**Pressure Relief and Plumbing Features**

**Cryostat Return-End**

**1.8K Magnet Test-Facility at LBL**

This paper lists the basic functions and system features required of the
return-end plumbing in a horizontal cryostat used for 1.8K testing of
1.3 meter superconducting magnets at LBL's Magnet Test Facility. Four
functions are required. Their associated features and purposes are
listed below:

  ---------------------------------- ------------------------------------
  **[Feature]{.underline}**          **[Purpose]{.underline}**

  ---------------------------------- ------------------------------------

**[Cryostat cool-down gas-return]{.underline}**:

+---------------------------------+------------------------------------+
| 1.  Gas exit: top , return-end  | Removal of the warmest gas from    |
|                                 | the cryostat.                      |
+---------------------------------+------------------------------------+
| 2.  Warm seals: vacuum-tight    | Clean "pump and purging" of the    |
|                                 | cryostat.                          |
+---------------------------------+------------------------------------+
| 3.  Radiation shields: Al,      | Radiation & thermal-acoustic       |
|     flapper-valve style         | block; R-V conductance.            |
+---------------------------------+------------------------------------+
| 4.  Radiation shields: external | Easy repair of design oversight or |
|     removal & repair            | quench damage.                     |
+---------------------------------+------------------------------------+
| 5.  Thermal interception:       | Controllable cold-gas heat         |
|     metered gas-outflow         | interception.                      |
+---------------------------------+------------------------------------+
| 6.  He return valve: cold       | "Cross-over" (T \< 30K) control.   |
|     operation                   |                                    |
+---------------------------------+------------------------------------+

Cryostat pressure-relief:

+---------------------------------+------------------------------------+
| 1.  Plumbing: large diameter    | Low gas-flow impedance.            |
+---------------------------------+------------------------------------+
| 2.  He-II R-V: spring-loaded    | Auto-close, auto-pressure-relief.  |
|     (10 psi)                    |                                    |
+---------------------------------+------------------------------------+
| 3.  He-II R-V: low super-fluid  | Minimize heat leak to cold volume. |
|     & heat-leak                 |                                    |
+---------------------------------+------------------------------------+
| 4.  He-II R-V: manual,          | Quick, easy over-ride of the       |
|     lever-controlled            | auto-close feature                 |
+---------------------------------+------------------------------------+
| 5.  psi R-V: automatic          | Safety: baseline pressure relief.  |
+---------------------------------+------------------------------------+
| 6.  psi R-V: automatic,         | Safety: back-up, high conductance  |
|     flat-plate                  | pressure relief.                   |
+---------------------------------+------------------------------------+
| 7.  psi R-V: double-captured    | Reliable resealing.                |
|     O-ring                      |                                    |
+---------------------------------+------------------------------------+
| 8.  Manual R-V: easy access, 2  | Quick insurance against frozen R-V |
|     inch ball valve             | seals.                             |
+---------------------------------+------------------------------------+
| 9.  All valve seals: Teflon     | Cold seal-able, no cracking, no    |
|                                 | sticking.                          |
+---------------------------------+------------------------------------+
| 10. All valve bodies: copper or | Facilitate torch resealing of      |
|     brass                       | leaking seals.                     |
+---------------------------------+------------------------------------+
| 11. All valves: downward        | Safety: no water condensation      |
|     venting                     | failures, easy reach.              |
+---------------------------------+------------------------------------+
| 12. All valves: vent deflectors | Safety: freezing gas away from     |
|                                 | people or equipment.               |
+---------------------------------+------------------------------------+

Liquid helium removal:

+---------------------------------+------------------------------------+
| 1.  Insertion valve: warm, ball | Quick insertion capability.        |
|     valve                       |                                    |
+---------------------------------+------------------------------------+
| 2.  Insertion valve seals:      | Cold operation capability.         |
|     Teflon                      |                                    |
+---------------------------------+------------------------------------+
| 3.  Insertion seal: beveled     | Reliable, cold- sealing            |
|     Teflon                      | capability.                        |
+---------------------------------+------------------------------------+
| 4.  Insertion seal location: 2  | Safety, permits sealing before     |
|     inches above valve          | opening the valve.                 |
+---------------------------------+------------------------------------+
| 5.  Super-fluid seal: Teflon    | Easy, inexpensive, removable,      |
|     plug on end of tube         | replaceable seal.                  |
+---------------------------------+------------------------------------+
| 6.  Radiation shield: Nema-G    | Easy, inexpensive radiation        |
|     spaced Al rods              | blockage.                          |
+---------------------------------+------------------------------------+
| 7.  Heat intercept: gauged      | Controllable heat interception.    |
|     gas-outflow                 |                                    |
+---------------------------------+------------------------------------+

Diagnostic:

+---------------------------------+------------------------------------+
| 1.  Return pressure: Bourdon,   | Facilitate valve operation         |
|     30" - 15 psi                | decisions.                         |
+---------------------------------+------------------------------------+
| 2.  Cryostat pressure: Bourdon, | Facilitate valve operation         |
|     30" - 15 psi                | decisions.                         |
+---------------------------------+------------------------------------+
| 3.  Cryostat transient          | Permit remote recording.           |
|     pressure: electronic        |                                    |
+---------------------------------+------------------------------------+
| 4.  R-V heat intercept          | Facilitate controlled heat-leak.   |
|     flowmeter: 20 CFH           |                                    |
+---------------------------------+------------------------------------+
| 5.  LHe, F/O heat intercept     | Facilitate controlled heat-leak.   |
|     flowmeter: 5 CFH            |                                    |
+---------------------------------+------------------------------------+
| 6.  All gauge mounts: warm      | Avoid excess temperature and/or    |
|     stems, upward               | water condensation.                |
+---------------------------------+------------------------------------+
| 7.  All gauge locations: away   | Avoid torch damage to glass,       |
|     from R-V's                  | plastic, or wiring.                |
+---------------------------------+------------------------------------+
| 8.  All gauge orientations:     | Facilitate visibility to           |
|     toward control room         | operators.                         |
+---------------------------------+------------------------------------+
| 9.  Fiber-optic seals: 2,       | Easy sealing/installation/removal  |
|     short,potted-pipe type      | of fragile cables.                 |
+---------------------------------+------------------------------------+

**Valve Status**

**Standard Operations**

+------------------+------+-------+---------+-------+------+---------+
| **[Operatio      | **[  | **[M  | **[He   | *     | *    | **[LHe] |
| n]{.underline}** | He-I | anual | Gas]    | *[R-V | *[LH | {.under |
|                  | I]{. | ]{.un | {.under | ]{.un | e]{. | line}** |
|                  | unde | derli | line}** | derli | unde |         |
|                  | rlin | ne}** |         | ne}** | rlin | **[     |
|                  | e}** |       | **[     |       | e}** | Access] |
|                  |      | *     | Return] | **    |      | {.under |
|                  | *    | *[R-V | {.under | [Flow | **   | line}** |
|                  | *[R- | ]{.un | line}** | ]{.un | [Flo |         |
|                  | V]{. | derli |         | derli | w]{. |         |
|                  | unde | ne}** |         | ne}** | unde |         |
|                  | rlin |       |         |       | rlin |         |
|                  | e}** |       |         |       | e}** |         |
+------------------+------+-------+---------+-------+------+---------+
| Cool-down (300 - | Open | C     | Open    | 0     | 2    | Rod     |
| 30K)             |      | losed |         |       | CFH  | Plug^5^ |
+------------------+------+-------+---------+-------+------+---------+
| Cool-down (30K - | Open | C     | Cl      | \<10  | \<2  | Rod     |
| 4.3K             |      | losed | osed^1^ | CFH   | CFH  | Plug    |
|                  |      |       |         |       |      |         |
|                  |      |       | seque   |       |      |         |
|                  |      |       | ntially |       |      |         |
+------------------+------+-------+---------+-------+------+---------+
| 4.3 Liquid       | Cl   | C     | Closed  | 10    | 2    | Rod     |
| Accumulation     | osed | losed |         | CFH   | CFH  | Plug    |
+------------------+------+-------+---------+-------+------+---------+
| Quench (P \< 20  | Cl   | C     | Closed  | Large | N/A  | Rod     |
| psi)             | osed | losed |         |       |      | Plug    |
+------------------+------+-------+---------+-------+------+---------+
| Quench (P \> 20  | Ope  | Op    | Open^2^ | 0     | N/A  | Rod     |
| psi)             | n^3^ | en^4^ |         |       |      | Plug    |
+------------------+------+-------+---------+-------+------+---------+
| Super-Fluid (T   | Cl   | C     | Closed  | 10    | 2    | Rod     |
| \< 2.17K)        | osed | losed |         | CFH   | CFH  | Plug    |
+------------------+------+-------+---------+-------+------+---------+
| Liquid Helium    | Cl   | C     | Closed  | C     | Cl   | LHe     |
| Recovery         | osed | losed |         | losed | osed | Stinger |
+------------------+------+-------+---------+-------+------+---------+

1.  Cross-over (30K to 4.3K): It is preferable to close a return valve
    that is warm (as far as practical from the cryostat), in order to
    reduce the damage to the sealing surfaces and give better control
    over the valve's conductance.

2.  Quench Recovery: He gas-return is opened at the start of the
    lead-end's 20 psi R-V blow-off.

3.  Quench Recovery: He-II R-V is opened to keep the lead-end's 20 psi
    R-V from getting too cold.

4.  Quench Recovery: Manual R-V is opened to keep all "warm" R-V's from
    getting too cold.

5.  Rod plug: A rod, having the same diameter as LHe removal stinger, is
    composed of a series of Nema-spaced Al radiation blocks. It has a
    beveled Teflon tip for the bottom super-fluid seal.

**Super-Fluid Seals**

**Reliability and Low Thermal Conductance Issues**

+-----------------------------------+-----------------------------------+
| **Assertion**                     | **Conclusions**                   |
+-----------------------------------+-----------------------------------+
| 1.  All seals will leak some      | Expect helium accumulation above  |
|     superfluid.                   | the seal.                         |
+-----------------------------------+-----------------------------------+
| 2.  Heat-flow to the "1.8K"       | Minimize by reducing:             |
|     system is controlled by the   |                                   |
|     temperature and the flow-rate | a)  the inlet temperature to the  |
|     at which normal helium is     |     leak,                         |
|     driven through the leak.      |                                   |
|                                   | b)  the mass flow-rate through    |
|                                   |     the leak.                     |
+-----------------------------------+-----------------------------------+
| 3.  The normal helium flow-rate   | Minimize by reducing:             |
|     is governed by the pressure   |                                   |
|     difference across the leak,   | c)  the pressure difference       |
|     and the leak's conductance.   |     across the leak,              |
|                                   |                                   |
|                                   | d)  the conductance through the   |
|                                   |     leak.                         |
+-----------------------------------+-----------------------------------+
| 4.  A leak's conductance is       | Minimize by providing:            |
|     minimized by minimizing the   |                                   |
|     leak's cross-sectional area,  | e)  mating or auto-matching       |
|     maximizing the leak           |     surfaces.                     |
|     path-length.                  |                                   |
|                                   | f)  a long leak-path.             |
+-----------------------------------+-----------------------------------+
| 5.  Differential contractions can | Use the same thick, isotropic     |
|     distort surfaces.             | material for all pieces.          |
+-----------------------------------+-----------------------------------+
| 6.  It is unlikely to duplicate   | Expect the leak-rate to vary with |
|     the exact mechanical          | each actuation of a valve.        |
|     conditions after each valve   |                                   |
|     actuation.                    |                                   |
+-----------------------------------+-----------------------------------+
| 7.  The leak's inlet temperature  | Arrange the geometry to           |
|     is controlled by the system's | facilitate stratification:        |
|     ability to stratify helium at |                                   |
|     the leak's inlet.             | g)  Intercept any heat-leak at    |
|                                   |     the 4.3K layer.               |
|                                   |                                   |
|                                   | h)  Feed any leak from the 2.2K   |
|                                   |     layer.                        |
+-----------------------------------+-----------------------------------+

**Return-End Plumbing**

**Schematic Diagram**