Packing Leakage of Control Valves

The control valve is one of the terminal control components of the automatic control system. As there are many high-temperature and high-pressure working conditions in chemical plants, some media are highly corrosive and toxic, and flammable and explosive. When the valve packing leaks, it will not only cause waste of raw materials, but also cause serious pollution to the environment, and even cause life-threatening safety accidents such as fire, explosion and poisoning. Therefore, enough attention should be paid to the leakage of control valve packing. It is very important to select sealing packing reasonably in design and selection. According to the packing seal of control valves, combined with many years of working experience and related data, through the structural analysis of the control valve packing box, the characteristics and application occasions of PTFE and flexible graphite packing are introduced, and the reasonable selection of packing for the control valve is briefly introduced.
 
Function and Classification of Control Valve Packing
 
The control valve is composed of valve internals and the valve body. The internals of the valve includes a valve core, a valve stem, a stuffing box and an upper bonnet, etc. Among them, the stuffing box is used to seal the valve stem, which is an integral part of the valve body. It is an elastic method to prevent the process medium from leaking on the surface of the valve stem through reciprocating or rotating movement. Almost all valve stem seals are realized by stuffing boxes.
 
The control valve packing is the packing material of the dynamic seal, which is usually installed in the packing box of the upper bonnet, and its function is to prevent the leakage of the controlled medium caused by the movement of the valve stem.
 
Commonly used fillers are mainly divided into two categories according to their materials: polytetrafluoroethylene and flexible graphite.
 
1. Polytetrafluoroethylene (PTFE)
 
Polytetrafluoroethylene (PTFE) is a polymer compound made by polymerization of tetrafluoroethylene, which has excellent chemical stability, corrosion resistance, sealing, high lubrication and non-viscosity, electrical insulation and good anti-aging ability. Its corrosion resistance even exceeds that of glass and ceramics, and it has good corrosion resistance even to strong acid, strong alkali and strong oxidant, so it is an ideal sealing material. However, its temperature resistance is poor. PTFE begins to crack slightly above 200℃, and it will creep under pressure and heat, which will affect the sealing performance. It is not suitable for molten alkali liquor or fluoride occasions. Commonly used PTFE fillers are:
 
(1) Teflon molding woven packing: It is made by weaving and pressing Teflon in bulk. It is an open material with good flexibility, durability, good sealing effect and easy replacement, and is the most widely used material.
 
(2) V-shaped PTFE packing: Generally, it is made of PTFE bar by turning, and the packing structure is V-shaped. Under the condition of pressing both ends, PTFE has the advantages of low friction coefficient, lubrication and good sealing performance. The V-shaped packing ring is characterized in that by the pressure of the medium in the valve. The lip of the packing outer ring always clings to the inner wall of the packing box when it is opened, thus realizing static sealing. Under the same pressure, when the lip of the packing inner ring is opened, it always clings to the valve stem to ensure the dynamic seal, so that even if the valve stem moves up and down, it can also ensure the tightness.
 
(3) PTFE-graphite filler: By adding some glass fiber, graphite and molybdenum disulfide, the creep resistance and thermal conductivity of PTFE are improved, but the hardness is increased, and the corrosion resistance and sealing performance are decreased.
 
2. Flexible graphite
 
Flexible graphite is a non-fibrous material, which removes impurities from natural flake graphite. And then it is treated by strong oxidizing mixed acid to form graphite oxide. Graphite oxide is decomposed by heat to release carbon dioxide, and its volume expands rapidly, turning into flexible graphite with loose texture, softness and toughness. It is a material with long service life and good sealing performance.
 
Characteristics
 
(1) Excellent heat resistance and cold resistance
The physical properties of flexible graphite are almost unchanged from the ultra-low temperature of -250℃ to the high temperature of +600℃.
 
(2) Excellent chemical corrosion resistance
Flexible graphite has corrosion in nitric acid, concentrated sulfuric acid and other strong oxidizing media, but almost no corrosion in other acids, bases and solvents.
 
(3) Good self-lubricity
Flexible graphite, like natural graphite, is easy to slide under external force, so it has self-lubricating property and good sealing performance.
 
(4) Because its porosity is loose and curled, it has good resilience and flexibility. Or when the radial circular run out of the important official shaft sleeve occurs due to eccentricity in manufacturing and installation, it has sufficient floating performance. Even if the graphite cracks, it can be well sealed, thus ensuring the tight sealing and preventing leakage.
 
Types of graphite fillers

(1) Filamentous or braided: This packing has good elasticity, which can better wrap the valve stem and purify the surface of the valve stem. Braided graphite filler can overcome the leakage of other types of graphite filler due to wear, but the braided graphite filler is easy to penetrate because of the void caused by the filamentous structure.
 
(2) Sheet graphite: A packing ring pressed by several graphite plates, whose texture is perpendicular to the surface of the valve stem, forming a square section, so the fluid is not easy to penetrate. It also reduces the ratio of axial pressure to radial pressure, so it needs a larger pressing force to seal.
 
(3) Bent graphite: Its grain is parallel to the axial direction of the valve stem, and it is made of several graphite sheets. The pressure of bent graphite in pressing is much higher than that in working, so it will not shrink in the stuffing box. When the high temperature is known by its principle, it is easy to penetrate. However, it is widely used because of its easy processing and low price.
 
Main causes of packing leakage
 
There are many reasons for packing leakage of control valves in actual use on site. In site maintenance, specific analysis and judgment should be made according to the actual use of valves, so as to fundamentally eliminate the causes of leakage and solve potential safety hazards. According to the law of leakage, the main reasons are summarized as follows:
 
(1) Improper selection of packing material and type: if tetrafluoroethylene packing is selected at 150℃-200℃, tiny creep will occur when the packing works for a long time under critical conditions, and the sealing performance of the packing will decrease under the action of the high-pressure medium; Another example is that woven packing is used in liquid ammonia, tar, fuel and other highly permeable media. These situations are affected by high-temperature and high-pressure media and are prone to leakage.
 
(2) Improper packing installation method: After packing is loaded into the packing box, axial pressure is exerted on it by the gland. Due to the plasticity of packing, it generates radial force, and it is in close contact with the valve stem. For example, the packing is tightened and loosened during installation. The stress is uneven and some parts of it are loose or even not, resulting in packing leakage.
 
(3) When the control valve is in use, there is relative movement between the valve stem and the packing, which is called axial movement. During use, with the influence of high temperature, high pressure and fluid medium with strong permeability, the control valve stuffing box is also the part with more leakage. The main cause of filler leakage is interface leakage, and there will be leakage for textile fillers (pressure medium leaks outward along the tiny gap between 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 and loss of elasticity of the packing itself, etc. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
 
(4) Frequent action of control valves, bending of the valve stem, abrasion, corrosion, and decrease of smoothness cause packing abrasion and leakage.
 
(5) The operating conditions of the control valve change. For example, in the process of heating up the device, the control valve changes from a cold state to a hot state, and the hot medium changes greatly. The valve stem is affected by thermal expansion so that the packing gap becomes larger and the packing leakage is serious.
 
(6) The packing gland is not tight, skewed, or the control valve is installed horizontally, which will cause the valve stem to contact the packing poorly, and the clearance is too large or too small.
 
Countermeasures to prevent packing leakage
 
(1) To facilitate packing, chamfer the top of the stuffing box, and place an erosion-resistant metal protection ring at the bottom of the stuffing box (the contact surface with the stuffing cannot be inclined) to prevent the stuffing from being pushed out by the medium pressure.
 
(2) Improve the surface accuracy and smoothness of the valve stem and the stuffing box. If the sum of the friction coefficients between various movable parts is zero, the force acting on the packing gland can be uniformly transmitted to the whole packing without any attenuation. The actual total friction force exists, and it can't 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 is, the greater the pressure attenuation will be. Once the sealed pressure exceeds the force on the packing, it begins to leak. Therefore, the valve stem and stuffing box should be free from scratches, pitting and wear during maintenance, and the smoothness should be good.
 
(3) Selecting suitable materials must have the ability to resist temperature change, creep resistance, relaxation resistance and oxidation resistance. In general, tetrafluoroethylene is preferred when the conditions are met, and graphite is preferred when the conditions are not met. Mixed filler can also be selected: first, the graphite filler and the tetrafluoro filler are mixed; Second, the O-ring and the V-packing are mixed. The graphite filler can be selected for the medium with strong permeability.
 
(4) When packing, it should be added one by one, and pressed one by one with a pressing tool, so that the packing is stressed evenly. Openings should be staggered by 90° or 120° from top to bottom, and the number of laps of fillers should be suitable for no leakage. If the packing is missing, the gland will enter the stuffing box, which is easy to cause leakage.
 
(5) For PTFE packing with spring action, gland screws should be tightened symmetrically, without deflection. Other types of packing don't have to be screwed too tightly so as not to leak.
 
(6) After the new valve or the newly repaired control valve is put into service, check whether there is any leakage of the packing. If there is any, handle it in time to prevent the leakage of the packing from getting bigger and bigger.
 
Problems needing attention in using
 
(1) Cut the molding packing with a 45° incision, and each circle of the incision is staggered by 90° or 120° during installation.
 
(2) When PTFE molding filler is used under high pressure, attention should be paid to cold flow characteristics.
 
(3) The flexible graphite ring used alone has a poor sealing effect, so it should be used in combination with the graphite compression packing.
 
(4) The graphite packing can't be used for strong oxidants, such as concentrated sulfuric acid, concentrated nitric acid and other media.
 
(5) The dimensional accuracy and surface roughness of the stuffing box and the valve stem are the key factors affecting the sealing performance of the molding packing.
 
Conclusion
 
In many years of field applications, there are many reasons for the leakage of the control valve packing. As long as we are diligent in observation and carefully analyze the causes of leakage, the problem of packing leakage can still be eliminated. However, it should also be noted that the causes of the packing leakage of some control valves are complicated. There are many leakage factors, which need to be continuously analyzed and explored in future practice.