Daya Bay Near Experiment Hall RPC Gas System
 Daya Bay Near Experiment Hall RPC Gas System Procedure for operation of the RPC gas system (1/31/2011) DocDB #6076‐V3 Changguo Lu, Princeton University 1 Procedure for operation of the RPC gas system
1. Purpose
The Daya Bay RPC gas system provides predefined gas mixture to the RPC
modules. Each experimental hall has its own RPC gas system. The gas system is designed
to be robust and well protected from fault operation. However if the operator does not
follow the operating instruction, it may interrupt the normal operation of entire RPC
system. Resuming the nominal gas mixture may take hours or even days. Although the
routine maintenance and system debug are the tasks for gas experts, the gas cylinder
replacement and the system daily check-up are supposed to be the routine tasks for the
trained personnel, the basic operation concept of the gas system is also important
information for the shifter in case of emergency they will know what action they need to
take immediately. This procedure provides the basic information of the gas system and
the daily operation of the RPC gas system. Shift taker should read through this procedure
carefully. Since the RPC gas mixture has flammable gas - isobutane in it, extra precaution
must be taken when you are operating the system.
2. Scope
This document covers the basic information about the RPC gas system.
3. Policy
Princeton University has provided the RPC gas system, is responsible for the
safely running this system, however the Daya Bay experiment needs a team to operate
and maintain the normal operation of this system. The procedure mentioned in this
document should be the routine tasks for this team.
4. References
The RPC gas system user’s manual (DocDB #5441
is the most comprehensive resource for those who want to know the details of the RPC
gas system. This document is just a short version of the user’s manual.
5. Definitions
6. Precautions and Limitations
This procedure intends to provide the basic information for the operation of the
RPC gas system. The execution of this procedure is the task of the Daya Bay RPC gas
team only, not for every shift taker. During the experiment if there is any warning
message appeared on the DCS screen, the shift taker can obtain detail description of the
warning from this document or the RPC gas system user’s manual. In emergency case
please be advised that contact gas expert (Changguo Lu, Tel. 1-609-258-1288, Jilei Xu,
Tel.13426305618) to get immediate advice before taking any action.
2 7. Parts and Equipment
8. Procedure
8.1 Introduction of the Daya Bay RPC gas system and its operation procedure
The gas system building blocks are shown in figure 1.
Fig. 1. Daya Bay RPC gas system building blocks.
8.2 Description and operation of the RPC gas system
Gas cylinders and cylinder switchover panel
Four gases are used for Daya Bay RPC gas mixture: Argon 65.5%, R134A 30%,
Isobutane 4% and SF6 0.5%. To eliminate the down time when the depleted gas cylinder is being
replaced, for each gas line a gas switchover panel is equipped. For the argon gas an automatic
gas switchover panel is used, it is shown in figure 2 and 3. For the other three gases the manual
switchover panel is employed, shown in figure 4.
Automatic switchover panel
Fig. 2. Argon cylinder automatic switchover panel. (a) Front; (b) Rear; (c) Side.
3 Operation procedure of this panel is as follows:
The illustration of various parts and the mechanical sketch of this switchover panel is
sketched in figure 3.
Fig. 3. Illustration of various parts and mechanical sketch of argon automatic switchover panel.
4 When the supply side pressure drops below 100 psig, the selector regulator is then
internally switched to the other side of the gas supply. The respective inlet pressure
gauges show two sides gas pressures, therefore the operator will know which side is
depleted, and can replace the bottles.
To change the depleted bottles please refer to the “Procedure for replacement of
the argon cylinder”, DocDB #xxxx-Vx. Manual switchover panel
Two types of manual switchover panel are used, one with the solenoid valve at
the outlet as shown in figure 4, is used for Isobutane gas; the other one w/o solenoid
valve is used for both R134a and SF6.
Vent2 Vent1 Vs1 Vs2
Vp1 Fig. 2. Manual gas switchover panel used for isobutane.
The isobutane gas switchover panel is installed inside the gas cabinet, its photo is
shown in figure 5.
5 To Gas mixing panel
To vent 6 5 Primary supply 1 2 3 4 Backup supply Fig. 3. Isobutane gas switchover panel mounted inside of the gas cabinet.
The solenoid valve controls the isobutane gas outlet. In emergency the gas control system
will automatically shut off the power of this valve, therefore no isobutane gas will flow
into the gas mixing panel. To replace the depleted Isobutane cylinder please refer to the
“Procedure for replacement of the isobutane cylinder”, DocDB #6071-V3.
The manual switchover panel for SF6/R134A is similar to Isobutane panel, only
difference is without the solenoid valve before the outlet port. To replace the depleted
SF6/R134A cylinder please refer to the “Procedure for replacement of the SF6 and
R134A cylinder”, DocDB #6072-V3.
8.2.2 Gas mixing and control system
The outlets of the switchover panels for four gases are connected to the inlets on the gas
mixing panel via ¼” Swagelok fittings. This system consists of gas mixing panel and four crates:
flowmeter crate, pressure crate, power supply crate and gas status crate. Gas mixing system Gas mixing panel
The picture of the gas mixing panel is shown in figure 6. Figure 7 shows the
schematic of the mixing panel.
6 Fig. 6. Gas mixing panel (Penn Fluid System, ASY-550).
7 To GC Reserved for venting mixing panel (normally capped) From gas cylinders Fig. 7. Schematic of the gas mixing panel.
Table 1. Gas flow controller's parameters.
for N2
Set the rear
Control Pot
at flow rate
This mixing panel can mix 4 gases, including 0.5% of SF6. There are two additional
outlet ports on the top-right corner in figure 7. One port is connected to a GC system to check the
8 gas mixing ratio once every two hours. The other port is reserved for venting the gas mixing
system in certain circumstance. Four mass flow controllers are controlled by MKS 247D, see
figure 8.
Fig.8. MKS 247D four-channel readout box, (Top) front panel, (Bottom) rear panel.
9 Detailed information on using this box can be found from “MKS Type 247D
Four-Channel Readout Instruction Manual”. Here we just give a brief description of this
control box.
The operation of MKS 247D is set in the ratio mode, the channel #1 (Isobutane) is
the master channel; the other three channels are slaves. On the front panel the Set Point
Source Switch is set at “Ratio” position for channel #2, 3, 4, and at “Flow” position for
channel #1. If you need to change the total flow rate and maintain the mixing ratio
unchanged, you only need to change the flow rate of the first channel by adjusting its
potentiometer “Set point” on front panel of 247D.
The Varian 430 GC system is used to verify the mixing ratio. Water bubbler
The water bubbler is used to add ~4000ppm water vapor into the gas mixture. The
gas mixture is split into two branches: one branch of the gas will be bubbling through
water. The water vapor saturated gas mixture is then mixed with the dry gas mixture from
the other branch. By adjusting the flow rate in two branches we can easily change the
water content in the gas. Figure 9 shows a picture of this system.
Fig. 9. Water bubbler.
The left tank is the water bubbler, gas mixture is flowing through a sintered
metallic bubbler head and entering the water, getting saturated water vapor in it before
flowing out the tank. The right tank is used as a water reservoir. There is a tube
connecting two tanks through a valve. There are marks on each tank to indicate the
required water level, the water level in the reservoir always higher than the level in the
water bubbler. Anytime if we need to add water into the bubbler, we only need to open
the valve and let the water flows from reservoir to bubbler. If the water ceased flowing,
lift the water reservoir will help the water flow. Be advised that use the distilled water to
fill the tanks only.
Gas control system
The gas control system includes four crates, they are: Gas Flowmeter crate; Gas
Pressure crate; Power crate and Gas System Status crate. Gas flowmeter crate
The gas flowmeter crate is used to display the gas flow rate from four mass flow
controllers and three flow meters. The later are for the water vapor control branches: total,
dry branch and water vapor added branch. A normal flow rate band is defined for each of
seven flow meters, if the flow rate is out of this preset band, the build-in relay of the Simpson
controller will be acting accordingly. This signal will be sent to gas pressure crate, which will
control the solenoid valve. The abnormal flow rate/pressure signal will shut off the solenoid
valve, which in turn will close all gas inlets to the mixing panel. In reality the gas flow
rate/pressure fluctuation due to power supply glitch and/or atmospheric pressure sudden
change is possible during certain season. To eliminate this type of interfere there is a
“BYPASS” timer push button, push this button can bypass the status check logic for 5
minutes (meanwhile a buzzer will be actuated to remind you that the bypass is acting), keep
the solenoid valves open for such time duration. Usually within this period the environmental
fluctuation would calm down and return to normal, therefore the gas system will continually
operate w/o interrupt. When you see all flow rates and pressures show normal value, the
Simpsons show no warning signs on the right side bar, you should push the RESET/START
button, and the “Flow Interlock OK” green light should be back on. This “BYPASS” button
is also very useful during the cylinder replacement operation. Push this button before the
cylinder replacement can temporarily disable the interlock mechanism, thus avoid the
unwanted system interlock that may be caused by the cylinder replacement.
There is a RESET/START button on the front panel. When the entire gas system is just
powered up, push and hold this button down until all seven flow rates displayed on the
Simpson meters reach normal value (no warning sign on any of the meters), then release the
button. By doing this the system will lock itself in a normal operation mode until the next
abnormal state occurs. After you push this button, the green light labeled as “Flow Interlock
OK” should be on. Also the water branches green light should be on, red light should be off.
The water branches are used for adding water vapor into the gas mixture. There are three
Simpson meters to display the flow rate for branch #1, #2 and the total. Branch #1 gas
mixture will be bubbling through water, then combine with the dry gas mixture from branch
#2. The metering valves on branch #1 and #2 can adjust the flow ratio between them, thus
adjust the water vapor content in the mixture.
Figure 10 shows the picture of the front and rear panels of the flowmeter crate.
11 A B
Fig. 10. Gas flowmeter crate, A. Front; B. Rear. Gas pressure crate
The gas pressure crate is used for displaying the gas pressure at the upper stream
of the flow controller for each gas. The ambient pressure will read as 0 psi. When any gas
shows lower than preset pressure limit, this crate will generate warning signal, shut off
the solenoid valve that will in turn shut off all gas inlet ports on the gas mixing panel.
The pressure can be adjusted by four gas regulators on the gas mixing panel (you
are NOT permitted to adjust the regulators!). All gas pressures are maintained around 20
psi, at higher pressure isobutane might be liquefied at lower temperature, the orifice of
mass flow controller for the Isobutane might be clogged to cause it malfunctioning. If it
happens call gas expert immediately and get guidance.
The front and rear panels for the pressure crate is shown in figure 11.
A B Fig. 11. Gas pressure crate, A. Front; B. Rear. Gas system power supply crate
The power supply crate provides the DC power to all flow meters, solenoid valves.
12 There are four lights on the front panel: +15V, -15V, +24V, (solenoid) Valves
open. In normal case all of them should be on.
Figure 12 shows the front and rear panels of the power supply crate.
A B Fig. 12. Gas power supply crate, A. Front; B. Rear. Gas status crate
Fig. 13 shows the gas status crate.
The front panel of the gas status crate is divided into two regions: the right region
displays RPC gas system status and the left region is fire safety system status.
Gas system status region shows: 1, Actual weights of isobutane, R134A and SF6
gas cylinders and their Weight Low Warning LEDs. Any of these three gas cylinders
hits its preset low weight limit, this warning red LED will light up to indicate it is the
time for replacing the depleted gas cylinder. 2, Relative humidity of the fresh gas
mixture (flowing into the RPC module) and the return gas mixture (flowing out the
RPC module). Two gas sub-branches are monitored. By rotating the Gas Humidity
switch the % of relative humidity for channel #1 or channel #2 will be displayed on
the Simpson controllers.
Fire safety system region displays % of LEL measured by HAD #1 and #2, gas
cabinet air ventilation velocity. HAD sensor #1 is installed at lower section inside the
gas cabinet, HAD sensor #2 is mounted outside the gas cabinet. They have been
calibrated by 0.9% of isobutane in air, which is 50% of isobutane LEL. When the
measured level reaches 10% LEL, the warning LED will turn on. If the level reaches
25% LEL, the solenoid valve that controls the opening of all four pneumatic valves in
front of each gas inlet will shut off, thus all gas inlet ports on the gas mixing panel will
shut off. The gas cabinet air ventilation is monitored by a Pitot tube and a Digihelic
differential pressure meter, which measures air velocity. We set the ventilation
warning limit at 0.005” water.
13 A B Fig. 13. Gas status crate, A. Front; B. Rear. Emergency shutoff system
There are two emergency response mechanisms constructed in the gas system:
hardware push button and software clickable button on the detector control gas system
monitoring page.
Hardware solution: There is an emergency shutoff switch installed on the outside wall
of the gas room as shown in Fig. 14. This switch is enclosed under a spring loaded Plexiglas
cover. In an emergency case people can lift the cover and push the button, it will shut off the
entire RPC gas system immediately. All gas inlets on the gas mixing panel will be closed, the
solenoid valve in the Isobutane gas cabinet will also close, therefore no more Isobutane gas
flows out the gas cabinet. At the same time RPC interlock will be activated, HV will be
automatically reduced to a safe value to protect RPC modules. The remaining gas mixture
upstream of RPC modules will continue flow into the modules until the gas pressure of the
mixture reaches the ambient atmospheric pressure. The RPC modules are still protected by
the output gas bubblers, which keeps the RPC modules’ gas pressure about 3cm W.C. higher
than the ambient pressure.
Software solution: On the slow control gas system monitoring page, there is a
clickable emergency button, when this button being clicked, a popup
window will ask for your confirmation for shutting down the gas system,
if it is conformed, then it will function just like what the hardware push
button does.
Fig. 14. Emergency shut off switch. To use the switch lift the cover and push the button.
14 8.2.3
Gas distribution/digital bubbler system
The Daya Bay near hall gas distribution/digital bubbler system consists of seven
panel-branches. A schematic of this system is shown in figure 15.
Fig. 15. Daya Bay near hall RPC gas distribution system (part).
The schematic diagram of one panel-branch of gas distribution/digital bubbler
system is shown in Fig. 16(A), its photo is in Fig. 16(B).
Fig. 16(A). One panel-branch of gas15 distribution/digital bubbler system
Fig. 16(B) Gas distribution (bottom) and digital bubbler (top) panels.
For a gas detector system the oil bubblers are always used at the outlets of the
chambers to isolate the gas chamber from air. Besides this basic function the digital gas
bubbler can provide additional quick on-line diagnosis of gas flow. In this system the
bubblers are instrumented with photogates. Its working principle is illustrated in figure 17.
Without gas bubble the light reaches the photogate through oil without interruption.
When a bubble passes, it will reflect partial light, and the light intensity at that moment
would be reduced, thus generates a pulse signal to the photogate PC board.
16 To digital bubbler readout board (A) (B)
(C) Figure 17. Digital gas bubbler. (A) Mechanical structure of the bubbler (for illustration
purpose only, not the same as used in our gas system); (B) Working principle of the
photogate; (C) The digital bubbler output signal recorded by microcontroller.
A host PC is supervising the microcontroller readout board through RS232 port. Figure
18 is a photo of this system.
Through flat cables connect to bubbler photogate PC boards Host PC and its USB port Digital bubbler readout crate USB extender with 100 ft CAT‐5E cable 7‐Port USB hub with 7 USB to RS232 converters 17 Fig. 18. Digital bubbler readout crate and its host PC with USB extender/100ft long CAT-5E cable. (In the
photo the flat cables are disconnected from the digital bubbler readout boards.)
The cable interconnection of the digital bubbler system is shown in figure 19.
Fig. 19 Interconnection of the digital bubbler system.
The host PC will be in the gas room, the digital bubbler hardware and readout
crate will be located on the RPC module supporting platform, the distance between two
locations is around 20m. Use a pair of USB extenders and a 100 ft CAT5E cable to link
them together. The interconnection of the digital bubbler system is shown in Fig. 19.
This section is evolving with the running experience; your feedback is most welcome.
All gas flow
meters show
zero flow
Possible cause
1, The solenoid valve doesn't open, all
pneumatic valves are closed.
2, Nitrogen gas cylinder does not have
enough gas pressure (< 5 atm ).
18 1, Check if all four gases’ pressures are
higher than low limits or their flow rates
are higher than preset lower flow limit
(check 247D).
2, Replace a full N2 cylinder.
Bypass button has been pushed
After 5' it will turn off automatically.
Bubbling rate
show all zero
COM port lost connection.
Valve Open
LED not on,
no any other
warning light
on, but there is
no gas flow in
all 4 channels.
Check with Detector Control System to
make sure they are not activating the
emergency shut off state and the wall
mounted fire emergency push button is
not on.
Check COM port communication:
Start|Control Panel|System|Hardware|
Device Manage, click Ports(COM &
LPT) to see if all required COM ports
are there and in correct Order. Sometimes
the electric interference, such as plug
a new device into the same power strip,
it may disturb the COM port communication.
Unplug the USB cable, then plug in,
it may solve the problem.
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