Today’s chemical process industry is gearing itself toward safer technology for a better environment. With increasing safety awareness in the industry and society, it has become every process engineer’s responsibility to design a plant that provides minimum damage to the environment. The emphasis is often to minimize waste of materials and energy due to plant leaks. Apart from pollution due to leakage of toxic materials, every drop leaked adds to the production cost. These leaks come from various sources typically pumps, valves and flange joints. This
can be reduced considerably, if not eliminated totally, by various steps. Valve leakage can be reduced by:
1) proper choice of valves with respect to material of construction, size and design conditions;
2) proper installation;
3) periodic and preventive maintenance. In spite of taking all precautions, there is a possibility of fluid leakage from the valve gland.
A bellows seal valve (Fig. 1) remedies this problem.
Why bellows seal valves? Leakage
from the valve gland is often a problem for the maintenance engineer. Losses incurred due to this type of leakage are considerable. Steam at 150 psi escapes through a gland with a clearance of 0.001 in. at a rate of 25 lb/hr, This means a loss of$I.2/8-hr shift and $1,100/yr. A tiny drop of O.4-mm diameter/sec results in a waste of about 200 l/yr of costly oil or solvent. Apart from this high cost of energy and material loss, 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. A bellows seal valve can be a simple solution to all these problems. It is essential to understand the construction, operation, and types of bellows seal valves. Further, these valves should suit the process conditions and requirements with respect to operations and materials of construction.
The process fluid is contained in the annular region between the valve bonnet and bellows.
Design improvements. Clearly, the bellows is subject to metal fatigue and fatigue-induced weld failure. Bellows fatigue life is affected by materials of construction, fabrication technique, stroke length and stroke frequency. Additionally, as usual, parameters such as temperature, pressure and process fluid also affect bellows performance, To avoid corrosion and reduce fatigue failure, bellows can be fabricated from materials such as inconel 625 or hastelloy C221C276.Inconel 625 improves fatigue strength and corrosion resistance compared with stainless
steels. Hastelloy C22/C276 offers greater corrosion resistance and fatigue strength than inconel 625. Fatigue resistance can be improved by using a multiply bellows system and reducing stroke length. A slight reduction in stroke length can significantly increase bellows service life.
As discussed earlier, the bellows can be welded inside the valve in two ways. If the process fluid is outside the bellows (i.e., bellows is welded to the stem at the bottom), the process fluid will exert external pressure on the bellows. Bellows will naturally withstand external pressure better than internal pressure.
- Type: Bellows seal valves are normally gate and globe types due to their internal construction ana stem
- Pressure rating: These valves are available in the range of ASA 150 to 2500 class.
- Materials of construction: Depending on process condition, materials of construction can be selected from carbon steel, stainless steel or even exotic materials like inconel625 or hastelloy C276.
- Size: Based on available information, currently the size range is 6 mm NB to 400 mm NB.
Construction and operation. In bellows seal valves, conventional gland packing is replaced by a bellows cartridge welded to the valve bonnet and stem. A bellows is a long accordion-like tube. As the valve stem strokes, the bellows expands or compresses with the stroke movement. There are no sliding or rotating seals through which process fluid can pass. A bellows is the heart of this type of valve. There are two types of bellows: formed and welded leaf. A formed bellows is made by rolling flat sheet into a tube and longitudinal fusion welding.
This tube is then mechanically or hydraulically formed into bellows with rounded and widely spaced folds. In welded leaf-type bellows, washerlike plates of thin metal are welded together. Welding is done on both inner and outer circumferences of the washerlike plate. A welded leaf bellows has more folds per unit length. Both formed and welded leaf bellows have the same travel per fold. Thus, for the same stroke length, mechanically formed bellows are two to three times longer than their welded leaf counterparts. Reportedly, mechanically formed bellows fail at random spots, while the welded leaf usually fails at or near a weld. To ensure full valve thickness, it is advisable to fabri-cate bellows by full penetration micro~plasma welding. For handling higher pressures, ‘–IFg–.’-.·
-1-A-e=-obw-1s1 seaIvaIve can soIve multi-ply bellow.s design is recom leakage problems. mended. In multi-ply bellows, pressure that bellows can withstand can be increased by using two or three plies of metal wall. A two-ply bellows can increase its pressure rating by 80% to 100% compared to a single-ply bellows of the same thickness. Alternately, if a single-ply bellows of thickness equivalent to pressure rating of a two-ply
bellows is used, stroke length is reduced. Thus, a multi-ply design offers a distinct advantage over a single-ply bellows. The bellows can be sealed to the valve in two different ways. In the first, the bellows 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 bellows. Or, the bellows can be sealed to the valve stem at the bottom and the body on the top.
Since bellows seal valves eliminate the risk of process fluid leakage, they are ideally suited to applications where valve leakage cannot be tolerated. Hence, in highly hazardous material service hydrogen, ammonia, chlorine and other similar poisonous or explosive fluidsthese
valves are a boon for the process engineer. Additionally, apart from hazard risk, leakage cannot be tolerated due to the high cost of materials being handled. In such applications, economics will also be favorable for bellows seal valves. Obviously the higher cost of these valves will be justified by equivalent savings on fluid cost. …..In hot oil transfer, there is always risk of fire due to hot oil spillage on highly inflammable chemicals. This leakage can be stopped by bellows seal valves . …..Because of stringent legislation preventing additional emissions to the environment, it is becoming difficult to expand existing premises. With the use of bellows seal valves, expansion without additional environmental damage is possible.
Before installing. Bellows seal valves can be of great use in the chemical industry. However, some considerations are equally important before choosing these valves:
- In actual practice, any bellows can, eventually fail. So it becomes necessary to safeguard against leakage caused by such failure. A stem packing or backup system should be installed above the bellows to prevent release of emissions to the environment. Alternately, a detection system can be provided by having a monitoring port between the bellows packing.
- Due to the bellows structure, it is likely that solid particles are trapped in the spacing between the bellows. The trapped solids can create stresses. Deposition of corrosive particles or stagnant corrosive liquid inside the bellows cavities can lead to corrosion.
- Cost of bellows seal valves is higher than ordinary gate or globe valves of the same size and service. Hence, before opting for a bellows seal valve, factors such as economics and additional safety must be duly considered.