5.5.3.4  Gas system

General description

 The gas Cerenkov counter is an apparatus which will be filled with gas during a long period. It is important to measure along the time if there are no leakage of gas. An easy way to detect gas leakage is to record the entrance and the exit volume in a gas circulation maintained with a constant flow. Figure illustrate this principle. A flexible vessel (symbolised by a cylinder in the figure) is connected to the gas exit of the Cerenkov box. This box is able to store up to 200 l of gas and will restitute it to the Cerenkov box, if needed, due to a rapid change of the ambiant temperature or atmospheric pressure. The gas flow recorder is a device which garantee a precision within 10^-2 but below a low flux such as 15 l/h this precision goes down.
The minimum flux for the gas circulation of the Cerenkov counter is 15 l/h.

  

Figure: Shematic description of the gas flow.

The counter is operated at atmospheric pressure with a very small over-pressure due to the gas flow maintained to detect gas leackage. The over pressure inside the Cerenkov box is limited by the maximum value of his volume which assure that the windows don't touch other plane of detectors.
The value of the maximum over-pressure in the Cerenkov is 5.5 mbar.
In the paragraph detailed informations are given about the variation of the volume with pressure.
A safety valve placed on the top of the Cerenkov box, assure that the inside pressure don't go up to 6 mb.

Gas rack description

 Our gas system is divided in two parts;

-

the upper part which is outside the Hall and stay near the gas bottles (near the gas shed),

-

two gas racks associated to each Cerenkov counter.

The upper gas rack is the place where gas is extracted from bottles and is expanded from the bottle pressure (52 bar at T=15C, 74 bar at 31 C = critical point) to a pressure between 9 and 10 bars. The distribution to electron and hadron arm is made at this place with a Tee valve so that you can open or close the gas line you want to select. The shematic view of this part is showned in figure . The numerotation of following items is reported on the same figure.

1.

There are two bottles of CO2 gas which provide a high capacity CO2 storage. This correspond to a volume of . With the flow of 20l/h this will give an autonomy of 42 days.

2.

A system for automatic bottle switching with also expansion valve which decrease the bottle pressure to a pressure of 10 bar.

3.

A stop valve

4.

A Tee valve for the distribution in the two gas lines, electron and hadron arm.

  

Figure: The gas rack for the gas distribution in the electron and/or the hadron Cerenkov counters.

The gas rack which stay near the Cerenkov counter include the gas flow monitor and control the inside pressure of the box. The place of this rack is just near the last stairs which go on the platform of the detector house. The following list describe the elements of this rack wich is shematised on figure .

1.

An expansion valve from bar to 0.5 bar. The gas flow recorders are devices which are working at this pressure.

2.

A gas flow selector. We can have two different flow inside the Cerenkov, a high flow for gas filling and a low flow during physics experiments.

3.

A pair of gas flow recorders, recording both entrance and exit gas flow. This provide a high sensitivity leak diagnostic within a few 10^-3.

4.

a flexible vessel connected to the gas exit, able to store up to 200 l of gas and to restitute it if needed due to a rapid change of the ambiant temperature or atmospheric pressure.

  

Figure: The gas rack for gas flow tunning and the control of the inside pressure in the Cerenkov counter.

n the final position of the gas setup, final pipes and connections with the Cerenkov Box, we have no gas leakage. The total volume of gas which came in, recorded with the entrance gas meter, was within a few percent equal to the volume recorded at the exit.

Filling Cerenkov counters with gas

We decide to fill the volume five times in order to consider that is well full of CO2 gas. With the highest flow of 150 l/h we need

-

100 hours to fill one time the electron Cerenkov (V=2780 + 200 l)

-

66 hours to fill one time the hadron Cerenkov (V=1849 + 200 l)

With this high flow we need to have a heater attached to the system of switching bottles to avoid condensation on pipes. During experiment low flow is running ( 30/h) and you can swich off the heater. There is one heater per bottle and they are fixed on the upper gas rack which is outside near the gas shed.

Window deformation with pressure

 Gas Cerenkov counters are working at the atmospheric pressure with a small over-pressure (few mbar). The windows are very thin (see paragraphe ) and can be deformed with a small variation of the pressure. We have study the deformation of the windows as a function of the small over-pressure in the Cerenkov box. The volume difference wich correspond to this deformation is given by the formula:

where

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H=2457mm is the high of the window

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lⁱⁿ=605 mm is the width of the entrance window

-

l^out=838 mm is the width of the exit window

-

fⁱⁿ and f^out are the ``arrows'' which represent the window deformation (see figure ).

  

Figure: The volume deformation, due to the inside pressure, was measured through the measurement of the``arrows'' of each entrance

  

Figure: Volume variation for the hadron Cerenkov counter in function of the pressure.

To measure these ``arrows'', different over-pressures inside the Cerenkov counters was applied, from 1 to 6 mbar. This was maintained in the box without a gas flow by closing the output valve. For each over-pressure, window's deformation was measured (``arrows'' in figure ) and this operation was repeated several times in order to obtain asymptotic value of the deformation. For over-pressures going from 1 to 6 mbar, the minimum and maximum values of the ``arrows'' , in millimeters, are:
e^- Cerenkov:

hadron Cerenkov:

The figure represent the volume variation for the hadron Cerenkov counter in function of the pressure.
The windows bend is within tolerancies for the maximum flow.
We have made a test in order to know the maximum pressure that can support the windows. The inside pressure on the Cerenkov box was increased until the windows was torn. We have maintained this pressure without gas flow by closing the output valve on the top of the Cerenkov box. The value of this over-pressure is .

Empty bottle signal

There are two bottles of CO2 gas which provide a high capacity CO2 storage. This correspond to a volume of . With the flow of 20l/h this will give an autonomy of 42 days.

Two signals, one for each bottle, are going from the upper gas rack (see paragraph ) to the counting house. When a bottle is empty, a signal can be seen from the user in the counting room. An automatic swiching bottle is installed in the upper gas rack, so that you are not obliged to move immediatly to change the bottle. There are only two things to do:

-

to change the ``arm'' direction by hand on the upper-gas rack. The pressure gauge under this device will go from 8 to 10 bar.

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command a new bottle for the next time (the autonomy is at least 40 days during experiment when low flow is running).

Check list before experiment

This list is a summary of all procedures for gas filling and control which has to be done before an experiment.

Upper gas rack(see figure ):
If the gas is already in circulation you don't have to do the points 1 to 6. There are necessary only if you need to install a new bottle. Points 7 to 12 are the only things that you have to check if gas is already installed.

1.

close bottle and close orange button

2.

open the purge valve (black button on the side)

3.

open slightly the bottle. You should hear the gas outgoing from the purge valve.

4.

close the purge

5.

open completly the bottle

6.

open the orange valve

7.

At this time the two bottles are open, the two orange valves also and the two purge are closed.

8.

the swich valve (``piece of metal with a black button at the extremity) in high or low position indicate which bottle we are using

9.

when a bottle is empty, the device (CENTRALE LF50) will automaticaly swich to the other bottle. In the pressure gauge instead of a pressure of 10 bars you will read a lower pressure of 8 bars. To recover the pressure of 10 bars at this point we have to change the direction of the swich valve by hand to the direction of the bottle which is now in operation.

10.

open the right valve provide gas to the rack number 1 for electron arm

11.

or open the left valve provide gas to the rack number 2 for hadron arm

12.

or both

13.

be sure that the valve of the Cerenkov counter you don't use is closed.

gas rack number 1 or 2(see figure ):
The pressure gauge at the buttom of the rack indicate the over-pressure ( in mbar) which is inside the Cerenkov counter. This value should always be below 5.5 mbar during all operations (see paragraph ).

1.

tune the expansion valve at a pressure of maximum 0.5 bar

2.

before manipulating the three channels valve (low flow on the left, closed for position up, and high flow on the right) be sure that the two valves for the gas flux tunning are closed

3.

for high flux, the maximum gas flow is 150 l/h which correspond to a pressure gauge of 5.4 mbar and a pressure inside the Cerenkov of (3.1 mbar (there is pressure lost in the 50 meters of pipes)

4.

for low flux, the minimum is around 25 l/h, because the gas flow recorders become less precise below 15l/h, and the pressure inside the Cerenkov counter is 1mbar

Each time you can, when you go into the Hall A to make something in the detector house, take the numbers from the two gas meter (in and out) of the rack n⁰1 for the electron and n⁰2 for the hadron and report these numbers in the experiment book. The table is an example of what will be useful to control that there is no gas leakage.

 

date V_in V_out gas flow l/h l/h l/h mbar

9/18/96 11h10 a.m 52531 37407 30 1

9/18/96 12h25 a.m 52571 37447 30 40 40 1

9/18/96 4h20 p.m 52694 37571 30 123 124 1