What Is A Bellow Valve
What Is A Bellow Valve
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.
The need for a completely different approach required for the desired sealing & redundancy has led to the development of Bellow seal globe Valve.
Table of Content
1. What Is A Bellow Valve
2. Construction of Bellow Seal Globe Valve
3. How Does Bellow Seal Globe Valve Work
4. Advantages of Bellow Seal Globe Valve
5. Comparison of Bellow Seal Globe Valve with Normal Valve
6. Bellows Selection Guide
7. Application Ranges of Bellow Seal Globe Valve
8. Precaution for Bellow Seal Globe Valve
9. Reason of Leakage for Bellow Valves
10. Summary
Construction of Bellows Seal Globe Valve
Metal bellows are the main feature of a Bellows globe seal valve. There are two types of bellows:
i. Formed Bellow – They are made by rolling the flat sheet into a tube and longitudinal fusion welding. After this, the tube is mechanically formed into bellow with rounded and widely spaced folds.
ii. Welded leaf Bellow – In this type, the washer-like plates of thin metal are welded together. The welding is performed on the inside and outside of the washer-like plate. The folds per unit length in welded leaf bellows are more.
The amount of movement per fold is the same for both formed and welded-leaf bellows. So, for the same stroke rate, the mechanically formed bellows are two to three times longer than their welded leaf equivalent.
The lower end and the stem assembly is welded by automated seam welding, while the upper end is welded by automated seam welding on the connecting plate, creating a metal shield between the fluid medium and the environment, ensuring zero leakings from the valve stem.
The sealing surface of the valve disc and the valve structure is precisely ground, so once the installation has been done, it is confirmed that it has 100 % passed a pressure test and the possibility of any kind of leakage is eliminated. As an auxiliary seal, the stuffing box is always held on the bellows, which improves durability and protection. The metal bellows of the high-pressure valves have a multi-layer construction which will guarantee an open/close life of 10000 times at the highest pressure by hydroforming.
How Does Bellow Seal Globe Valve Work
A Bellows Seal Globe Valve is a compact structure that is a kind of control valve which uses bellows for sealing the valve stem components. The bellows valve has a bellow inside, and the lower end of the stainless-steel bellows is welded to the stem to prevent the system fluid from eroding the stem. The other end is positioned between the body of the valve and the cover of the valve to create a permanent seal. This double seal configuration eliminates leakage i.e., even if the bellows collapses, the stem packaging will prevent further leakage. The bellows are welded to the valve stem to ensure stable operating efficiency and prevent the vibration of the valve stem induced by the movement of the valve insert.
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.
Comparison of Bellows Seal Globe Valve with Normal Valves
Ø 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.
Ø 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.
Ø 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.
Ø 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.
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.
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.
10. SummaryAPI
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.
STV Valve is a professional China bellow seal valve manufacturer, don’t hesitate to contact us if you have any inquiry