As a key component, the compressor impeller needs to undergo multiple Non-Destructive Testing (NDT) during the entire manufacturing process as well as before and after putting into use to ensure its quality and safety. The purpose of non-destructive testing is to find potential defects or damage to ensure the reliability and life of the impeller under high pressure and high speed operation conditions.
- compressor impeller processing method
- casting, finishing after casting (milling + grinding)
- integral milling: for open or semi-open impeller (some ultra-wide closed impeller), you can use five-axis machining center for integral milling, integral milling impeller strength is high, less defect rate.
- two-piece welding: for closed impellers, most of the forgings are processed into two pieces and then two pieces of welding + post-welding finishing. Most of the closed impeller of a famous compressor manufacturer in China adopts the method of two-piece welding.
- 3D printing additive manufacturing: Some small impeller, can also use 3D printing additive manufacturing method, it is said that a famous compressor factory can print the diameter of the impeller has reached more than 400mm, and the printed finish, strength and other indicators compared with the two pieces of welded impeller, there is little difference. And now the printing efficiency is significantly improved, 7 days to print an impeller.
- electrical corrosion: the use of electrical spark corrosion of the impeller internal flow path processing. This method is inefficient.
2. API617-2022 requirements for impellers
- if it is a welded impeller, the impeller must be welded after the post-welding heat treatment in order to release the welding stress.
- All welded surfaces of the impeller shall pass visual + magnetic particle inspection or visual + liquid penetration inspection. The standard does not specify whether nondestructive testing should be performed before heat treatment or after heat treatment.
- Non-destructive testing after welding usually needs to be carried out before or after post-welding heat treatment according to the specific material and welding conditions. For materials prone to delayed cracking, such as certain low-alloy high-strength steels, it is usually recommended to perform non-destructive testing at least 24 hours after welding is completed in order to detect possible delayed cracking. For welds with a tendency to reheat cracks, such as some chrome-molybdenum alloy steels, non-destructive testing should be carried out after heat treatment to ensure the accuracy and integrity of the test results.
- the cast impeller hub and roulette should be ray or ultrasonic non-destructive testing of the hub and roulette before finishing.
- The impeller needs to be overspeed test at 1.15 times the maximum continuous speed, and the size of the impeller needs to be checked after the overspeed test to see if there is any deformation. Any repair of the impeller after the overspeed test must be notified to the owner and the owner’s written consent.
- The impeller should be non-destructive testing (either magnetic particle testing or penetration inspection) after the overspeed test.
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