Control valve is one of the automatic control system of the terminal control components, due to the chemical plant, there are many high temperature, high pressure conditions, some media have strong corrosive and toxic, and flammable and explosive, when the valve packing leakage, not only cause waste of raw materials, but also cause serious pollution to the environment, and even cause fire, explosion, poisoning and other life-threatening safety accidents. Therefore, the control valve packing leakage problem should be paid enough attention to, in the design selection of a reasonable choice of sealing packing is very important. In view of the control valve packing seal, combined with years of work experience and related information, through the control valve packing box structure form analysis, introduces the characteristics and application of PTFE and flexible graphite packing, and the reasonable choice of control valve packing are briefly introduced.

1. control valve packing role and classification:

Control valve: The valve part is composed of the inner parts of the valve and the valve body. The inner parts of the valve include the valve core, the valve stem, the stuffing box and the upper valve cover. The stuffing box part is used to seal the valve stem. The elastic method prevents the process medium from leaking on the surface of the valve stem through reciprocating or rotating movement. It is an integral part of the valve body. The valve stem sealing of the valve is almost all realized by stuffing box.

Control valve packing is a dynamic seal of the filling material, generally installed in the valve cover of the stuffing box, its role is to prevent the accused of medium due to the movement of the valve stem caused by leakage. How much.

Commonly used fillers are mainly divided into two categories according to the material: PTFE and flexible graphite. How much.

2. features:

(1) has excellent heat resistance and cold resistance. Flexible graphite from -250 degrees C ultra-low temperature to 600 degrees C high temperature, its physical properties have little change;

(2) Excellent chemical resistance. Flexible graphite in addition to nitric acid, concentrated sulfuric acid and other strong oxidizing medium corrosion, in other acids, alkalis and solvents almost no corrosion;

(3) Good self-lubrication. Flexible graphite is the same as natural graphite, when subjected to external forces, it is easy to produce sliding, so it has self-lubrication and good sealing performance;

(4) because of its porous porous and porous and curled, so its elastic flexibility is good. When the shaft or sleeve has radial circular runout due to eccentricity in manufacturing, installation, etc., it has sufficient floating performance, and even if the graphite has cracks, it can also be closely fitted, thus ensuring tight adhesion and preventing leakage.

Type of 3. graphite filler:

(1) filamentous or braided: this packing has good elasticity and can cover the stem and purify the stem surface. The braided graphite packing can overcome the leakage caused by wear of other types of graphite packing, but the braided graphite packing is easy to penetrate due to the voids caused by the filamentous structure. How much.

(2) flake graphite: a packing ring made of a number of graphite plates, the texture and the surface of the stem are perpendicular to form a square section, so the fluid is not easy to penetrate, but it also reduces the ratio of axial pressure to radial pressure, and requires greater pressing force to seal. How much.

(3) bent flake graphite: its texture is parallel to the axial direction of the valve stem and is processed with several graphite sheets. The pressure of bent flake graphite during pressing is much higher than the pressure when it works, so it will not shrink again in the stuffing box. At high temperature, it is easy to penetrate by its principle. However, due to easy processing and low price, it is widely used at present.

Main causes of 4. packing leakage:

Control valve in the actual use of a variety of reasons for the leakage of the sample, on-site maintenance should be based on the actual use of the valve for specific analysis, judgment, and then fundamentally eliminate the causes of leakage, to solve the security risks. According to the law of leakage, the main reasons can be summarized as follows:

(1) Improper selection of filler material and type: if tetrafluoro filler is selected under the working condition of 150 ℃-200 ℃, slight creep will occur when the filler works for a long time under critical working condition, and the sealing performance of the filler will decrease under the action of high pressure medium; Another example is that woven fillers are selected for media with strong permeability such as liquid ammonia, tar and fuel. These conditions are affected by high temperature and high pressure media and are prone to leakage. How much.

(2) packing improper installation method: packing into the stuffing box, the gland on its axial pressure, due to the plastic packing, make it produce radial force, and close contact with the stem, such as packing in the installation of tight loose, uneven force, so that the contact is very uneven, some parts of the contact loose, or even no contact, resulting in packing leakage. How much.

(3) control valve in use process, there is a relative movement between the stem and packing, this movement is called axial movement. In the process of use, with the influence of high temperature, high pressure and strong permeability of the fluid medium, the control valve stuffing box is also a leakage phenomenon more parts. The main reason for the leakage of the filler is the interface leakage, and the leakage of the textile filler (the pressure medium leaks out along the tiny gap between the filler fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the contact pressure of the packing, the aging of the packing itself, the loss of elasticity and other reasons. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem. How much.

(4) control valve frequent action, stem bending, wear, corrosion, finish caused by packing wear and leakage decreased. How much.

(5) The control valve changes in operating conditions. For example, in the process of heating up the device, the control valve has a cold state to a hot state. The hot medium changes greatly, and the valve stem is affected by thermal expansion, which makes the packing gap larger and the packing leakage serious. How much.

(6) The packing gland is not tight, skewed, or the control valve is installed horizontally, which will cause poor contact between the valve stem and the packing, and the gap is too large or too small. How much.

5. to prevent filler leakage countermeasures:

(1) In order to make the packing easy to load, chamfer the top of the stuffing box, and place a metal protection ring with appropriate erosion-resistant clearance at the bottom of the stuffing box (the contact surface with the packing cannot be inclined) to prevent the packing from being pushed out by the medium pressure. How much.

(2) improve the stem and stuffing box surface accuracy, finish. If the sum of the coefficients of friction between the various movable parts is zero, the forces acting on the packing gland can be transmitted uniformly throughout the packing without any attenuation. The sum of the actual friction force exists, and it is impossible to be zero. The radial force acting on the packing decreases with the increase or decrease of the distance from the gland. The greater the friction force, the greater the pressure attenuation. Once the pressure being sealed exceeds the force on the packing, leakage begins. Therefore, in the maintenance of the valve stem and stuffing box can not have scratches, pitting and wear, the finish is better. How much.

(3) The selection of suitable materials must have the ability to resist temperature changes, creep resistance, relaxation resistance and oxidation resistance. In general, tetrafluoro is preferred when the conditions are met, and graphite is selected when the conditions are not met. You can also choose a mixed filler: one is a mixture of graphite filler and tetrafluoro filler; the other is a mixture of O-rings and V-shaped filler. The medium with strong permeability can be filled with graphite. How much.

(4) When loading the packing, it should be added one by one, and pressed one by one with a pressing tool to make the packing force uniform. The opening filler shall be staggered by 90 ° or 120 ° up and down, and the number of turns of filler shall be suitable for no leakage. Add less packing gland will enter the stuffing box, easy to cause leakage. How much.

(5) For the PTFE packing with spring action, the gland screws should be tightened symmetrically and cannot be skewed. Other types of packing do not have to be screwed too tightly to leak so far. How much.

(6) the new valve or just maintenance of the control valve after the commissioning, should check whether the packing leakage, if there is a leak to deal with in time, in order to prevent the packing leakage is more and more big.