4.4   Cerenkov Gas System

This gas system is designed to purge the six Cerenkov detectors, recirculate and filter the radiator gas, and recover the gas for storage. The gases used are Nitrogen, as a purge gas and Perfluorbutane (C4F10), as the radiator gas. Liquid Nitrogen is used as the source of the purge gas and is also used as a coolant for the system's distillation unit. C4F10 is a very dense, non-flammable, non-toxic, colorless gas with no discernible odor. It's commercial use is as a fire extinguishing agent. C4F10 is easily liquefied and is delivered as a liquid in 25 psig tanks. This system is split between the Hall B gas shed (96B) and the in-hall equipment mounted on the forward carriage. The system features two supply lines and two return lines. The two supply lines deliver Nitrogen and C4F10 to the hall. A three-way valve upstream of the flow control valve selects which gas is fed to the detector. The outlet from the each detector is fed to a three-way valve which selects whether the effluent from a detector is directed to the recirculation loop or to the distillation system. Flow rate into each detector is controlled by a mass flow control valve downstream of the three-way supply valve. The mass flow control valves are controlled by a MKS 647B controller in the gas shed control room. Pumps, in the gas shed draw the gas from the hall. The system is protected from over and under pressure by three systems. The primary system, two electronic pressure controlled proportional valves downstream of each buffer tank, act as pump throttle valves for the recirculation and distillation lines. The second protection is a set of pressure switches that measure the pressure in both buffer tanks. The position of these pressure switches is shown by indicator lights on the door to the by-pass switch box on the second level of the forward carriage. An overpressure in either tank will shut-off the C4F10 supply in the hall. An underpressure in either buffer tank will close the solenoid valve in the return line from that tank to the gas shed. The third protection is over and under pressure bubblers mounted on the second level of the forward carriage.

4.4.1   System Operation

In normal operation the detectors are brought on-line one at a time. The detectors are purged of air by flowing nitrogen through the detectors. The nitrogen is vented through vent valves in the hall. Caution: The overpressure pressure switches do not shut-off the nitrogen supply. The nitrogen purge is intended to be used only when the system is manned. After the detector is adequately purged, the flow into the mass flow control valve is switched to the C4F10 supply and the return should be directed to the distillation unit. The distillation pump must be turned on before switching the detector to the distillation return line. The C4F10 will be trapped in the distillation unit, the Nitrogen will pass through it. When the detector is filled with C4F10 the return line should be switched to the recirculation loop. The recirculation pumps must be turned on before switching the detector to the recirculation line.

4.4.2   Safety Considerations

Oxygen Deficiency Hazard and Cryogens Liquid Nitrogen is used as the source of purge gas and is also used as a coolant for the system's distillation unit. Both of these are supplied from 160 liter dewars that are mounted outside the gas shed. A small amount of nitrogen and liquid Nitrogen passes through gas shed. The gas shed features an active ventilation system. Refer to the Jefferson Lab Safety Manual for information on handling cryogens. The detectors themselves represent a large volume of gas. C4F10 is approximately 10 times the density of air. In the event of a catastrophic failure of all six detector windows simultaneously, an ODH hazard would be created to a depth of 2 inches on the floor of the hall. Special attention would needed for low points within the hall such as the Photon Tagger Beam Dump in the event of such a failure. Refer to the Jefferson Lab Safety Manual for information on Oxygen Deficiency Hazard.

Window Failure The windows of these detectors, front and rear, have a large area and are very thin. They have very little pressure differential tolerance. The high density of this gas creates a significant head pressure within the detectors themselves. In addition, while the pressure safety equipment is as close as possible to the detectors, due to space restrictions, it is not connected directly to the detectors. There is a pressure drop, albeit small, between the detectors and the point where the pressure is measured. This pressure differential increases with flow rate. The large volume of these detectors makes responses to changes in operating conditions very slow. One should not increase flow rates to the detectors unless the detectors can be monitored for several hours afterward to insure a safe equilibrium has been reached at the new flow rate. There are several possible indications that the detectors are in an over or under pressure condition. An overpressure or an underpressure would indicate a failure in the pressure control systems or too high of a flow rate into the detectors. The possible indications of an out of range pressure are; 1. The pressure readouts on the buffer tank pressure controllers indicating too high. 2. The displacement of the liquid in the bubblers is too high. (The bubblers can be monitored from a camera mounted on the forward carriage.) 3. The detector windows are either bowed out more than normal or in at all. 4. Any pressure switch indicator lights on the forward carriage is off.

Over or Under Pressure Conditions. 1. Check the pressure indicator lights. An overpressure in either buffer tank will cut off the C4F10 gas supply. This can happen if the distillation loop is off-line and the pressure rises in that tank. If this occurs, the buffer tank overpressure light will be off and all actual flow rates as read at the 647B mass flow valve controller will read zero. To solve this, run the distillation pump for a few minutes to lower the pressure in the buffer tank. The pressure switch will reset and allow gas to flow. 2. Check that the flow rates set on the 647B in the control room are not too high for this system. 3. Check that the pumps are operating. 4. Check that the handles of the three-way valves (supply and return) are fully in the desired position. These valves do have a off position in the center.

Liquid in the System C4F10 is easily liquefied, so an effort must be made to insure it does not liquify where undesired. Of particular importance is the outside tank and the raceway connecting the gas shed to the hall. Both are insulated and heated. One must insure those heaters are operating to prevent the gas from liquefying. If the liquid reaches the mass flow control valves in the hall, unstable operation of those valves will result. There are also heaters upstream of the pumps to prevent liquid incursion into the pumps. The tanks that the C4F10 is delivered have both liquid and gas withdrawal ports. The ports are unmarked and they differ in type and position from tank to tank. The regulator must be connected to the gas side only. If the system is connected to the liquid side of the tank, liquid will enter the make up pump, which will cause immediate damage. The only way to insure that the system is connected to the gas side is to slightly open each port to find out if liquid or gas comes out.

Leak Checks This system should be periodically rechecked for leaks. Per the manufacturer of the C4F10, Freon leak detectors have a five times increase in sensitivity, relative to Freon, when used with C4F10. Several hand held leak detectors are available in the gas shed for this purpose.