medium pipelines, flammable gas alarm and toxic gas alarm should also be set up in accordance with the requirements of relevant standards and norms to ensure that accidents can be detected at the first time.
- (8) Strengthen the training of operators, make a good plan for dealing with the leakage accidents of flanges of dangerous media, avoid the spread of accidents and cause major damage.
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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