What are bellows valves
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Bellow valves are a type of valve that use a flexible bellow element to control the flow of fluids or gases. The bellow element is typically made of metal or other durable materials and is designed to expand and contract in response to changes in pressure or temperature. This movement allows the valve to open or close, regulating the flow of the fluid or gas through the system. Bellow valves are commonly used in applications where precise control of flow is important, such as in chemical processing, pharmaceutical manufacturing, and other industrial settings. A Bellows Valve is a specially designed valve suitable for use in industries dealing with a hazardous medium where even minute leakages can be extremely critical. With the exponential growth of new industries such as the chemical and nuclear industry, there is a growing abundance of flammable, hazardous, extremely poisonous, and toxic media flowing through the pipelines. This creates a high risk of leakage, explosion, and health hazards. So, the valves used in such pipes and systems have specific stringent leakage and sealing requirements.
What is the costruction of bellows seal valves
A bellows valve is a type of industrial valve that uses a bellows element to create a hermetic seal. The construction of a bellows valve typically includes a valve body, a bonnet or cover, a stem, a handwheel or actuator, and the bellows element itself.
The bellows element is typically made of metal, such as stainless steel, and is designed to flex in response to changes in pressure or temperature. This flexibility allows the bellows element to compensate for thermal expansion and contraction, as well as pressure fluctuations, while maintaining a tight seal.
The stem of the valve connects the handwheel or actuator to the valve disc or plug, which is used to control the flow of fluid or gas through the valve body. The bonnet or cover is used to protect the internal components of the valve, including the bellows element and stem, from external damage or contamination.
Overall, the construction of a bellows valve is designed to provide reliable and precise control of fluid or gas flow while maintaining a hermetic seal to prevent leaks or contamination.
The types of bellows seal valve
Bellows seal valves are a type of industrial valve that use a flexible metal bellows to create a hermetic seal between the valve stem and the valve body. They are commonly used in applications where leakage cannot be tolerated, such as in chemical processing, pharmaceuticals, and food and beverage production.
There are several types of bellow seal valves, including:
1. Globe valves
2. Gate valves
3. Check valves
4. Ball valves
5. Butterfly valves
Each of these types of valves has its own unique design and characteristics, but they all use a bellows seal to prevent leakage and ensure reliable operation.
Advantages of Bellows Seal Globe Valve
1. The most prominent advantage of any bellows valve is the Double seal design (bellows + packing). So, even if the bellows fails, the stem packing will prevent any kind of leakage to meet the international sealing standards.
2. It minimizes any loss of medium fluid and ensures the safety and environmental protection. This will ultimately improve the safety of the plant where is bellows valve has been installed.
3. It can act as an energy saver as it contributes to minimizing energy losses due to leakage.
4. Since the leakage is minimized, it prevents frequent maintenance of the valve. So, it leads to longer valve life.
5. Owing to longer service life, operating and maintenance cost is considerably reduced.
6. The rugged bellows seal design ensures zero leakage from the valve stem, thereby providing the maintenance-free conditions.
7. The safety of working personnel is improved with the installation of a bellows valve as emissions of inflammable, toxic, and hazardous chemicals can be minimized
The difference between common valve and bellows seal valves
Bellows globe valves and common valves are two types of valves used in industries to control the flow of fluids or gases.
The main difference between these two types of valves is that bellows globe valves have a bellows assembly that separates the fluid or gas flowing through the valve from the valve stem, whereas common valves do not have this feature.
When valves with gland packing are used such as gate valves or ball valves in the system for heated oil transfers (permeability of heated oil is about 50 times that of steam), chances of hot oil spillage are quite high and can cause loss of equipment and property. In such cases, due to its construction, the bellows valve can ensure absolutely nil leakage.
In ordinary valves, the shape of graphite packing cannot provide 100% effective sealing which is the main reason for frequent leakage of packing valve. As ordinary fillers are manufactured with graphite, if the purity of graphite is not adequate, its oil resistance would below. When heating oil being transferred is soaked in a graphite filler, certain graphite impurities get separated and flows along with it.
When a ball valve is used in the system with heated oil, if the internal PTFE is not resistant to high temperatures, it will quickly deteriorate and cause valve leakage.
The internal parts of the bellows valve are made of stainless steel, which can withstand a temperature of 425°C. Considering the life span of Bellows Seal Globe Valve, the duration of the general bellows valve is more than three years. Therefore, to maintain the reliability of output and to reduce the direct running costs, a bellow shut-off valve is preferred over ordinary seal valves in case of a system where heated oil is being transferred.
The bellows assembly is made of a series of convoluted metal plates that expand and contract with changes in pressure or temperature, allowing the valve to operate smoothly and without any leaks.
In contrast, common valves rely on a packing material, such as graphite or PTFE, to prevent leaks around the valve stem. This packing material can wear out over time, leading to leaks and increased maintenance costs.
Overall, bellows globe valves are generally considered to be more reliable and longer-lasting than common valves, especially in applications where leakage is not acceptable.
Selection Guide for Bellows
1. Choose the inner diameter of the bellows according to the diameter of the valve stem. The inner diameter of the bellows should be greater than the stem diameter of 115 mm.
2. The outer diameter of the bellows is selected according to the inner diameter of the bellows. The outer diameter and the inner diameter are related by the ratio of the outer diameter to the inner diameter. In general, the ratio of the outer diameter to the inner diameter is 113 to 115.
3. The wall thickness, number of layers, number of corrugrtions, and length of the corrugated pipe must meet the requirements of the pressure resistance, stroke, and cycle life of the corrugated pipe. Under normal circumstances, greater the pressure, more thickness of the bellows is required. To increase the displacement or reduce the stiffness of the bellows and increase life, the thickness of the single layer can be reduced and the number of layers of the bellows can be increased.
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4. For handling higher pressures, the multiply design of bellows is recommended. The pressure that bellows can withstand can be increased by using 2 or 3 piles of the metal wall as in case of multiply design of bellows. Pressure rating by using a two-ply bellow can increase by 80% to 100% than single-ply bellow of the same thickness. Alternately, the stroke length is reduced if single-ply bellow of a thickness equivalent to the pressure rating of two ply bellow is used. So, we get a distinct advantage of multiply design over the single-ply thick bellows.
5. To prevent the distortion of bellows, the designed guide structure must be strictly followed.
6. To prevent wear and premature failure, local contact between the valve stem or bellows must be avoided.
7. The design should be such that twisting and torsional deformation of the bellows is prevented.
8. The valve design should be limited by the extension stroke and compression of the bellows. This is to prevent displacement of the bellows and to withstand compression and tensile extension beyond the defined cycle life test. Under normal conditions, the stroke of the bellows should not exceed 25% of the free length. If this is not fulfilled, it will affect the cycle life of the bellows, especially bellows valves under high temperature and high-pressure environment.
9. The bellows adopt the same pressure and temperature grade as the Bellow seal Globe Valve. The bellows must withstand the pressure test of the nominal pressure of the main valve at 38 ℃ and 115 times the nominal pressure at 38 ℃. During the pressure test, the weld should not crack or leak, and the bellows should not twist.
10. The material of the bellows components should be selected within the allowable temperature range, and it should have good formability, welding performance, and compatibility with the medium.
Application range of Bellows Seal Globe Valve
· Bellows valves eliminate the risk of leakage of the process fluid and are therefore ideally suited for applications where leakage cannot be tolerated from the valves. So, places where highly hazardous materials such as hydrogen, ammonia, chlorine, and other similar poisonous or explosive material are handled, these valves are preferred.
· It is suitable for use as a shut-off valve with heat-conducting oil, toxic, flammable, highly permeable, environmentally polluting, and radioactive fluid pipelines. It can be considered as a high-quality product for creating a leak-free factory.
· To name a few, bellows seal globe valve is highly used in petrochemical industries, electronics industry, various plants, vacuum industries, etc.
Precautions for Bellows Seal Globe Valve
Ø To avoid damage or injury to personnel or equipment, always read all warnings and instructions. Unprofessional re-conditioning, the use of foreign replacement parts, or the incorrect maintenance steps, may cause a loss of efficiency or lead to personnel injury or damage to parts.
Ø Use double seal bellows in case of a highly corrosive chemical environment so that you get two protective layers of sealing.
Ø If you find that there is a smell of media in the air or the valve cover is wet, it is generally because the bellows have been damaged by the medium.
Ø You can replace the bellows by yourself or get it replaced by makers. Replace it to avoid prolonged negligence, a threat to life, and damage to the entire valve.
Ø An excessively tightened gland nut can cause excessive packing wear and can hinder the free movement of the plug stem.
Reasons for Leakage of Bellow Seal Glove Valve
There are many reasons for the leakage of a Bellows Seal Valve. Bellows valve leakage is generally divided into internal leakage and external leakage.
1. It is because the liquid medium contains solid impurities that damage the sealing surface and cause failure.
2. Combined with the analysis of the on-site usage, the reason for the leakage of the valve stem is mainly related to the valve structure, working environment, and operation mode.
3. The valve stem adopts a double sealing structure of bellows and packing. The bellows are welded with the inner lining ring and the outer lining ring to form a bellows assembly and then welded with the valve stem and the guide body to block the leakage of the medium through the valve stem. The valve stem of the bellows valve moves up and down by the flat key, until the upper-end surface of the flat key contacts the lower end surface of the lower packing. Since the lower packing is made of PTFE material, if the valve is opened too much, the flat key will be embedded in the packing, the bellows valve can no longer be opened and closed, and the bellows assembly is knocked to death, causing the medium to leak along the valve stem.
4. Tensile elongation and compression of the bellows valve according to the line determination process can contribute to leakage. If tension or compression exceeds the limit, the bellows may be damaged, resulting in small series hose rupture, seal failure. Therefore, it is necessary to increase the limit device for opening and closing of the bellows valve, so that the bellows always expand and contract within the design range during the operation of the valve.
Summary
The unique design of Bellows Seal Globe Valve and its efficiency to provide leak-proof operations makes it the best-suited valve for a wide range of applications, especially hazardous materials & where system media is at high temperatures. Durability, maintenance-free operations and cost-effectiveness is an added benefit with bellows valves.
Leakage at various points in pipelines found in chemical plants creates emissions. All such leakage points can be detected using various methods and instruments and should be noted by the plant engineer. Critical leakage points include flanged gasket joints and the valve / pump gland packing, etc. Today the chemical process industry is gearing itself towards safer technology for better environmental protection and it has become every process engineer's responsibility to design plants that limit damage to the environment through the prevention of leakage of any toxic chemicals.
Leakage from the valve gland or stuffing box is normally a concern for the maintenance or plant engineer. This leakage means..
a) Loss of material b) Pollution to the atmosphere c) Dangerous for plant employees.
For example, take the case of a steam leakage through the valve gland. At 150 PSI, a clearance of just 0.001" through the gland will mean a leak at the rate of 25 lb/hour. This equates to a loss of USD 1.2 per eight hour shift, or USD 1,100 per year. Similarly, a tiny drop of 0.4 mm diameter per second results in a waste of about 200 litres per year of costly oil or solvent. This leakage can be reduced considerably by using the bellow seal valve. This article will now consider the construction and operation of the bellow seal.
The bellow cartridge is welded to both the valve bonnet and the valve stem. The bellow cartridge has a number of convolutions and these convolutions become compressed or expanded depending upon the movement of valve stem. (Scientifically speaking the bellow gets compressed when the valve is in the open position and expanded when the valve is in the closed condition). It is important to properly install the valve bodies. The bellow can be sealed to the valves in two different ways. Firstly, the bellow can be welded to the Valve stem at the top and the valve body on the bottom. In this case the process fluid is contained inside the bellow or in second method the bellow is welded to the valve stem at the bottom and the body on the top. In this case the process fluid is contained in the annular region between the valve bonnet and bellow (from the outside).
The bellow is a critical component and forms the heart of the bellow seal valves. To avoid any twisting of the bellow the valve must have a stem with linear movement only. This can be achieved using a so-called sleeve-nut at the Yoke portion of the valve bonnet. A handwheel is fitted onto the sleeve-nut which effectively transfers a rotary motion of the handwheel into a linear motion in the valve stem.
There are two main types of bellow.. the Forged Bellow and the Welded Bellow. Formed-type bellows are made from rolling a flat sheet (thin wall foil) into a tube which is then longitudinally fusion welded. This tube is subsequently mechanically or hydrostatically formed into a bellow with rounded and widely spaced folds. The welded leaf type bellow is made by welding washer-like plates of thin metal together at both the inner and outer circumference of the washers - like plates. A welded leaf bellow has more folds per unit length as compared to forged bellows. Thus, for the same stroke length, forged bellows are two to three times longer than their welded leaf counterparts.
Reportedly, mechanically forged bellows fail at random spots, while the welded leaf usually fails at or near a weld. To ensure full penetration of bellow ends and end coller welding it is advisable to fabricate using micro plasma welding.
The multi-ply bellow design is preferred for handling higher pressure fluids (generally two or three plies of the metal wall). A two ply bellow can increase its pressure rating by 80% to 100% as compared to a single ply bellow of the same thickness. Alternatively, if a single ply bellow of a thickness equivalent to a pressure rating of a two ply bellow is used, the stroke length is reduced. Thus, a multi-ply bellow design offers a distinct advantage over a single ply bellow. It is clear that the bellow is subject to metal fatigue and this fatigue can induce weld failure. The bellow fatigue life is affected by the material of construction, fabrication technique, stroke length and stroke frequency, in addition to the usual parameters such as fluid temperature and pressure.
The most popular stainless steel bellow material is AISI 316Ti which contain Titanium to withstand high temperatures. Alternatively, Inconel 600 or Inconel 625 improve fatigue strength and corrosion resistance as compared with stainless steel bellows. Similarly, Hastalloy C-276 offers greater corrosion resistance and fatigue strength than Inconel 625. Fatigue resistance can be improved by using a multiply bellows system and reducing the stroke length; this can significantly increase the bellow service life.
The most common valve types to be fitted with bellow seals are the gate and globe designs (see Figure 1).These are very suited for use with bellows due to their internal construction and axial movement of the valve stem.
Based on available information, it seems that current bellow seal valves range in size from 3 mm NB to 650 mm NB. Pressure ratings are available in from ANSI 150# to 2500#. Material options for the valves include carbon steel, stainless steel and exotic alloys.
Applications
Heat Transfer media.. hot oil is commonly used in industries such as synthetic fibres / POY (Partially Oriented Yarn). However, there is always a risk of fire due to hot oil spillage on highly inflammable chemicals. Here, bellow seal valves can stop the leakage.
Vacuum / ultra high vacuum.. some applications require a vacuum pump to continually extract air from a pipeline. Any conventional valves installed on the pipeline can allow external air to enter the pipeline thorough the valve stuffing box. Hence the bellow seal valve is the only solution to prevent air from passing through the stuffing box.
Highly hazardous fluids.. for media such as chlorine (see Figure 2), hydrogen, ammonia and phosgene, the bellow seal valve is an ideal design as leakage through the gland is totally eliminated.
Nuclear plant, heavy water plant.. in instances where radiation leakage is to be prevented at all times, the bellow seal valve is the ultimate choice.
Costly fluids.. in some applications leaks need to be avoided simply because of the high cost of the fluid. Here, an economic assessment often favours the use of bellow seal valves.
Environmental standards.. around the world, standards regarding emissions and the environment are getting more stringent day by day. It can therefore be difficult for companies to expand within existing premises. With the use of bellow seal valves, expansion without additional environmental
damage is possible.
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