5.5.7.2  Overview

The detector systems in both HRS's (FPP and VDC) are expected to use a mixture of Argon and Ethane in roughly equal proportions, plus about 1% ethanol. The Argon and Ethane are supplied from high-pressure gas bottles. They are combined in the desired proportion by a mixing system and this mixture is passed through a bath of isopropyl alcohol which is maintained at a fixed temperature. See Figure  for a schematic diagram of the gas supply / mixing system.

The gas mixture is delivered to gas distribution racks in the Hadron Spectrometer and the Electron Spectrometer. The transmission lines and the distribution plumbing have been designed as if the FPPs and VDCs were actually independent systems using different gas mixtures. This design was chosen in order to ease the expected transition to such a system in the future. Also supplied is a source of purge gas, currently pure argon. The distribution racks provide, for each detector, selection of either operating gas or purge gas, flow control and metering, overpressure relief to protect the detector components, exhaust flow measurement, and backflow prevention.

Bulk Gas Supply

The bulk gas supply consists of two bottles each of Argon, Ethane, and Carbon-Dioxide. Except for fittings which vary by type of gas the three supplies have identical plumbing. One bottle of each gas will be on-line during system operation while the second bottle serves as a ready reserve (connected to the manifold, but valved off). The two bottles are connected through check valves and manual valves to a high-pressure manifold. The pressure in this manifold is sensed by a pressure transducer whose signal is available to the slow-controls computer for monitoring. The pressure is also indicated locally by a mechanical pressure gauge attached to a two-stage regulator. A pressure regulator for each type of gas reduces the pressure to approximately 45 psig. This is the pressure at which gas is supplied to the gas shed. It may be monitored by the outlet pressure gauges (PG-021, -022, -023) on the pressure regulators and, inside the gas shed, on gauges PG-131, -132, -133. Prior to entering the shed the gas passes through manual valves (MV-031, -032, -033), Excess Flow Valves (XF-041, -042, -043), and Solenoid Valves (AV-051, -052, -053).

The Excess Flow Valves automatically close if the flow rate exceeds about 4 slpm at 45 psig. These valves must be manually reset after they trip. Refer to the section Resetting a closed Excess Flow Valve for this procedure The solenoid valves are electrically operated (24 VDC) normally-closed valves. Power must be supplied to the solenoids in order for gas to flow. Valve power is supplied, when interlock conditions are satisfied, by the Gas Interlock System. Note that one of the required interlock conditions is that there be ample gas pressure downstream of the solenoid valves. System operators must use the manual "Low Pressure Override" pushbutton on the interlock panel (in the mixing room) in order to initially bring up the 45 psig supply pressure. The pushbutton circuit automatically re-arms after ample pressure is detected by the pressure switches (PS-111, -112, -113). These switches are located immediately above gauges PG-131, -132, and -133. Just below these gauges are overpressure relief valves (RV-121, -122, -123) which have been set to release if the supply pressure exceeds about 60 psig. After passing through check valves which prevent backstreaming the three gas supplies enter the mixing system.

  

Figure: Gas Shed Schematic Diagram