Research and application of high temperature valve packing
The operating temperature of the valve is 425 ~ 550℃ for high temperature ⅰ grade (referred to as PI grade). The main material of PI class valve is “high temperature grade Ⅰ medium carbon chromium nickel rare earth titanium quality heat-resistant steel” in ASTMA351 standard CF8 as the base. Since PI grade is a specific term, the concept of high temperature stainless steel (P) is included here. Therefore, if the working medium is water or steam, although also available high temperature steel WC6(t≤540℃) or WC9(t≤570℃), in sulfur oil, although also available high temperature steel C5(ZG1Cr5Mo), but here can not be called them PI grade.
Research and application of high temperature valve packing
Valve is a common mechanical product in modern industry. As a key control component in the fluid transmission system, it is mainly used in boiler, steam pipeline, oil refining, chemical industry, fire and metallurgy, because of its cut-off, regulation, pressure regulation, shunting and other functions. Modern industry has put forward higher and higher requirements for the reliability of valve seal. Sealing performance is an important technical index for evaluating the quality of valve products.
High temperature valve refers to the valve whose working temperature is higher than 250℃. The filler sealing technology of the stem of high temperature valve has been a prominent problem that has not been solved for many years, and it is also one of the weak links to improve the reliability of the valve. Common high temperature valve stem packing seal generally exist insufficient or excessive seal, valve stem in the long run easy to leak, the leakage of inflammable, explosive, poisonous and other dangerous objects not only plant shutdown and economic losses, but also cause environmental pollution, and even personnel casualty accidents, to device run great risks.
First, the principle of valve packing seal
The sealing performance of valve is an important index to evaluate the quality and performance of valve. Now most of the control valve or general valve stem and packing seal for contact seal, because of its simple structure, easy assembly and replacement, low cost and is used.
Valve stem and packing leakage is a common phenomenon. The reason why packing can play the role of sealing, its principle now there are two main sealing views, respectively bearing effect and maze effect.
Packing bearing effect refers to the packing between filler and stem, squeeze packing and under the influence of external lubricant, because of the tension in the contact area of the stem to form a layer of liquid membrane, make packing and stem form similar to that of sliding bearing, the relationship between such packing and stem won’t because of excessive friction and wear, because liquid film exist at the same time, the packing and the valve stem moment in a sealed state.
Labyrinth packing effect is refers to the smooth degree of the stem cannot reach the micro level, packing and the valve stem is only partly joint and do not fully fit, the packing and always there is a very small gap between the valve stem, and because of incision asymmetry between packing assembly, these gaps formed a maze with together, medium, in which multiple throttling, step-down, and reach the role of sealing.
Of labyrinth effect refers to the valve stem packing seal surface level degree cannot reach the micro level, the tiny gap between stem and packing this is objective existence, cannot eliminate, if from this aspect to carry on the packing design, often the effect is not very ideal, which is causing the basic conditions of space leakage or power leakage. Sealing media through packing and stem leakage mechanism has many forms: corrosion gap leakage mechanism, porous leakage mechanism, power leakage mechanism, etc. In this paper, the improvement design of valve packing seal structure under high temperature condition is based on the above mentioned various leakage mechanisms, and the practical improvement scheme is put forward.
Two, the current common packing type and application
1, teflon pan root
Polytetrafluoroethylene pan root is made of pure POLYTETRAFluOROethylene DISPERsing resin as raw material, first made of raw material film, and then through twisting, weaving into a strong pan root. This KIND OF disk root without other additives, can be used in food, pharmaceutical, papermaking chemical fiber and other high cleanliness requirements, and have strong corrosive medium on the valve, pump.
Scope of application: Use temperature not more than 260℃, use pressure not more than 20MPa, pH value: 0-14.
2, expanded graphite disc root
Expanded graphite disc root is also known as flexible graphite disc root using flexible graphite wire woven through the heart. Expanded graphite disc root has the advantages of good self-lubricity and thermal conductivity, small friction coefficient, strong versatility, good softness, high strength, and protective effect on shaft and rod.
Scope of application: Use temperature not more than 600℃, use pressure not more than 20MPa, pH value: 0-14.
3. Enhanced graphite coil root
The enhanced graphite coil is woven by glass fiber, copper wire, stainless steel wire, nickel wire, caustic nickel alloy wire and other materials reinforced by pure expanded graphite wire. With the characteristics of expanded graphite, and strong versatility, good softness, high strength. Combined with general braided roots, it is one of the effective sealing elements to solve the problem of high temperature and high pressure sealing.
Scope of application: Operating temperature not more than 550℃, operating pressure not more than 32MPa, pH value: 0-14.
The disc root is an enhanced version of the expanded graphite disc root, which is a very good sealing material.
The above lists several common types of packing disc root. In the actual production process, there will be other kinds of packing disc root developed for special working conditions. For example, good chemical resistance of aramid fiber coil root; Suitable for high load rotation axis arylon carbon fiber mixed coil root, etc., this paper is limited to space, not a detailed introduction.
Three, common valve packing structure and selection
The common stem packing seal structure is mainly composed of pressure plate, gland, spacer and packing. In order to achieve good sealing effect, the packing is generally required to have dense structure, good chemical stability, low friction coefficient. In general, the temperature is lower than 200℃, the filler is often selected polytetrafluoron disc root, which has the characteristics of high lubrication, non-viscosity, electrical insulation and good aging resistance, and is used in petroleum, chemical, pharmaceutical and other fields.
Graphite disc root is selected for its high temperature resistance, self-lubrication and low friction coefficient at temperatures ranging from 200 to 450. Graphite disc has been developed according to the use of various classification, in practical application, fillers can be selected according to the actual working conditions of the appropriate type of graphite disc, such as 250℃, low pressure conditions can choose expanded graphite disc, medium and high pressure can choose enhanced graphite disc or a combination of both.
Four, high temperature valve packing structure leakage analysis
Under high temperature conditions, such as the selection of graphite disc root seal structure, it is easy to appear leakage. The reasons are as follows:
The graphite disc root is packed into the packing box, and the axial pressure on the packing is applied by tightening the fastening bolt on the packing gland. Because the packing has a certain degree of plasticity, axial pressure after radial pressure and micro deformation, the inner hole and stem closely fit, but this fit is not uniform up and down. According to the distribution of packing pressure and packing sealing force, it can be seen that the pressure of the upper packing and the lower packing in the packing box is not uniform. The plastic deformation of the two parts of the packing is not consistent directly, and it is easy to have excessive or insufficient sealing between the packing and the valve stem. At the same time, the friction between the packing and the valve stem will be much when the radial compression force near the gland is much, and the valve stem and packing are easy to wear here.
In the case of high temperature, the higher the temperature, the greater the expansion of graphite disc root, friction also increases, the heat dissipation caused by high temperature is not timely, accelerate the wear rate of stem and packing, which is also the main reason for high temperature valve packing leakage.
Five, high temperature valve packing structure improvement design
Valve packing under high temperature conditions is especially prone to leakage, and high temperature packing is generally based on expanded graphite disc. The self-lubricity and swelling of expanded graphite packing is good, the rebound coefficient is high, but the disadvantage is fragile, poor shear resistance, generally installed in the middle part of the packing box, to prevent the expansion of graphite packing by packing gland and bottom pressure pad extrusion damage; The enhanced graphite disc root can be installed at the top and bottom because it contains nickel wire and is strong and extrusion resistant.
Although the combination of expanded graphite and enhanced graphite disc solves part of the packing leakage at high temperature. But for the valve action is more frequent working conditions, graphite disc root wear rate is relatively high, the use of a period of time after the need to tighten the fastening bolts on the stuffing box, for manual and inspection have brought a big problem. Based on considerations of the above problem, we combined with literature at home and abroad and the experience accumulated in recent years to develop a compensating valve packing structure, especially for different working conditions, high temperature and low pressure and high temperature and high pressure, the development of targeted different high temperature packing structure, solve the valve under the condition of high temperature easy leakage.
High temperature and low pressure type, using special compensatory ring spring and combined graphite disc root combination.
The working pressure is not high, so the packing sleeve is cancelled. The special compensatory ring spring is added to the bottom of the stuffing box. When installing, the bolts need to be tightened with a certain preload. Even if graphite packing and stem appear frictional wear, the ring spring can immediately make corresponding compensatory adjustment to ensure the leakage of the valve.
Type high temperature and high pressure, this is a kind of advanced packing system, adopts the disc spring and cast spring external double compensation structure, can avoid the advantage of high temperature disable spring, this kind of condition, especially in high temperature, high pressure compensation point failure in one area, another group of compensation is still effective, both non-interference, single compensation but at the same time for packing work. The disc spring seal also facilitates use in harsh outdoor conditions, and the external structure of the two compensation points facilitates replacement without removing the entire stuffing box, improving efficiency and ease of operation. After long-term user tracking, this type of packing structure for high temperature, high pressure stem sealing to prevent leakage effect is obvious, long service life.
Post time: Sep-27-2022