edm collaboration meeting, february 14-15, 2007 valve bucket-dewar tests steve williamson uiuc

EDM Collaboration Meeting, February 14-15, 2007

Valve Bucket-Dewar Tests

Steve Williamson

UIUC


Goals of the Bucket Dewar TestsTest valve components

(not complete valves) Test concepts for seat and stem

Materials Geometry

Develop technology for making seat and stem (e.g. polishing Teflon)

Verify that valve seat/stem design sufficiently seals LHe II Determine required seating force Test seal for many cycles Test bellows for many cycles Test LHe “hydraulic” actuator concept (?)


How Good?

Two types of valves: V1, V2, V4, and V5 seal “gas” of

dilute 3He in purified 4He Use molecular flow model:

average velocity (vav), ballistic mean free path (), leak time constant ()

Assume some allowed degradation

Calculate allowed leak area (atoms hitting leak area leak out)

Evaluate conductance and flow of gas under test conditions.

V3 and V6 seal liquid He II Max V3 leak rate determined by

allowed pressure change during m

Max V6 leak rate determined by allowed heat load during regeneration

cm01.84cell

cell A

V

cm/s39948

*3

m

TKv b

av

leak

cell

Av

A

av

s10)(9.0)()( 400

mmm nnnn

2

nn

dt

dn

2

nn

dt

dn

tnntntn exp)()()(

00

25cellleak cm1008.4)9.0ln(

mavv

AA

μm72leak d

cc/satm105.1K,77 Torr,45 1300 K

v Q Tp

Example: Cell Isolation (V1) Valve


Two Types of Valves

1.4.6 Volume Displacement

1.4.11.1 Volume Displacement Interconnect

Plumbing

1.4.10.1 Cell Isolation (V1)

Valve Actuators and Interfaces

1.4.5 Purifier

1.4.10.6 Purifier isolation (V6)

Valves

1.4.1 3He Atomic Beam Source

(ABS)

1.4.2 ABS Interface

1.4.3 Polarized 3He

Collection System

1.4.8 Pressurizer

1.4.10.5 Purifier

Control (V5) Valve

1.4.10.4 Collection

Isolation (V4) Valve

1.4.10.3 Pressurizer

Standoff (V3) Valve

1.4.10.2 Pressurizer

Isolation (V2) Valve

1.4.11.2 Collection/

Purifier Interconnect

Plumbing

1.4.11.3 Pressurizer/Cell

Interconnect Plumbing

Seals He II liquid

Seal 3He “gas”


Some LimitsValve Limit

Test Conditions

Qm

(mol/s)

Qv at 300K

(atm-cc/s)

V1Cell Isolation

10% drop in signal over m=104 s

77 K

p = 45 Torr6.0710-6 0.150

V2Pressurizer isolation

5% drop in signal over 500 s fill time

77 K

p = 45 Torr1.8810-4 4.63

V3Pressurizer stand-off

1% drop in pressure over m=104 s

0.5 K

760 Torr4.5510-6 0.112

V4Collection Isolation

3He concentration drops by 5% during 500 s storage time

77 K

p = 45 Torr 4.7910-6 0.118

V5Purifier Control

Signal drops by 5% during 500 s fill time

77 K

p = 45 Torr9.4110-5 2.32

V6Purifier Isolation

1 mW heat load during regeneration

0.5 K

p = 3 Torr6.8810-6 0.169

Note: these are “whistlers” (very high leak rates)


First step: Build Bucket Dewar Valve Tester

Gas Handling Circuit for valve seat testing DGH 8/13/06

to helium gas source

to vacuum pump

to vacuum pump

to leak detector

Baratron 1 Baratron 2

Valve being tested

Liquid helium level

This is the tube that the actuator goes through

This volume should be as small as possible to reduce evacuation and filling time Valve seat assembly

Valve actuator

Vacuum/motion feed-through

Heat shields

Supply tube

Exhaust/actuator tube

LHe level sensor


Bucket Dewar Test Insert Completed in early December Used for room temp, LN2, LHe, and LHe

II tests since then.

Top Flange (pretty crowded)The Insert Installed in Dewar


Valve Seat/Stem Geometry*

Benefited from experience of John Doyle’s group at Harvard.

Ball designs are insensitive to rotations – but fail if seat/ball are out of round.

Flat designs are insensitive to XY rotation, but fail if surfaces are not flat.

* From notes of Nathan Brahms

Harvard Choice


More on Geometry*

Empirically, conductance (C) depends on the geometry.

Smaller r, larger F is better. (Bolted flanges can produce enormous force.)

(T) is basically constant below 77 K Two regimes for w:

Knife-edge Insensitive to dirt Not very repeatable (edge deforms other

surface)

Large area Need smooth surfaces Need higher closing force Use soft material to deal with dirt.

eC wr

FT

w

Tr

)(

* From notes of Nathan Brahms

wr


Valve Materials Valves don’t seal because of:

Surface roughness Dirt

Two types of materials: Hard: glass, metal, Al2O3*, Vespel,

Kapton, Torlon Often come with good surface

(Kapton, glass) Do not accommodate dirt

Soft: Teflon* (PFTE), Kel-F, PFA (fluorocarbons)

Difficult to polish Can be deformed by closing

pressure

Composite materials are out (too hard to polish).

Seat

Boot

Soft Hard

Soft Biological valves.

Imprint of seat on stem must not be misaligned.*

Hard No hole in polished hard material.

e.g. Engine valves. Fail if held open by dirt.


Our First Shot

Torlon 5030 with Kapton gasket Problems:

Composite material is too hard to polish (used stainless steel)

Kapton and Torlon (or steel) are both “hard” materials (dirt problem). Also tried a Teflon gasket.

A gasket seals on TWO surfaces. Use Teflon boot.

Torlon 5030 piston (but initially acrylic or

aluminum)

Torlon 5030 outer wall and seat

Torlon 5030 gasket clamp

Kapton gasket

Boot

Se

at S

ea

t

Kapton Gasket


Lessons Dirt is a REAL problem. Sintered filters

seem to make it worse. Our initial w was too small. Try larger w

smaller r If it doesn’t seal at room temperature, it

won’t seal when colder. Use simple apparatus on leak detector

to check seat/boot Allows much faster tests – no cryogenic

turn-around.

Seat

Teflon “Boot”

Load Cell

Inlet to leak detector

Ball Joint

Screws for compression

clean room


Polishing Teflon Teflon is soft – tends to “gouge” when

polished. Harvard Technique

Sand by hand on granite with 400, 600, 1000, 1200, 1500 grit sandpaper.

On lathe, 1500 grit, then “plastic polish” with polishing cloth.

Hours of grad student labor. Another technique (light-guide machining):

Diamond machine on lathe (slow feed, high RPM).

Surface is improved if Teflon is “frozen” by cooling in LN2 before machining.

Another idea: Teflon “melts” at 327° C (begins to decompose

around 350 ° C) Use optical flat pressed against surface “cast” a

flat surface.

Diamond Tool


Soft Seat, Hard Boot Teflon tends to retain

deformation after applied pressure. After cycling, small misalignment

can cause leak Make seat out of Teflon, instead

of boot. Less sensitive to misalignment Edges of hard material do not

contact soft No need to fabricate hole in hard

material (e.g. ceramic, glass) Smaller Teflon surface to polish 510-9 Atm-cc/sec (leak detector

limit) with optical flat or cast acrylic boot at room temperature.

But larger volume of Teflon can break due to thermal shock.

Teflon

Teflon T

eflo

n

Cold Shocked Polished Teflon


A “Self Cleaning” Valve? Dirt continues to be a problem.

Contributes to non-reproducibility of tests. May originate in test assembly or vacuum system – despite good vacuum technique. Dirt will also be a problem for the experiment.

Geometry issues Sealing on an “edge” increases closing force per unit area Sloping surface allows boot + gravity to move the dirt – provides a place for dirt to go. Departure from Harvard flat-on-flat design: wear may be an issue

Room temperature tests with Teflon seat and an edge: Ball was difficult to polish while retaining roundness. High seating force required (>50 lbs on 3/8” diameter seat) All suffered from “porous” Teflon (need better quality?)


Return to Harder Materials Use Vespel seat and boot – hard plastic

Try cone-on-chamfer (cork-in-bottle) geometry Cones are more easily fabricated (lathe) Self-centering (if stem can comply) Wedge shape multiplies force (10° 6)

Reproducible over a few (<10) cycles D=3/8”

Sealed (leak det. limit) at 10 lbs at room temp. 30 lbs required at LN2, LHe

And at LHe II with p 500 Torr

D


What’s Next Complete tests of large diameter Vespel valve.

D=1” Already found to seal at 30 lbs at room temp. Low temperature tests – to be done. Closing force versus number of cycles – to be measured.

Hard-on-hard valve with more closely matched conical seat.

Need to understand constraints on materials from polarized 3He relaxation studies.

Design and build complete valve test apparatus.

100 Cycle Test (0-50 lbs seating force)

edm collaboration meeting, february 14-15, 2007 valve bucket-dewar tests steve williamson uiuc

Documents

leak area atoms

leak time constant

iimax v3 leak rate

mmax v6 leak rate

collection isolation

s storage time77 kp

s77 kp

purifier isolation v6