CAUSE ANALYSIS OF FANGE LEAKAGE OF HIGH PRESSURE VALVE AND TREATMENT UNDER PRESSURE

Analysis of leakage causes in common parts of high pressure valves

During the process of alumina production, there are a large number of high and medium pressure mud valves. After many years of use, the valve flanges generally have some slight leakage due to erosion and wear of the transported mud, various stress changes, and operating vibration. If the sealing element is not replaced or replaced, the problem of disintegration of the leak cannot be solved.

 

Leakage analysis of valve connection flange surface

The connecting flange of a valve is the most common and important connection form for connecting pipes, fittings, valves, instruments, and equipment. The sealing of the flange is realized through the synergetic action of fastening bolt, flange and gasket. Gaskets are necessary to achieve the core of the seal and tighten the bolts to keep the flange sealed to the gasket. The minimum pre-tightening force is only sufficient bolt pre-tightening force to maintain sufficient friction between the flange and the gasket to prevent the flange gasket from maintaining an effective seal between blowouts. Although the flange connection has the characteristics of easy disassembly and no heat generation, due to the structural characteristics of this flange, it relies heavily on the fastening method and the fastening bolts. The torque is friction at the end of the bolt, which consumes 40% of the friction torque on the thread, and only about 10% of the torque is used to generate the flange pretension.  

 

At the same time, even after bolt fastening, the flange still has the influence of various stresses and the plastic deformation of gasket. When the pressure in the pipeline system increases, the flange between the valve and the pipeline shows a trend of separation. The gasket between the bolt and the flange will gradually produce elastic or plastic deformation under pressure and stress. The tightening force on the bolt will reduce the tightening force on the flange and the gasket on the flange. When the pressure acts on the gasket, the cross-section is reduced to equal the pressure inside the piping system, keeping it sealed. At this point, the remaining compression force on the gasket is the effective fastening force of the gasket. Leaks occur when the effective tightening force is reduced to less than the pressure in the piping system.

 

Valve body flange leakage analysis

Compared with valve flange leakage, valve connection flange leakage is much less, and it is an important part of valve leakage, and the valve flange is not a standard part, and its shape is irregular, so it is more complicated to handle valve flange leakage. The non-metal gasket is used between the flange and the valve, and has been assembled in the factory and in the experiment. Caused by pipeline stress, plastic deformation and vibration of the valve. However, the valve flange shape is not completely regular round. So it's the same leak but it's not handled the same way.

 

In general, flange leakage can be handled by retightening the bolt. However, in fact, the effect of fastening bolt to deal with leakage is not optimistic, the main reasons are:

 

a. When the gasket is pressed from the flange for a long time, plastic deformation will inevitably occur. The deflection of the gasket is closely related to the squeeze of the gasket. The greater the pressure between the flange and the gasket is, the more severe the deformation is. The compression rate and elastic acceleration rate of the gasket must also have a great impact, and severe deformation will make the gasket lose its original function of the gasket.

 

b. The stress in the pipeline causes the pressure on the gasket to be reduced by the flange. The bolt is also a process of continuously lengthening the flange while pre-tightening the flange between the valve and the pipeline. This will make this process more difficult. When the gasket is plastically deformed, the pressure on the gasket can only be supplemented by further reducing the flange gap. This will undoubtedly increase the stress conductivity of the pipeline. The pre-tightening force of the bolt on the flange is increasing, but the pressure of the flange on the gasket has little effect.

 

c. The leakage of impurities causes the gap between the sealing surfaces to not completely fit. The medium in the valve will remain between the flange and the gasket during the leakage process, and the medium will be scratched during the leakage. The blue sealing surface causes penetrating injuries to the flange and the gasket. If a penetrating injury occurs to the sealing surface, the entire flange sealing surface will lose its effect.

 

Common ways of plugging under pressure

There are many methods of plugging under pressure. Depending on the processing method, there are support methods, top pressure method, clamp method, gland method, packing method, hood method, expansion method and other processing methods and according to the site conditions. Processing capacity, can also be a reasonable combination of the above two or more methods of plugging, and in the valve pressure under pressure treatment, the valve body punching injection method, fixture plugging method, cover method.

 

a. Valve and pipeline flange leakage treatment

Plugging with clamps is mainly used for leaks in valve flanges. It is simpler to handle the leakage of flanges between valves and pipelines with clamps because these connecting flanges are round and are standard parts. The manufacturing and installation of the fixture is relatively simple, and the leakage of the valve body flange is more complicated because the flange is not a regular circle and is not a standard part.

 

In the technology of plugging under pressure, the quality of the design and manufacture of the fixture has a very important role. The fixture is not used to plug directly, but it plays an auxiliary role. The clamp has two semicircles. Tightly clamp the two semicircles on the outside of the leaking flange, and fix them. The arc of the clamp matches the outer arc of the flange, and a plastic injection hole is reserved in the clamp. After the clamp is fixed, Through special injection tools, the leak-tight sealing sealant is injected into the fixture and the flange to form a dense filling between the flange and the fixture, and a new seal is formed on the outer ring of the flange. Play a role in plugging the leak point.

 

b. Valve body flange leakage treatment

Valve flanges generally do not use the method of plugging the installation fixture, but instead use flange gap butt welding to weld the flange outer ring. Due to the continuous leakage of the valve medium, full welding cannot be performed. Constantly weld the plug valve or screw cap to ensure that the leaked medium can be continuously discharged. It cannot form pressure during welding. After the circle is fully welded, connect the injection tool and the plug valve to start the injection to make the leakage area reappear. Sealed to prevent leaks.

 

Leak plugging under pressure is an efficient, convenient, and low-cost leakage treatment method. Although the leaked parts treated with pressure plugging can generally guarantee no leakage for six months or years, plugging under pressure can only be a temporary. For the measures to be taken, the leaked valve must be used to stop the replacement of worn spare parts or replace the valve with a new one.

 

Jaze is a company that has long been committed to supplying flanges, butt welding fittings, castings, valves and other related products. If you have any professional knowledge about the above products, you can feel free to consult.

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