1. Cryogenics – The Basics
Lesson 6
Buffer Dewar Operation and Recovery
D. Kashy

2. Lesson 6 - Topics Supply from ESR
Valve Box 4K source reminder
Hall B Distribution can for Helium Supply
Buffer dewar flow paths and controls
4K Supercritical Gas and filling the dewar
Cold return and back pressure control
Warm return
Return gas path to ESR
Off normal operation
ESR shutdown
Quench or Fast Dump of Torus
Recent issues
What to look for if called in and what to do next
Questions

3. Special NOTES
The slides in this presentation are a snapshot in time. They give general principles of operation.
The PID control loop settings may not be up to date so the operators must use the relevant spread sheets to verify recent and most proper settings.
M:\hallb_eng\CLAS12\Magnets\Operations\Cryogenic Operations

4. ESR Valve Box (part 2) and 10,000 liter Dewar
Helium Dewar With HX to cool the 4.5K supply
CTD672 Warm return temperature from All 3 Halls (Important upon CD or Recovery)
Small Polishing HX in the VB

5. ESR Valve Box (part 1)
Nitrogen Sub-cooler to send 4atm liquid at 77K (subcooled liquid)
4K supply flow meter (note sometimes reads about double actual flow, normal is ~6-7g/s)
4K Return Line with Cold Return Valve
CEV6721B (must be shut upon long ESR outages to facilitate Hall B magnet recovery

6. Hall B 500 liter Buffer Dewar
The buffer dewar supports target operation in Hall B
The dewar has multiple relief valves in parallel located on its neck and these are set to 15psig (2atm).
The dewar is located on level 2 of the space frame for standard cryotarget operation, but it has long flexible u-tubes that allow it to be moved to level 1 if needed for alternate target operations.

7. Hall B Distribution Box (DBX)
No supply valve in the 4K supply in the DBX. Also notice there is no variable temperature supply valve. This is different than the Solenoid or Torus.
Blue line is 4K supply, it feeds Torus, Solenoid and Buffer Dewar

8. Hall B Buffer Dewar Main Control Screen
Total Hall supply flow CFI6711B
ESR valves CEV6711B and CEV6721B
Return Pressure should read ~ (broken)
TD8210 is after the supply valve so it should read 4.5K (bad calibration) but if it reads 3.6 then liquid is being made at the “JT” valve EV8210
LL8210 is a superconducting level probe controlled by an AMI 135 controller on Level 1 of the space frame
Exhaust temperature TD8211 is a useful control parameter to help keep the u-tubes cold
PI8210 is the buffer dewar pressure and it is this pressure that drives liquid from the dewar through EV8214 or EV8215 or both
The supply valve EV8210 uses Cascaded PID Loops more detail later

9. Hall B Buffer Dewar Recent issue
LL8210 was reading around set point of 60% but jumping +/- 5%
The AMI level controller was cycled from Update to Sample and back and this cleared the problem
The Level quickly dropped to zero and then recovered.
This was caused by high pressure in the dewar. If the dewar is not reading properly it may be the pressure is too high. At 1500mBar it should be OK but at it can get flakey.
/entry/

10. Dewar Instrumentation Control Hardware – Space Frame Level 1 - Beam Left
Helium Level Controller
Warm Return valve EV8210CD Controller
Temperature sensor Module
Cold Valve Controllers
Update/Sample switch on level controller when in sample the sensor current is not continuously powered

11. Supply Controls EV8210 primary loop controls on level
A loop controls valve min position on exit temperature to assure U-tubes have flow
B loop controls valve max based on dewar pressure to throttle supply valve it dewar is blowing its relief
C loop is in Manual mode and set Max of B loop
Primary and A loops have long sample times because these control variables change slowly.

12. Back Pressure Control
EV8210BP primary loop controls on dewar pressure PI8210
Pressure needs to be above the targets helium circuit pressure to transfer liquid from the dewar to the target (Saclay or HD Ice or other)

13. Liquid Withdrawal Controls
Two liquid withdrawal valves allow filling of two separate systems. This was done to allow filling of both a target reservoir and a superconducting magnet during the original CLAS running.
At present we only use EV8214 to fill the Saclay Target, but in the future we may need the second valve to support other loads.

14. Hall B 500 liter Buffer Dewar – Possible issues and recovery
Upon ESR trip it is possible and even likely that the dewar will be exposed to high pressure and blow its relief valve. To get the relief valve to reseal may require the following:
Determine the pressures in other parts of Hall B and get info from cryo as to the ESR plant status. (When will compressor be running, when will pressures recover, when will cold flow be available…)
When the dewar pressure is depressurized (below 8 psig or ~1500mbar) the leaking relief valve and others that have reseated should be gently warmed using a propane torch (per JLab/Hall B procedures, contact work coordinator if in doubt)
IF the pressure is not dropping it may be necessary to shut the supply valve EV8210 by putting it in manual mode.
The Helium level probe may need to have its probe current cycled as discussed earlier in this talk
Always check to see there is a return path for dewar helium especially when the 4K cold return valve CEV6721B at ESR is shut. This path is typically through either the Torus and or Solenoid cold and warm returns back to the ambient vaporizer (next slide)
There is a cooldown line that can be used but it has not been needed during CLAS12 operations thus far, and if used it could make recovery more difficult so using the cold return U-tube to the DBX is preferred (next slide)
Because the dewar holds lots of liquid it can last for days before it goes empty. Typically after the relief valves have been sealed the dewar will recover without changing any PID settings.

15. Hall B Distribution Box (DBX)
No supply valve in the 4K supply in the DBX. Also notice there is no variable temperature supply valve. This is different than the Solenoid or Torus.
Blue line is 4K supply, it feeds Torus, Solenoid and Buffer Dewar

16. Warm Return Valve – EV8210CD
The warm return valve is a ball valve that is mounted to the dewar. It has an actuator that allows remote actuation, instructions for use are written on paper taped to the actuator
This valve has not been opened during CLAS12 operation except during system purification, and except in an emergency or very off normal situation it should remain closed
To know that the valve is closed one can look at the flats on the valve stem and see if they are horizontal if they are the valve is closed. If the valve is open then there will likely be ice on the piping at the valve

17. Questions?