Cryogenic Vacuum Pumps

Transcription

1 Oxford Instruments Austin Cryogenic Vacuum Pumps Decontamination Procedure

2 Only use helium gas with at least % purity for decontamination procedure described in this document. Decontamination procedures #1 and #2 will remove contaminants from the helium gas circuits of the coldhead/compressor system. One of the major sources of helium contamination is the use of helium gas with insufficient purity. Helium gas for leak detection or welding is not sufficiently pure. It contains impurities (chiefly nitrogen, neon and water vapor) that could cause performance degradation of and/or damage to the coldhead/compressor equipment. Another major source of contamination is from leak(s) in the closed-loop helium circuit. In this case, atmospheric air could be introduced into the helium circuit during system operation. Contamination of the helium-gas circuit in the cryopump is usually indicated by sluggish or intermittent operation (ratcheting) or a grinding noise in the cold-head drive mechanism. Such symptoms of contamination could also be accompanied by the rise or fluctuation of coldhead temperatures, and/or the rise of pressure level in the vacuum chamber. In severe cases of contamination, the cold head drive motor will seize and fail to operate, indicating a large amount of contaminants may have been frozen inside the coldhead hence overloading the drive motor. If there is an observation window on the cold head drive motor, look for clear drops, gold- or silver-colored metallic specks or black powders on the inside of the window. If substantial amount of such substances are present, it may indicate that the coldhead components may have severe wear and tear, or damaged. In this case, stop operating the pump immediately and arrange to have the cryopump overhauled or repaired. Continued operation may cause severe damage to the cryopump (such as broken motor shaft). Doing decontamination at this stage may not be sufficient to return the cryopump to its normal operation. Possible explanation(s) of substances seen in the drive motor observation window: Clear drops: compressor. Gold-colored specks: Silver-colored specks: Black powders: Excess oil in bearings or oil carry-over from the Linear bearing wear. Crank failure or bearing wear. Bearing or displacer seal failure. Procedure #1 should be used to decontaminate a coldhead. Decontaminating a coldhead during Procedure #1 will also clean the compressor if the overall contamination is not severe.

3 Decontamination Procedure #2 is rarely required, but can be used to clean up a compressor unit whenever: a. Maintenance has been performed on the helium-gas circuit of the compressor unit. b. It is suspected that the compressor unit has become severely contaminated. Note: Routine replacement of the adsorber in the compressor does not contaminate the compressor. As shipped, each replacement adsorber is charged with high purity helium gas and is provided with self-sealing couplings that enable installation without contaminating the helium circuit. Oxford Instruments Austin does not recommend evacuation using a mechanical vacuum pump to decontaminate a coldhead. Never operating the coldhead or compressor when under vacuum, doing so will damage the coldhead drive motor or the compressor motor. PROCEDURE #1 DECONTAMINATION OF THE COLDHEAD OF A CRYOGENIC HIGH VACUUM PUMPS Note: For this procedure, a maintenance manifold (p/n 10134) is required. Contact Oxford Instruments Austin ( ) to acquire this tool. CAUTIONS: 1) WHEN CONNECTING OR DISCONNECTING A SELF-SEALING COUPLING, ALWAYS USE TWO WRENCHES TO AVOID LOOSENING THE BODY OF THE COUPLING FROM ITS ADAPTER. 2) ALWAYS CHECK THAT THE FLAT RUBBER GASKET IS IN PLACE BEFORE CONNECTING A SELF-SEALING COUPLING. 3) IF THE COLD HEAD STARTS TO RATCHET DURING STEP 1 (BELOW), PROCEED IMMEDIATELY TO STEP 2. STEP 1 Cool down the cryopump and operate it for at least one hour (preferably for 3 hours or longer). Note: As a result of this step, the contaminants in the system will become concentrated and frozen inside the coldhead. The longer the cryopump is operated before the following steps, the greater the amount of contamination that can be trapped in the coldhead. STEP 2 Shut down the cryopump Note: If more than one cryopump is running off the compressor and only one pump needs to be decontaminated, disconnect the coldhead drive power cable from the affected cryopump, the compressor and the rest of the cryopump(s) can continue to run.

4 However, it is strongly recommended to decontaminate ALL cryopumps that are connected to the same compressor. STEP 3 IMMEDIATELY disconnect the helium supply and return lines from the gas supply and gas return connectors on the compressor unit, leave the lines attached at the coldhead would decontaminate the lines with the coldhead. Note: It is imperative that the helium lines are disconnected IMMEDIATELY after shutting down the compressor so that the trapped contaminants cannot migrate back into the compressor. CAUTION IN THE FOLLOWING STEP, DO NOT REDUCE THE PRESSURE BELOW 30 PSIG. TO DO SO MAY INTRODUCE CONTAMINATION INTO THE COLD HEAD. STEP 4 Attach the flexible helium supply and return lines to the maintenance manifold. STEP 5 Reduce the pressure in the coldhead to between psig, by slowly open the discharge valve (with red handle) on the maintenance manifold. STEP 6 Warm up the cryopump. Allow sufficient time for the second stage of the cold head to warm up to room temperature (~ 290 Kelvin), this usually takes 3~4 hours. The warming up time can be shortened by using heated dry nitrogen gas to purge the vacuum side of the cryopump through the gas purge port on the cryopump, and/or using a heater blanket on the pump vessel (with heater temperature limited to 150 F or less). Note: if the cryopump is not warmed up to room temperature, the trapped contaminants may not be fully released into the helium gas stream therefore rendering the decontamination procedure ineffective. STEP 7 Attach a helium gas cylinder, regulator and charging line to the maintenance manifold and purge the regulator and line of air as follows: a. Install a gas regulator (0-400 psig) on the helium bottle b. Open the regulator by turning clockwise on the handle until it engages the diaphragm and turns approximately 1/8 to 1/4 turn beyond engagement c. Open any other valves on the regulator d. Open the valve on the helium bottle e. IMMEDIATELY back off on the regulator handle until helium stops flowing CAUTION: Failure to open the regulator before opening the helium supply bottle valve will result in contamination of the helium gas supply. f. Loosely attach the 1/4 flair line between the regulator and the charge port (with small Swagelok valve with black knob) on the maintenance manifold. g. Purge the line for at least 15 seconds before tightening the flare nut.

5 STEP 8 a. Pressurize the coldhead with helium gas to the desired static charge pressure (see Table 1 for different cryopump/compressor configurations), by opening the charge valve (small Swagelok valve with black knob) b. Close the charge valve on the maintenance manifold when the desired pressure level is reached c. Slowly open the discharge valve (with red handle) and depressurize the coldhead to psig d. Repeat steps 8a through 8c for four more times e. Pressurize the cold head back to the static charge pressure by opening/closing the charge valve on the maintenance manifold, run the coldhead drive motor for 1 to 2 minutes by turning the compressor on/off CAUTION: Only run the coldhead/compressor when system is charged to proper static pressure. Operating system at lower pressure or in vacuum will damage the equipment. STEP 9 Perform STEP 8 for four more times. Note: There will be a total of 25 helium gas purges and a total of 5 drive motor runs from STEPS 8 and 9. STEP 10 Verify that the coldhead and compressor is charged to the static charge pressure shown in Table 1. STEP 11 Disconnect the helium supply and return lines from the maintenance manifold using two wrenches for Aeroquip connectors. STEP 12 Reconnect the helium return and supply lines to the corresponding gas-return and gas-supply connectors on the compressor using two wrenches for Aeroquip connectors. CAUTION: Check that the flat rubber gaskets of the self-sealing couplings (Aeroquip connectors) are in place and are not damaged before reconnecting. Missing or damaged gaskets will cause the connectors to leak, and could result in further contamination. STEP 13 Leak check the complete system (cryopump, compressor and lines) using a handheld or a portable leak detector (if available). STEP 14 Restart the compressor and the cryopump.

6 Table 1 Static Pressure* for Different Cryopump/Compressor Configurations Cryopump Compressor Static Pressure [PSIG; ± 5 PSIG] CT100 ASC M125 CTI SC, 8, 8300 ASC Conversion of CTI SC, 8, 8300 CT-7 CP-8/CT-8 CP-10/CT-10 CP-16/CT-400 CT-500 ASC M125 CTI SC, 8, 8300 ASC Conversion of CTI SC, 8, 8300 ASC M125 CTI SC, 8, 8300 ASC Conversion of CTI SC, 8, 8300 ASC M450 ASC M600 CTI 1020, 8500, 8510 CTI 9600 CTI 9700 ASC M450 ASC M600 CTI 1020, 8500, 8510 CTI 9600 CTI 9700 ASC M450 ASC M600 CTI 1020, 8500, 8510 CTI 9600 CTI *These pressure levels are valid when both the cold head and compressor are at nominal room temperature (60 ~ 80 F) 225

7 DECONTAMINATION PROCEDURE #2 Note: For this procedure, a maintenance manifold (p/n 10134) is required. Contact Oxford Instruments Austin ( ) to acquire this tool. Only connect the helium gas supply line of the compressor to the maintenance manifold. Do NOT connect the return line of the compressor to the manifold, doing so will damage the compressor during decontamination. When the level of contamination within the helium gas circuits of a cryopump is not severe, Procedure #1 will generally decontaminate the compressor unit as well as the cold head. This is because contaminants in the compressor will have become isolated inside the cold head during the procedure. Use decontamination Procedure #2 to decontaminate the compressor unit when the following conditions exist: a. Maintenance has been performed on the helium gas circuit or the oil circuit of the compressor unit (for example, replacing a filter or an orifice). For this condition, commence the cleanup with STEP #3 below. b. It is suspected that the compressor unit has become SEVERELY contaminated. For this condition, commence the cleanup with STEP 1. CAUTIONS: 1. WHEN CONNECTING OR DISCONNECTING A SELF-SEALING COUPLING, ALWAYS USE TWO WRENCHES. THIS IS TO AVOID LOOSENING THE BODY OF THE COUPLING FROM ITS ADAPTER. 2. ALWAYS CHECK THAT THE FLAT RUBBER GASKET IS IN PLACE BEFORE CONNECTING A SELF-SEALING COUPLING. STEP 1 Shut down the cryopump and compressor. STEP 2 IMMEDIATELY disconnect the helium supply and return lines from the gas supply and gas return connectors on the coldhead, leave the lines attached at the compressor would decontaminate the lines with the compressor. Disconnect the adsorber inside the compressor if it is to be replaced. Note: If the cold head is contaminated, use Decontamination Procedure #1 (STEPS 4 through 14) STEP 3 Connect the maintenance manifold in-line on the supply side of the compressor, using the available helium gas supply line of the compressor. Leave the helium gas return flex line unconnected. CAUTION: Connecting both supply and return lines of the compressor to the maintenance manifold for decontamination could seriously damage the compressor.

8 STEP 4 Follow the steps below to attach a helium gas cylinder, regulator and charging line to the maintenance manifold and purge the regulator and line of air as follows: a. Install a gas regulator (0-400 psig) on the helium bottle b. Open the regulator by turning clockwise on the handle until it engages the diaphragm and turns approximately 1/8 to 1/4 turn beyond engagement c. Open any other valves on the regulator d. Open the valve on the helium bottle e. IMMEDIATELY back off on the regulator handle until helium stops flowing CAUTION: Failure to open the regulator before opening the helium supply bottle valve will result in contamination of the helium gas. f. Loosely attach the 1/4 flair line between the regulator and the charge valve (small Swagelok valve with black knob) on the maintenance manifold. g. Purge the line for at least 15 seconds before tightening the flare nut. STEP 5 a. Pressurize the compressor unit to its proper static charge pressure (see Table 1) by opening the charge valve (small Swagelok valve with black knob) on the maintenance manifold b. Close the charge valve on the maintenance manifold when the desired pressure level is reached c. Slowly open the discharge valve (with red handle) and depressurize the compressor to psig d. Repeat the purge procedure 4a through 4c four times. e. With the compressor unit pressurized to the static charge pressure, run the compressor for 1 to 2 minutes f. Reduce the static pressure of the compressor unit to a level of between 30 to 50 psig by slowly opening the discharge valve (with red handle) on the maintenance manifold. STEP 6 Repeat STEP 4 for four more times. Note: There will be a total of 25 purges and a total of 5 runs of the compressor unit. STEP 7 If the adsorber was disconnected, replace or reconnect the existing unit. STEP 8 Verify that compressor unit is charged to proper static charge pressure shown in Table 1.

9 CAUTION: Check that the flat rubber gaskets of the self-sealing couplings (Aeroquip connectors) are in place and are not damaged before reconnecting. Missing or damaged gaskets will cause the connectors to leak, and could result in further contamination. STEP 9 Reconnect the helium return and supply lines to the gas-supply and gas-return connectors on the compressor unit. STEP 10 Restart the compressor and cryopump (after cryopump is also decontaminated, see Procedure #1). Note: As the cryopump continues to operate, any residual contamination of the compressor unit will have become isolated in the cold head and can be readily removed from the cold head by using Decontamination Procedure #1. STEP 11 If further decontamination of the coldhead is desired, wait after coldhead running for 48 hours or more before conducting Decontamination Procedure #1 (starting from STEP 2 in Procedure #1).