There are different points in a pipeline which are found in a chemical plant that creates emissions. All the points where the leakage occurs can be detected by using different methods and instrumentation which a plant engineer need to note. The critical points where leakage occurs include the flanged gasket joints as well as the pump/valve gland packing, etc. Nowadays, most of the process industries are moving towards the use of safer technology so as to enhance the environmental protection and it has become a duty for the engineers to design plants which will helps in limiting the damage which occur in the environment by the prevention of leakage of oil, gases and other forms of toxic chemicals.
The leakage from the stuffing box and valve glands is a major concern for the plant or maintenance engineer, this is because the leakage will result to atmospheric pollution, loss of materials and it is also dangerous for employees of the company.
For instance, if there is a steam leaking through the valve gland, a space of 0.001″ via the gland will result to a rate of 25 lb/hour; this will result to a loss of about 1,100 per year. in addition to that, a tiny drop of 0.4mm diameter per second will result to a waste of about 200 liters per year of costly solvent, gas or oil. However, the use of bellow seat valve can be used to reduce this leakage.
The bellow cartridge is welded to both the valve stem as well as the valve bonnet. There is a number of convolutions in the bellow cartridge and they become expanded or compressed based on the valve system movement. The bellow also gets compressed whenever the valve is opened and expanded whenever the valve is closed. It is also crucial to install the body of the valve properly. You can then seal the bellow to the valves using two different ways. In the first one, the bellow can be welded at the top of the valve stem and the valve body on the bottom. In this case, the fluid which is processed will be contained in the bellow and the second method involves the welding of the bellow to the valve stem which is at the bottom and the body to the top. In this case, the fluid which has been processed is contained in the annular region which is between the bellow and the valve bonnet.
The bellow serves as an essential component and also forms the heart of the bellow seal valves. It is crucial for the valve to have a stem with linear movement only in order to avoid any form of twisting in the bellow valves. A sleeve-nut at the yoke portion of the valve bonnet can be used to achieve this. A hand wheel would be fitted to the sleeve-nut and this will effectively transfer a rotary motion of the hand wheel into a linear motion in the valve stem.
The welded bellow and the forged bellow are the two major types of bellow which exist. The formed type of bellow is constructed by rolling a flat sheet which is a thin wall foil into a tube which is in the form of a longitudinal fusion welded. The tube can then be formed hydrostatically into a bellow with rounded and wide folds. The bellow with a welded leave is constructed by welding a washer-like plate with a thin metal together at the inner and outer circumference of the washer such as plate. There are more fold per unit in a welded leaf bellow when compared to a forged bellows. Therefore when the same stroke length is used, a forged bellows are two or three times longer than their counterparts which is welded leaf.
It has also been reported that bellow which are forged mechanically usually fail at some points while the welded leaf usually fails near or at a weld. So as to ensure a full protection of the end coller and bellow ends welding, it is essential to fabricate with the aid of micro plasma welding.
The design of the bellow is generally preferred for handling of higher pressure fluids such as two or three piles of the metal wall. A bellow which consist of two ply can get its pressure rating increased by 80 to 100% when compared to one ply bellow which have the same thickness to a pressure rating of a bellow with two ply, the length of the stroke will be reduced. It is also being seen thatsuch bellow would have metal fatigue which may later lead failure of the weld. The life of the bellow fatigue is affected by the technique which is used for the fabrication, construction material, stroke frequency as well as length of stroke, in addition to the normal parameters like fluid pressure and temperature.
The commonest material which is used for stainless steel type of bellow is AISI 316Ti; this contains Titanium which helps to withstand extreme temperature. In addition to that, Inconel 625 or Inconel 600 can be used so as to improve the fatigue strength as well as corrosion resistance when compared to the use of bellows made with stainless steel. Hastalloy C-276 likewise offers a great corrosion resistance as well as fatigue strength than 625. The use of multiple bellow system and reduction in the length of the stroke can be used to improve the fatigue resistance which will also enhance the service life of the bellow.
The valve type which is commonly fitted with bellow seals are the gate and globe designs. They are very suited for use with bellows due to the presence of their internal construction as well as the axial movement of the valve stem. Based on the provided information, the size of the bellow seal valves falls between 3 mm NB to 650 mm NB and the minimum pressure rating is 150# while the maximum is ANSI 2500#. The material which can be used for the valves include stainless steel, carbon steel as well as exotic alloys.
Heat transfer media: One of the applications is the heat transfer media where hot oil is commonly used in industries like Partially Oriented Yarn or synthetic fibers. But there is high tendency of fire accident which may occur due to hot oil spillage on chemicals which are highly inflammable. The leakage can be stopped with the use of bellow seal valves.
There are some kinds of applications which need the presence of a vacuum pump so as to extract air from the pipelinecontinuously. Any kind of conventional valves that has been installed on the pipes can accommodate external air to enter the pipeline via the valve stuffing box. Thus, the bellow seal valve is the best solution so as to prevent air from passing through the stuffing box.
Highly hazardous fluids:
The bellow seal valve is the best design which can be used to prevent the total leakage through the glands for media like phosgene, ammonia, hydrogen as well as chlorine.
Heavy water plant, nuclear plant:
In situation where it is essential to prevent radiation leakage at all time, the best choice is the bellow seal valve.
There is need to prevent the leakage in some applications due to the fact that the fluid is costly, in this case, a thorough assessment usually give room for using this kind of valve.
Good environmental standards:
All around the word, there is an improvement about the level of emission in the environment. This serves as a challenge for companies that want to expand within the existing premises. However, expansion can occur without causing any damage to the environment by using bellow seal valves.