Oil Trap Cleaning Procedure

Transcription

1 Oil Trap Cleaning Procedure R. Bunker, D. Noble, and R. Schnee Department of Physics, Syracuse University, Syracuse, NY 13244, USA (Dated: November 28, 2012) 1

2 FIG. 1. The RES pumping station and its anti-backstreaming oil trap. I. INTRODUCTION The Radon-Emanation System s (RES) pumping station includes an oil-based Edwards RV8 rotary-vane pump. To help prevent oil that has been converted into vapor (by the heat of the pumping action) from back diffusing into the RES s plumbing, the pumping station includes an anti-backstreaming oil trap (connected directly to the pump s input; see, e.g., Figure 1). This filter is nothing more than a cylindrical vacuum chamber packed with copper wool. The copper wool has a large surface area that is effective at catching oil vapor as it passes through the chamber. Much of the collected oil will drip back into the pump. Nevertheless, the trap s efficiency will gradually degrade over time as the copper wool becomes saturated with oil (perhaps a few years), allowing oil to eventually break through the trap. The relatively simple procedure outlined below has been found to be an effective method for cleaning the copper wool and thus restoring the trap s efficiency. It s also worth noting that if the pump is turned off (or loses power) while its input is still connected to a vacuum space, oil can be pulled out of the pump and through the 2

3 FIG. 2. The RES pumping station s isolation valves. anti-backstreaming trap. In this case, liquid oil (rather than vapor) is being sucked through the trap and into the vacuum space, a process facilitated by the absence of a pull from the opposite direction (i.e., the action of the pump). This is bad! The trap is much less effective at preventing liquid oil from passing through it and may therefore allow sensitive equipment attached to the pump to be contaminated with oil. Furthermore, the trap s likely to become saturated much more quickly several hours or days rather than years and will thus require maintenance more frequently. To help prevent this situation, always make sure to isolate the pumping station from the vacuum space to which it s attached before turning it off. This is done by closing the ball valve attached to the other side of the trap (see, e.g., the primary isolation valve in Figure 2). Also, never leave the RES unattended while pumping on one of its chambers or radon traps; if the power goes out (and it has), it s important to have someone nearby to close both isolation valves. 3

4 II. CLEANING PROCEDURE If you find that the trap has allowed oil to breakthrough, it s probably time to clean it. The only other option is to replace it, which will likely cost a few hundred dollars (e.g., K.J. Lesker offers an equivalent Metal Wool Foreline Trap with KF25 flanges for $200.00; part number TAS4CS100QF). The following procedure is considerably cheaper, requiring only 1/2 gallon of 70% isopropyl alcohol (IPA). 1. Remove the trap from the pumping station, allowing any excess oil to drip back into the pump. 2. Cap the end that connects directly to the pump with a KF25 blank-off flange. Use a proper rubber o-ring and KF clamp. 3. Stand the trap upright, with its open end at the top and the capped end at the bottom. The trap will not stand up on its own. Locate (and clean if necessary) one of the large plastic beakers in the 3rd floor lab. The diameter of the trap is small enough for it to just fit inside one of these beakers, allowing the trap to stand upright and also providing a catch for any IPA that leaks through the blank-off cap. 4. Completely fill the upright trap with IPA and let sit for 1 day. 5. Remove the blank-off flange and allow the contents of the trap to collect in the plastic beaker. The IPA should now have an orange tint. Dispose of the contaminated alcohol in a properly labeled hazardous-waste container (located under the fume hood). 6. If the IPA from the previous step was clear, the trap is clean. Otherwise, repeat steps 2 through 5 until the IPA comes out clear. 7. The trap should be allowed to dry out before being put back into use: Either uncap both ends and allow to air dry for several days, or Attach a dry-nitrogen gas line to the top of the trap (via a KF25 to 1/4 " Swagelok adapter) while leaving its bottom port open. Stand the trap upright in the (empty) plastic beaker and allow 5 10 psig of nitrogen to run through the trap for several hours. 4

5 The efficiency of the RES pumping station s anti-backstreaming trap should now be restored. If you observe oil breaking through the trap directly after cleaning, it might be time to purchase a new trap! 5