Process design of vacuum system in paper machine wet end

f Technical report Vacuum system!

Process design of the vacuum system in the wet end of paper machine Zhao Sheng (China Light Industry International Engineering Design Institute, Beijing, 100026) Air-water separator and vacuum piping. Explained the design of front and rear air-water separators, space layout, and the selection of vacuum pumps. The necessity of recycling the working fluid of vacuum pumps to save fresh water resources and reduce product costs, and the collection, circulation, filtration, cooling and reuse of working fluids. Engineering program. Put forward the principles of equipment layout and pipeline design, noise prevention measures and system automatic control scheme.

Engineers; engaged in pulp and paper machine wet paper machine is a paper machine engineering consulting and design. The main embodiment of preparation level.

1 System Components The equipment and accessories for dewatering, drainage, and venting (vapor) in a papermaking plant relying on vacuum pressure differentials can all be classified as vacuum systems in paper machines. In addition to the dehumidifier vacuum system, it also includes the condensate discharge vacuum device at the end of the steam condensate system and the supporting vacuum device for the stem Vac.

The wet end vacuum system mainly includes components such as a vacuum element, a pre-stage gas-water separator, a vacuum pump (or fan), a post-stage gas-water separator, a vacuum pipe, and the like, as shown.

2 System Components Configuration and Arrangement 2.1 Vacuum Element Forming Section The vacuum components are all set for the dewatering and forming of the web. The form and quantity of the vacuum components are determined by the shape of the former.

Circular net formers with a speed less than 1006/min (ie, rotary net formers) are not equipped with vacuum dewatering elements. Rotary net cage formers developed from circular wire mesh formers, such as vacuum cylinder formers, Bristol formers, super formers, etc., are equipped with vacuum cages, vacuum suction boxes, etc., and their working speed can reach 500m. /min is even higher.

The long wire formers are generally equipped with vacuum suction boxes, vacuum rolls and other vacuum components. Low-speed paper machines are often equipped with non-vacuum dehydration components such as case rollers and cutting boards. However, as the vehicle speed increases, the intensity of the pressure-vacuum pulses caused by the dehydration process of the case roll and the cutting board drastically increases, which seriously affects the web forming quality. In the high speed paper machine, the case roll and the chopping board are gradually replaced by vacuum dewatering plates and vacuum suction boxes.

The net shaper is also equipped with a vacuum dehydration element such as a vacuum suction box, a vacuum (volt) roller, etc. However, due to its short forming area, the number of vacuum dewatering elements is much smaller than that of the long-wire former.

Thus, in several major formers, the vacuum dewatering element is more used for the long wire former. For long wire formers, the higher the speed, the more vacuum dewatering elements are. The degree of vacuum of the vacuum dewatering element is higher and higher in the running direction of the web.

Press section vacuum elements include a vacuum suction roll for web transfer, a vacuum press roll for web dewatering, and a vacuum suction box for felt cleaning and dewatering transfer. The vacuum of the press section vacuum is higher than that of the forming section.

The composition of the vacuum components in the wet section of a typical multigrip paper machine is shown in Table 1. 2.2. The first-stage gas-water separator The first-stage gas-water separator The gas-water mixture from which the vacuum components are discharged is shown in Table 1 Typical Multidraw Papermaking Machine Wet-end vacuum elements Composition Table Vacuum Element Name Vacuum Degree/kPa Quantity 1 Surface Layer Vacuum Dewatering Plate 5'102 Vacuum Suction Box 3022 Lining Mesh Vacuum Dewatering Sheet 5'102 Vacuum Suction Box 302 Transfer Suction Box 2013 Core Network Vacuum Dewatering Board 5'107 Low vacuum suction box 10'153 Transfer suction box 201 Vacuum suction box 3024 Bottom net vacuum dewatering board 5'102 Transfer suction box 202 Vacuum suction box 30'335 Vacuum roll roller 46, 6025 Press section Vacuum suction roll 26, 664 Wool cloth water absorption The tank 508 may be separated to prevent white water from entering the vacuum piping, thereby increasing the working efficiency of the vacuum system. After the air-water mixture is separated by a separator, the white water overflows through the water-sealing tank or is pumped into the white sink under the net, and then returns to the white water system of the plant. The wet air is discharged by the vacuum pump through the vacuum pipeline.

The volume of the preceding gas-water separator is determined by the volume of gas-water mixture discharged by the vacuum unit per unit time. The volume of the gas-water mixture and the gas-water volume ratio determine the diameter of the gas-water separator drain and exhaust pipe. In the forming section, the ratio of air/water volume increases with the dryness of the paper web in the running direction of the web, which can increase from 0.25 to 125. In the press section, the air/water volume of the mixture discharged from the vacuum suction box of the felt is removed. Compared to the structure of the suction box and the water content of the felt, the air-water volume ratio of the mixture discharged from other vacuum components is also related to the dryness of the web.

A vacuum separator with the same degree of vacuum can share a gas-water separator. A gas-water separator can be used for more vacuum elements in the front section of the forming section where the ratio of gas to water is small. However, in the rear section of the forming section and the press section where the volume of gas and water is relatively large, a gas-water separator cannot be used for too many vacuum elements.

When arranging the separator, under the premise that the inlet of the separator is lower than the outlet of the vacuum element, the height of the separator should be increased as much as possible to ensure a sufficient height of the water leg, and at the same time, the effective liquid level of the water seal groove and the effective volume of the white pool under the net should be increased ( see). If the height of the water leg cannot be ensured due to the space and the structure of the paper machine, a drainage pump should be provided. Under normal circumstances, the separator used for the vacuum roller and the vacuum suction box under the press section needs a drainage pump.

In order to ensure the stability of the system, the water leg of the separator should be inserted vertically into the water seal tank, and an overflow baffle plate should be installed in the water seal tank to ensure the liquid level is stable. Tilting of the water leg will cause the air leg to retain air, causing fluctuations in vacuum to affect web formation.

2.3 Vacuum pump and exhaust fan For the vacuum element with a lower vacuum (5~15kPa) at the beginning of the forming zone, a centrifugal fan or a Roots blower should be used as the vacuum generating device. For other vacuum components, Roots vacuum pumps or water ring vacuum pumps are used depending on the degree of vacuum. The Roots vacuum pump is mechanically degassed and is noisy. Due to mechanical wear, the maintenance workload is large. However, the working fluid is not required and the system is simple. The water ring vacuum pump has a small maintenance workload, and the power consumption is less than the former, and the stand-alone capacity is large. However, the working fluid is required and the system is relatively complicated. The vacuum system of large paper machines mostly uses water ring vacuum pumps. In the following discussion, vacuum pumps are all referred to as water ring vacuum pumps.

When selecting a water ring vacuum pump, a pump dual-chamber vacuum pump with a partition can be used. The two chambers correspond to the vacuum elements required by different vacuum degrees, and work under different vacuum degrees. The reduction of the number of vacuum pumps not only saves equipment investment, but also reduces the total installed capacity of the motor to reduce energy consumption and system operating costs. At the same time, it reduces the floor space and saves infrastructure investment.

The pumping speed of the vacuum pump is determined based on the exhaust speed of the vacuum element, the vacuum degree, the average pressure in the pipeline, the pipe diameter, and the pipe length. Taking into account the reasonable pipe diameter and the economic pressure drop of the pipeline, the vacuum pumping speed is calculated according to formula (1).

The vacuum pump type can be determined according to the required suction speed S+ and vacuum component vacuum requirement. When arranging the vacuum pump, concentrate as much as possible on the area close to the vacuum element, shorten the length of the vacuum pipeline and reduce the energy consumption of the system.

2.4 The second-stage gas-water separator The second-stage gas-water separator separates the air-water mixture discharged from the vacuum pump and has a noise-reducing effect. Air is exhausted through the exhaust pipe. The working fluid (for water ring vacuum pumps only) is discharged from the system or recycled after being processed. For smaller vacuum systems, use one steel-grade air-water separator or multiple vacuum pumps for each vacuum pump. When the scale is large, the steel-grade gas-water separator is not configured, but the air-water mixture is discharged into the trench, and gas and water are separated in the pit.

The displacement of the gas-water separator in the final stage is mainly determined by the amount of working fluid in the vacuum pump. In systems without a pre-stage water-gas separator, drainage also includes white water discharged from the vacuum element, which has a higher fiber content than the former. In small paper machines, the displacement is small and most of the water is directly drained. In large paper machines, the displacement can be up to 20m3/t paper or even higher, regardless of the previous level of the air-water separator before the 1015 gas-water separator in the previous stage, and the air-water separator exhaust pipe 1520 before the first stage. Air-Water Separator Drainage Pipe 1 After-stage gas-water separator 1517 Recycle of 2.5 working fluid in the air-water separator of the second-stage gas-water separator The complete working fluid circulation system includes: collection tank, circulation pump, filter, cooling tower, etc. As shown.

A collection sink must be set up for collection. Collecting the sink is an extension of the trench and deepening.

The subsequent gas-water separator and collection sink merge into one.

Selecting the appropriate media flow rate is particularly important in designing the trenches and the size of the collection sinks. The flow rates of the media at each location are given in Table 2. Circulation pump pipe position flow rate Pump the working fluid to the cooling tower on the cover of the collection sink (pit). The water circulation method of the circulating pump can be designed as a suction pipe with a bottom valve or a self-priming tank. The self-priming tank covers a large area, but is safer and more reliable than the bottom valve.

The working fluid discharged from the vacuum pump (or after the gas-water separator) contains a small amount of solids such as fibers, fillers, etc., which can cause fouling of the vacuum pump rotor and the cooling tower. Therefore, a filter must be installed in front of the cooling tower. Due to the short fibers and small particle size, pressure screens and multiple disk filters that are widely used in the paper industry are not suitable for this purpose. Equipment such as inclined screens and pipeline filters can be used according to the water quality. A system with a pre-stage gas-water separator is generally provided, and a pipeline filter may be used. Sloped screens should be used in systems without a pre-stage gas-water separator.

In order to improve the paper web dehydration forming and increase the temperature of the paper machine flow system and the slurry of the web part, the temperature of the air and the saturated steam mixture entering the vacuum pump will increase, thereby increasing the working fluid temperature. Thermal energy generated during the operation of the vacuum pump is also absorbed by the working fluid.

The increase of working fluid temperature will lead to the reduction of vacuum pump working efficiency. The vacuum pump will only work if the working fluid temperature is lower than the temperature of the suction air and saturated steam mixture. The air cooling tower can be used to reduce the temperature of the working fluid to recycle. The appropriate working fluid temperature is in some vacuum systems. In addition to the external circulation shown in the configuration, the vacuum pump is divided into two groups according to the vacuum degree requirements, and the collection height is high. The working fluid discharged from the vacuum group is directly used as the working fluid of the low-vacuum group without cooling, and is referred to as working fluid internal circulation. The internal loop will make the system too complex, not easy to control and change the system parameters, so generally only the outer loop is set.

2.6 Vacuum System In the vacuum state, the distance between gas molecules increases, and the collision of gas in the pipeline decreases. The gas flow belongs to the viscous flow or molecular flow, or the intermediate flow state between the two. In the condition of low vacuum (vacuum: 0101.19kPa), it belongs to the state of viscous flow. In the high vacuum state, it belongs to the molecular flow state.

The degree of vacuum required for the vacuum components of the paper machine is in the low vacuum range. Under vacuum, in theory, the vacuum pipe diameter is a function of the conductivity, the average pressure in the pipe, and the pipe length. The shorter the vacuum pipe, the better. In practical engineering design, one or more exhaust pipes can be used to direct gas from the pit to the outside during public hours. The exhaust pipe can also be made of concrete or brick. When the air is discharged through the subsequent air-water separator, the exhaust pipes are brought together into one or several exhaust mains and led outside. The height of the outdoor exhaust manifold can reach the roof, and the silencer installed at the end of the exhaust manifold is arranged on the roof.

2.8 Noise Control The vacuum system is one of the major noise sources in paper mills. The noise mainly comes from the vacuum element of the paper machine body, the vacuum pump rotor, the vacuum pump motor, and resonance. In order to reduce noise pollution, the vacuum pump is arranged in a separate room, and the room doors, windows, and walls need to be treated with sound insulation; a flexible short circuit is provided between the vacuum pump and the pipeline to prevent noise transmission; and a post-stage air-water separator is reasonably designed. , Select a reasonable media flow rate to prevent resonance; install a silencer on the exhaust pipe exhausted to the outside of the vacuum pump to reduce noise pollution on the outdoor environment.

3 System automatic control system Automatic control mainly includes: Adjustment of system vacuum degree The vacuum degree of vacuum element can also be adjusted on the vacuum pipeline by adjusting the adjustable air intake device equipped with its own. If the vacuum pump is equipped with a variable speed drive system, the system vacuum can be adjusted more economically by adjusting the speed of the vacuum pump.

Interlocking of the system with the paper machine The vacuum pump can only be closed and operating normally at the main drive point of the corresponding paper machine. Only when the spray pipe associated with the vacuum element is normally sprayed with water can it be started. When the main drive point of the paper machine associated with the vacuum element stops operating, the vacuum pump is interlocked and shut down. Reduce the abnormal wear of the forming net and felt and extend its service life.

Level control of the preceding air-water separator: The liquid level of the separator with the drainage pump is interlocked with the drain pump motor and the liquid level is stabilized by adjusting the regulating valve on the outlet pipe of the drainage pump. Other separators stabilize their state by stabilizing the liquid level of the water-sealing tank overflowing the separator.

The liquid level of the collection tank is interlocked with the circulating water pump motor, and the regulating valve on the outlet pipe of the circulating water pump is adjusted to make the liquid level stable.

The temperature control of the circulating water is achieved by adjusting the speed of the cooling tower motor.

DCS has become a popular choice for modern papermaking workshops, and even becomes a necessary condition for the normal operation of production. The control of this system should be incorporated into the DCS as a separate group to automatically start and stop.

4 Conclusions Among the main factors affecting the configuration of the vacuum system in the wet end of a paper machine, the structure of the wet end of the paper machine, the composition of the vacuum elements and their requirements are the input conditions, and it is the basic basis; the layout of the paper machine determines the space for equipment layout; the wet end is ensured. Paper quality is the purpose; economic efficiency is one of the criteria for checking the system's reasonableness.

Single-layered and low-yield paper machines may not have a pre-stage gas-water separator due to space limitations, but as the output increases, the power consumption of vacuum pumps will increase significantly. In a large-scale vacuum system, the gas-water mixture is discharged into the trench, and the gas is separated and discharged in the pit.

The recycling of the working fluid has nothing to do with the stability of the system. It will not affect the quality of the paper in the wet end. It only affects the fixed investment and product cost. In the case of large-scale vacuum systems, the recirculation of the working fluid increases the fixed investment and power consumption, but it reduces the production cost of the final product due to the decrease in the amount of water used. For a small-scale vacuum system, it is possible to determine whether or not to reuse the working fluid after comparing the impact of the increased cost and the cost of water withdrawal on the cost of the final product after the recycling of the working fluid. However, in the face of a serious shortage of water resources, the recycling of working fluids is a development trend.

Air Pneumatic Cylinder

â‘  Single acting cylinder: only one end of the cylinder has a piston rod. Air is supplied from one side of the piston to generate air pressure. The air pressure pushes the piston to produce thrust to extend and return by spring or self weight.

â‘¡ Double acting cylinder: supply air alternately from both sides of piston, output force in one or two directions.

â‘¢ Rodless cylinder: general term for cylinders without piston rod. There are magnetic cylinder and cable cylinder.

The reciprocating cylinder is called swing cylinder. The inner cavity is divided into two parts by the blade, and the air is supplied to the two cavities alternately. The output shaft swings, and the swing angle is less than 280 °. In addition, there are rotary cylinder, gas-liquid damping cylinder and stepping cylinder.

Compared with the electric actuator, the cylinder can work reliably under harsh conditions, and the operation is simple, and the basic maintenance free can be realized. Cylinder is good at reciprocating linear motion, especially suitable for industrial automation in the most transmission requirements - workpiece linear handling. Moreover, only adjusting the one-way throttle valve installed on both sides of the cylinder can simply achieve stable speed control, which has become the biggest feature and advantage of the cylinder drive system. Therefore, for users without multi-point positioning requirements, most of them prefer to use cylinder from the perspective of convenience. At present, most of the applications of electric actuators in the industrial field require high-precision multi-point positioning, which is the result that it is difficult to achieve with the air cylinder.

The electric actuator is mainly used for rotating and swinging conditions. Its advantage lies in the fast response time and the precise control of speed, position and torque through the feedback system. But when it needs to complete the linear motion, it needs to be transformed by mechanical devices such as toothed belt or screw rod, so the structure is relatively complex, and there are high requirements for the working environment and the professional knowledge of the operation and maintenance personnel.

advantage

(1) The requirement for users is low. The principle and structure of the cylinder are simple, easy to install and maintain, and the requirements for users are not high. Electric cylinder is different, engineering personnel must have certain electrical knowledge, otherwise it is likely to be damaged due to misoperation.

(2) The output force is large. The output force of the cylinder is proportional to the square of the cylinder diameter; The output force of the electric cylinder is related to three factors: cylinder diameter, motor power and screw pitch. The larger the cylinder diameter and power, the smaller the screw pitch, the greater the output force. The theoretical output force of a cylinder with a diameter of 50 mm can reach 2000 n. for the electric cylinder with the same diameter, although the products of different companies are different, they are basically no more than 1000 n. Obviously, the cylinder has more advantages in terms of output force.

(3) Strong adaptability. The cylinder can work normally in high and low temperature environment, and has the ability of dustproof and waterproof, which can adapt to all kinds of harsh environment. Because of a large number of electrical components, the electric cylinder has high requirements for the environment and poor adaptability.

Air Cylinder, Pneumatic Cylinder, double acting cylinder, single acting cylinder, rodless cylinder

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