The primary purpose of the MuID Gas System (see gas diagram below) is to provide CO2+9%iC4H10 or 25%iC4H10 pure mixture to the MuIDs (North and South arms) at the correct pressure. Also, it will supply pure Nitrogen using the separate supply line for keeping the in-panel preamp dry condition and dilute the possible working gas leak into the environment. Gas system parameters are as follows:
|Detector volume||50 000 litres|
CO2 + (9 ± 0.1%) Isobutane
|Compressor pressure||50 – 150 mbar|
|Internal detector pressure||20 – 25 mbar|
|Recirculation flow||300 – 1800 l/h|
|Flow through detectors||2 400 – 4 000 l/h|
|Purge flow||7 500 l/h|
|Nitrogen purge flow||3 000 l/h|
|Fresh mixture||1- 20 l/min|
|Oxygen content||< 250 ppm|
|Water content||< 50 ppm|
The system operates nominally as a open circuit gas system with the small amount mixture recirculating through the MuID. During normal operation a fresh mixture is added and an equivalent quantity of the existing mixture is vented. The mixture circulation rate through the small membrane compressor is 15LPM at 5PSIG. The gas system contains two compressors(SC1,SC2), one active and one spare. The mixture from the compressor goes to the supply line through the check valves CV6,CV6a and flow indicator FI8, if the purifier and dryer are not used. A bypass valve (MV5) is manually adjusted to support stable flow at the correct output compressor pressure.
Two flow indicators (FI11 and FI12) will measure the recirculating flows: North and South MuID arms. The purity and composition of recirculating mixture is monitored using oxygen, carbon dioxide and humidity analyzers. All recirculating mixture can be passed through a purifier and dryer to remove moisture and contaminants as needed.
A computer driven data acquisition/control system monitors all of the process variables. The computer system flags quantities which fall outside of predefined limits and initiates corrective action. However, where the safety of equipment or personnel are affected, a relay based system connected to redundant set of sensors control the pressure levels of all key based controls fail.
A rapid change in atmospheric pressure is typical preceding storms and hurricanes for BNL area. To assure that the MuIDs follow a fast rise in atmospheric pressure, a relatively large flow of inert gas will admitted into the chambers in the event that normal pressure controls fail to keep up with "falling" internal pressure. The vent lines, associated valves and bubblers are sized to allow for rapid venting of the MuIDs mixture to prevent a high internal pressure in the case of the fast barometric pressure fall.
There are two sources of pressure in the system, the first is the compressors located at the exit of the MuIDs. The second is the flow of fresh mixture through the mixing manifold. Nominally the MuIDs differential pressure is controlled by maintaining a constant flow upstream of the MuIDs via the mass flow meters (FM1, FM2). On a longer time scale the flow of fresh mixture is constant. Basically, the inside MuIDs pressure will be determined with a pressure resistance of output manifolds and flow indicators and will not exceed 2" WC at the purging mode for each arm. At the normal operation the differential pressure will not exceed 1"WC. Maximum differential pressure may be set with the oil level in the bubblers connected to the supply lines and to the vent line. In the case if the MuIDs supply pressure will exceed the set point level PC control system will stop flow through FM1 and FM2 by closing SV2,SV3. The same action will be done by the pressure indicating switch PI11 with two set points , if the PC control system fails. Also, the negative pressure in the MuIDs will be prevented with the PC control system or PI11 by opening the solenoid valve SV1.
Pressure transmitters PT5, PT6 monitor the pressure drop in the each MuIDs arm and mixture flows to the vent line. Theirs flow readings may be used to estimate MuIDs leak. Downstream of the mixer the pressure is controlled with the back pressure regulator BPCV1 and supported at 25mBar to supply the mixture to the analyzers and through the thermostat containing the isopropyl alcohol.
A fine 1 micron filter F3 is installed in the main supply line. Its plugging is checked with the pressure transmitter PT2. Two small membrane compressors are used to recirculate the mixture through the MuIDs.The output from the compressors SC1, SC2 will be at the maximum pressure of 150mBar, if the purifier/dryer are turned off. In the case when the purifier/dryer are in the operation the output compressors pressure may be 130-140mBar. It does not influence strongly on the MuIDs inside pressure. The flow through the compressors and the pressure before them are adjusted by means of the flow indicators FI9, FI10 and manual bypass valve MV5. They will be very stable for a long term operation.
The quantity of fresh mixture may be changeable in the range of 1.-35l/min with the mass flow controllers (FM1 and FM2). To purge the MuIDs the quantity of fresh mixture can be increased up to 250 l/min by using the flow indicators (FI1 and FI2). To have the stable content of fresh mixture the CO2 mass flow controller (FM1) operates the i-Butane one (FM2). It means that FM1 is a master and FM2 is a slave. These units are normally PC controlled. The quantity of fresh mixture are monitored with PC data acquisition/control system.
The Nitrogen supply line are supported at 20 mBar pressure with the pressure regulator PCV1 and monitored by PC data acquisition/control system using the pressure transmitter PT7. In the case of the supply line over pressure PC control system or pressure indicating switch will stop Nitrogen flow by closing the solenoid valve SV4.
Two flammable gas detectors are installed on the output manifolds to check LEL content of i-Butane in Nitrogen flows.
In the event of a power failure, the solenoid valves SV1 and SV2 will open, or remain open and SV3 will close, causing 50 l/min of inert gas to flow through the MuIDs. This flow rate is adequate to assure that fluctuations in the atmospheric pressure will not result in the creation of over or negative pressure inside the MuIDs. Also, SV4 will stay opened and the Nitrogen will purge the volumes. The computer data acquisition /control system will measure the atmospheric pressure with a barometer (PT-B) to have the absolute pressure data.
Three temperature transmitters (TT3, TT4 and TT5) are used to measure the MuIDs mixture temperature. The data of measured mixture temperature are logged for later use in data reduction.
Along with automated valve control, the gas system’s dedicated computer controlled data acquisition provides constant monitoring of the mixture composition via measuring the mass controllers output signals. CO2 analyzer will be used periodically to check CO2 content in the mixture. The mixture ratio is fixed by the Teledyne mass flow controllers (FM1, FM2) with the inert gas "slaved" to the i-Butane flow controller. The stability of the flow controllers is sufficient to make variation in the mixture content negligible. Mashed mixer is used to prepare the stable mixture content.
The gas system is equipped with Oxygen, Moisture and CO2 analyzers plumbed such that each section of the gas system can be selected separately for evaluation. All analyzers data are read and archived by the computer data acquisition system and used to control the gas system. Using the reference mixture gives a possibility to check the CO2 analyzer condition.
A mixture dryer and purifier withdraws about 15 l/min of the reciculating flow. They are used only as needed. The dryer is made from the a stainless steel tube containing 3 lbs of molecular sieve(zeolite 13X) as adsorbent. This amount permits the removal of about 1 lbs of water vapor to a level 5 ppm at room temperature. Filters are installed upstream and downstream of the adsorbent to prevent particles from entering to the mixture stream. A heating element is placed inside the dryer. As the thermal insulation a fiber glass insulation is used.. The dryer is regenerated by heating to 350-400 C with purging of CO2. The purge gas enters at the top of the dryer and exits at the bottom carrying with it the water vapor. A temperature transmitter installed inside the dryer is connected to the temperature controller (TIC2) that supports the dryer regeneration temperature on the set-pointed level. A moisture analyzer is used to measure the quantity of the water in the circuit before and after the dryer to determine when the adsorbent is saturated.
The purifier is similar to the dryer except that it is filled with a catalyzer that permits the oxidization of i-Butane by Oxygen, present as an impurity, to form alcohol. The alcohol is subsequently removed by the dryer. The catalyzed oxidization process takes place at 220 C that is supported with the temperature controller (TIC1). This purifier does not require regeneration but must work in conjunction with the dryer. Solenoid valves (SV9 and SV10) installed at the inlet and outlet of the purification loop isolate the unit from the main circuit when it does not in use. If the inside pressure of purifier/dryer will be more than 10 PSI the check valve CV7a works as the safety valve and prevents the purifier/dryer from a damage. A 10 micron filter is installed after the purifier/dryer to prevent dust from passing into the main mixture supply line. A differential pressure transmitter (PT1) is used to check the filter’s plugging.
There are two ways to connect the dryer/purifier output to the main supply line. If the isopropyl alcohol is not used, the output flow is directed to the main supply line with manual valve MV1c. In the case of isopropyl alcohol presence in the mixture, the dryer/purifier is connected to the thermostat by MV1b to add the isopropyl alcohol, because the dryer/purifier flow does not have the one.
Computer Control and Data Acquisition
The gas system includes a computer driven data acquisition and control system. The controlling computer is a dedicated PC running Windows 2000. It reads the data and operates the gas system via SCXI data acquisition and control system. The solenoid valves, compressors, light and sound are controlled by PC through the SCXI and solid state relay board. This computerized system is programmed to acquire the signals from the various temperature, pressure, flow and content measuring devices and issue warnings and/or take corrective action in the event that predetermined levels are exceeded. All acquired values can be selected and viewed on the terminal. There is a possibility to look the gas system parameters using World Wide Web. The control software was developed using Borland Delphi and is almost identical with TEC/TRD, MuID and STAR TPC software.
A microprocessor driven interlock/alarm system is installed to warn of fault conditions and to take corrective action automatically if specified limits are exceeded. These actions include stopping the gas system compressor, flammable gas flow and shutting off the high voltage to the MuID detector. The interlock/alarm system is parallel and, in many cases, redundant to the computer control system but it directly reads the gas system parameters and takes the action immediately if specified limits are exceeded the set-pointed level.