The following posts highlight solutions we have offered our customers for common issues.
Aerospace customer noticed that a cryopump installed on a large vacuum chamber would not pump down below 10-5 range. Additionally he found oil contaminants on the 80K array. While there is oil used in the cryogenic system in the compressor and it can contaminate the helium circuit, this oil can not physically reach the vacuum vessel. Large “wet” pumps such as piston pumps are major contributors of oil back streaming into the vacuum chamber. This is due both to design and the large size of the pump’s inlet when compared smaller sized inlets of rotary vane pumps. In this specific case, the customer was using a large Piston pump to provide rough vacuum to his chamber at about 15 Microns. PTB’s recommendation: Increase the roughing to 50 microns from 15 (whatever rough pump is used) and do not let the rough pump, rough for any length of time to lower pressure. However for a long term solution, consider a different type of rough pump. A dry pump would be optimal, but a rotary vane would be better than a piston pump for this application. If the Piston pump is the only option, then PTB recommends an inlet oil trap, these traps are typically available from the OEM.
Customer called asking inquiring about adding helium to a 1020R compressor as he believed that there was not enough pressure in the helium circuit causing ratcheting in his CT 10. Following the technical support call provided by PTB it was established that the helium circuits pressures were in normal operating range. PTB advised a regeneration on the cryopump, after which the ratcheting ceased. This is indicative that the pump is in need of a rebuild. This CT 10 was 2.5 years old, typical mean time between repair for this model is 18 to 24 months.
“Due to the nature of the process we are using, the cold head is not being used during the regen. The noise from the cryo is not the ratcheting we get when the lines are contaminated but rather mechanical clunk.” Customer sent pump back after 8 months in service. On arrival it should be noted:
- The pump (vacuum side) was laden / covered with green process material. See photo.
- There was a significant scratch on the flange. See photo.
Because of the reported problem, PTB’s standard practice is to perform a decontamination process is enacted upon receipt of pump. Further, a helium sample was taken immediately from the pump for future analysis, if required. After decontamination and the pump recharged with helium, it was put on test. The problem (mechanical sound) was duplicated. Upon disassembly of the refrigerator, signs of contamination were seen inside the cylinder and on both stages of the displacer. The displacer and the yoke were removed and inspected. The yoke and the securing pin both had signs of wear. It appears this pin could have sheared at any time. If unattended the pin would have sheared causing the pump to stop abruptly.
The contamination in the helium, it appears, caused enough friction to increase the motor torque to overcome it. This increased torque, in turn strains / overloads the yoke to cause its bore to become elongated and its securing pin to wear. This deformation no longer secured the displacer firmly inside the cylinder, but allowed it to make contact with the cylinder top, producing the mechanical noise. As this failure was not due PTB workmanship or parts, it will be deemed as a non-warranty. That stated, the helium side of pump will be rebuilt as a courtesy to the customer. The vacuum side, as noted, is heavily coated and should, in our opinion be rebuilt, as well. PTB Sales will contact customer as to the best way to proceed with this. Warranty Status Not Approved This is not an approved warranty. Refrigerator of pump will be rebuilt as a courtesy to the customer. Customer should enact decontamination procedure on the helium system which drove this pump.
Customer sent pumps back after 9 months in service. Pump was operated alongside an OB10. After 5 days of running, the 10 inch cyro would start to warm up (2nd Stage up to 23K when 1st stage set at 80K). The OB250F (RMA Number: 31241-002) has been sent to PTB for analysis to test whether it is starving the neighboring OB10 of helium. An exchange OB250F was sent / operated, and the 10 inch cryo experienced same temperature rise. The OB10 was sent back to PTB as well and tested with the original OB250F. On arrival it should be noted:
- Both pumps exhibited heavy / pungent odors associated with arsenic.
- The OB250F had over 9300 service hours upon receipt.
- Regen pump to base pressure (for both pumps) was set for 200 microns. Recommended setting is should set at 50 millitor.
Both the original 250F and OB10 pumps were put on test via a networked configuration for over a week. Both pumps operated satisfactorily during this time frame with T1 temperatures at 80K and second stage hovering at 14K. Please see graphs below of the OB250F.
Pump 2 (CTI On-Board 250F SN: TK9638603) Left Axis: Temperature 1 / Right Axis: Temperature 2 This part of the test was discontinued and the pump's refrigerator section was removed and tested via PTB's automated valve timing station. The inlet/exhaust valve timing was confirmed and passed. Helium leakage test was also performed. The refrigeration unit passed this test as well. We were unable to duplicate the problem stated, further we see no workmanship or material failures. That stated there was evidence of helium contamination seen in the cylinder. See photos.
Warranty Status Not Approved As we are unable to duplicate the problem and no workmanship / material problems were found, this will be deemed a Non-Warranty return. This pump is also outside the warranty period. At this point PTB Sales will contact customer as to appropriate next steps for this pump.
The pump was received and evaluated. Upon applying cold head power to the pump there was a clunking sound coming from the motor. The customer’s reported problem was duplicated. The refrigerator was discharged and the motor was removed from the assembly. The motor was disassembled and identified that the bearing was damaged. After removing the bearing from the housing, the bearing was disassembled. The bearing’s outer ring was misaligned relative to the inner ring within the housing’s steel race. This misalignment caused the bearing to have irregular wear producing excessive heat resulting in premature failure. The pump was further evaluated and discovered that the diode leads were disconnected from the feedthrough. The varnish and heat shrink were also missing. Damage to the diode and feed though wires occurs when rapid pressure excursions in the cryopump occur. This is typical with pumps that are exposed to a high oxygen process when regenerations are not performed frequently. The arrays and shield are also highly contaminated. Due to the highly contaminated arrays and shield, a complete rebuild is recommended. All motors will be checked for rear bearing alignment by hand and then once assembled using an arbor press.
The reported problem was not duplicated: 1st and 2nd stage temperatures increasing. The cryopump was connected to the test system and successfully passed several regenerations.
We concluded that the cryopump probably had a small vacuum leak at the vacuum vessel flange. It was indicated that this is plausible since they do not change the flange O-Ring each time they replace the cryopump. PTB recommends installing a new flange gasket / O-ring when installing a cryopump onto the chamber.
The reported problem was duplicated; the motor shaft is broken. The cryopump was taken apart and inspected; specifically, the refrigerator was disassembled for signs of mechanical fatigue / failure. There were no indications of mechanical failure that would cause the motor shaft to break. The most likely cause of failure is helium contamination. When the helium system is contaminated, more torque is required to drive the displacer. If the contamination is not removed from the helium system then mechanical failure will occur; in this case the motor shaft. PTB recommends compressor decontamination before installation of new cryopump.
Brooks CTI On-Board IS-320F: After a complete regen the pump only cooled down to 18K. T1 – 99K; T2 – 18K rising to 20K in about 20 minutes
We were unable to duplicate the reported problem: T2 - 18K and rising. Below is a summary of the activities we performed in an attempt to duplicate the reported problem.
- 1. Cooled down the cryo and recorded bottom out temperature. 1st Stage ~ 48K; 2nd Stage ~ 8.7K
- 2. Set Temperature Control to 65K / 13.5K
- 3. Set T1 Temperature Control to 100K and disabled T2 temperature control
In all three test cases, the cryopump performed as expected. An e-mail was sent to the customer advising him that we could not duplicate the reported problem and we need to explore other potential areas to determine the cause of the failure. Our Engineer responded on as follows: “We believe there was a vacuum leak … The last three things on the tool that were completed were;
- P1 was changed.
- The beamline turbo was changed.
- A bushing was found to be installed possibly off center.”
Based on the testing performed by PTB Sales and the acknowledgment by Engineering of a vacuum leak explains why the cryopump would not cool down below 18K on the second stage.
The reported problem was duplicated. The motor shaft was broken due to possible contamination in the helium circuit. The refrigerator was disassembled / inspected and there were no signs of mechanical failure or fatigue that would cause the motor shaft to break. We spoke with the customer and explained how important it is to perform a decontamination on the helium circuit at the first signs of the cryopump making a “racketing” sound. It is highly recommended that the customer perform a helium decontamination on the helium circuit.