Powder pneumatic conveyor is an important part of the production equipment of powder as raw material. Whether it can accurately and stably transport raw material is the key to ensure the normal production of the equipment. In order to improve the economic benefits of enterprises, safe, stable and long-term continuous production is crucial, so it is necessary to choose the correct and reasonable control scheme under the condition of ensuring the normal operation of equipment |.

The pneumatic conveying of powder is closed conveying, with good sealing performance and long conveying distance. Fully enclosed operation, no dust flying. The operation site is clean. The system has few transmission parts and is easy to maintain and overhaul.

The design principles of the control system for a specific powder medium are discussed below.

1 Process Introduction

Powder pneumatic conveyor is the use of nitrogen or air high-speed airflow, through the injector to make the powder in the suspended state in the pipeline, the powder in the suspended state is driven by the high-speed airflow along the closed pipeline to the end user. The process operation process: 1) First start the roots blower, Roots blower control can be completed by a complete set of controllers; 2) Open the bag filter, and the bag filter blows compressed air to the bag in pulse form according to the set program to remove the dust on the bag surface; 3) Open the shaft seal valve XV-4101 of the rotary feed valve (air shutoff valve). After completing the above steps, check whether the wind pressure meets the requirements. If the requirements are met, the feeding process can be started by starting the rotary feed valve 4X01, and the powder enters the injector from the feed bin through 4X01. At this time, the powder is suspended in the pipeline under the high-speed air flow, and the powder is transported forward along the closed pipeline into the 1# or 2# powder receiving bin. Access to the receiving bin 1# or 2# can be achieved via the reversing valve XV-3101, which is equipped with a valve position switch. Each receiving bin is equipped with upper and lower limit capacitive level switches to determine whether the bin is allowed to be filled.

2 Comparison of control system design schemes

According to the requirements of process operation, there are two options: 1) use conventional instruments for control, and use conventional relays for interlocking sequence control; 2) Small PLC system is used for control.

For scheme 1), relay contacts may stick together, and the more relays that make up the system, the greater the probability of contact adhesion, and the more contacts of the wire, the more fault points increase at the same time. The advantage is that the investment is relatively low, but it is not conducive to normal production and maintenance. After a fault occurs, it is difficult to locate the fault. For scheme 2), in terms of the number of relays and the number of wire contacts, it is better than scheme 1), but the investment is higher than scheme 1). Considering all the factors and the performance price of PLC in recent years, the performance-price ratio of scheme 2) is significantly better than scheme 1).

3. Determination of dosing system design scheme

There are fewer I/O points in the feeding system of the powder pneumatic conveyor, so the small PLC system can fully meet the needs of control and operation. From the perspective of system security and reliability, the communication card, power supply unit, controller and IO card used in the control loop are redundant.

PLC system is powerful, with operation status display, history, interlocking logic control and sequence control and various control and calculation functions, convenient and fast configuration software, so that operation and maintenance engineers are easy to complete the system configuration and modification, in addition, PLC has a friendly man-machine operation interface, operators can easily complete a variety of operations and monitoring.

4 Feeding system and field instrument design principles

According to the site situation and process requirements, the site is a dust explosion-proof environment, and the field instrument is flameproof. The field instrument signal is sent to PLC for status indication and interlock control, PLC performs interlock control according to the stored interlock control program, starts or closes the related valves on and off, sends the switching signal to start and stop the bag dust collector, rotary feed valve (air shut-off)4X01 and Roots blower, etc. The switching signal is fed back to PLC. Interlocking control loops are introduced to ensure that various devices are in the correct state. Finally, the powder is transported to the bin outside the boundary area by compressed air along the closed pipe.

Main field instrumentation equipment: fan outlet pressure transmitter PT-4101; Rotary feed valve (air shut-off) shaft seal valve XV-4101; Y-type reversing valve XV-3101; Y-type reversing valve valve position switch ZSS, ZSD; Bunker level switch LSH, LSL; The key of field instrument selection is the reasonable selection of Y-type reversing valve and the upper and lower level switch of bunker.

5 Interlock sequence control main program box

The program runs as follows.

Start the loading button in the field or control room to start the interlock control loading procedure: 1) First determine whether the 1# bin can be filled, such as the 1# bin can be filled, the PLC gives the sound and light prompt, the loading starts. 2) Start the following devices in turn. Set the reversing valve XV-3101 in S direction, start the bag filter 1Z01 control program, open the XV-4101, and then start the Roots blower 5A01 control program. 3) Determine whether the wind pressure is normal. If the wind pressure is normal, the PLC will give the sound and light prompt and start the rotating feed valve (air shut-off). 4) In the feeding state, continuously detect whether the wind pressure is normal until the 1# bin is full. If the wind pressure is abnormal during the feeding period, the feeding will be stopped. At the same time, the sound and light alarm will be carried out, the rotating feeding valve (air shut-off) will be stopped, and the blower will be stopped after a delay of 30 seconds. 5) If the 1# warehouse is full, the PLC gives the sound and light prompt, and then judge whether the 2# warehouse can be filled, if it can be filled, the PLC gives the sound and light prompt, the loading process is the same as above. 6) If the 1# and 2# bins are full, the audible and visual alarm displays, stop feeding, stop rotating the feed valve (air shut-off), and stop the blower with a delay of 30s.

The whole feeding process can be completed in a fully automatic situation, as long as the PLC detects the feeding signal given by the subsequent process, the interlock control program automatically executes the feeding process.

After the automatic/manual switching function is added to the interlock control program, manual operation can be realized.

This paper only discusses the case where there are two receiving silos. The situation of multiple receiving silos is similar. As long as the reversing valve is correctly combined, the receiving condition of multiple receiving silos can be realized. It will not be detailed here.

6 Feed interlocking logic

7 Closing remarks

With the development of automation technology, the reliability and safety of PLC system have been greatly improved, and the price is more reasonable. The powder gas conveyor adopts a small PLC system, which can improve production efficiency, reduce the labor intensity of the operator, and reduce unnecessary losses caused by the operator's mistakes.