If any measuring instrument fails, it will cause trouble to the entire production process, or even cause interruption of the production process or cause safety problems. So, how to effectively maintain the huge production system to avoid the failure of the instrument control system?
Common small problems of the instrument:
1. The flow signal of the vortex flowmeter is unstable. On-site inspection. Are the gaskets concentric during installation? The processing result is re-installation.
2. The level instrument takes the source components, it can be clearly seen that this instrument cannot be installed.
3. The mass flowmeter is installed, the pipeline is not centered, reinstall.
4. The core wire connected to the liquid level gauge is bent to short-circuit the core wire and the outer wall of the joint, and the display shows +E. The core wire is not connected to the connector, and the display shows -E or 000L.
5. Reasons for errors during temperature measurement:
1) The error caused by improper installation, the depth of thermocouple insertion should meet the requirements, the thermocouple and the power cable should not be installed in the same conduit, so as to avoid errors caused by interference.
2) Insulation deterioration introduces errors. Poor insulation between the poles and the disadvantages of the thermocouple will cause the loss of thermoelectric potential and introduce interference, thereby causing errors.
The 77 common pits, instrument people enter carefully...
1. Never mix signal cables and power cables with a multi-core cable!
2. Maintenance of oxygen pipeline instrument equipment must not be oily, and oil-free transmitters and pressure gauges should not be mixed with ordinary meters.
3. When repairing the disconnection of the instrument, be sure to wrap the thread end to prevent short circuit!
4. The cable should not have an intermediate connector.
5. The shielding ground of the point is generally shielded and grounded on the control room side.
6. The protective hose must be lower than the inlet of the instrument to prevent water from entering the instrument.
7. The leaky meter should be equipped with a meter protection box or wrapped with nylon plastic bag.
8. When the cable is laid in the trough (public number: pump housekeeper), the intrinsically safe cables, power cables, and signal cables should be separated by a partition.
9. When wiring, the compensation wire cannot use the wiring nose (piece) to avoid contact between two different conductors and cause measurement errors.
10. During production, if the instrument has to deal with problems, including indoor and outdoor, it must be handled in accordance with procedures or procedures, especially to notify the operator, and sometimes a written signature is required.
11. In case of lightning protection, the field instruments are connected to the safety barrier after the surge protector, and then connected to the DCS, SIS and other control systems. In order to avoid unnecessary cabinet wiring, the surge protector in the field cabinet and the corresponding circuit safety The grid should be installed on the same side as possible in the cabinet.
12. The control room must take measures to prevent small animals, because the mouse urinates on the ESD card to cause the entire device to stop, and the loss can be described as large.
13. The unit must be completed before the instrument is installed. After the installation is completed, the circuit debugging must be completed before the joint debugging.
14. When the device is running, technicians must be present to maintain the instrument. At this point, remember that something is not trivial.
15. The on-site maintenance of the instrument must be contacted with the technical personnel, and the process status must be turned off when the instrument with power supply is disassembled, and then use the multimeter to confirm whether the power is turned off, and know that life is your own.
16. When designing the flow meter, it is necessary to select the appropriate flowmeter type according to the measurement medium, temperature, and pressure, and do a good job in flow compensation. Pay attention to various special requirements of flow meters during installation.
17. When the instrument is designed to enter the slot plate of the control room, in order to prevent rainwater from entering the control room, it must be considered to bend up and down, and be well sealed.
18. When the instrument wind is introduced from the main pipe, the valve must be above the center of the pipeline, preferably at a 90-degree position above the pipeline, to avoid stolen goods in the wind line: enter the instrument valve.
19. The shielding layer must not be grounded at both ends; outdoor cable protection nozzles should have rain-proof measures; explosion-proof environment pay attention to the sealing of the nozzle.
20. Alarm instruments and audio equipment must be well maintained and put into normal use, otherwise, once an accident occurs in the process, the instrument will have to delaminate if it does not die. The reason is because the alarm is broken and the operator did not find it.
21. In situations involving ammonia, copper and copper alloys are forbidden; DCS system power supply should be designed with dual power supply entry!
22. Thermal resistance temperature measurement, long-distance transmission can not use two-wire system.
23. The insulation resistance of the cable should be greater than 5 megohms; the turning radius of the cable should generally be greater than 10 times the cable diameter and 15 times the optical cable; the instrument cable and the electrical cable should be laid parallel to maintain a certain distance (greater than 0.8 meters), and the equipment and pipeline The spacing is greater than 150 mm.
24. Instrument pipeline hydraulic test. When performing experiments on austenitic stainless steel pipelines, the chloride ion content in the water should not exceed 25 PPM (parts per million). The instrument grounding should be less than 1 ohm, and the other grounding should be less than 4 ohms.
25. The protection of the meter should use fireproof cloth "asbestos cloth". Plastic bags should not be used.
26. The instrument used in the hydrogen unit must meet the explosion-proof level and protection level requirements, which are indispensable. Intrinsically safe signals (cables) and explosion-proof signals (cables) cannot enter the same field junction box.
27. The insulation resistance of the cable should be greater than 5 megohms; the turning radius of the cable should generally be greater than 10 times the cable diameter and 15 times the optical cable; the instrument cable and the electrical cable should be laid parallel to maintain a certain distance (greater than 0.8 meters), and the equipment and pipeline The spacing is greater than 150 mm.
28. Instrument pipeline hydraulic test. When performing experiments on austenitic stainless steel pipelines, the chloride ion content in the water should not exceed 25PPM (parts per million). The instrument grounding should be less than 1 ohm, and the other grounding should be less than 4 ohms.
29. The instrument used in the hydrogen unit must reach the explosion-proof level and protection level requirements, which are indispensable. Intrinsically safe signals (cables) and explosion-proof signals (cables) cannot enter the same field junction box.
30. When designing the FF bus, a terminal device (resistor and capacitor connected in series) must be installed at the terminals of the power regulator and the field bus junction box.
31. The solenoid valves used for interlocking should be of the fail-safe type, which is energized under normal circumstances and is powered off when interlocked.
32. When designing and installing the instrument, if the pipeline installed by the temperature instrument is below DN80, expand the pipeline with a thermometer to expand the pipeline to above 80.
33. Flow measurement components do not participate in the power plant hydraulic test.
34. To measure the steam flow, when the condenser is used for the positive and negative pilot lines, the installation height of the two condensing tanks should be kept the same.
35. Cold simmering is used for the simmering of the on-site instrument pressure guiding tube, and hot simmering such as gas welding cannot be used.
36. In the process of designing and selecting control valves, designing pipelines, and determining pressure distribution, the occurrence of flash evaporation must be fully considered. From the perspective of the control valve, the following items should be noted.
Improve the hardness of the material.
Reduce the flow rate of the fluid.
Select the appropriate control valve type and flow direction. For example, for fluids that are easy to vaporize, high-pressure recovery balls should not be used
37. For valves and butterfly valves, single-seat valves with low pressure recovery can be selected. Measures to eliminate and reduce the actual occurrence:
Control the pressure drop so that cavitation does not occur. For example, a multi-stage pressure reduction method is used to divide the pressure drop of the control valve into several stages.
Reduce the impact of cavitation. Use a similar method to prevent flash evaporation. For example, increase the hardness of the material, reduce the flow rate, etc., so that the impact caused by cavitation is reduced.
Reasonably distribute pipeline pressure and increase downstream pressure.
38. Issues to be noted when cables and wires are laid overhead into the control room:
Before entering the control room, a fixed fulcrum must be given to the slot plate to prevent climate change and stress from acting on the indoor equipment;
Before the slot plate enters the control room, it must have a slope of more than 1/100, and the slope should be outdoors to prevent rainwater from flowing into the control room along the slot plate;
The wall through the control room should be sealed to prevent the entry of rats and mosquitoes.
39. The choice of local pressure gauge must clearly distinguish the nature of the pressure source: whether it is an impact load or a general pressure, when there are not many gauges, the replacement can be miserable. The installation size of the in-situ thermometer must be communicated with the process. When measuring the liquid of the dissatisfied tube, the size must be selected; the design of the high temperature and high pressure material must be different from the normal temperature and pressure. Do not dismantle and build equipment while electrified. Carry a test pencil with you to prevent the risk of electric shock.
40. Do not dismantle and build equipment with electricity, bring a test pencil with you to prevent the risk of electric shock.
41. DCS must do anti-static work, do not cause accidents due to static electricity.
42. The maintenance of chain equipment must be forced by DCS before it can act.
43. The solenoid valve coil must not be unplugged when it is powered, otherwise it will burn the coil.
44. When the rotor and wheel flowmeters are installed vertically, we must pay attention to the fluid from bottom to top.
45. After the newly installed regulating valve, its air source pipeline must be emptied for a period of time before connecting to the valve positioner to prevent damage to the positioner due to the entry of oil.
46. Copper for acetylene gas should also be banned, so when using acetylene gas, in addition to the requirements of explosion-proof level, it should be noted that some device nameplates will be marked "not applicable to acetylene gas".
48. To heat the steam of the instrument pipeline, it is better to use the heating pipe above 12O.D. Otherwise, once the route is long, it is easy to appear that the steam is not hot and the heat tracing effect is not good.
49. Correct a mistake that is easy to make in a design. Some people add a safety barrier regardless of the kind of anti-riot system. In fact, safety barriers are used in intrinsically safe explosion-proof systems. Intrinsically safe explosion-proof system:
On-site instruments must be intrinsically safe;
There must be a safety barrier on the side of the control room.
The cable in the middle must be an intrinsically safe signal cable.
50. Isolation can be used for signal isolation for increased safety instruments and explosion-proof instruments if necessary.
51. Zener safety barrier must pay attention to the grounding problem.
52. In fact, the instrument and the process are inseparable. The online maintenance of the meter must pay attention to reduce the interference to the process as much as possible.
53. The choice of flow meter: when the conductivity of the measured process medium is low, the electromagnetic flowmeter cannot be used; when the factory-level measurement requires high measurement, the mass flowmeter should be selected.
54. When measuring the pressure of the medium, when the temperature of the measured medium is greater than 60 degrees, a condensing pipe or siphon gas should be added.
55. After installing the pneumatic control valve, be careful not to reverse the function setting of the air-opening and air-closing types on the DCS.
56. The indication value of the temperature instrument system suddenly changes to the maximum or minimum, which is generally a failure of the instrument system. Because the measurement lag of the temperature instrument system is large, there will be no sudden changes. The cause of the fault at this time is mostly caused by the thermocouple, thermal resistance, disconnection of the compensation wire or the failure of the transmitter amplifier.
57. Be sure to mark the wire number when connecting the instrument! At first, I made such a low-level mistake, and it is troublesome to recover. If the primary component is installed at the corner of the pipeline or installed at an angle, it should be reversed.
58. If there is a pressure point or temperature point on the same pipeline at the same time, the pressure point should be on the upstream side of the temperature point.
59. The rotameter must be installed vertically on the pipe, and the flow direction of the medium must be from bottom to top.
60. Straight pipelines require 5DN on the upstream side and 3DN on the downstream side (DN is the diameter of the pipeline).
61. When installing the electromagnetic flowmeter, pay attention to the positive and negative directions of the flowmeter or the direction of the arrow should be consistent with the flow direction of the medium.
63. The requirement for the smallest straight pipe section is 5DN on the upstream side and 2DN on the downstream side.
64. The arrow of the regulating valve must be consistent with the flow direction of the medium. Angular pressure regulating valve for high pressure drop, the flow direction is along the bottom of the valve core into and out.
65. When installing a small-diameter regulating valve with threaded connection, a removable connection must be installed.
66. The regulating valve should be firmly installed. Large-sized regulating valves must be supported. The operating handwheel should be in a convenient position.
67. The condensate discharge of the heat tracing system of the instrument must be designed reasonably, and the traps should be used as little as possible, which is easy to block.
68. It is strictly forbidden to touch the shaker before it stops rotating or before the device under test is discharged. When removing the wire, do not touch the metal part of the lead. During the shake test, no one can work on the device under test.
69.
(1) The power line and the signal line must be separated, and the same cable cannot be used to prevent interference;
(2) The shielding layer must not be grounded at both ends, otherwise electric difference will be formed;
(3) The cable should not have an intermediate connector;
(4) 220V power supply and 24V power supply should be separated;
(5) Power ground and signal ground (PLC) ground must be separated;
(6) The flow rate measurement must consider the compensation problem, etc...
70. When using radar liquid level timing, it is necessary to see whether the dielectric constant of the medium is too low, otherwise the radar liquid level gauge cannot be used. When using guided wave radar, the material of the guided wave cable (pole) meets the requirements for corrosion protection. When using the horn radar, pay attention to the installation requirements, not too close to the wall, otherwise there will be false echoes.
71. The dismantling and inspection of the instrument valve must be completed after the process is processed and confirmed, otherwise the consequences will be at your own risk; the maintenance of the interlocking instrument must be processed after the relevant ticket is unlocked; the disassembly of the single (double) flange instrument must be slow Slowly remove the bolts to prevent splashes from hurting people; the selection of the instrument flange must pay attention to the sealing surface, pressure level, material and standard to prevent the wrong type.
72. Radar level gauge is especially suitable for high pollution or high viscosity products, such as asphalt. The repetitive accuracy of the radar level gauge measurement is high, without regular maintenance and re-calibration, the measurement accuracy is also high, but the price is high, and it is difficult to measure the oil-water interface.
73. The isolated transmitter is mainly used for the special measured medium. If the measured medium leaves the device, it will crystallize, while using the ordinary type transmitter needs to remove the medium, which will block the pressure guiding tube and the capsule chamber. To prevent it from working properly, it must be isolated.
Isolation type is usually made into flange installation, that is, the opening of the device under test is added with a flange so that the sensor diaphragm is part of the device wall after the transmitter is installed, so that it will not take out the measured medium and generally will not cause crystal blockage . When the measured medium needs a high crystallization temperature, a structure with a protruding diaphragm can be used, so that the sensing diaphragm can be inserted into the device, so that the temperature of the sensed medium will not decrease, so the measurement is guaranteed, The plug-in flange transmitter is selected.
74. The main basis for the selection of the pressure/differential pressure transmitter: the property index of the measured medium shall prevail, with reference to saving money and easy installation and maintenance. If the measured medium is high viscosity, easy to crystallize and strong corrosion, you must choose an isolated transmitter.
75. When selecting the model, the corrosion of the membrane box metal must be considered, and the material of the membrane box must be selected, otherwise the outer diaphragm will be corroded and the flange will be corroded and the equipment will be damaged. And personal accidents, so the choice of materials is very important. Transmitter membrane box materials include ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum membrane box material, etc.
76. The temperature of the measured medium should be considered when selecting the model. If the temperature is generally 200°C to 400°C, the high temperature type should be used, otherwise the silicone oil will produce vaporization expansion, making the measurement inaccurate.
77. The working pressure level of the equipment should be considered when selecting the model. The pressure level of the transmitter must be consistent with the application. From an economic point of view, the material of the outer membrane box and the insertion part is more suitable, but the connection flange can use carbon steel and chrome plating, which will save a lot of money.