# Presentation: 432248

## SF6 contamination by air in the Pelletron

- Recycler Departmental Meeting
- November 5, 2008
- A. Shemyakin

## Statistics of Pelletron accesses 

- There were 11 Pelletron access from Oct 2007 to Nov 2008 while only 6 from Oct 2005 to Sep 2007
- Could be just an effect of aging
- Another possibility is an effect of air contamination in the tank

- Number of Pelletron accesses per calendar quarter in 2005 - 2008. On the horizontal axis 1 is the 2nd quarter of 2005, and 12 – 15 corresponds to 2008.

## Results of the gas analyses performed in MI-31 assembly

- Three consecutive tests of the gas in the Pelletron tank gave similar results: SF6 is heavily contaminated by air
- The same company
- 2007 test was good

- July 17- repair of the storage tank leaks was finished

- Jun 10, Jul 28, Aug 21 & 29- five bottles of SF6 were added

- Oct 8- repair of the Pelletron tank leaks was finished

## What might have contributed

- There were leaks in the Pelletron tank
- Between Jan and Oct 2008, the SF6 loss was 1.5 – 2 psi/month (out of equivalent of ~100 psia in both tanks)
- The leak added air in the time of gas transfers with a negative pressure in the Pelletron tank
- Decrease of the total SF6 amount forced to leave the very minimum relative pressure in the storage tank
- There was a large leak in the storage tank
- The most probable candidate because of scale of the leak
- The storage tank pressure gauge was found reading +3 psig at atmospheric pressure instead of zero
- Temperature variations provided a good opportunity for sucking the air into the leaking storage tank
- Before transfers, the gauge was reading from 0 (on 11-Apr-08) to 5 psig
- With a large amount of manually operated valves to set during transfers, it’s difficult to rule out completely a human error
- Until Jun-08, there were no readings storage tank P and T, so it is impossible to trace changes of PV/T during transfers

## Effect of the Pelletron tank leak

- One of possibilities is an air contamination in the time when SF6 is pumped out of the Pelletron tank, and the tank is already at a negative (vs atmosphere) pressure
- Estimation from the following data:
- min achievable pressure, 0.3 Torr in summer months
- Pipe between the gauge and the pump with ~100 l/s conductivity in the molecular regime
- Pumping in this mode for ~ 1 hour
- This effect adds only a small amount of air, equivalent of ~0.5 psi per one transfer (out of ~ 85 psia)

## Air intake through the storage tank

- Until 10-Jun-08, the storage tank pressure and temperature were not read by ACNET. Immediately after connection, a large leak was found in the pressure relief valve. The leak could have been there for a long time.
- It means a large loss of SF6 in the time of transfers and possibility to suck in air both in the pumping the gas into the Pelletron and  in the time of temperature variations

- Storage tank temperature and pressure after connecting the gauges to ACNET. SF6 was added to the storage tank two days before the connection.

- Adding SF6 after repairing the storage tank leak. The mass stayed constant after that.

## Calculated mass during the transfers

- The table shows  the sums of PV/T in both tanks before the transfer, with all gas in the storage tank, and after transferring gas back to the Pelletron
- Except the transfer on 11-Apr-08, numbers before the transfer and after the transfer back to the Pelletron are consistent within 1%, and amount calculated for the gas in the storage tank is by ~7% lower (error in calibration of gauges).
- On 11-Apr-08, ~6% was added in the time of pumping into the storage tank.
- Unclear how it might happen
- Storage tank pressure gauge read zero before the transfer (that corresponds to -3 psi)

- Data (P, T) for transfers: storage tank - from technicians’ manual records in the log books, Pelletron tank – from data logger D44

## Consequences

- Stability of HV and recirculation has not changed
- Most likely, due to the fact that we are running a nominally 6 MV machine at 4.3 MV
- Humidity is kept low by the alumina-filled dehumidifier
- Burst of the lost current are frequent only after tank openings
- Large absolute pressure of oxygen may result in a fire inside the tank
- On the other hand, amount of oil-like material is small, and recently there were no sparks
- Oxygen is said to deteriorate reliability
- Grease burning and contact oxidation
- Purification of so heavily contaminated SF6 is difficult
- According to companies Karl Williams have contacted, it can be only fully replaced, ~100 k$ value
- A possibility to remove only oxygen are being looked at
- We may have to live with this gas and with a likely increased frequency of failures

## Summary

- Tests showing a large air contamination in the system are most likely correct
- We are discussing sending a sample for testing in a different company
- Significant portion of air might have come through a large leak in the storage tank
- Only one instance, the transfer on April 11, 2008,  reliably shows a increase of PV/T (apart of adding SF6 from bottles)
- Still there is no complete explanation for almost 40% of air in the gas
- Presently, there are no detectable leaks in the system
- There is no direct impact of the contamination on the cooler operation
- However, oxygen may deteriorate reliability of the Pelletron
- No cheap way to have the gas with no oxygen has been found yet