Overall introduction of valve leak plugging technology with pressure
Valve leakage often occurs on packing, flange seal and valve body. Valve leakage for a long time can cause erosion of valve stem and flange seal surface, and eventually lead to valve scrap, coupled with loss of medium fluid, which increases power plant consumption, increases cost and reduces economic benefits. If the medium fluid is poisonous, flammable, explosive, corrosive and other leakage occurs, it is prone to poisoning, fire, explosion and other casualties and accelerate the corrosion rate of plant equipment, shorten its service life, seriously pollute the surrounding environment, destroy power production, and damage people's health. The existence of leakage seriously threatens the safety of production and increases the number of unplanned shutdown accidents.
Forms and factors of valve leakage
1. Leakage and Cause of Valve Packing
During the operation and use of the valve, there is relative movement between the stem and the packing, which includes rotation and axial movement. With the increase of the number of switches, the number of relative motion increases, as well as the influence of temperature, pressure and the characteristics of fluid medium, valve packing is the most vulnerable part of leakage. It is due to the gradual weakening of contact pressure of fillers, aging of fillers themselves, loss of elasticity and other reasons. At this time, the pressure medium will leak out along the contact gap between the filler and the valve stem, which will blow part of the filler away and wash the valve stem out of the groove for a long time, so as to expand the leakage.
2 Flange Leakage
The flange seal of the valve mainly relies on the pre-tightening force of the connecting bolt to achieve sufficient sealing specific pressure through the gasket to prevent the leakage of the sealed pressure fluid medium. There are many reasons for its leakage, such as the insufficient pressure of sealing gasket, the roughness of the joint surface does not meet the requirements, the gasket deformation and mechanical vibration, etc. will cause the leakage of sealing gasket and flange joint surface. In addition, bolt deformation or elongation, gasket aging, resilience decline, cracking, etc. will also cause flange surface seal lax and leakage. There are also human factors that can not be neglected in flange leakage, such as the deviation of sealing gasket, insufficient tightening force of local sealing pressure, exceeding the design limit of sealing gasket, uneven force in flange tightening process or deviation of the center line of two flanges, which are prone to leakage.
3. Leakage and Causes of Valve Body
The leakage of the valve body is mainly caused by casting or forging defects in the production process of the valve, such as sand holes, gas holes, cracks, etc. The erosion and cavitation of the fluid medium are also common factors causing the leakage of the valve body.
Principle and advantages of leak plugging under pressure
1. Principle of leak plugging under pressure
The principle of leak plugging under pressure is based on the sealing mechanism of solid sealing material in liquid medium under dynamic conditions. The method is to install special equipment in the leakage part, use the chamber formed between the sealing part and the special equipment, use special high-pressure injection tool to inject the sealant into the chamber, and fill the whole chamber space, so that the pressure of the sealant is balanced with the pressure of the leaking medium, and establish a new sealing structure to plug the channels of the leaking pore and block the leakage of the medium.
2 Advantages of Leakage Stoppage with Pressure
(1) There is no need to shut down or isolate the system.
(2) There is no need to decompress the system.
(3) Save a lot of energy and manpower.
(4) It greatly reduces the power loss caused by equipment isolation or shutdown.
(5) Reducing social and economic losses.
Introduction of Several Pressure Blocking Methods for Common Leakage Points
According to the specific situation of the factory production site, the general leakage can be eliminated by replacing valves, replacing valve fillers, replacing flange gaskets or welding holes. But for the valves in production and operation, in the case of no isolation, it is necessary to take appropriate technical measures to eliminate leakage, in order to ensure the normal operation of the unit safety in production.
1. Leakage plugging treatment of valve packing chamber in power plant under pressure
The injection pressure plugging technology is a relatively safe and reliable technical means at present. It uses special fixtures and hydraulic injection tools to inject sealant into the sealing cavity formed by the external surface of the fixture and the leakage part, and quickly compensates for various complex leakage defects. Under the condition that the pressure of injection is greater than the pressure of leaking medium, the leakage is forcibly stopped, and the injection itself changes from plastic body to elastic body in a short time, forming an elastic sealing structure and maintaining a certain specific pressure of working seal, so as to achieve the purpose of re-sealing. At present, there are two kinds of sealants produced and used at home and abroad: one is thermal curing sealant, which can only be changed from plastic to elastomer at a certain temperature, and the other is non-thermal curing sealant, which is suitable for dynamic sealant operation at room temperature, low temperature and high temperature. Most of these sealants are rod-shaped. The body or two-component greasy material, after being installed in the high-pressure injection gun, has good injection technicality and filling property under certain pressure, and does not lose the function of valve switch. Here are two common methods:
(1) When the wall thickness of the valve packing box is more than 8 mm or so, when the defect is eliminated by using the injection pressure plugging under dynamic conditions, the injection hole can be directly opened on the wall of the valve packing box. The sealing cavity is the valve packing box itself, and the function of the sealing agent injected into the valve packing box is the same as that of the filler. Firstly, drills with diameters of 10.5mm or 8.7mm are used to drill holes in the proper position of the outer wall of the valve packing box. The holes can not be drilled, leaving about 1-3mm. Remove the drill bit, tap with M12 or M10 taps. After tapping, screw the special plug valve for injection and keep it in the open position. Drill the remaining valve packing wall with a long rod drill with diameters of 3mm. At this time, the leakage medium will follow. In order to prevent the high temperature, high pressure, poisonous or corrosive medium from spraying out and hurting people during drilling, a baffle can be used before drilling small holes. Firstly, a 5 mm diameter hole can be drilled with a bit on the baffle so that it can be sleeved on a long bit, and the remaining wall thickness of the baffle drill will not be dangerous. After drilling through the small hole, take out the bit, screw the special cock valve for injection to the closed position, cut off the medium and connect the high pressure injection gun for injection sealing operation. If the medium pressure in the valve packing box is low, the small hole can also be drilled directly with a long drill with a diameter of 3 mm, and then sealed injection operation can be carried out.
In June 2003, the self-sealing Grand Leakage of No. 3 electric main steam valve in Panzhihua Iron and Steel Power Plant solved the leak problem of the valve by using this plugging technology, and avoided a non-stop operation.
(2) For the valve with thinner packing wall, auxiliary fixture can be used for dynamic sealing operation. The auxiliary fixture is just to make up for the insufficiency of the wall thickness of the valve packing box, which is equivalent to a special joint fixed on the outer wall of the valve packing box to connect the high-pressure injection gun. It is difficult to get the ideal local fitting surface by the fixture machining method. When the conditions permit, the outer wall of the valve packing box can be repaired properly to make it fit better with the auxiliary fixture. If the shape of the outer packing wall of the leak valve is complex or the dressing condition is not allowed, an asbestos rubber plate or rubber plate can be padded at the bottom of the auxiliary fixture, and the clamp bolt can be tightened so that the rubber plate padded below can well plug the gap of the fitting surface. The auxiliary fixture should have a thread patch matching the injection cock valve. The next step is the same as the wall thickness of the packing box. After the whole sealing operation is completed, do not switch the valve immediately until the sealing agent solidifies, it can be put into normal use.
In mid-November 2002, the balance valve flange of the high-pressure heater intake valve leaked. Because the filling wall of the valve was thin, the auxiliary fixture was used to seal the valve dynamically, which solved the leakage problem of the valve.
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