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60 Precautions In Instrument Inspection And Maintenance
Dec 24, 2018


60 precautions in instrument inspection and maintenance



1. Do not mix a signal cable with a power cable for a multi-core cable!

2. Oxygen pipeline instrument maintenance should not be sticky, oil-free transmitters and pressure gauges should not be mixed with ordinary watches.

3. When repairing the instrument, you must pay attention to wrap the thread to prevent short circuit!

4. The cable should not have an intermediate connector.

5. Point shield grounding, generally shielded grounding on the control room side.

60 precautions in instrument inspection and maintenance

6. The protective hose must be lower than the instrument inlet to prevent the meter from entering the water.

7. Leaky meters should be equipped with instrument protection boxes or wrapped in nylon plastic bags.

8. When the cable is laid in the rack, the intrinsically safe cable, power cable, and signal cable should be separated by a partition.

9. When wiring, the compensation wire can not be connected with the nose (piece) to avoid contact between two different conductors, causing measurement errors.

10. At the time of production, if the instrument is to deal with problems, including indoors and outdoors, it must be handled according to procedures or procedures, especially to the operator, and sometimes must be signed in writing.


11. In case of lightning protection, the field instrument is connected to the safety barrier and then connected to the control system such as DCS and SIS through the surge protector. In order to avoid redundant wiring between cabinets, the safety of the surge protector and the corresponding circuit in the cabinet room. The grid is installed on the same side as possible in the cabinet.

12. The control room must take measures to prevent small animals. Because the rats pee on the ESD card, the whole device stops, and the loss is large.

13. The instrument must be completed before the installation of the instrument. After the installation is completed, the circuit must be completed before the adjustment.

14. When the device is running, the maintenance of the instrument, the craftsman must be present. Keep in mind that if something goes wrong, it is not a trivial matter.

15. On-site maintenance of the instrument must be in contact with the craftsman. Ask the process status. When disassembling the instrument with power supply, be sure to turn off the power first. Then use the multimeter to confirm whether the power is off. You must know that life is your own.

60 precautions in instrument inspection and maintenance

16. When designing the flow meter, it is necessary to select the appropriate flow meter type according to the measured medium, temperature and pressure, and do the flow compensation. Pay attention to the special requirements of the flow meter when installing.

17. When the meter is designed into the slot of the control room, in order to prevent rainwater from entering the control room, it is necessary to consider the up and down bend and seal it.

18. When the instrument wind is introduced from the main pipe, the valve must be above the positive center of the pipeline, preferably at 90 degrees above the pipeline to avoid the dirt in the wind line: enter the instrument valve.

19. The shielding layer shall not be grounded at both ends; the outdoor cable protection nozzle shall have rainproof measures; the explosion-proof environment shall pay attention to the sealing of the nozzle.

20. Alarm instruments and audio equipment must be maintained and used normally. Otherwise, if there is an accident in the process, the instrument professional will not have to peel off the skin. The reason is that the alarm is broken and the operator has not found it.


21. In the case of ammonia, copper and copper alloys are banned; for DCS system power supply, two-way power supply should be designed to enter!

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 cable turning radius should be greater than 10 times the cable diameter, and the cable should be 15 times; the instrument cable and the electrical cable should be laid parallel to each other (greater than 0.8 m), with equipment and piping. The spacing is greater than 150 mm.

24. Instrument pipeline hydraulic test, for the experiment of austenitic stainless steel pipeline, the chloride ion content in the water shall not exceed 25PPM (parts per million). The working ground of the instrument shall be less than 1 ohm, and the other grounding shall be less than 4 ohms.

25. The protection of the instrument should be done with a fireproof cloth "asbestos cloth". Do not use plastic bags.


26. The instrument used in the hydrogen unit must meet the requirements of explosion-proof grade and protection level. Intrinsically safe signals (cables) and explosion-proof signals (cables) cannot enter the same field intermediate junction box.

27. The insulation resistance of the cable should be greater than 5 megohms; the cable turning radius should generally be greater than 10 times the cable diameter, and the cable should be 15 times; the instrument cable and the electrical cable should be laid parallel to a certain distance (greater than 0.8 m), with equipment and piping The spacing is greater than 150 mm.

28. Instrument pipeline hydraulic test, for the experiment of austenitic stainless steel pipeline, the chloride ion content in the water shall not exceed 25PPM (parts per million). The working ground of the instrument shall be less than 1 ohm, and the other grounding shall be less than 4 ohms.

29. The instrument used in the hydrogen unit must meet the explosion-proof grade and the protection level requirements. Intrinsically safe signals (cables) and explosion-proof signals (cables) cannot enter the same field intermediate junction box.

 30. FF bus design, the terminal is installed in the terminal of the power regulator and field bus junction box (resistance and capacitor are connected in series).


31. The solenoid valve for interlocking should adopt the fail-safe type, which is charged under normal conditions and de-energized when interlocked.

32. When designing and installing the instrument, if the temperature instrument is installed below the DN80, use a thermometer to enlarge the pipe to extend the pipeline to more than 80.

33. The flow measurement component does not participate in the hydrostatic test of the power plant.

34. Measure the steam flow. When using the condenser in the positive and negative pressure guiding pipelines, the installation height of the two condensing tanks should be consistent.

35. The field instrument pressure guiding tube is cold-twisted and can not be used for heat welding.


36. The occurrence of flashing should be fully considered in the design and selection of control valves, design piping, and determination of pressure distribution. From the perspective of the control valve, the following should be noted.

(1) Improve the hardness of the material.

(2) Reduce the flow rate of the fluid.

(3) Select the appropriate control valve type and flow direction. For example, for fluids that are easy to vaporize, it is not advisable to use a ball with high pressure recovery.

37. Valves and butterfly valves, available with a low pressure recovery single seat valve. Eliminate and reduce the measures that actually occur:

(1) Control the pressure drop so that cavitation does not occur. For example, a multi-stage step-down method is used to divide the pressure drop of the control valve into several stages.

(2) Reduce the effects of cavitation. A method similar to that to prevent flash evaporation is used. For example, the hardness of the material is increased, the flow rate is lowered, and the like, so that the influence of cavitation is reduced.

(3) Reasonably distribute pipeline pressure and increase downstream pressure.

 38. Cables and wires overhead laying problems to enter the control room:

(1) Before entering the control room, a fixed fulcrum should be given to the trough plate to prevent climate change and generate stress on the indoor equipment;

(2) Before the slot plate enters the control room, there must be a slope of 1/100 or more, and the slope should be outdoors to prevent rainwater from flowing into the control room along the slot plate;

(3) The wall that enters and exits the control room should be sealed to prevent rats and mosquitoes from entering.

 39. The choice of local pressure gauge must distinguish the nature of the pressure source: whether it is an impact load or a general pressure, when there are not many meters, the replacement can be miserable. The installation size of the local thermometer must be communicated with the process. When measuring the liquid that is not full, 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 take power to dismantle the equipment, carry a test pencil with you to prevent the risk of electric shock.

40. You can't take power to dismantle the equipment, and bring a test pencil with you to prevent the danger of electric shock.

60 precautions in instrument inspection and maintenance

41. DCS must be done anti-static work, do not cause accidents due to static electricity.

42. Overhaul chain equipment must be forced by DCS to act.

43. The solenoid valve coil must not be unplugged, otherwise the coil will be burned out.

44. When the rotor and the wheel flowmeter are installed vertically, be sure to pay attention to the fluid from the bottom to the top.

45. After the newly installed regulating valve, the air source pipeline should be emptied for a period of time and then connected to the valve positioner to prevent the oil from entering and causing damage to the positioner.


46. The instrument for acetylene gas should also be banned from copper. Therefore, in addition to the requirements of explosion-proof grade when using acetylene gas, it should be noted that some device nameplates will be marked as “not applicable to acetylene gas”.

47. Current signal between DCS and electrical. Because the electrical supply is generally active, it is best to isolate it through an isolator. On the one hand, it will not string the instrument I/O card into the electrical circuit. On the other hand, if the isolator is not used, the two sides may not be able to debug.

48. The steam of the instrument pipeline is accompanied by heat. The heat pipe is preferably used with 12 O.D. or above. Otherwise, if the route is long, the steam is not hot and the heat tracing effect is not good.

49. Correcting a mistake that is easy to make in a design, no matter whether the system is the kind of anti-riot system, the uniform is added to the safety barrier. In fact, the safety barrier is used for intrinsically safe explosion-proof systems. Intrinsically safe explosion-proof system:

(1) Field instruments must be intrinsically safe;

(2) There must be a safety barrier on the control room side.

(3) The cable in the middle must be an intrinsically safe signal cable.

50. If the Zengan instrument and the explosion-proof instrument are necessary, the isolators can be used for signal isolation.

60 precautions in instrument inspection and maintenance

51. Zener safety barriers must pay attention to grounding problems.

52. In fact, the instrument and process are inseparable. On-line maintenance of the instrument must pay attention to reduce the interference to the process as much as possible.

53. Selection of flow meter: When the conductivity of the tested process medium is low, the electromagnetic flowmeter cannot be selected; when the measurement of the factory-level measurement is very high, the mass flowmeter should be selected.

54. When measuring the medium pressure, add a condenser or siphon when the temperature of the measured medium is greater than 60 degrees.

55. After the pneumatic control valve is installed, pay attention to the effect of the air-opening type and the air-closed type on the DCS.


56. The indicator value of the temperature meter system suddenly changes to the maximum or minimum, generally the instrument system failure. Because the temperature meter system measures a large lag, no sudden changes will occur. The cause of the fault at this time is mostly caused by thermocouple, thermal resistance, compensation wire breakage or transmitter amplifier failure.

57. When wiring the meter, be sure to mark the line number! At the beginning of work, I have committed such a low-level mistake, and it is very troublesome to recover. If the temperature is once installed on the bend of the pipe or installed obliquely, it should be reversed.

58. If there is a pressure point or a 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 mounted vertically on the pipe and the flow direction must be from bottom to top.

60. Straight pipe requires 5DN on the upstream side and 3DN on the downstream side (DN is the diameter of the pipe).