5.5.7.6  Gas Delivery into Hall A

Between the gas shed and the two Hall-A shield houses are several gas line runs of about 700 feet in length. These are shown schematically in Figure . Three gasses (inert, VDC, and FPP) are supplied to the Hadron Arm through 1/2-inch OD polyethylene tubing. Two similar tubes are teed into these near the beamline entrance to Hall-A and they supply VDC and inert gas to the Electron Arm shield house. The pressures in all of these lines is nominally 15 psig.

Distribution in the Shield Houses

Inside each shield house there is a gas distribution panel which controls the gas flows to the individual wire chambers in that detector stack. Figure  shows a diagram of the shield house gas systems.

Each gas supply is first filtered and fed to a visual pressure gauge (PG-401-A/B and PG- 501-A/B/C) so that the supply pressure can be locally verified. Inert gas (for purging detectors) and operating gas (either VDC or FPP gas) is manifolded to a series of three-way valves Ñ one for each detector flow circuit. These valves are labeled MV-411, -412 in the Electron Arm, and MV-511 Ð MV-516 in the Hadron Arm.

The three-way valve associated with each detector may be used to select either operating or purge gas independently of the other detectors. The selected gas is supplied to the inlet of a needle-valve / rotameter combination (labels MV-42x and MV-52x) which is to be used to set and observe the gas flow to each detector. The rotameters are sized for reasonably accurate metering of 5 slph Argon-Ethane and purging at about ten times this rate. (Note that the gas mixer will supply a total of only 60 slph Argon-Ethane, limited by the capacity of the Ethane mass flow controller).

On its way from the rotameter to the detector the gas passes by an overpressure relief bubbler which is basically a manometer filled with mineral oil. The overpressure bubblers are set to release at a pressure greater than about 30 mm of water (33 mm mineral oil). This pressure is sufficient to allow purging at the desired rate. Gas returning from the detector passes through an electronic mass flow meter and through a low pressure oil bubbler. This bubbler prevents backstreaming of waste gas into the detectors. The flow meter reading is indicated locally on a LCD display and is available as an analog signal for connection to the slow controls computer. Note that these digital flowmeters are factory- calibrated for Nitrogen. To correct the readings for Argon multiply by 1.45; for Ethane multiply by 0.5; for CO₂ multiply by 0.74.

Waste Gas Collection and Venting

Gas coming from the chamber exhaust bubblers is collected in a manifold and routed back to the gas shed through a large (1-inch OD) polyethylene tube. There are separate manifolds and exhaust lines for the VDC and FPP systems. Back-pressure in the exhaust manifolds is monitored by PHOTOHELIC(R) pressure switches. If more than about 1-inch H₂O backpressure develops in an exhaust manifold the gas supply interlock system is tripped, turning off the gas supply at the solenoid valves outside the gas shed. The purpose of this particular interlock is to protect the detector windows from overpressure.

  

Figure: Gas Distribution inside Hall-A