Abstract:
In the petrochemical industry, filter screen is widely used in the process of fluid transportation.
In recent years, the Johnson screen has more and more replaced the traditional woven net and orifice plate, which is mainly determined by the unique structure and many advantages of the Johnson screen.
In a certain range of pores, the fluid resistance drop of the Johnson screen is obviously smaller than that of the woven screen and orifice plate.
The calculation and comparison of the resistance drop in the design and selection of filter materials are of great significance for the selection of filter materials.
Keywords: Johnson screen, woven screen, resistance drop.
The filter element of chemical pipeline filter, the grid of reactor, dryer, etc. all need to use screen. When the fluid passes through the screen, there will be a resistance drop. The magnitude of resistance drop is not only related to energy consumption and production cost but also directly affects the status of the processing system, thus affecting production efficiency and product quality. Therefore, without affecting the structure of the equipment, reducing the resistance drop of the filter screen can not only reduce the energy consumption and production cost but also improve the production efficiency and product quality.
1. Characteristics of woven screen
The composite structure of metal wire woven filter screen and the porous plate is widely used in the industry. Due to the large contact area between solid particles (catalyst, molecular sieve, or various filter materials) and wire mesh, the particles are easy to block or get stuck in the pores of metal wire mesh or porous plate, resulting in increased pressure drop and expensive media loss after bending deformation, even making the device unable to operate and maintenance more difficult.
2. The characteristics of Johnson screen
Johnson screen is a kind of metal wire which is similar to a V-shaped shape. It is welded on the vertical support rod by resistance welding to form a continuous and uniform gap. There are only two points of contact between the filter and the screen surface. Its structure is shown in Figure 1.
The unique structure of Johnson Net determines that it has many characteristics that are not found in the woven screen.
(1) Each intersection of the wire and the support rod has high welding accuracy and accurate gap.
(2) High strength and durability.
(3) With the increase of the opening range, the opening rate, and the effective flow area increase.
(4) The gap is continuous and the mesh surface is smooth, which makes the catalyst flow smoothly with less wear, and the particles are not easy to crush.
(5) Anti-blocking.
(6) Wear resistance.
3. Comparison of Johnson screen and woven screen
The comparison of the wedge wire screen and woven net in practical work is shown in Figure 2.
It can be seen from Fig. 2 that the anti-clogging performance of the two filters is obviously different. And the strength of the woven net is low, in the process of use generally needs to be combined with the porous plate. Therefore, the opening rate of the composite structure is the product of the opening rate of the woven net and the porous plate, and the opening rate is greatly reduced. According to GB 10612-89 and GB / t5330.1-2000, when the opening ratio of the woven screen and the perforated plate is 60% and 60% respectively, the product value is only 36%. Because of the different structures of the two kinds of filters, even without considering the blockage and other factors, the resistance drop of fluid passing through the two kinds of filters is quite different in a certain range of pores.
4. Calculation formula of fluid resistance drop
The calculation formula of the mass flow rate of the microplate is as follows:
5. Comparison of calculated values of fluid resistance drop between Johnson screen and woven screen
The following is the calculation and comparison of the fluid resistance drop of the two kinds of filters in the case of no blockage.
Taking a basket filter in an ethylene plant expansion project as an example, this paper calculates and compares the resistance drop of the composite structure of woven screen and perforated plate and that of Johnson screen. The technical parameters and structural dimensions of the blue filter are shown in Table 1.
Table 1: Technical parameters of basket filter.
Diameter/inch 32 Design pressure/Mpa 3.3
Slot/mm 3.15 Density/(kg.m-3) 475.11
Dynamic viscosity / PA · s 78.67 × 10-6
Flow rate / (kg · h-1) 1003852 14 Shell ASTM A671 CC65
Design temperature/C 60 Filter SS304
5.1 calculation of resistance drop with woven mesh and perforated plate structure.
According to the technical parameters table, the inner diameter of the filter cartridge is 1000mm and the height of the filter cartridge is 960mm.
Ap=3.14x(1/2)2+3.14x1x0.96=3.799m2
Considering the mesh, orifice plate, and other related factors, the filtration accuracy provided by the technical parameter table is 3.15mm. According to GB 10612-89 and GB / T 5330.1-2000, the liquid flow area coefficient is 30%.
5.2 calculation of resistance drop using Johnson screen
According to the technical parameters table, the inner diameter of the filter cartridge is 1000mm and the height of the filter cartridge is 960mm.
Ap=3.14x(1/2)2+3.14x1x0.96=3.799m2
The opening rate of the Johnson screen is calculated as follows:
Opening rate=slot size/slot size+width of the wedge wire
The standard 90 mesh wire is selected:
Wedge wire width = 226mm, height = 3.5mm, clearance angle = 13 °.
Opening rate=3.15/(3.15+2.26)=58.2%.
Af=3.799×58.2%=2.21m2
It can be seen that the resistance drop of Johnson Net is obviously smaller than that of the woven screen. Therefore, without affecting the structure of the equipment, Johnson screen not only reduces the energy consumption, but also reduces the production cost, and has obvious advantages over woven screen in improving the production efficiency and product quality.