Double casing concentrated phase positive pneumati

1.Double casing dense phase positive pneumatic ash conveying system general introduce

Double-casing concentrated phase positive pneumatic conveying system is an advanced pneumatic conveying technology, which is developed in abroad from the late of the 1980^th.  

    It is widely used in power plant fly ash, cement, limestone powder, aluminum powder, gypsum powder, pulverized materials, such as pneumatic conveying industry. Double casing concentrated phase positive pneumatic conveying system mechanism is different from the conventional positive transmission .Conventional positive pneumatic conveying system uses the suspension delivery, the transmission concentration is low, high-speed, easy to wear and possession of Plugging; Double casing concentrated phase positive pneumatic conveying system uses the hydrostatic transmission, has a high ash/air tatio, and large efforts, low-velocity, small wear, etc., is the ideal solution to transport the high wear and efforts, heavy wear materials (such as boiler fly ash) dense phase conveying, It’s on behalf of today's advanced technology level . In the normal running conditions, the system automatically adjusts the double casing to loose the big fly ash, so that make sure the fly ash transported in steady-state and prevent the tube from blocking.

2.Double casing concentrated phase positive pneumatic conveying system work principle

Double casing concentrated phase positive pneumatic conveying system’s typical structure characteristic is its conveying pipe, the characteristic is that install a inner propellant tube in the mother tube of double casing, the propellant tube has a special designed slope mouth every distance, in the middle of the mouth there is a round piece with hole. To convey the fly ash in the turbulent state by the pipe automatic adjustment .At the same time, air passes through the double casing mother pipe and inner propellant tube, the air into the is inner propellant tube then affected by the specially designed slope mouth, make the fly ash form the ion of turbulence as more as possible, so that the air and fly ash will be in full flow and mixing, and the fly ash will be accumulated and separated into pieces, its main principles are as follows:

In the normal state of transmission, with the pipe length increasing, the state of turbulent flow of the ash will be destroyed, and ash will accumulate on the pipe bottom, the mother tube of the double casing pipes will tend to be blocked while more and more accumulation of the material occurs, then the local resistance increases, which cause the local high Air pressure, making the air through bypass into the inner propellant increases, forming a more turbulent at the tow ends of the fly ash cake ,it can loose the accumulated fly ash cake and eliminate the plug. This cake is expected to be split, moved, dispersed continually, and be transported. Double casing dense phase positive pneumatic conveying system’s transmission mechanism is based on a very simple principle of physics: the air is always flowing to the smallest resistance direction.

The wear caused by high-speed is a difficult problem in the pneumatic conveying system. Due to the special nature of gas-solid flow, The calculation of the convetional system’s velocity is based on air flow rate, and can not really determine the flow speed of the materials, but can determine the velocity of the materials from the mechanism of transmission system, the conventional positive conveying system transmission mechanism is Suspension transmission ,the materials are suspended in the air in the campaign and the movement resistance is small, so the velocity is close to the air movement speed; the double-casing  system is hydrostatic transmission mechanism, the materials move in a half embolism condition, and in the upper the double casing propellant tube is shunting flow ,so the material movement velocity is greatly lower than the air . Comparing with the conventional conveyor systems, when calculate velocity, even if the same system, the velocity of the material is also much lower than the conventional positive conveyor systems. As we all know, the wear velocity to the other material objects is proportional three times square to the material itself’s velocity, double-casing concentrated phase low-speed transmission system compared with the conventional system, the delivery of materials operates inreal low-speed state and therefore The wear to the bend can be mostly minimized.

3.Double casing concentrated phase positive pneumatic ash conveying system process

Double casing concentrated phase positive pneumatic ash conveying system is composed of air system, transmission system, ash warehouse and control system.

3.1Air system is composed of air compressor, after processing equipment (freeze-drying, non-renewable sources of dry heat, pre-filter, post-filter), cylinders, pipes and valves.

Air compressors supply gas used to transmission systems and instrument control. As the compressed air discharged from the air compressor contains a lot of water, oil and impurities, if not use the post-processing equipment to purify the gas, it is likely to result in agglomeration of fly ash, causing difficulties in transmission or Plugging, so the system usually set up after the Processing equipment to make sure that the treated transmission gas and instrument control gas can meet the system requirements. The main role of the storage tank is to store the clean treated transmission gas and instrument control gas, to meet gas requirements when the peak air consumption or unloading or parking a short period of time.

3.2 Transmission system

Transmission system is composed of dry ash transmitter, feed valve, balance valve, discharging valve, inlet valve group, the boosters, double casing, warehouse top Switch valve, pipe Annex, and so on.

Double casing concentrated phase positive pneumatic conveying system can work at a relatively low transmission speed, low transport air pressure, high ash/air ratio condition. Especially in the large ash volume and long transmission distance projects, its’s advantages are more obvious. Double casing dense phase pneumatic conveying system ’s initial velocity is about 4-6m/s, at the end of transmission is about 8-10m/s. The system adjusts the conveying gas pressure by the relief valve, and adjusts the gas flow rate by the air hole plate to ensure the system operate economically and stably.

3.2. Double casing concentrated phase positive pneumatic ash conveying system start conditions

★Compressed air supplier of the conveying system work normally;

★Material level of the  warehouse is in normal;

★The bag filter on the top of the warehouse operates normally;

★Total inlet valve is on;

★The prepare work of putting the system into run is in normal.

3.2.2 Transmission processing

Hopper bottom balance valve open→ hopper bottom feed valve open →feed the unit dry ash transmitter→ dry ash material transmitter is full and send signal or the control time is coming → hopper bottom feed valve close → hopper bottom balance valve close→ the unit discharge valve open →the unit inlet air valve open→ the main inlet air valve of the booster open → conveying pipe transmission→ warehouse → discharge the remaining gas into the atmosphere

3.2.3 Operation

A.The system is in the feeding condition, at this time the feeding valve and the balance valve open, the inlet air valve and the discharge valve close, the fly ash fall into the dry ash transmitter under the role of the gravity.

B.When one or more high levels of the dry ash transmitters send signal or achieve the settled loading time, the feeding valve and the balance valve close.

C.Open the discharge valve, inlet air valve and the booster one after the other.

D.When the transmission pressure in the pipe is lower than the setting, then the booster,Inlet air valve and the discharge valve close, after a few time delays, reset the cycle and wait for the next cycle.

3.2.4 System interlock

The compressed air supply work in normal, as well as the prepare work of putting the system into run is in normal, and then the system can be put into run.

The dry ash transmitter balance valve open, and the feeding valve open.

The unit dry ash transmitter is full of material and send signal, the dry ash transmitter feeding valve close, the balance valve close.

The unit dry ash transmitter feeding valve close, the balance valve close, discharge valve open, the unit booster open.

The unit transmission low level send signal, dry ash transmitter inlet air valve close, the unit booster inlet air valve close.

The unit booster inlet air valve close and the unit discharge valve close.

3.3 Warehouse system

Warehouse system is composed of warehouse top bag filter, vacuum pressure release valve, level, dry discharge loader, wet mixer, warehouse gasification equipment (air gasification, electric heater, gasified chute), pipeline valves,etc.

The ash conveying pipe of the double casing concentrated phase positive pneumatic conveying system is directly connected into the warehouse; the discharging air is purified by the bag filter and then flow out. The gas velocity of the bag filter is no more than 0.8m/min，the concentration of the emission meets the requirement of GB16297. The bag filter chooses pulse anti-draught method to blow the bag ,using the air of the quality for the instrument control .

On the top of the warehouse set a vacuum pressure release valve, to prevent the warehouse from being in high pressure that causedby the jam of the bag filter.

The warehouse has low level and high level; some also has continuous level to monitor fly ash level in the warehouse.

The warehouse has gasifying equipment, the gasifying pan provides air source, which is heated by the air heater, then heat and flow the fly ash, helping the dry discharge loader or the wet mixer unload.

The dry discharge loader carries the fly ash from the warehouse to the transport vehicles. Dry ash can is widely used in building materials industry.

The wet mixer mixes the ash with the water, and then the transport vehicles carry them to the landfill.

3.4 The control system

The double casing concentrated phase positive pneumatic conveying system uses the PLC+PC control model; automatically control the equipments that are in the control of the project. Setting a located control device near the dry ash transmitter, can operate all the valves of every single device of the system locately.

The system has two kinds of model: automatic control and manual on the spot.

Automatic control: The equipment that needs control is run by the PLC automatic control according to the pre-determined procedures process.

Manual on the spot: on the located control box which is beside the equipment in control, set a manual-maintenance-automatic switch and indicator light, if necessary, we can control the valve and maintenance manually on the spot.

The operating mode is manual; you can start and stop the valve device one-to-one.

User’s PLC output should provide relay and isolation.

The power of the level of the dry ash transmitter is provided by the user, the voltage is AC 220V .The power of all the solenoid valves is provided by the user, the voltage is AC 220V . Single-coil valves, each valve box provides a line of 2A capacity power.

4. The characteristics of double casing concentrated positive pneumatic ash conveying systems

★The system operation is reliable, not possession of Plugging

Using a unique double -casing pipe transport technology with its own stability / regulation, the conveyed gas produces orderly adjustment turbulence in the double –casing, particularly in the delivery process, at the site that tend to be plugged, the turbulence will strengthen automatically to eliminate the plug, this improves the security and reliability while the operation of the system; in the trial operation period, when the transport is cut for the air compressor failure and does not start the back-up machine ,you can restart the Air Compressor and blow the blocked pipe, less than 10 minutes all the pipeline can be blown and then recover.

★Power exhaust is small

Double casing concentrated positive pneumatic ash conveying system ＇ash/air is high, its conveying concentration is more than double of the conventional positive conveying system, it’s effective to reduce the capacity of the air compressor and the post-processing equipment as well as the warehouse top bag filter, the system greatly reduce the energy consumption and operating cost, so its running cost are far lower than the conventional transmission system;

★Long-distance, large load

The characteristic of double casing system determines that the system fits for long-distance transmission, the transmission geometry distances even can be more than 2,000 meters, and the load can reach to 200t / h or more;

★Low velocity, low wear and tear, little  maintenance workload

As the material handling speed of the transmission system is low (in the head is 4-6m/s, at the end is 8-10m / s), a low operate pressure (about 1.5-2.5Kg/cm²), higher ash/air ratio ( up to 40-50kg ash/ Gas kg), reducing the pipeline valves and other equipment＇s wear and tear (and the wear rate approx three times square of the speed of the flow of material), so the system is very durable, and the maintenance workload is low;

★Advanced dedicated valve

The feeding valve uses the sealing dome valve, effectively prevent the sealing surface and turning components of the valve from contact wear and increase the tightness of the closed valve, the valve endures wear, and can detect leakage itself; The unit discharge valve is expected to adopt a Double-sealed wearable valve, effectively cut off the positive and negative pressure, make the system trouble-free operation for a long time;

★Adequate efforts of the system

While the system calculating, consider the first electric field as the stand-by when failure occurs, ensure the system effort.

★reasonable means to eliminate the lack ;

When the fly ash mixed with screws, nuts and other metal foreign body, they will fall into the transmitter and then automatically settle in the specially designed box (three way discharge), the user only need to clean regularly by manual. So that the system operates more Safely;

★Optimized system configuration

In the transporting the branch can mostly hang 8 dry ash transmitters at the same time, so the system can delivery all at one time, then after this delivery it can deliver another branch immediately , which can reduce the unload operation time of the transmission air compressor, resulting in improved air transport efficiency;

Set one discharging valve on each delivery branch, greatly reducing the number of the discharging valves, further ganrantee the normal operation of the system;

The most less pipe number;

The least possible leakage points

The minimum operate frequency of the valves, equipment and the system.

Easy to operate the system;

★Intelligent Control

Use PLC + PC to control, the system control model is to monitor the air transporting work pressure. Save most part of the levels, balance valves, pressure vessel heaters, insulation and other equipments,

Further reduce the system's energy consumption and investment in equipment and make the system more reliable and easy to maintenance.

5. Double casing concentrated positive pneumatic ash conveying systems’s　dedicated valve introduction

5.1 The pneumatic dome valve

The pneumatic dome valve can be used as feeding valve、discharging valve in double casing concentrated positive pneumatic ash conveying systems , can also be used as the balance valve or the storehouse top switch valve. The pneumatic dome valve has an excellent performance and high reliability. Different from the traditional valve and butterfly valve, pneumatic dome valve can be able to rotate through the wearing and tearing bulk materials and viscous mixture materials, such as any. The pneumatic dome valve can close in one step and form a perfect air lock seal. The front edge of the valve can lock the large particle materials when the pneumatic dome valve closed. The dome of the pneumatic dome valve can do not contact with any flow of materials, which avoids the valve wearing their own. The pneumatic dome valve is applicable to high intensity operations, one overhaul cycle per 100 million times.

The dome valve uses soft rubber seal ring; ring automatically decompression and rebound when the valve opens, avoiding direct contact fray between the seat and the spool of the valve. Only when the dome is completely closed, the dome valve will automatically be inflated.

The main features of the dome valveisas follows:

★ Friction-free gate, free to open and difficult to jam, and ensure the sealing surface in the process of headstock without friction wear and tear;

★ After the closure, inflate the air to seal, under the high pressure the flexible seal ring is firmly attached to the dome, the joint is striped, has a good sealing performance.

★The unique structure of the spool can cut off the material column horizontally.

★Crankshaft designed, it is wide cross-section flow when the valve is opened and can protect the sealing surface of the Dome.

★Automatically monitor the sealed air pressure and has the alarm function to ensure a good seal.

★The design life is millions of times; the average life of wearing parts is 2 years.

★ For temperature higher than 300 ℃ or other special occasions,

The seal ring will be attached with the water cooling inside system.

5.2 balk cargo anti-abrasion valve

The balk cargo anti-abrasion valve can be used in fly ash、concrete、limestone powder delivering outlet or discharging valve, and can also be used in high abrasive、caustic materials delivery system that transported by gravity or pressure. The valves are adjusted automatically by double hard sealed structure, solving the seal problem of the positive and negative pressure on the valve of the conveying pipeline.

The valve seat and plate are resistant steel plate, can endure the temperature of 450 ℃.The body material is cast iron material. And it is driven by compressed air.

5.3 switch valve of the warehouse top

The switch of the fly ash can be realized by the switch valve of the warehouse top, this kind of valve adopts slippage pneumatic trial distributor valve, mainly used in the delivery of the solid dry loosen material, the hermetic unit adopts the metal seal which can be replaced, and easy to maintenance. The principle is by changing the position of the valve baffle to interconnect the valve to direct path or side path.

The characteristic of the switch valve of the warehouse top：

★Compact in structure

★Can bear 200℃ while delivery  

★Aerodynamic power or by hand

★Hard material seal

★No need for cleaning