Start-up failure of the nitrogen compressor during commissioning

2021-07-04

1 unit overview

Bayi Iron & Steel Co., Ltd. built a new 2×40000m3 /h oxygen plant, and its 2# 40000m3 /h oxygen plant liquefaction plant is equipped with a nitrogen pressure equipment produced by Cooper, USA, model 4MSGEP-5/30. Design flow 26000m3 / h, pressure 2.1MPa (absolute pressure), motor is Siemens motor, power 4500kW, rated speed 2988r / min rated current 300A, frequency 50Hz. The unit is a 4-stage H-type centrifugal turbine compressor, compressor The 1st and 2nd stages share a gear shaft, and the 3rd and 4th stages share a gear shaft, which are respectively driven by the gearbox of the speed increaser, and the large gear and the motor are connected by a laminated coupling.

2 Start fault diagnosis and resolution process

2.1 On September 17, 2009, before the compressor was started, the compressor, motor and other ancillary equipment were thoroughly inspected according to the manufacturer's instructions and the relevant specifications for compressor operation and operation. The conditions were rectified.

(1) Inspection of the installation quality of the compressor;

(2) Inspection of intake pipes and filters;

(3) Inter-stage pipe expansion joint interface clearance verification and stress detection;

(4) Inspection of the conveying gas pipeline;

(5) Installation and inspection of cooling water pipelines, water supply test;

(6) Lubricating oil system, inspection of oil product test;

(7) Motor-compressor alignment check;

(8) Reset the interlocking conditions and perform the corresponding interlock test;

(9) Inspection of local PLC cabinet and DCS communication;

(10) Inspection of high voltage power distribution system and soft start system.

2.2 compressor startup process

After careful inspection and rectification, the nitrogen press was started on September 19, 2009, and was started twice on the same day. Both were jumped due to the first-stage shaft vibration.

2.3 Start failure analysis

Careful observation of the entire start-up process revealed that the start-up linearity of the solid-state soft-start cabinet was poor during the speed increase process, and the speed increase process was ladder-like.

During the whole process of speed increase, it is obvious that the two speed stagnation processes I, II, over the stagnation phase, during the re-acceleration under the control of the soft start cabinet, the first-stage rotor bearing vibration is high and jumps. In the car, the start-up process is obviously accompanied by the situation that the motor is dragged and the linearity is bad. In order to ensure the safe operation of the equipment and reduce the number of starts, the high-voltage soft start cabinet start setting and the first-stage shaft vibration start setting are made. Contacted Cooper's US manufacturing headquarters, Siemens motor related professionals to discuss the decision to reset the soft start cabinet start value (buck 70%, constant pressure 65%, time 18′′), and start the first stage shaft vibration jump The value setting is adjusted to 2.5 times (125μm) from the original 2 times normal operation value. After the adjustment is completed, the vehicle starts on September 16 and the startup is completed, reaching the rated speed. The cycle nitrogen pressure machine startup time and the speed corresponding curve are reset and then started. The shaft vibration condition during normal operation is 5m, and the other levels are normal except for the fourth stage vibration value. The soft start cabinet has a startup failure.

Through the analysis of the corresponding relationship curve of the compressor start-up time D speed, all the electrical components in the solid-state soft-start cabinet are inspected and no abnormalities are found. Through the low-voltage test, the soft-start PLC is controlled to control the constant voltage output percentage and the actual voltage. There is a large deviation in the output, which leads to poor linearity of the compressor starting process, excessive starting current, and no soft start effect. After the PLC program optimization, after the low-voltage test again, the start-up conditions can be met. Start the compressor again and the starting process is linear.

3 Analysis and processing of the fourth stage vibration fault

3.1 oil pressure adjustment

In view of the fact that the initial setting of the lubricating oil supply pressure is higher than the design value, the actual adjustment value is 0.2 MPa, the design value is 0.16 to 0.18 MPa, and the oil pressure is adjusted to 0.16 MPa to observe the situation. The vibration at each level is not large, and the vibration at the 4th level is 34.7. The μm is reduced to 33.8 μm, and the vibration value is still high, and the effect is not obvious. High oil pressure is only a secondary cause of high vibration values.

3.2 Re-review motor-host alignment

In order to eliminate the possibility of the motor-host alignment affecting the 4-level vibration, the installer is required to remove the coupling and re-check the alignment of the unit. The verification values ​​are as shown in Table 4 below. The alignment is in full compliance with the design requirements. In the later spectrum analysis, the host doubles the frequency without harmonics, and eliminates the influence of the centering on the vibration).

3.3 vibration test

After judging and eliminating the factors that may cause the vibration value to be high in all aspects, the focus of the fault is finally placed on the poor balance of the 4-stage rotor.

In order to accurately measure the vibrator of the nitrogen compressor, the collector of the CameronCompressionSystems is used to collect the data of the four-stage shaft vibration for spectrum analysis. As can be seen from Fig. 3, the vibration of the fourth stage vibrates at a frequency of 27774r/min. The value is 0.71 mils or 18 μm, and it has been observed that the unit is in an upward trend with the four-stage vibration during the lifting of the load. Judging that the poor dynamic balance of the four-stage rotor is an important cause of the high vibration value.

3.4 rotor dynamic balance check

According to the negotiation with the manufacturer, the 4-stage rotor dynamic balance test will be carried out. There are two ways to balance the dynamic test:

(1) After removing the 3-4 rotors, they are airlifted to the Shanghai dynamic balancing machine for testing and calibration.

(2) The dynamic balance test on the equipment line shall be corrected by replacing the 4-stage rotor impeller lock nut and the weight block.

In order to save time and avoid affecting normal production, and the device online balancing method can observe the vibration value of the unit under full load operation, it has a high implementation significance. Finally choose the second way.

3.5 processing effect

After dynamic balancing on the compressor line, the vibration level of the unit 4 is improved. The data acquisition of the 4-level shaft vibration is performed by the CameronCompressionSys-tems collector. The spectrum analysis, the vibration value of the 4-level vibration at the host frequency of 27774r/min From 18μm to 6μm. Continuous observation for 3 days, no upward trend, the effect is more obvious.

4 Conclusion

In addition to the comprehensive inspection of the nitrogen compressor in the debugging process, in addition to the equipment before the commissioning, there is a comprehensive inspection, as soon as the problem is found to be rectified, it is necessary to carefully analyze, summarize and make accurate judgments during the commissioning process. In the process of analyzing and judging the cause of failure, the principle of grasping should be from simple to complex, from easy operation to difficult to implement to formulate a treatment plan, in order to appear more orderly, in order to avoid more problems in the judgment of the problem point. Detours are entangled in the same problem. More importantly, before using new technologies and equipment, you should have a comprehensive understanding of its performance and operation methods. In the case of fully grasping its working principle and structure, you must pay close attention to each of its operations. The details can be changed to control the condition of the equipment as a whole.

Nitrogen compressor is one gas compressor used to increase nitrogen pressure. Normal nitrogen pressure at the outlet of nitrogen generator is  5-10 bars. For some customers, they need high puressure nitrogen such as 30 bars or 150 bars or 200 bars. Piston nitrogen compressor is the most popular type.

Nitrogen compressor specifications:

Code	Item	Unit	Specification
1	Model No.		VW-45/6-32
2	Structure		Two stage, skid mounting
3	Medium		Nitrogen
4	Rated suction pressure	MPa(g)	0.6
5	Rated outlet pressure	MPa(g)	3.2
6	Capacity	Nm3/h	45@ suction pressure=0.6MPa(g)
7	Rotation speed	rpm	680                   ※
8	Motor power	kw	5.5                    ※
9	Cooling type		Wind cooling
10	Drive type		Belt
11	Inlet temperature	℃	≤35
12	Outlet temperature	℃	≤55
13	Inlet joint		DN25
14	Outlet joint		DN15
15	Control		Automatic
16	Weight	KG	360                   ※
17	Dimension(length*width*height)	mm	1200*900*950          ※
18	Installation		Flat concrete ground

Nitrogen Compressor, Oilless Nitrogen Compressor, Nitrogen Air Compressor, High Pressure Nitrogen Compressor
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