Automatic monitoring of instrument air supply syst

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Automatic monitoring of instrument air supply system

instrument air supply system is the power source of pneumatic actuator. If DCS is the brain of the automatic control system, then the air source system is the heart of the automatic control system. If the air source is normal, the automatic control system will operate flexibly; If the air source fails, the automatic control system will be paralyzed. In order to ensure the normal operation of the air supply system, an automatic monitoring device can be equipped. This paper analyzes the structure and principle of a set of monitoring device in the imported air source system, and introduces the problems in use and their solutions

1 introduction to instrument air supply system and monitoring device

the instrument air supply system is composed of air dust hood, lubricating oil compressor, double drying tower, dry and wet gas tank, three-stage filter, power distribution equipment and integrated automatic monitoring device, which is used to generate clean compressed air with a pressure of 800kpa and a dew point of -30 ℃ (see Figure 1)

1- filtration; 2-silencing filter; 3-damper; 4- oil compressor; 5-oil gas separation tank; 6- air cooling; 7- temperature control valve; 8-solenoid valve; 9- pressure gauge; 10- solenoid valve; 11- air cooling; 12- temperature switch; 13- pressure switch; 14- upper limit of pressure; 15 pressure upper limit; 16- low pressure limit; 17- filter; 18- moisture tank; 19- filter; 20- flow limiting valve; 21- flow limiting valve; 22- pressure gauge; 23- pressure gauge; 24- shuttle valve; 25- drying tower; 26- shuttle valve; 27- solenoid valve; 28- solenoid valve; 29- wet exhaust silencer; 30- wet exhaust silencer; 31- filter; 32- dry gas tank; 33- pressure transmitter

Figure 1 Composition of instrument air source system

the monitoring devices of compressor and drying tower are used for startup, shutdown, over temperature and over pressure maintenance, outlet air pressure regulation, main and standby machine, dual tower switching, alarm, etc., which are discussed in detail below. In addition, the dust removal process of air filter 1 itself can be started by differential pressure or manually; The contamination procedure of filters 17, 19 and 31 can be monitored with a differential pressure gauge; The pressure of the dry gas tank is measured by transmitter 33 and sent to the main control room

2 compressor unit

the unit is composed of silencing filter 2, oil-gas separation tank 5, air cooling 6 and 11, main and standby compressor 4, measurement and control components and instrument panel. Ingersoll - Rand lubricating oil compressor mh45 is adopted, with motor power of 45KW, which can generate 1MPa wind pressure

for the small air source system without buffer tank, the inlet air valve 3 should work in the "modulate" mode. At this time, the outlet pressure 9 is maintained at a constant value by the automatic adjustment of the air valve opening. For the air supply system with buffer tank in this example, damper 3 works in "on/off" mode. At this time, if the compressor is in the "normal" state, the damper 3 and outlet solenoid valve 10 are fully open, the solenoid valve 8 is closed, and the gas is sent out; If the compressor is in the "unload" state, the damper 3 is closed (the damper leaves a gap and will not be closed), the outlet solenoid valve 10 is also closed, and the solenoid valve 8 is opened. The compressor can only suck a small amount of air from the gap of the damper 3, which is basically idling to save electricity

the compressor is in the "no-load" state under the following four conditions

① normal startup

before startup, manually select the "no load" status. After startup, a small amount of air enters compressor 4 through the gap of damper 3, and then enters separator tank 5 after being compressed. Since the solenoid valve 10 is closed, all compressed air returns through the solenoid valve 8, forming a 5 → 8 → 3 → 4 → 5 cycle, and driving the lubricating oil into a 5 → 7 → 6 → 3 → 4 → 5 cycle; The pressure of the separator tank 9 gradually rises below the goal of environmental protection and lightweight of the vehicle. When it rises to 200KPa (or two minutes later), it can be manually switched to the "normal" state and enter the loaded operation. If you do not follow the above steps, take 2 significant digits of the straight elongation and start the machine in the "normal" state, the oil circuit is often poor, resulting in overheating

② normal shutdown

before shutdown, manually select "no-load" operation to make the compressed air circulate and depressurize and gradually discharge from muffler 2, while the lubricating oil gradually converges to the bottom of separator tank 5. When the pressure 9 of the separation tank drops to 200KPa, it can be shut down manually. If you do not follow the steps and directly shut down in the "normal" state, since the compressor no longer sucks air from the inlet, the high-pressure air in the oil-gas separation tank and the rear pipeline will inevitably force the lubricating oil to spray violently from the silencer 2. If it is due to the sealing gasket, a large amount of oil leakage will be caused. The higher the pressure generated by the air source system, the more serious the oil leakage caused by gas backflow

③ normal operation

at the high pressure limit (see section 4 for the meaning), the monitoring device automatically switches the compressor from "normal" to "no load", and a small amount of compressed air and lubricating oil enter the circulation, ready to be loaded at any time, which saves power

at the low pressure limit (see section 4 for the meaning), the monitoring device will automatically switch from "no load" to "normal", the gas will be sent out, and the lubricating oil will still circulate normally

④ sudden power failure during operation

this is similar to direct shutdown under "normal" state, which will lead to serious leakage of lubricating oil. For the safety after the next power call, the monitoring device automatically makes the compressor enter the "no-load" state, but the normal cycle process is missing

3 drying tower unit

the monitoring device of this unit is used to guide the two drying towers for drying/regeneration cycle

the special instrument panel is composed of two cams, two groups of contacts, timers and power switches driven by a constant speed motor (see Figure 2). The two cams A and B rotate at a uniform speed coaxially to drive the two groups of contacts a and B to open and close periodically. Contacts a and B are the power switches of "normally open" solenoid valves 27 and 28 respectively, so the two solenoid valves will be opened and closed periodically, and then guide the two towers to carry out drying/regeneration alternately. The specific analysis is as follows

Fig. 2 control mechanism of drying tower

it is advisable to set the relative position of cam and contact as shown in Fig. 2. Contact a is open and B is closed, and solenoid valve 27 is open and 28 is closed. According to Fig. 1, tower a regenerates and Tower B works (drying). The cam keeps turning anticlockwise, the contact a is closed and the solenoid valve 27 is closed. Due to the low pressure of tower a, part of the dry gas will be charged to tower a from the upper flow limiting valve (20), so tower a is in the preparation stage; At this time, contact B and solenoid valve 28 are also closed, and Tower B is still working. The cam continues to rotate, the contact B opens, the solenoid valve 28 opens, and the Tower B enters the re


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