Electrodialysis equipment for wastewater treatment

Abstract

The invention relates to the technical field of wastewater treatment, in particular to electrodialysis equipment for wastewater treatment, which comprises a wastewater electrodialysis soft water tank, a blocking and cleaning assembly and a disturbance assembly, an end cover is arranged at the upper end of the wastewater electrodialysis soft water tank, and before wastewater enters an electrodialysis membrane group, the blocking and cleaning assembly is communicated with the disturbance assembly. The wastewater electrodialysis water softening tank is firstly used for treating hardness ions such as calcium and magnesium, and the wastewater is filtered and distributed through the distribution disc with the filter assembly after entering the wastewater electrodialysis water softening tank, so that the wastewater can be uniformly and cleanly sprayed on adsorption resin, the contact effect of the adsorption resin is improved, and the adsorption efficiency is improved. Then a plugging cleaning assembly is arranged below the distribution disc and used for automatic cleaning and maintenance treatment of the filtering assembly, meanwhile, during cleaning and maintenance, the distribution disc cannot be polluted and blocked, operation is easy and convenient, in addition, adsorption resin can be turned over through rotation movement control of the plugging cleaning assembly in cooperation with a disturbance assembly, and the adsorption efficiency is improved. The utilization rate and the treatment effect are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of wastewater treatment, and particularly to an electrodialysis device for wastewater treatment. Background Art

[0002] Electrodialysis is a technology for separating wastewater by using the selective permeability of ion exchange membranes and is widely used in the field of wastewater treatment;

[0003] The utility model with the existing publication number CN210367096U discloses a metal surface treatment wastewater electrodialysis water recovery device, belonging to the technical field of wastewater treatment. Through a touch screen controller, an electric current can be applied to graphite plates and stainless steel plates, thereby forming an anode and a cathode. And under the drive of an electric field, by using the selective permeability of ion exchange membranes, anions and cations move towards the anode and the cathode respectively, so as to achieve the purposes of solution desalination, concentration, refinement or purification, etc. Through a temperature controller and a temperature sensor, the heating rod can be controlled to heat, thereby accelerating ion migration and further improving the efficiency of electrodialysis water. However, due to the complex components in the wastewater, if the water entering the electrodialyzer contains a large amount of hardness ions such as calcium and magnesium, during the electrodialysis process, due to the effects of ion migration and concentration, these hardness ions are likely to form scale on the membrane surface and the electrode surface, such as calcium carbonate, magnesium hydroxide, etc. The formation of scale will affect the ion permeability, reduce the efficiency of electrodialysis, and may also lead to a decline in the performance of the membrane and shorten the service life of the membrane. Therefore, the wastewater needs to be pretreated with soft water before electrodialysis treatment of the wastewater. However, the existing pretreatment equipment is easily blocked by sundries such as suspended matter during use, and the utilization effect of the internal adsorption resin is poor. Summary of the Invention

[0004] The purpose of the present invention is to provide an electrodialysis device for wastewater treatment to solve the problems raised in the above background art.

[0005] To achieve the above purpose, the present invention provides the following technical solution: An electrodialysis device for wastewater treatment, comprising:

[0006] A wastewater electrodialysis soft water tank, the upper end of the wastewater electrodialysis soft water tank is provided with an end cover, a distribution plate is horizontally arranged above the end cover inside the wastewater electrodialysis soft water tank, a resin filling area is arranged below the distribution plate in the wastewater electrodialysis soft water tank through a water permeable partition board, a central control box is vertically arranged at the center of the distribution plate, a wastewater inlet pipe is movably sealed inside the central control box, a filtering component is arranged at the lower end of the wastewater inlet pipe, and the filtering component includes an annular filter cover;

[0007] A plugging and cleaning component, a protective outer cover is arranged at the center of the lower end of the distribution plate, the plugging and cleaning component is arranged inside the protective outer cover through a control component, the plugging and cleaning component includes a plugging ball and a cleaning slot, and the control component includes a cleaning control box and two U-shaped control frames;

[0008] A disturbance component, which is arranged in the resin filling area. The disturbance component includes a stirring shaft, and the upper end of the stirring shaft penetrates through the protective outer cover and is connected to the lower end of the cleaning control box.

[0009] Preferably, a piston groove is formed at the upper end inside the waste water inlet pipe. Air inlet joints are respectively arranged at the upper and lower ends of the piston groove. A piston disc is horizontally arranged at the upper end inside the piston groove where the waste water inlet pipe is inserted. A filtering groove is formed at the lower end inside the waste water inlet pipe. A distribution groove is formed in the distribution disc, and the distribution groove is communicated with the filtering groove. The annular filter cover is fixedly arranged at the lower end inside the filtering groove where the waste water inlet pipe is inserted through a bracket, and the annular filter cover is arranged corresponding to the distribution groove.

[0010] Preferably, the cleaning control box is arranged at the center of the lower end inside the protective outer cover. A ball groove is formed at the upper end of the cleaning control box. A plugging ball is rotatably arranged in the ball groove. The lower end of the waste water inlet pipe is inserted into the protective outer cover and is provided with a sealing ball groove, and the sealing ball groove is communicated with the filtering groove. One side of the plugging ball is hermetically inserted into the sealing ball groove.

[0011] Preferably, lap joints are horizontally arranged at both ends of the plugging ball. The two lap joints are coaxial and correspond to the center of the plugging ball. A support ring is arranged at the upper end of the cleaning control box, and the two lap joints respectively rotatably penetrate through both sides of the support ring.

[0012] Preferably, shifting gears are sleeved on both sides of the lap joint penetrating through the support ring. Storage grooves are respectively formed at the lower ends of the cleaning control box where the shifting gears are located. Jacking piston grooves are respectively formed at the lower ends of the cleaning control box inside the storage grooves. Jacking air supply grooves are respectively communicated and opened at the lower ends of the cleaning control box inside the jacking piston grooves. Jacking piston rods are vertically and movably inserted between the storage grooves and the jacking piston grooves. Jacking pistons are arranged at the lower ends of the jacking piston rods inside the jacking piston grooves. A U-shaped control frame is vertically inserted into the storage groove and is connected to the upper end of the jacking piston rod.

[0013] Preferably, the width between the two vertical sections of the U-shaped control frame is greater than the diameter of the shifting gear. A number of reversing teeth are symmetrically arranged on one side of one vertical section. One side of the shifting gear is meshed and connected with the reversing teeth, and the reversing teeth of the two U-shaped control frames are arranged in opposite directions. When one U-shaped control frame descends to the lowest position, the other U-shaped control frame ascends to the highest position.

[0014] Preferably, a cleaning slot is formed on one side of the plugging ball. The diameter and height of the cleaning slot are respectively greater than the diameter and height of the annular filter cover. An annular diversion groove is formed on one side of the plugging ball close to the cleaning slot. A number of counter punching holes are communicated with the cleaning slot in the annular diversion groove. When the annular filter cover is placed in the cleaning slot, the number of counter punching holes corresponds to the annular filter cover.

[0015] Preferably, both sides of the protective outer cover are penetrated by strip-shaped clearance grooves, and the two lap shafts are movable through the two strip-shaped clearance grooves respectively. The center of one of the lap shafts is connected to the annular diversion groove to form a liquid inlet groove, and the center of the other lap shaft is connected to the lower end of the cleaning slot to form a sewage discharge groove.

[0016] Preferably, the stirring shaft is placed in the resin filling area and is symmetrically provided with a plurality of turning rods on one side, and the upper end of the stirring shaft passes through the lower end center of the protective cover through a bearing and is connected to the lower end center of the cleaning control box.

[0017] Preferably, a swing groove is provided on one side of the stirring shaft in the protective outer cover, arc-shaped piston grooves are provided on both sides of the swing groove, a toggle block is provided on the side of the stirring shaft in the swing groove, arc-shaped piston rods are provided on both sides of the toggle block, and one side of the two arc-shaped piston rods are movably inserted into the two arc-shaped piston grooves and are provided with pushing pistons.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] Before the wastewater enters the electrodialysis membrane group, it is first treated with hardness ions such as calcium and magnesium through the wastewater electrodialysis softening tank. After entering the wastewater electrodialysis softening tank, the wastewater is filtered and distributed through a distribution plate with a filter component, so that the wastewater can be sprayed evenly and cleanly on the adsorption resin, thereby improving the contact effect of the adsorption resin. Then, a plugging and cleaning component is provided under the distribution plate for automatic cleaning and maintenance of the filter component. At the same time, during cleaning and maintenance, the distribution plate will not be contaminated or blocked, and the operation is simple and convenient. In addition, through the rotation activity control of the plugging and cleaning component, the adsorption resin can be turned over in cooperation with the disturbance component to improve the utilization rate and treatment effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the structure of the present invention;

[0021] Figure 2 It is a schematic diagram of a partial side section structure of the present invention;

[0022] Figure 3 For the present invention Figure 2 Schematic diagram of part A;

[0023] Figure 4 For the present invention Figure 3 Schematic diagram of part B;

[0024] Figure 5 This is a schematic diagram of the installation structure of the cleaning control box and the stirring shaft of the present invention;

[0025] Figure 6 A schematic diagram of the control structure for selecting the stirring shaft of the present invention;

[0026] Figure 7 This is a schematic diagram of the sealing ball installation structure of the present invention.

[0027] In the figure: wastewater electrodialysis soft water tank 1, end cover 2, distribution plate 3, resin filling area 4, central control box 5, wastewater inlet pipe 6, distribution groove 7, annular filter cover 8, protective outer cover 9, sealing ball groove 10, cleaning control box 11, blocking ball 12, lap shaft 13, toggle gear 14, storage groove 15, jacking piston groove 16, jacking piston rod 17, U-shaped control frame 18, flip teeth 19, jacking air supply groove 20, cleaning slot 21, annular diversion groove 22, backwash hole 23, liquid inlet groove 24, sewage discharge groove 25, strip-shaped give way groove 26, stirring shaft 27, flip rod 28, swing groove 29, toggle block 30, arc piston groove 31, arc piston rod 32. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Please refer to the attached Figure 1-7 , this application provides the following technical solutions.

[0030] Embodiment 1: An electrodialysis device for wastewater treatment, comprising a wastewater electrodialysis soft water tank 1, an end cap 2 is provided at the upper end of the wastewater electrodialysis soft water tank 1, a distribution plate 3 is provided horizontally at the upper end of the end cap 2 in the wastewater electrodialysis soft water tank 1, the wastewater electrodialysis soft water tank 1 is located below the distribution plate 3 and is provided with a resin filling area 4 through a water-permeable partition, a central control box 5 is vertically provided at the center of the distribution plate 3, a wastewater inlet pipe 6 is movably sealed in the central control box 5, a filter assembly is provided at the lower end of the wastewater inlet pipe 6, and the filter assembly includes an annular filter cover 8, a lifting groove is provided at the upper end of the wastewater inlet pipe 6, and an air inlet joint is provided at the upper and lower ends of the lifting groove, respectively, and the wastewater inlet pipe 6 is plugged into the horizontally provided upper end of the lifting groove. There is a piston disk, a filter tank is provided at the lower end of the wastewater inlet pipe 6, a distribution tank 7 is provided in the distribution plate 3, the distribution tank 7 is connected with the filter tank, an annular filter cover 8 is fixed to the lower end of the wastewater inlet pipe 6 plugged into the filter tank through a bracket, and the annular filter cover 8 is arranged corresponding to the distribution tank 7, the resin filling area 4 is filled with adsorption resin, and the wastewater that needs to be electrodialyzed enters the filter tank through the wastewater inlet pipe 6 before being sent to the electrodialysis equipment. At this time, the wastewater in the filter tank is filtered through the annular filter cover 8 and then enters the distribution tank 7 of the distribution plate 3 for distributed spraying on the resin below. At the same time, the wastewater filtered by the annular filter cover 8 can be further treated after internal solid contamination.

[0031] A plugging and cleaning assembly is provided to clean the annular filter cover 8, aiming to achieve continuous high-quality operation. A protective outer cover 9 is provided at the center of the lower end of the distribution plate 3. The plugging and cleaning assembly is arranged in the protective outer cover 9 through a control assembly. The plugging and cleaning assembly includes a plugging ball 12 and a cleaning slot 21. The control assembly includes a cleaning control box 11 and two U-shaped control frames 18. The cleaning control box 11 is arranged at the center of the lower end inside the protective outer cover 9. A ball groove is opened at the upper end of the cleaning control box 11. The plugging ball 12 is rotatably arranged in the ball groove. The lower end of the waste water inlet pipe 6 is inserted into the protective outer cover 9 and is provided with a sealing ball groove 10. The sealing ball groove 10 is communicated with the filtering groove. One side of the plugging ball 12 is hermetically inserted into the sealing ball groove 10. When the annular filter cover 8 is used for normal filtration, one end of the plugging ball 12 plugs the sealing ball groove 10. At this time, the waste water in the filtering groove can only enter the distribution groove 7 of the distribution plate 3 through the annular filter cover 8.

[0032] Lap joints shafts 13 are horizontally arranged at both ends of the plugging ball 12. The two lap joints shafts 13 are coaxial and correspond to the center of the plugging ball 12. A support ring is provided at the upper end of the cleaning control box 11. The two lap joints shafts 13 respectively rotatably penetrate through both sides of the support ring. Dial gears 14 are sleeved on both sides of the lap joints shafts 13 penetrating through the support ring. Receiving grooves 15 are opened at the lower ends of the cleaning control box 11 where the dial gears 14 are located. Jacking piston grooves 16 are opened at the lower ends of the cleaning control box 11 inside the receiving grooves 15. Jacking air supply grooves 20 are communicated and opened at the lower ends of the cleaning control box 11 inside the jacking piston grooves 16. Jacking piston rods 17 are vertically and movably inserted between the receiving grooves 15 and the jacking piston grooves 16. Jacking pistons are provided at the lower ends of the jacking piston rods 17 inside the jacking piston grooves 16. The U-shaped control frames 18 are vertically inserted into the receiving grooves 15 and are connected to the upper ends of the jacking piston rods 17. The width between the two vertical sections of the U-shaped control frames 18 is greater than the diameter of the dial gears 14. A number of reversing teeth 19 are symmetrically provided on one side of one vertical section. One side of the dial gear 14 is meshed and connected with the reversing teeth 19. The reversing teeth 19 of the two U-shaped control frames 18 are arranged in opposite directions. When one U-shaped control frame 18 descends to the lowest position, the other U-shaped control frame 18 rises to the highest position. Through the up-and-down alternating movement control of the two U-shaped control frames 18, the rotation control of the plugging ball 12 can be realized. When high-pressure gas is sent into the jacking piston groove 16 below one U-shaped control frame 18, the gas connection of the other jacking piston groove 16 is interrupted. At the same time, as the U-shaped control frame 18 rises, the other U-shaped control frame 18 will be pushed downward under the action of the reversing teeth 19 arranged in the opposite direction. In this way, the two reciprocate, and the rotation control of the plugging ball 12 can be completed, realizing the plugging operation of the sealing ball groove 10.

[0033] The cleaning slot 21 is opened on one side of the blocking ball 12, and the diameter and height of the cleaning slot 21 are respectively greater than the diameter and height of the annular filter cover 8. An annular diverter groove 22 is opened on one side of the blocking ball 12 near the cleaning slot 21, and a plurality of recoil holes 23 are opened in the annular diverter groove 22 to connect to the cleaning slot 21. When the annular filter cover 8 is placed in the cleaning slot 21, the plurality of recoil holes 23 correspond to the annular filter cover 8. Strip-shaped clearance grooves 26 are opened on both sides of the protective outer cover 9, and the two lap shafts 13 are respectively movable through the two strip-shaped clearance grooves 26. The center of one of the lap shafts 13 is connected to the annular diverter groove 22 and an entry groove is opened. The liquid tank 24, the center of the other overlapping shaft 13 is connected to the lower end of the cleaning slot 21, and a sewage discharge tank 25 is opened. When the sealing ball 12 rotates and the cleaning slot 21 corresponds to the sealing tank, the wastewater inlet pipe 6 controls the wastewater inlet pipe 6 and the annular filter cover 8 to descend through the piston disk. At this time, the annular filter cover 8 is inserted into the cleaning slot 21 and corresponds to the recoil hole 23. When the liquid inlet tank 24 transports high-pressure water to the annular diversion slot 22, under the further pressurization of several recoil holes 23, it recoils to the annular filter cover 8, and cleans the pollutants gathered on the inner circumference of the annular filter cover 8. The cleaned sewage is discharged from the sewage discharge tank 25 to the cleaning slot 21.

[0034] Embodiment 2: On the basis of embodiment 1, a disturbance component is provided to stir the adsorbent resin in the resin filling area 4 to improve the contact effect between the wastewater and the resin. The disturbance component is provided in the resin filling area 4. The disturbance component includes a stirring shaft 27. The upper end of the stirring shaft 27 penetrates the protective cover 9 and is connected to the lower end of the cleaning control box 11. The stirring shaft 27 is placed in the resin filling area 4 and is symmetrically provided with a plurality of flip rods 28 on one side. The upper end of the stirring shaft 27 penetrates the lower end center of the protective cover 9 through a bearing and is connected to the lower end center of the cleaning control box 11. A swing groove 29 is provided on one side of the stirring shaft 27 in the protective cover 9. The swing groove Arc-shaped piston grooves 31 are provided on both sides of 29, and a toggle block 30 is provided on one side of the stirring shaft 27 located in the swing groove 29, and arc-shaped piston rods 32 are provided on both sides of the toggle block 30, and one side of the two arc-shaped piston rods 32 is movably inserted into the two arc-shaped piston grooves 31 and is provided with a pushing piston. When high-pressure gas enters one of the arc-shaped piston grooves 31, the arc-shaped piston rod 32 will push the toggle block 30 to rotate toward the arc-shaped piston groove 31 on the other side. At this time, the toggle block 30 can control the stirring shaft 27 and the cleaning control box 11 to rotate, so as to realize the flipping of the resin in the resin filling area 4 by the flipping rod 28, thereby improving the adsorption utilization rate of the resin.

[0035] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An electrodialysis device for wastewater treatment, characterized in that: include: A wastewater electrodialysis softening tank (1), wherein an end cover (2) is provided at the upper end of the wastewater electrodialysis softening tank (1), a distribution plate (3) is horizontally provided at the upper end of the end cover (2) in the wastewater electrodialysis softening tank (1), a resin filling area (4) is provided in the wastewater electrodialysis softening tank (1) below the distribution plate (3) through a water-permeable partition, a central control box (5) is vertically provided at the center of the distribution plate (3), a wastewater inlet pipe (6) is movably sealed in the central control box (5), a filter assembly is provided at the lower end of the wastewater inlet pipe (6), and the filter assembly includes an annular filter cover (8); A plugging and cleaning component, wherein a protective outer cover (9) is provided at the center of the lower end of the distribution plate (3), the plugging and cleaning component is arranged in the protective outer cover (9) through a control component, the plugging and cleaning component comprises a plugging ball (12) and a cleaning slot (21), and the control component comprises a cleaning control box (11) and two U-shaped control frames (18); A disturbance component is arranged in the resin filling area (4), and the disturbance component comprises a stirring shaft (27). The upper end of the stirring shaft (27) passes through the protective cover (9) and is connected to the lower end of the cleaning control box (11).

2. The electrodialysis equipment for wastewater treatment according to claim 1, characterized in that: A piston groove is provided at the upper end of the wastewater inlet pipe (6), and air inlet joints are provided at the upper and lower ends of the piston groove respectively. A piston plate is horizontally provided at the upper end of the wastewater inlet pipe (6) plugged into the piston groove. A filter groove is provided at the lower end of the wastewater inlet pipe (6). A distribution groove (7) is provided in the distribution plate (3), and the distribution groove (7) is connected to the filter groove. An annular filter cover (8) is fixed to the lower end of the wastewater inlet pipe (6) plugged into the filter groove through a bracket, and the annular filter cover (8) is provided corresponding to the distribution groove (7).

3. The electrodialysis equipment for wastewater treatment according to claim 2, characterized in that: The cleaning control box (11) is arranged at the center of the lower end of the protective outer cover (9); a ball groove is provided at the upper end of the cleaning control box (11); a sealing ball (12) is rotatably arranged in the ball groove; the lower end of the wastewater inlet pipe (6) is plugged into the protective outer cover (9) and is provided with a sealing ball groove (10); the sealing ball groove (10) is connected to the filter groove; one side of the sealing ball (12) is sealed and plugged into the sealing ball groove (10).

4. The electrodialysis equipment for wastewater treatment according to claim 3, characterized in that: Both ends of the blocking ball (12) are horizontally provided with lap shafts (13), the two lap shafts (13) are coaxial and correspond to the center of the blocking ball (12), and the upper end of the cleaning control box (11) is provided with a support ring, and the two lap shafts (13) rotate and penetrate the two sides of the support ring respectively.

5. The electrodialysis equipment for wastewater treatment according to claim 4, characterized in that: The lap shaft (13) passes through the support ring and is sleeved with a toggle gear (14) on both sides; a storage groove (15) is provided at the lower end of the toggle gear (14) of the cleaning control box (11); a lifting piston groove (16) is provided at the lower end of the storage groove (15) in the cleaning control box (11); a lifting air supply groove (20) is provided at the lower end of the lifting piston groove (16) in the cleaning control box (11); a lifting piston rod (17) is vertically movably inserted between the storage groove (15) and the lifting piston groove (16); a lifting piston is provided at the lower end of the lifting piston rod (17) located in the lifting piston groove (16); and a U-shaped control frame (18) is vertically inserted into the receiving groove (15) and connected to the upper end of the lifting piston rod (17).

6. The electrodialysis equipment for wastewater treatment according to claim 5, characterized in that: The width between the two vertical sections of the U-shaped control frame (18) is greater than the diameter of the toggle gear (14), a plurality of flip teeth (19) are symmetrically arranged on one side of one of the vertical sections, one side of the toggle gear (14) is meshed and connected with the flip teeth (19), and the flip teeth (19) of the two U-shaped control frames (18) are arranged in opposite directions, and when one U-shaped control frame (18) descends to the lowest position, the other U-shaped control frame (18) rises to the highest position.

7. The electrodialysis equipment for wastewater treatment according to claim 6, characterized in that: The cleaning slot (21) is provided on one side of the blocking ball (12); the diameter and height of the cleaning slot (21) are respectively greater than the diameter and height of the annular filter cover (8); an annular diversion groove (22) is provided on one side of the blocking ball (12) close to the cleaning slot (21); a plurality of recoil holes (23) are provided in the annular diversion groove (22) and connected to the cleaning slot (21); when the annular filter cover (8) is placed in the cleaning slot (21), the plurality of recoil holes (23) correspond to the annular filter cover (8).

8. The electrodialysis equipment for wastewater treatment according to claim 7, characterized in that: Both sides of the protective outer cover (9) are penetrated by strip-shaped clearance grooves (26), and the two overlapping shafts (13) are respectively movable and penetrate the two strip-shaped clearance grooves (26). The center of one of the overlapping shafts (13) is connected to the annular diversion groove (22) and is provided with a liquid inlet groove (24), and the center of the other overlapping shaft (13) is connected to the lower end of the cleaning slot (21) and is provided with a sewage discharge groove (25).

9. The electrodialysis equipment for wastewater treatment according to claim 8, characterized in that: The stirring shaft (27) is placed in the resin filling area (4) and is symmetrically provided with a plurality of turning rods (28) on one side. The upper end of the stirring shaft (27) passes through the lower end center of the protective cover (9) through a bearing and is connected to the lower end center of the cleaning control box (11).

10. The electrodialysis equipment for wastewater treatment according to claim 9, characterized in that: A swing groove (29) is provided in the protective outer cover (9) on one side of the stirring shaft (27), arc-shaped piston grooves (31) are provided on both sides of the swing groove (29), a shifting block (30) is provided on one side of the stirring shaft (27) in the swing groove (29), arc-shaped piston rods (32) are provided on both sides of the shifting block (30), and one side of the two arc-shaped piston rods (32) are movably inserted into the two arc-shaped piston grooves (31) and are provided with push pistons.

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