Views: 3576 Author: Site Editor Publish Time: 2020-02-19 Origin: Site
Valve is the most commonly used equipment in chemical enterprises. There are all kinds of valves in chemical plant, such as ball valve, ball valve for steam, butterfly valves, check valves, and forged steel valves. Control valve is one of the automatic control system of the terminal control components. There is a lot of working condition of high temperature and high pressure in the chemical plant, some medium have strong corrosion resistance and toxicity, and they are also flammable and explosive. When the filler valve leak, not only cause the waste of raw materials, but also cause serious pollution to the environment, and even cause fire, explosion, poisoning and other life threatening accident Therefore, enough attention should be paid to the leakage of control valve packing, it is very important to choose seal packing reasonably in design and selection.
The valve part is composed of valve internal parts and body, valve internal parts including the valve core, valve stem, stuffing box and the upper cover and so on. The stuffing box is used to seal the stem, is to prevent the process medium through reciprocating or rotary movement in the stem surface leakage, it is an integral part of the body, the valve stem seal is almost all the use of stuffing box to achieve.
The control valve packing is the filling material of the moving seal, generally installed in the packing box of the upper bonnet, to prevent the controlled medium from leaking due to the movement of the valve stem.
Commonly used fillers are mainly divided into two categories by material: polytetrafluoroethylene and flexible graphite.
There are various reasons for packing leakage in the actual use of the control valve on the site. In the site maintenance, specific analysis and judgment should be made according to the actual use of the valve, so as to fundamentally eliminate the causes of leakage and solve safety hidden dangers. According to the law of leakage, the main reasons can be summarized as follows:
(1) Improper Selection of Packing Material and Type: for example, teflon packing is selected in the condition of 150℃-200℃, the packing will undergo micro-creep for a long time under the critical working condition, and the sealing performance of the packing will decline under the action of high pressure medium; And such as liquid ammonia, tar, fuel and other permeable media choose woven type filler, these conditions are affected by high temperature and high pressure medium, easy to produce leakage.
(2) Improper Packing Installation Method: after packing into the stuffing box, the gland of the axial pressure, due to the plastic packing, make its produce radial force, and closely contact with the valve stem, such as packing under installation to tighten the loose, force not divide evenly, make this contact is very uneven, some parts loose contact, didn't even contact, causing packing leakage.
(3) Control valve in the use of the process, there is a relative movement between the stem and packing, this movement is called axial movement. In the use process, with the influence of high temperature, high pressure and strong permeability of the fluid medium, the control valve stuffing box is also the place where more leakage occurs. The main cause of packing leakage is the interface leakage, for textile packing will also appear leakage (pressure media along the tiny gap between the packing fibers leakage outward). The leakage of the interface between the stem and the packing is caused by the gradual attenuation of the packing contact pressure, the aging of the packing itself, loss of elasticity and other reasons, then the pressure medium will leak out along the contact gap between the packing and the stem.
(4) Frequent action of the control valve, bending of the valve stem, wear, corrosion, and decline in finish result in packing wear and leakage.
(5) The operating conditions of the control valve change. For example, during the heating process of the device, the control valve has a process from cold state to hot state. The hot state medium changes greatly.
(6) Packing gland not tight, deflection, or control valve horizontal installation, will cause the stem and packing contact is not good, clearance is too large or too small.
(1) For the convenience of packing, chamfering at the top of the stuffing box and placing appropriate metal protection rings (the contact surface with the packing box shall not be inclined) for anti-erosion clearance at the bottom of the stuffing box to prevent the packing from being pushed out by medium pressure.
(2) Improve the surface precision and finish of valve stem and packing box. If the sum of the friction coefficients between the various moving parts is zero, the force acting on the packing gland can be uniformly transmitted to the whole packing without any attenuation. But the actual sum of the friction force exists, can not be zero, the radial force acting on the packing with the distance from the gland increases and decreases, the greater the friction force, the greater the pressure attenuation. As soon as the pressure sealed exceeds the force on the packing, it begins to leak. Therefore in the maintenance of the stem and stuffing box can not have scratches, pitting and wear, the finish should be good.
(3) The selection of appropriate materials is important. It must be capable of resisting temperature changes, creep resistance, relaxation resistance and oxidation resistance. Generally, tetrafluoride is preferred when conditions meet, and graphite is not. Also can choose mixed packing: one is graphite packing and tetrafluoride packing mixed use; The other is the mixed use of o-ring and v-type packing. Graphite packing can be used for permeable media.
(4) When packing, it should be added round by round, and press one by one with something.to make the packing force uniform. The open packing shall be staggered up and down 90° or 120° and the number of turns added to the packing shall be appropriate to no leakage. Add less packing gland will enter the stuffing box, easy to cause leakage.
(5) For the ptfe packing acted by the spring, the gland screws should be tightened symmetrically without deviation. Other types of fillers do not need to be screwed too tightly so as not to leak.
(6) After the new valve or just overhaul of the control valve into service, should check whether there is any leakage of packing, if there is leakage to timely deal with, in order to prevent more and more leakage of packing.
In the field of many years of practical application, the control valve occurs packing leakage reasons are varied, as long as we are diligent observation, carefully analysis of the causes of leakage, the problem of packing leakage can be eliminated; But also should see, some control valves produced packing leakage reasons are more complex, leakage factors are more, but also need to continue to analyze and explore in the future practice.