A seemingly simple compressor bearing operating temperature problem was completely solved using two different methods!

Zhou Fujun

The No. 1 Utilities Department of Lanzhou Petrochemical has two centrifugal compressors that experienced bearing-temperature problems and oil leakage during operation. Bearing operating temperature is the most comprehensive operating parameter among the factors reflecting bearing condition. It reflects the combined thermal effects of internal friction, rolling, vibration and impacts under bearing load, speed, oil lubrication, abnormal unit operation and ambient temperature. Temperature can therefore indicate lubrication condition and help diagnose bearing faults associated with heat accumulation and temperature rise.

1, the function of bearing temperature in the unit control system

The temperature measurement signal of large compressor bearings is usually connected to the unit protection and alarm system to stop or alarm the compressor unit. For example, the thrust bearing is one of the key parts of the steam turbine unit. The thrust bearing tile temperature is the only means to monitor the operating status of the thrust pad. This temperature signal must be connected to the protection system. When the thrust tile temperature reaches 80℃, the system issues an audible and visual alarm; when the thrust tile temperature reaches 95℃, the protection system triggers the compressor unit stop operation command to safely stop the compressor.

2, bearing temperature oil leakage causes and consequences

2.1 Cause of oil leakage

The protective structure of platinum thermal resistance is generally composed of polytetrafluoroethylene sheath and steel wire braided pin. Platinum thermal resistors are prone to wear due to long-term erosion of lubricating oil inside the body and vibration of the body. The temperature inside the machine body is relatively high, so the lubricating oil vapor generated in this situation uses the damaged sheath as its outer channel. Finally, it condenses at the terminal head and becomes droplets. Over time, the condensed oil droplets accumulate more and more, eventually causing the lubricating oil to overflow into the surrounding environment.

2.2 Consequences of an oil spill

(l) There is oil leakage at the thermal resistor terminals, which can easily cause poor contact, wire core aging, etc., thereby triggering malfunction of the temperature interlock, causing the compressor to stop operating, causing significant losses to production;

(2) The temperature measurement probe and the lead wire have been immersed in the turbine oil flowing along the rotation direction of the unit for a long time, and are subject to the radial shear force of the oil flow and the up and down shear force of the unit vibration. It is easy for the connection between the PT100 component and the lead wire to break, making it impossible to realize normal monitoring of the bearing temperature, and it is also easy to cause interlock malfunctions;

(3) Oil and gas leakage pollutes the surrounding environment and also causes a waste of lubricating oil;

(4) Leaking oil can easily cause smoke, fire and other unsafe phenomena in high-temperature parts of the machine body, threatening the normal operation of the unit, and in serious cases may even lead to forced shutdown. Once the bearing temperature is confirmed to be a fault, it can only be replaced during the shutdown of the compressor unit for maintenance, so its reliability requirements are relatively high.

3, treatment measures and effect comparison

3.1 processing measures and effects during production

In view of the particularity of the production period, we have mainly taken the following measures to ensure the safe operation of the unit:

(1) Further seal and reinforce the leads at the sealing joint of the casing lead outlet to try to prevent oil and gas from escaping inside the body;

(2) Raise the height of the platinum thermal resistance wiring pipe to a level higher than the lubricating oil level inside the body, and try to make the condensed droplets fall back to the inside of the body in the middle of the wiring pipe without overflowing; the effect of this measure: the oil leakage problem still exists and cannot be solved.

3.2 Renovation measures and effects during the first overhaul

To address the cause of oil leakage, we mainly took the following measures during the first maintenance period:

Peel off the Teflon sheath of the platinum thermal resistor and the integrated dense mesh shielding layer inside the lead at the sealed joint of the casing lead outlet, and take sealing reinforcement measures at the sealed joint.

The effect of this measure:

Achieve the function of preventing lubricating oil leakage during compressor operation. However, this measure will also have some negative effects:

(1) The installation and processing workload is large, and the wire core is easily broken during the wire stripping and sealing joint tightening and installation processes, resulting in a waste of resources and an extension of the construction period;

(2) During use, as the oil mist corrodes the wire core, the wire core ages and becomes brittle, and is easily broken during maintenance and disassembly. In order to ensure the reliability of instrument measurement, replacement must be mandatory every maintenance cycle, resulting in increased costs.

3.3 transformation measures and effects during the second overhaul

In order to make up for the shortcomings of the first transformation measure, we took the following measures: design and purchase an integrated package temperature measurement element, that is, from the micro pin-mounted temperature measurement thermal resistor to the lead, it is an integrated package structure, and it is equipped with a sealed joint with an oil-resistant rubber plug.

This structure has the following advantages:

(1) Long life: With the improvement of equipment production technology, the annual continuous operation time of the unit is extended, and the interval between maintenance cycles is also increased. This platinum thermal resistor is produced by integrating a miniature platinum wire temperature measuring element and a steel pin core, and is equipped with a sealing joint with an oil-resistant rubber plug, which extends its service life;

(2) Anti-vibration: The integrated molding process of all-steel pin packaging and production makes it highly resistant to vibration and reduces the probability of breakage of the temperature measuring element and the root of the lead;

(3) Oil resistance: Due to its one-piece molded structure, it solves the problem of platinum resistance leads cracking or breaking during long-term use of turbine oil;

(4) Anti-oil seepage: The integrated molding process makes the structure ensure that high-temperature oil and gas cannot penetrate from the inside of the body through the leads, thus achieving the purpose of anti-oil seepage.

The effect of this measure:

After long-term operation inspection of the compressor, this measure can completely solve the oil leakage problem.

4, conclusion

The advantage of the first modification measure is that it can make use of the existing inventory of ordinary platinum thermal resistors, without the need to wait for long production and procurement preparations, and the production process of the temperature measuring element itself is simple and the cost is relatively low; while the advantage of the second modification measure is that it can fundamentally solve the problem, but its production process is complex and the cost is relatively high, and customers often need to customize it individually. But for compressors, the necessary cost investment in the early stage can bring higher benefits to the long-term safe production in the later stage.

(Source: Compressor Network)

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