I. Introduction
Pumps in the smelting operation has a very important role, is the necessary equipment refining process. In large petrochemical refineries, there are thousands of pumps in service to keep the plant running smoothly. In smelting operations, centrifugal pumps are usually used to replace the original reverse pump and rotary pump. Improvements in manufacturing technology and metallurgy have led to a significant drop in the manufacturing costs of centrifugal pumps; and at the same time maintenance work has become simpler due to fewer moving or rotating parts in the pump.
Mechanical problems or inefficiencies in pump operation can have an adverse effect on the entire plant. Wear rings provide protection for the pump and control the fluid recovery cycle when the pump is operating under abnormal operating conditions. So its performance optimization is very important. The traditional wear ring is made of different materials of metal, but due to the characteristics of the metal material is limited, affecting the overall function of the centrifugal pump to further enhance.
Second, the role of wear rings
Wear ring role is to pump the rotating and stationary parts, high pressure and low pressure part of the separation. Over the years, designers have tried different materials to replace metal wear rings in order to obtain performance improvements. In the American Petroleum Institute Standard (API 610), wear rings are required in centrifugal pumps and the use of composite materials is permitted for the manufacture of these parts, as shown in Figure 1 and Figure 2.
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Third, the cause of wear-resistant metal ring failure
When the wear rings of the metal come in contact there is usually a high temperature, causing the metal to be welded together (friction welding) and causing the pump to run poorly. This situation drastically increases the power consumption, causing serious damage to the equipment and at the same time may cause the liquid to leak into the atmosphere, damaging the workers in the field. The following conditions wear rings may contact.
(1) When the pump is just starting and idling before the liquid has flowed through the blades (especially for vertical pumps).
(2) When the rotating shaft is deformed, the rotating blades deviate from the rotating shaft due to a sudden increase in pressure in the pump or other abnormal conditions.
(3) When the "slow rotation" during start or stop, the sleeve is deviated from the rotation axis due to vibration.
(4) bubbles appear in low-pressure steam or when treating low-density liquids.
(5) radial bearing failure, making the wear ring contact with each other.
Fourth, the metal wear ring other than the other options
After understanding the actual cost of using metal and composite parts, some manufacturers turned to alternative materials such as graphite / metal alloys. However, parts made of graphite / metal alloy are too brittle, impact-resistant, anti-vibration and impact-resistant. These features make it possible for graphite / metal alloy parts to break during shipping and installation, dramatically increasing the cost of replacing the components.
In the mid-1980s, DuPontTM DuPont developed a high-performance material, Vespel CR-6100, based on fluorinated resins and reinforced with specialty fibers. CR-6100 incorporates the chemical resistance and high temperature resistance of fluorinated resins (up to 288 ° C), as well as the strength and dimensional stability of specialty fibers.
The CR-6100 material has been used in pumps used in industries such as West Bank Purification, steel grinding, paper mills, chemical plants and food processing plants. It replaces metal and other composite materials and acts as a wear ring, throat bush, and sleeve bearing in the pump to dramatically improve pump performance.
Fifth, the use of CR-6100 material wear ring benefits
1. Reduce maintenance costs
The cost of repairing a wear ring is very high, especially for large pump repairs, which often cost up to hundreds of thousands of dollars a year and cost several weeks. Take an oil refinery in the United States as an example, there have been some problems with its brine cooling system. Included in this system are 4 pumps configured in parallel to share the backup system. The system costs more than $ 200,000 a year for repairs (five to six times per year) over and over again. When larger debris passes through the inlet, it will cause violent vibration due to blockage, resulting in an increase of the wear resistance of the wear ring. Eventually, damage to the blades, pump housing, casing, bearings, bearing bushings, connectors, etc. can result. In addition to the high maintenance costs, there is the potential for threats to the safety of pump operators and maintenance personnel. The company later decided to wear the wear ring and throat bushings of the CR-6100 material. After the first pump has been modified for normal use, a large debris gets stuck in the rotating blades, damaging the pump casing. However, the CR-6100 wear ring and throat bushing still support broken pump bushing and can continue to be used. One of the rotating blades and the shell is not damaged. After all pumps' wear-and-tear static parts were replaced with the CR-6100, annual maintenance costs were reduced to $ 40,000 and repair times reduced from weeks to days.
2. Efficiency improvement
The operating efficiency of a pump depends mainly on the amount of fluid that flows back through the wear rings; narrowing the gap between the rings can reduce the area through which fluid can pass between the rings. The use of metal wear ring, you need to maintain a large gap to reduce the wear ring touch. However, larger clearances can cause liquid backflow, resulting in lower pump efficiency. At the same time a larger operating gap will result in strong pump vibration, making the sleeve shift caused wear rings in contact with each other.
The CR-6100 has a low coefficient of thermal expansion (CTE), so stationary parts such as wear rings made of this material have a smaller operating clearance when installed, as shown in the table below. This results in the same throughput at lower output power, while also increasing efficiency by 2% to 5%. In addition, the vibration caused by these wear rings has also been greatly reduced, extending the service life of bearings and wear rings.
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3. Idling will not cause the operation is not smooth
CR-6100 wear rings can be used in idling conditions. The pump does not run smoothly without any liquid being ingested, operating conditions outside the design limits, slow rotation, or during pump start-up. Conversely, pumps with metal wear rings can cause the pump to get stuck in heat due to the same conditions, requiring downtime for disassembly and repair to troubleshoot problems.
In a condensation recovery operation, the centrifugal pump is operated at a temperature close to the vaporization temperature, so that "steam deadlock" and "idling" often occur. Common damage, including the wear ring due to high temperature and stick together, the rotor blades rupture, and the deflection plate deflection. In addition, the pipe sleeve bearing may also cause damage to the sleeve and can not be repaired, resulting in the entire lower half of the pump being scratched. The pump is serviced twice a year on average, and more seriously, many very expensive clotting agents are wasted every time the pump is repaired. The factory's service technician changed all static wear parts in one pump to the CR-6100 and tested its pressurization performance during operation. Due to the small clearance of the wear part of the CR-6100, the pressurization performance of the pump is slightly higher than that of the new pump. After the feed valve was closed 1min, and let the pump continued operation, and then re-open the valve. Pump does not appear to run the problem, its vibration and temperature are maintained very stable, after measuring its performance has not changed. Then the import valve closed 60min time, nor its performance caused a very serious impact. The pump was returned for routine maintenance after 3 years of continuous use. CR-6100 material parts can still continue to use. Now all static wear parts of the pump are made of CR-6100 material, failure rate from twice a year to failure, in 4 years there is no fault.
Sixth, the conclusion
Most centrifugal pump operating conditions, are likely to cause damage to the wear ring. However, refiners can effectively avoid this situation by selecting the parts with the best operating and functional characteristics. In this way, managers can improve the efficiency and safety of their operations while significantly reducing the company's operations
