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The chamber gas is mixed 63
-37
(by weight) Ar ethane. The gas is mixed
in the Hall A gas shed which is located next to the entrance to the
Hall A truck ramp. One needs key #8, which is located in a key box
in the Hall A counting house, to get inside the shed
where the gas mixing is done.
The argon and ethane
bottles which feed the gas mixing system are located outside the shed
and can be exchanged when they are empty. The mixed gas is sent down into
Hall A and to each of the detector huts. There are two each of argon and
ethane bottles connected to the gas system and a Matheson 8590 controller
switches between the two bottles when the gas pressure in the bottle
drops below a certain level. At this point the one bottle can be replaced
while the other is being used. The procedure for changing gas bottles
is outlined below:
- Warning: High pressure gas bottles contain significant
stored energy and are potentially hazardous. Handling of gas bottles
should be done only by qualified, trained personnel.
- For smoothest operation, used gas bottles should be
replaced before their internal pressure drops below the desired
regulator output pressure.
- Two possible cases exist in which a gas bottle needs
to be replaced: only one empty gas bottle on a system or
both bottles empty on a gas system.
- For case 1 the sequence of steps is as follows:
- Check in the Hall A Gas Shed. If all bottles have
sufficient pressure each of the Matheson 8590 controllers
will have one green "RUN" LED lit and one yellow "READY" LED lit.
A red "EMPTY" LED lit indicates a bottle with low pressure, the
corresponding bottle needs to be replaced. If a red "EMPTY" LED is lit
the central "ALARM" LED should also show red. Nothing further needs
to be done here; go outside to the Gas Bottle Pad.
- Visually verify that the corresponding pressure gauge on
the flex line is showing a low pressure. A low pressure is not
necessarily zero. Close the bottle valve for the empty bottle.
- Disconnect the empty bottle from the high-pressure flex-line.
The in-line check-valves will prevent gas escaping from the manifold.
Replace the bottle's cap, and move the empty bottle to the EMPTIES
storage rack. Note that ethane bottle fittings, type CGA-350, have
left-handed threads.
- Place a full bottle of gas in the on-line rack, remove the
bottle cap, and connect the bottle to the flex-line.
- Open the new bottle's valve, check for leaks at the bottle
fitting. The corresponding pressure gauge should now read full bottle pressure.
- The ALARM state of the Matheson 8590 controller should have
automatically reset. Check inside the Hall A Gas Shed. Each controller
should show a green "RUN" and yellow "READY" LED lit. If not,
re-check the installation of the gas bottle.
- For case 2 the sequence of steps is as follows:
- Check in the Hall A Gas Shed. If all bottles have sufficient
pressure each of the Matheson 8590 controllers will
have one green "RUN" LED lit and one yellow "READY" LED lit.
If a Matheson 8590 controller shows two red "EMPTY" LEDs lit and
the central red "ALARM" LED lit, both bottles of the corresponding
manifold need to be replaced. Nothing further
needs to be done here, go outside to the Gas Bottle Pad.
- Follow steps 2. through 5., as detailed immediately above, for
both bottles.
- The ALARM state of the Matheson 8590 controller should have
automatically reset. Check inside the Hall A Gas Shed. Each
controller should show two yellow "READY" LEDs lit. If not, re-check
the installation of the gas bottle. Press either of the two buttons
labeled "LEFT BANK" and "RIGHT BANK". The lit LED above the button
you pressed will change from yellow "READY" to green "RUN". You will
most likely need to reset the Low Supply Pressure shutdown at this point.
The four FPP straw chambers are connected in parallel to the gas system.
(see Figure 5.22).
Figure 5.22:
Drawing of the gas panel on the hadron detector stack.
![\includegraphics[width=13cm,,clip]{/data8/user/new_safety/fpp/gaspanel.eps}](img207.png)
1.
|
(The FPP chambers are also in parallel with the VDC chambers.)
All gas connections
are made using POLYFLO
tubing and TJNAF-specified connectors.
The chamber volumes range from approximately
120 to 220
.
Gas pressure in the chambers is typically a few Torr above atmospheric
pressure.
The gas flow through the chambers may be independently varied and is
typically set to 7
/hr, leading to a replacement of the chamber
volumes about every 15 - 30 hours.
Gas is exhausted from the FPP chambers through a bubbler containing
1 mm
of mineral oil. A typical chamber leakage rate at this flow rate is 25 - 50
.
The flow rate of 7
/hr when combined with the leak rate of
3
/hr results in a complete exchange of gas in the chambers
roughly every 1 - 2 days.
At this level of consumption, a full gas bottle connected to the FPP system
lasts approximately 10 days.
When a bottle is nearing empty (
90
), it should be changed since
there may be heavy contaminants in the gas.
Gas bottles may only be changed by authorized personnel.
Gas-handling Procedures
- Typically gas is continually flowing though the chambers.
If at all possible, gas flow should be continuously
maintained, even in no-beam time periods. This avoids time loss to
reconditioning and maintains the desirable steady-state operating
condition. If the chambers are not being used in an experiment,
the flowmeters for the front chambers
are set to 20 and the flowmeters for the rear chambers are set to 60.
When the chambers are used in an experiment the standard setting
for the front chambers is 40 and for the rear chambers it is 105.
- Gas pressure at the gas panel on the
detector stack should be in the range
13 - 15 psi. With the large leakage rate of the
FPP chambers, we typically run at near the limit of the capacity of the
gas mixer to supply the gas flow demanded by the FPP and VDC chambers.
Therefore it is possible to demand too much flow rate from the mixer.
If the gas pressure drops below 13 psi drop the flow to the FPP chambers
and contact Jack Segal or Howard Fenker to determine the cause and
remedy for the situation.
The status of the gas handling system should be monitored carefully
as well as logged at least once per 8-hour shift. Any substantial
deviation from the median parameters indicates a change in the
operational parameters of the FPP and should be immediately
investigated.
Next: Power Supplies and Electronics
Up: Operating Procedure
Previous: Operating Procedure
  Contents
Joe Mitchell
2000-02-29