Steam at a pressure of 150 psi escapes through the gland with a clearance of 111,000thof an inch at a rate of 25 pounds per hour, i.e., loss of Rs. 50,0001-per year. A tiny drop of 0.4 mm diameter every 10 seconds results in loss of 200 litre’s of costly oil or solvent per year. Apart from the high cost of loss of energy and material, the leakage can pose a serious environmental problem. Bellow Seal Valves offer a simpler solution to eliminate this leakage from valves. This article highlights the construction, working and availability of bellow seal valves. The maintenance, advantages and applications are also discussed.
INTRODUCTION
Today’s chemical process industry is gearing itself towards safer technology for better environment. With increasing safety awareness in- the industry and in. the society, it has become every process engineer’s responsibility to design a plant with minimum damage to the environment. The stress is often to minimize the wastage of materials and energy due to leakages in the plant. Apart from pollution due to leakage of toxic materials, every drop of leak adds to the cost of production of materials. These leaks can take place from various sources – typically pumps, valves, flange joints. This can be reduced considerably, if not eliminated totally, by various steps. As far as leakage from valves is concerned, it can be reduced by : i) proper choice of valve with respect to material of construction, size, design conditions ii) proper installation iii) periodic and preventive maintenance. In spite of taking all precautions; there is a possibility of leakage of fluid from the gland of the valve. Bellow Seal Valve is a remedy to this problem.
WHY BELLOW SEAL VALVES?
Leakage from gland of the valve is often a headache for maintenance engineer. The losses incurred due to this type of leakage are awesome if quantified. Steam at the pressure of 150 psi escapes through the gland with a clearance of 1/1,000th of an inch at a rate of 25 Pounds per hour. This means loss of Rs. 501- per 8-hour shift and Rs. 50,0001- per year. A tiny drop of 0.4 mm diameter every second result in annual wastage of about 200 litres of costly oil or solvent. Apart from this high cost of loss of energy and material, the leakage also results in serious environmental, ecological and often hazardous problems. The leaking fluid can be corrosive, explosive or poisonous. In any case, it has to be checked before it leaks. We have a simple solution to all these problems, viz. bellow seal valve.
The bellow can be sealed to the valve in two different ways. In the first, the bellow can be welded to the valve stem at the top and valve body on the bottom. In this case , the process fluid contained ins ide the bellow. Alternately, the bellow can be sealed to the valve stem at the bottom and the body on the top, the process fluid is contained in the annular region between the valve bonnet and bellow.
It is very essential to understand the construction, working and types of Bellow Seal Valves. Further,these valves should suit the process conditions and requirements with respect to operations and materials of construction.
DESIGN IMPROVEMENT IN BELLOW SEAL VALVES
It is clear from the foregoing that the bellow is subject to metal fatigue and fatigue-induced weld failure. The fatigue life of bellow is affected by materials of bellow, fabrication technique of bellow, stroke length and stroke frequency. Additionally, as usual, parameters, viz. temperature, pressure and process fluid also affect the performance of the bellow, To avoid corrosion and better endurance to fatigue, bellows can be fabricated from materials such as Inconel 625 or Hastelloy C22/C276. Inconel 625 improves fatigue strength and corrosion resistance in comparison with stainless steels, while Hastelloy C22/C276 improves corrosion resistance in addition to improved fatigue strength than Inconel 625. Fatigue resistance can be improved by using multiply bellow system reducing the stroke length. Slight reduction in stroke length can increase bellow service life markebly. As discussed earlier, the bellow can be welded inside the valve in two ways. If the process fluid is outside the bellow (i.e., bellow is welded to the stem at the bottom), the process fluid will exert external pressure on the bellow. Bellow will naturally withstand external pressure efficiently than internal pressure.
AVAILABILITY OF BELLOW SEAL VALVES
- Type: The bellow seal valves are normally used in gate and globe type of valves due to their internal construction and movement of stem.
- Pressure rating: These valves are available in the range of ASA 150 class to 2500 class.
- Materials of construction: Depending upon the
process condition, materials of construction can be selected from carbon steel, stainless steel or even exotic materials like Inconel 625 or Hastelloy C276.
- Size: Based on available information, currently available size range is 6mm NB to 300mm NB.
APPLICATIONS OF BELLOW SEAL VALVES
Since bellow seal valves eliminate the risk of leakage of process fluid, these are ideally suited for applications where leakage cannot be tolerated from the valves. Hence, in service of highly hazardous materials such as hydrogen, ammonia, chlorine and other similar poisonous or explosive material handling, these valves are like a boon for process
Additionally, apart from the risk of hazard, leakage cannot be tolerated owing. to high cost of material being handled. In such service, bellow seal valves are suitable. Moreover, in such applications, economics will also be favorable for bellow seal valves. This is obvious from the fact that higher
CONSTRUCTION AND WORKING OF BELLOW SEAL VALVES
In bellow seal valves, conventional gland packing is replaced by a bellow cartridge welded to the bonnet and the stem of the valve. A bellow is long accordion-like tube. As the valve stem strokes, the bellow expands or compresses with the movement of stroke. There are no sliding or rotating scales through which process fluid can pass.
Bellow is the heart of this type of valve. There are two types of bellows, viz. formed and welded leaf. A formed bellow, is made by rolling flat sheet into a tube and longitudinal fusion welding. This tube is then mechanically formed into bellow with rounded and widely. spaced folds. In welded-leaf type bellow, washer-like plates of thin\metal are welded together. The welding is done on both inner and outer circumference of the washer-like plate. A welded leaf bellow has more folds per unit length. Both formed and welded-leaf bellows have same travel per fold. Thus for the same stroke length, mechanically formed bellows are two to three times longer than their welded leaf counterpart. It is reported that mechanically formed bellows fail at random spots, while the welded-leaf usually fails at or near a weld. To ensure full wall thickness, it is advised to fabricate bellows by full penetration micro-plasma I welding. .
For handling higher pressures, multiply design of bellows is recommended. In multiply design of bellow, the pressure that bellow can withstand can be increased by using 2 or 3 piles of metal wall. A two-ply bellow can increase its pressure rating by 80% to 100% than single ply bellow of same thickness. Alternately, if single-ply bellow of a thickness equivalent to pressure rating of two ply bellow is used, the stroke length is reduced. Thus, multiply design offers distinct advantage over single-ply thick bellow.
cost of these valves will be justified by equivalent savings on cost of fluid.
• In transfer of hot oils, there is always risk of fire due to spillage of hot oil, on highly inflammable chemicals. This leakage can be stopped by bellow seal valves.
• Recent trend indicates that because of stringent legislation preventing additional emissions to the environment, it is becoming difficult to expand in the existing premises. With the use of bellow seal valves, expansion without additional damage' to the environment is possible.
ADVANTAGES OF BELLOW SEAL VALVES
Leak-free operation : Due to peculiar construction, bellow seal valves are leak-free in operation. Thus, although initial cost of installing these valves is high, over a period saving, due to leak-free performance, justifies the same.
Safety 0f personneI and 0perations : With the installation of bellow seal valves, emissions of inflammable, toxic and hazardous chemicals can be minimized. This in turn improves safety in operating the plant.
Environmental advantage: By containing the escape of pollutants through valves, these valves help in not damagingthe environment. This is significant specially in the wake of growing concern towards protection of environment.
Long life: As the leakage is avoided, it avoids frequent maintenance of the valve. Replacement of gland packaging is not required. Thus, it leads to longer valve life.
BEFORE INSTALLING BELLOW SEAL VALVE
- From the above discussion, it can be seen that bellow seal valves can be of great use in chemical process industry. However, some considerations are equally important before making a choice of these valves:
- In actual practice, any bellow can fail eventually. So it becomes necessary to safeguard against leakage caused by such failure. A stem packing or back up system should be installed above the bellows to prevent release of emissions to environment. Alternately, a detection system can be provided by having a monitoring port between the bellow packing.
- Due to structure of bellow, it is likely that solid particles are trapped in the spacing between the bellows. The trapped solids can create stresses. Deposition of corrosive particles 0 r stagnant corrosive liquid inside the cavities in bellows can lead to corrosion.
- The cost of bellow seal valves is higher than an ordinary gate or globe valve of same size and service. Hence, before opting for bellow seal valve, factors, viz. economies and additional safety, must be given due consideration.