Here is A Link to _(MSDS)
(HAWGS uses ARGON, ETHANE, ETHANOL and MINERAL OIL)
The Interlock System Indicates a Fault... What should I do?
(HAWGS)
OPERATIONS MANUAL
Two detector systems in the Hall-A spectrometers make use of potentially flammable gasses: the Vertical Drift Chambers (VDC) and the Focal Plane Polarimeter (FPP) straw tubes. This document is intended to address the practical need for a users manual for this gas supply system as well as the formal requirement for a written operating procedure. The reader specifically interested in safety systems and compliance with safety regulations is referred to the sections Overview , Interlocks , Checklist , Interlock System , Calculation of Hazard Class , Refrigerator Safety, and Call List .
As of this writing both detector systems (FPP and VDC) are using 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 ethyl alcohol which is maintained at a fixed temperature. See Figure 2.3-1 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.
The bulk gas supply consists of two bottles each of Argon and Ethane. Except for fittings which vary by type of gas the two supplies have identical plumbing. One of the bottles for each gas is connected to the gas shed supply at all times by the Matheson model 8590 controllers inside the Hall A Gas Shed. The pressure in each bottle is sensed by a pressure switch whose signal is used by the Matheson model 8590 controller to change from an empty cylinder to a full cylinder. The pressure of each bottle is also indicated locally by a mechanical pressure gauge on each flex hose.
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-022, -023) on the pressure regulators and, inside the gas shed, on gauges PG-132, -133. Prior to entering the shed the gas passes through manual valves (MV-032, -033), Excess Flow Valves (XF-042, -043), and Solenoid Valves (AV-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-112, -113). These switches are located immediately above gauges PG-132 and -133.
Just below these gauges are overpressure relief valves (RV -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.
The gas mixing system works by metering three gas supplies into a common mixing tank. The mixture is then bubbled through alcohol in a tank within a small refrigerator which has been modified for safe operation in a flammable gas system. Because the gas flow, not the pressure, is regulated by the metering system, pressure switches have been installed to monitor the mixer outlet pressure and provide feedback to the flow-control system. The mixing, bubbling, and pressure control systems are all built into the same relay rack. They are collectively referred to as the Gas Mixing Station. A flow diagram is shown in Figure 2.3-1.
The flow rate of each component gas is controlled by a mass flow controller which delivers a constant mass of gas per unit time. (Tylan General model FC-280AV) The mass flow is independent of pressure, although a minimum differential pressure across the controller is required for proper operation. The valves are factory-calibrated for Nitrogen (N2). The system controlling them is field-programmed to compensate for different gasses. Flow channel 1, currently assigned for CO2, has a mass flow controller calibrated to deliver a maximum of 100 sccm (standard cubic centimeters per minute) N2 (74 sccm CO2). Flow channels 2 and 3 have controllers with a full scale range of 1000 sccm N2. With the calibration factors taken into account the maximum flows are 500 sccm Ethane (channel 2) and 1450 sccm Argon (channel 3). Manual valves (MV-201 & 221, etc.) are provided which allow one to bypass the mass flow controllers and use a needle-valve / rotameter set (MV-211, -212, -213) if desired. The needle valve must be closed during normal operation using the mass flow controllers.
Figure 2.3-1 -. Gas Shed Schematic
[Click Image to view at Larger Scale]
Figure 2.3-2 - Block Diagram of Mass Flow Control System.
The mass flow valves are controlled by a Dynamass Flow Control System (Vacuum General, Inc. model DM-2401). This unit is outfitted with four flow-control channels (two model FM-8 two-channel modules) and could be upgraded to eight flow channels if desired. Refer to for a diagram of this system. Currently HAWGS has only three of the four channels instrumented. The FM-8 receives a flow measurement from its associated flow controller, adjusts it by the calibration factor for the gas being used, and displays the result on the front panel. If the measured flow differs from the desired flow as set in the FM-8 by an operator, a correction signal is sent to adjust the valve in the flow controller. The DM-2401/FM-8 system allows the user to define up to four mixture/flow settings. Refer to the Dynamass System manual and to section Setting a Flow Rate for more detail on operating the flow controllers.
The measured flows of the three component gasses are combined in a small blending tank in the back of the mixing station. The resulting mixture is delivered to the alcohol bubbler through a line which is teed to an overpressure relief valve (RV-271) set for 25 psig. This prevents overpressuring of the blending tank, the bubbler, or the delivery lines.
Figure 2.3-3 - Vapor Pressures of Isopropyl and Ethyl Alcohols calculated using the CRC Handbook parametrization.
Because the interesting alcohols for use in wire chambers have a feeble vapor pressure at room temperature, it is not convenient to purchase bottled gas with alcohol already added. A practical means of adding alcohol vapor to a gas is to pass the gas through a reservoir of the liquid alcohol which is maintained at a specified temperature. At a given temperature, the vapor pressure of the alcohol may be known, and this vapor pressure represents directly the partial pressure of the vapor in the gas mixture. The vapor pressures of organic compounds may be calculated from information in the CRC Handbook of Chemistry and Physics, where it has been parametrized as
Log10 P = (-0.2185 A/K) + B
where P is the pressure in Torrs, K is the temperature in Kelvin, and A and B are parameters provided in the Handbook for a number of compounds. For isopropanol within the temperature range -26.1°C to +232.0°C, the parameters given are A=10063.5 and B=8.996156. For Ethyl Alcohol the parameters are A=9673.9, B=8.827392.
At 0°C, for example, this formula gives the vapor pressure of isopropanol as 0.0115 Atm. (1 Atm.= 760 Torr). If the gauge pressure of the bubbler gas + vapor is 1 atmosphere (2 Atm absolute pressure), as intended for Hall A, then the fraction of alcohol vapor, by partial pressure, is about 0.57%. Fig. 2.3-3 shows the vapor pressures of these alcohols as a function of temperature.
Note that the bubbler temperature defines the vapor pressure and thus the "dew point" for the vapor in the gas. If the gas comes in contact with any surface which is colder than the dew point (the temperature of the bubbler) the alcohol vapor will condense on that surface. This is why it is important that all components of the gas system be maintained at a temperature above that of the alcohol bubbler. Because gas in the Hall A chambers is at about 1 atmosphere absolute pressure while that in the bubbler is at twice this pressure, the dew point for the gas in the chambers is lower than the bubbler temperature.
The bubbler system consists of a refrigerator, a bubbler tank, a cold reservoir, a warm reservoir, and a fill tank. A float valve automatically maintains the liquid levels in the bubbler tank and the cold reservoir. Alcohol enters the bubbler tank only from the cold reservoir so that its temperature has already been established. The warm reservoir, sitting above the refrigerator, is equipped with a sight glass and serves as the main on-line alcohol storage vessel. When the level of liquid in this tank becomes low it must be manually refilled from commercially supplied bottles using the fill tank.
The refrigerator used to maintain the alcohol bubbler temperature has been modified specifically to make it safe for containing flammable gasses and liquids. This is documented in section, Notes Regarding Refrigerator Safety.
Filling the alcohol reservoir is not trivial. Please refer to and carefully follow the procedure detailed in section Adding Alcohol of this manual.
Gas will be metered to each detector element through a needle valve. To achieve a constant flow through a needle valve a constant differential pressure must be maintained across it, so it is necessary to provide a fairly constant supply pressure out of the mixing station. This comes only at a price, as the mass flow controllers deliver a fixed flow rate regardless of pressure (within practical limits). If the detectors in Hall-A consume less gas than the mixer supplies, the pressure in the supply lines will increase. Similarly, if less gas is mixed than is consumed the pressure will decrease.
To provide a usefully constant pressure of about 15 psig in the supply line, a pair of pressure switches has been installed in the mixing station outlet. The first of these, the Primary Pressure Control Switch, is set to open at 16 psig and close again at 14 psig or below. When the pressure is low this switch is closed and the Flow Control System is commanded to use the flow rates set into its PROGRAM C ("high flow"). When the Primary Pressure Control Switch opens, PROGRAM-D ("zero flow") is selected. By setting PROGRAM-C to provide just a little more gas than required by the detectors, the supply pressure can be maintained at between 14 and 16 psig. with a cycle time of several minutes. This pressure variation will result in a flow rate variation of no more than about 15%, which should be of no consequence for the detectors.
A second pressure switch, the Overpressure Alarm Switch, is calibrated to open at 18 psig and re-close at 14 psig or below. If the delivery line pressure manages to exceed the 18 psig threshold it indicates a system failure of some sort and the gas interlock system is tripped by this switch. Manual operator intervention is then required to re-establish gas flow.
Wiring details for this system are documented in the appendix in Hall A Gas Mixer - Pressure Control Concept and Wiring.
Pressure control of the inert gas supply, used to purge the detectors, is provided by a conventional single-stage regulator (PR-301) mounted inside the delivery rack. This regulator receives 45 psig inert gas (the same gas delivered to mixer flow channel 3) and provides 15 psig gas to the INERT supply line to 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 . 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 just prior to the pivot cross over point and they supply VDC and inert gas to the Electron Arm shield house. The pressures in all of these lines is nominally 15 psig.
Inside each shield house there is a gas distribution panel which controls the gas flows to the individual wire chambers in that detector stack. 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 -- one for each detector flow circuit. These valves are labeled MV-411, -412 in the Electron Arm, and MV-511 - 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. Calibration tables for the rotameters, corrected for several likely types of gas, are contained in the section HAWGS Rotameter Calibration Data. 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. For more information see the Sierra Top-Trak Flow Monitor Operators Manual.
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® 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 2.6-1 - Gas Distribution inside Hall-A
There are a number of monitor points which provide signals to the gas interlock panel and which can cause the supply of gas to be interrupted. The primary purpose of this system is to facilitate the safe handling of a flammable gas. A secondary but equally important function is protection of the detector hardware. Finally, this system serves to help insure the integrity of the data collected by Hall A experiments by alerting the experimenters on shift if a condition arises which might affect the detector gas quality.
The conditions monitored are 1) flammable gas leak detection, 2) low main supply pressure, 3) high delivery pressure, 4) high exhaust line pressure, 5) forced airflow in gas shed, 6) over-temperature in gas shed or a shield house, and 7) house fire alarm. The "Kill Gas" buttons in the counting room and in the gas shed also feed into this interlock system. More detail about the interlocks is contained in Troubleshooting : Things to Check.
When a fault condition occurs, power to the solenoid valves controlling gas flow into the gas shed is turned off, closing the valves. An audible alarm sounds in both the gas shed and the counting room, and one or more red lights on the interlock panels in both locations indicate the specific fault detected. The audible alarm may be silenced by pressing the "Alarm Override" button. Note that this does not restore gas flow or clear a fault.
After the fault is cleared it is necessary to activate the "Low Pressure Override" circuit by pressing the corresponding button on the interlock panel in the gas shed. This circuit temporarily disables the "Low Pressure" fault circuit, allowing the solenoid valves to open up and restore gas pressure to the inlet pressure switches. When pressure is restored this circuit automatically resets itself.
Note that the Excess Flow Valves will almost always trip immediately after the solenoid valves are re-opened. This is because of the sudden high flow rate which occurs when the pressure in the gas line downstream of a solenoid valve is low and the solenoid valve is opened with full inlet pressure. To reset the Excess Flow Valves refer to section , Resetting a closed Excess Flow Valve .
Before initial use with a flammable gas or after a significant down-time the following checks should be made to insure the safety and integrity of the HAWGS:
1. Leak-Check the entire gas system using a "safe" gas such as Argon or Nitrogen.2. Calibrate over-pressure relief valves: RV122 and RV123 should release at 55-65 psig, RV271 must release at 20-25 psig).
3. Check calibration of Excess-Flow valves (should close at 4-5 slpm).
4. Check proper operation of each interlock circuit and that interlock system shuts off gas supply.
5. Measure the detectors leak rates and verify that each is below 7 slph (or current administrative limit - note that current physical limit is 500 sccm Ethane and 1450 sccm Argon based on flow controller full scales for N2 and correction factors for Ethane and Argon).
6. Verify that flammable gas leak sensors are appropriately calibrated.
1. Close gas shed outlet valves MV-302-A, -B, -C to isolate the mixing/delivery system from the spectrometer detectors.2. Activate the "Kill Gas" crash button. Interlock panel should alarm. Silence the alarm by pressing "Alarm Silence". Reset the Crash Button by pulling outward. If any fault conditions (red LED) other than "Low Pressure" and "Main Relay" are indicated, clear them by correcting the indicated fault.
3. Check alcohol supply and Bubbler temperature. Fill and/or adjust as necessary (see Adding Alcohol) .
4. Check that adequate supply bottles of appropriate gasses are attached to the high pressure supply manifolds and bottle valves are open on both bottles for each manifold.
5. Verify that all used main pressure regulators (outside) are set for 45-50 psig.
6. Select "Manual / Expert" pressure control using toggle switch on panel in rear of mixing station, behind flow controller. Note that while in "Expert" mode there is no automatic delivery pressure control! Overpressure is prevented only by the overpressure shutoff switch and the alcohol reservoir relief valve.
7. Verify that the Dynamass Flow Control System DM-2401 is in "Non-VG" mode by
- Press PROGRAM SELECT pushbutton `A'.
- Put the DM-2401 in "NO-MODE" mode (if it is not already) by pressing the MODE pushbutton until all LED's in the column of LED's on the left of the unit are off.
- Simultaneously press and release PROGRAM SELECT buttons `A' and `C'.
- Press the MODE pushbutton until the "OPT 1" LED is lighted. Verify that the display for channel 4 reads all zeros.
- Return the DM-2401 to "NO-MODE".
8. Set Dynamass Flow-Control System to Program "D: Zero Flow". Put the DM-2401 in "FLOW-MODE" and verify that all flow settings are at zero. (For detailed instructions refer to section , Setting a Flow Rate and the Dynamass manual).
9. With the DM-2401 in "FLOW-MODE", set up flow program "C" to provide the desired mixture at about 10% higher total flow than the detectors are expected to consume.
10. Return the DM-2401 to "NO-MODE".
11. Select "Auto" pressure control using toggle switch on panel in rear of mixing station, behind flow controller.
12. Actuate the Low Pressure Override on interlock panel. "Main Relay" light should become green.
13. Set Excess-Flow Valves and Manual valves at regulators to full OPEN, wait until the supply lines to come up to pressure (Low Pressure indicator on the interlock panels will turn green), then set the ethane manifold Excess Flow Valve to AUTO SHUTOFF. [3]. The argon manifold Excess Flow Valve should remain OPEN.
14. Verify proper operation of flow/mix control system and outlet pressure regulation by observing flow rates on the mixer control and outlet pressure at the supply rack (pressure gauges 301B, -301C). If the alcohol bubbler loop is valved on it may take several minutes for its volume to fill with gas and come up to pressure. Be patient. When the pressure indicated on gauges PG301-B/C reaches about 15 psig the DM2401 system should cycle to flow program D. To bleed down the outlet pressure in order to cause the pressure loop to cycle, you may crack valve MV-299 (located in the rear of the mixer rack). When the pressure drops back to about 13 psig the control system should switch back to program C. Re-close valve MV-299 when tests are complete.
15. Check/Adjust the Purge gas pressure regulator (PR 301) to insure that the delivery pressure of the inert gas (taken from gas supply 3 - nominally Argon) is about 15 psig as registered on pressure gauge PG-301A.
16. Slowly open gas shed outlet valves MV-302-A, -B, -C to bring up the pressure in the supply lines to the spectrometers. After pressure has equalized open these three valves fully. Note that one or more Excess Flow Valves will trip (close) if the total gas flow through the three rotameters associated with these valves exceeds (roughly) 150 units (full scale on one rotameter).
17. At each of the Hadron and Electron shield house gas distribution racks, verify the presence of supply pressures (gauges PG401A,B and PG501A,B,C) and set gas selection valves and needle valves to desired gasses and flow rates for each chamber.
Warning: High pressure gas bottles contain significant stored energy and are potentially hazardous. Handling of gas bottles should be done only by qualified, trained personnel.
For smoothest operation, used gas bottles should be replaced before their internal pressure drops below the desired regulator output pressure.
Two possible cases exist in which a gas bottle needs to be replaced: only one empty gas bottle on a system or both bottles empty on a gas system.
For case 1 the sequence of steps is as follows:
1. Check in the Hall A Gas Shed. If all bottles have sufficient pressure each of the Matheson 8590 controllers will have one green "RUN" LED lit and one yellow "READY" LED lit. A red "EMPTY" LED lit indicates a bottle with low pressure, the corresponding bottle needs to be replaced. If a red "EMPTY" LED is lit the central "ALARM" LED should also show red. Nothing further needs to be done here, go outside to the Gas Bottle Pad.2. Visually verify that the corresponding pressure gauge on the flex line is showing a low pressure. A low pressure is not necessarily zero. Close the bottle valve for the empty bottle.
3. Disconnect the empty bottle from the high-pressure flex-line. The in-line check-valves will prevent gas escaping from the manifold. Replace the bottle's cap, and move the empty bottle to the EMPTIES storage rack. Note that ethane bottle fittings, type CGA-350, have left-handed threads.
4. Place a full bottle of gas in the on-line rack, remove the bottle cap, and connect the bottle to the flex-line.
5. Open the new bottle's valve, check for leaks at the bottle fitting. The corresponding pressure gauge should now read full bottle pressure.
6. The ALARM state of the Matheson 8590 controller should have automatically reset. Check inside the Hall A Gas Shed. Each controller should show a green "RUN" and yellow "READY" LED lit. If not, re-check the installation of the gas bottle.
For case 2 the sequence of steps is as follows:
1. Check in the Hall A Gas Shed. If all bottles have sufficient pressure each of the Matheson 8590 controllers will have one green "RUN" LED lit and one yellow "READY" LED lit. If a Matheson 8590 controller shows two red "EMPTY" LEDs lit and the central red "ALARM" LED lit, both bottles of the corresponding manifold need to be replaced. Nothing further needs to be done here, go outside to the Gas Bottle Pad.Follow steps 2. through 5., as detailed immediately above, for both bottles.
6. The ALARM state of the Matheson 8590 controller should have automatically reset. Check inside the Hall A Gas Shed. Each controller should show two yellow "READY" LEDs lit. If not, re-check the installation of the gas bottle. Press either of the two buttons labeled "LEFT BANK" and "RIGHT BANK". The lit LED above the button you pressed will change from yellow "READY" to green "RUN". You will most likely need to reset the Low Supply Pressure shutdown at this point.
Warning: Never open gas flow into the alcohol bubbler without an outlet valve being open.
As long as the level in the RESERVOIR is such that some alcohol is visible in the sight glass the bubbler will be maintained at its normal fill. An effort should be made to prevent the RESERVOIR level from getting too low.
1. To fill the RESERVOIR close valves MV-243 and MV-244 to isolate the RESERVOIR from the pressure equalization line.2. Open valve MV-241 to vent the RESERVOIR.
3. Remove the cover of the REFILL CANISTER and fill the canister with alcohol. Put the cover back on but do not seal it (if you seal the cover at this point the flow of alcohol out of the REFILL CANISTER will be impeded).
4. Open valve MV-242 to let the alcohol into the RESERVOIR. The liquid level can be monitored in the sight glass on the side of the RESERVOIR. Fill until the liquid level is near the top of the sight glass then close MV-242. Do not overfill (to or above the top of the sight glass).
5. Close valve MV-241, then open valves MV-243 and MV-244.
6. Seal the cover on the REFILL CANISTER to prevent contamination.
The flow of each individual gas component, and therefore the final gas mixture, is controlled by the Dynamass DM-2401 System, the FM-8 Flow/Ratio Modules, and the Tylan General FC-280 Mass Flow Controllers. The DM-2401 accepts and stores programs for the set of FM-8's. Each FM-8 controls one or two Mass Flow Controllers.
To set or alter a flow rate: (Refer to Figures 2.3-1 and 2.3-2)
1. Prevent the mixed-gas outlet pressure from exceeding its 18 psig interlock trip level by either a) closing valves MV-201,2,3, or b) insuring that the detectors are consuming a sufficient quantity of gas to prevent this overpressure from occurring during the time it takes you to perform steps 3-11, below.2. Set the Auto/Expert pressure control switch (rear of mixer rack) to EXPERT.
3. Verify that the DM-2401 is in the NO-MODE mode, indicated by none of the LED's in the column on the extreme left of the unit being illuminated. If necessary, press the MODE pushbutton until this condition is achieved.
4. Press program select button "C" and verify that the corresponding LED illuminates.[4]
5. At the DM-2401 Keyboard module, press the MODE switch until the FLOW LED illuminates. The window value for channel 1 will begin to flash.
6. Select the channel you wish to alter by pressing the STEP button until the window value of the desired channel is flashing.
7. Press the UP or DOWN SET buttons to alter the value as desired. Legal flow values are 0.0-100.0 sccm for channel 1, 0-1000 sccm for channels 2 and 3.
8. If the Red "ON" LED for the desired channel (immediately to the right of the value window) is not lit, press the ON/OFF button for that channel to illuminate this LED.
9. Repeat steps 6-8 as necessary to program all desired gas flows.
10. Return the DM-2401 to NO-MODE as in step 3.
11. Return the Auto/Expert pressure control switch (rear of mixer rack) to AUTO.
12. Re-Open valves MV201, 202, 203, if closed in step 1.
13. Observe system flow and pressure control and verify that it is correct.
Response to a Gas System Alarm
|
Label |
Meaning |
Likely Remedy |
Sensor |
|---|---|---|---|
|
Low Pressure Note: This channel will always indicate a fault after some other problem has caused the solenoid valves to close. |
The secondary pressure of one or more of the mixer inlet gas supplies has dropped below the 45psi threshold. |
* Check for a closed "Excess Flow" Valve, Remedy the cause, and reset the valve * Replace empty supply bottle. |
3 pressure switches located on the "Delivery Rack" |
|
Gas Shed Airflow |
Gas Shed Exhaust Fan (in ceiling of "Isobutane Room") has failed. |
Restore forced ventilation. |
Vane Switch mounted just inside exhaust fan |
|
Overtemp Gas Shed |
Temperature in Gas Shed Too High (~110deg. F). |
*Take action to protect equipment.
|
Klixon mounted in rack |
|
Over-Pressure FPP Over-Pressure VDC Electron Over-Pressure VDC
Hadron |
VDC/FPP Exhaust manifold pressure at Hadron or Electron
Shield House Gas Rack too high. |
*Eliminate exhaust line blockage
|
PHOTOHELIC® gauges on gas panels in shield houses |
|
Gas Leak |
Flammable gas detection system has sensed a leak |
*Localize leak by referring to readings on GasMaster-4 system in Counting House
|
GASMASTER® Flammable Gas Detector heads in Gas Shed
and each Shield House. |
|
Overtemp Electron |
Temperature in Electron Shield House Too High (~110deg.
F). |
*Take action to protect equipment.
|
Klixon mounted in gas rack in Electron Shield House |
|
Overtemp Hadron |
Temperature in Hadron Shield House Too High (~110deg.
F). |
*Take action to protect equipment.
|
Klixon mounted in gas rack in Hadron Shield House |
|
Post-Bubbler Supply
Overpressure |
Gas pressure out of mixer/bubbler rack is > 18psi. |
*Reset Dynamass Controller
|
Pressure switch in rear of Mixing Station (mixer/bubbler
rack) |
Each Excess Flow Valve automatically closes if the flow rate through it exceeds about 4 slpm at 45 psig. The exact flow threshold varies somewhat depending upon the delivery pressure. The manufacturer's specification curves are shown in the figure below. The valves in use in HAWGS are of type "C".
Figure 3.4-1 - Excess Flow Valves: maximum flow rate graph and photo of a valve. The valve handle is RED.
These valves must be manually reset after they trip. This is done by rotating the red handle 90deg. CW (to OPEN/RESET ) and then 90deg. CCW (back to AUTO SHUTOFF. ). It will be necessary to keep the valve in the OPEN/RESET position until nominal pressure builds up downstream. You will know this has been achieved when the Low Pressure status light on the interlock panel turns to green. The ethane manifold excess flow valve must be returned to the AUTO SHUTOFF setting to insure system safety.
If the gas pressure in any enabled supply line to the mixer rack drops below about 40-45 psig, the interlock will sound an alarm and close all of the solenoid valves. This prevents the system from delivering a bad mixture to the detectors. After restoring the gas supply, for example, after replacing an empty gas cylinder, perform the following steps to restart the flow of gas.
1. Verify that no faults other than "Low Pressure" and "Main Relay" are indicated on the gas interlock panel.2. Insure that all high-pressure manifolds are pressurized and that the secondary pressures indicated by the gauges above the bottles are at about 40-45 psig (normally these should not need adjustment).
3. Press the Low Pressure Override button on the interlock panel in the gas shed.
4. Reset all Excess Flow Valves by turning their handles to OPEN/RESET, waiting until the lines pressurize (Low Pressure status light will turn green), then returning the ethane's Excess Flow Valve handle to AUTO SHUTOFF. The argon's Excess Flow Valve should remain in the OPEN/RESET position.
5. Verify that all faults are now cleared on the gas interlock panel.
Normally the gas control system is protected from power outages by an uninterruptable power supply (UPS). If the system is nevertheless disturbed by a brief loss of power then perform the steps outlined immediately above (Restoring flow after a "Low Supply Pressure shutdown). If this fails or if the power was off for an extended period of time, a full system reset as prescribed in the section Startup Procedure may be required.
Power sources that can affect the gas system include the following:
In the Gas Shed: AC power to the gas mixer control system and the interlock chassis (provided through a uninterruptable power supply (UPS)). AC power to the alcohol refrigerator (wall outlet). AC power for the room ventilation fan (hard-wired back to the circuit-breaker panel outside). AC power to the VME crate (wall outlet. Note that the VME crate serves only to monitor [not control] the gas system).
In the Counting Room: AC power to the GasMaster flammable gas sensor system (which has its own internal battery backup). AC power to the 12 VDC and 24 VDC power supplies which feed 1) the electronic flowmeters measuring gas flow out of the chambers in the shield houses (12v), 2) the exhaust manifold pressure switches in the shield houses (24v), and 3) the high-pressure transducers outside the gas shed which report the pressure in the high-pressure manifolds (bottle pressures).
The GasMaster flammable gas detection system is composed of a control unit in the counting room and a flammable-gas-sensitive detection head in each of the shield houses and one in the gas shed. Technical details may be found in the CROWCON Instruction Manual for GASMASTER® Systems.
In normal operation the control unit repeatedly scans the three sensors and displays the concentration of flammable gas in the area of each one. Channel 1 (C1) monitors the atmosphere in the gas shed. C2 is connected to the detector in the Electron Shield House and C3 is for the Hadron Shield House. The measurement units are percent of the Lower Explosive Limit of Ethane (%LEL Ethane). Each channel of the control system has two threshold levels defined. The low level is 10% LEL. The high level is 25% LEL. If a sensor reading exceeds the low level the control unit sounds an alarm and illuminates a red LED. No other action is taken by the system. The alarm may be silenced by pressing the "ACCEPT/RESET" pushbutton on the control system front panel. The status LED will stay in the alarm condition until the "ACCEPT/RESET" pushbutton is pressed after the reading drops below the alarm threshold.
If a gas concentration exceeding the HIGH threshold is detected the alarm sounds again and a fault condition is transmitted to the gas system interlock chassis, shutting off the gas supply by means of the solenoid valves outside the gas shed. It will not be possible to restore the flow of gas into Hall-A until the flammable gas concentration reading is reduced below the threshold value.
In any case the indicated condition should be noted in the log and action should be taken immediately to determine the cause of the alarm and to correct the problem. A gas system expert should be contacted.
Anytime work is done on any part of the gas system, or there is an occurrence that could possibly have damaged the gas system, the system should be carefully inspected and checked for leaks.
The flammable gas detector heads and control system should be tested periodically for proper operation, in accordance with the manufacturer's recommendations (section , CROWCON Instruction Manual for GASMASTER® Systems ) and the Thomas Jefferson National Accelerator Facility fire safety program.
Each sensor feeding the Gas Interlock Panel should be exercised at least annually for proper operation. The interlock system itself should be tested at the same time to insure that it interrupts the supply of gas when it is tripped.
The high-pressure manifold and bottle connections should be regularly checked for leaks and damage. In particular, the CO2 system (CGA-320) uses plastic seals at the bottle connection which must be replaced periodically.
The Tylan General mass flow controllers (mass flow valves) require periodic cleaning and calibration (see the Tylan General Mass Flow Controller Instruction Manual ). The first step in this process is to check the operation of the instruments and perform further work as necessary. This procedure should be planned and carried out whenever 1) there appears to be a problem with the operation of a flow controller, or 2) there is a lengthy break in the Hall-A program that would allow the gas system to be taken off-line for several weeks. If absolutely necessary the needle-valve/rotameter combinations plumbed in parallel with the mass flow controllers could be used to allow interim operation of the gas system while one or more flow controllers is removed for maintenance.
Additional Figures Relating to the Interlock System:
4/26/96
This module's purpose is to shut off the gas flow to the wire chambers in the case an error condition is detected such as low gas pressure, gas leaks, fire, etc. It allows one to over-ride the low gas pressure condition and turn on the gas flow relays. This over-ride automatically resets when the low pressure error has been corrected. This module also allows one to silence the alarm which will also reset once the error condition has been corrected. It also has a connector for a remote indicator box planned to be stationed in the counting house.
Figure 5.1-1 - Front Panel of the Gas Interlock System Master Chassis. This unit is installed in the Hall-A Gas Shed.
The front panel has red/green LEDs showing the status of each system as well as the "LOW PRESSURE" over-ride, the alarm silence, the alarm speaker, and the ON/OFF switch. See Fig. 5.1-1.
The back panel has a fuse for the main 120 VAC input power (2A fast blow), a fuse for the 24 VDC power (1.5A slow blow), the AC input cord connector, Banana plug connectors for the error interlock switches, and the 37 pin remote output connector. See Figures 5.1-2 and 5.1-3. Each red banana plug has the 24 V output which should be connected to the appropriate interlock switch so that when the switch is closed, there is no error condition. This sends the 24 V back to the black banana plug which then energizes the appropriate interlock relay. The pair of banana plugs labeled Master Relay supplies 24 VDC to open the gas solenoid valves when there are no fault conditions.
Figure 5.1-2 - Rear Panel of the Master Interlock Chassis showing the Master/Slave Interconnect Connector, the 14 Status Input Pairs, and the Power Fuses and Connector.
Figure 5.1-3 Pin Assignments for the Interconnect Cable Between the Master and Slave Units of the Interlock System.
The circuit diagram is shown in the figure below. All relays except the main control relay are 24V DPDT (Aromat type 9601A, these are not in the stockroom). The main control relays is a 24V 4PDT (Potter & Brumfield type KHAU-17D11-24, this is in the stockroom). All LEDs are the red/green type (Micro Lamps type MLED-59EGW, these are not in the stockroom). The "input" connection for all the interlock relays is connected directly to the appropriate black banana plug. Not shown: the red and green remote LED connections are connected directly to their respective local red and green LED pin on the appropriate relay.
[Click Image for an Enlarged View] Figure 5.1-4 Schematic Diagram of the Gas Interlock System Master Chassis. The LEDs in the Remote Chassis are connected in parallel with those in the Master and the Alarm Silence portion of the circuit is duplicated there as well.
To by-pass an interlock, such as those which are currently unused, one just needs to short together the red and black banana connectors (on the rear of the master unit) of the appropriate interlock relay. It is not suggested to by-pass any interlocks in use except in unusual circumstances.
The front panel of the Slave interlock module, located in the Counting Room, is similar to that of the Master unit with the exceptions that there is no power switch or low-pressure override button. Green/Red LEDs (normally green) indicate the status of each interlock circuit as determined by the Master in the Gas Shed. There is very little in this module except for the one local alarm silence relay which is connected in the same way as the alarm silence circuit in the master module.
From CEBAF Stockroom
Item Stock # Quant. Cost ea (as of June, 1996)* Sonalert 5920 50010 2 12.90
* Momentary switch 5930 20155 2 10.50
* Red lens for switch 5930 20165 2 0.64
* Power on/off switch 5930 10435 1 6.70
* Power plug socket 6110 10105 1 0.88
* Remote lamp socket 5935 30330 2 1.25
* AC Power cord 6150 10060 1 2.50
* Proto plugboard 5975 23020 1 19.05
* 14-pin sockets 5968 21174 16 0.40
* 24V power supply 2.4 A 6130 10220 1 51.11
* Valve relay 5945 13020 2 4.29
* Fuse holder 5920 80045 1 1.48
Order from Newark/Allied
* DPDT relays , Aromat/NAIS # DS2E-S-DC24V 25 5.97 ea.* Chassis, BUD HC-14101 2 153.10 ea.
* Green/Red LEDs, Micro Lamps #MLED-59EGW 3 5.60/pk of 10
* LED lamp holders, Micro Lamps #MLED-RTF-5010 30 0.37 ea.
* Double binding posts, ITT Pomona #4243-0 15 4.58 ea.
This analysis is based on the procedure outlined in the document "Storage and Use of Flammable Gases for Experiments at CEBAF" in effect at the time of this writing.
Overview
The Hall A Wire chamber Gas System (HAWGS) provides operating gas for the Vertical Drift Chambers (VDCs) in both the Electron Arm and Hadron Arm shield houses and the Focal Plane Polarimeter straw tube chambers (FPPs) in the Hadron Arm shield house. This analysis assumes that the gas delivered will be 50% Argon + 50% Ethane (C2H6) by volume. The two components are supplied in separate bottles and are combined by microprocessor controlled mass-flow mixing equipment in the gas shed.
HAWGS is distributed over five isolated geographic regions:
1) The bottle storage / high-pressure manifold area outside the gas shed2) The gas mixing room of the gas shed
3) The Electron Arm shield house
4) The Hadron Arm shield house
5) The main supply / return lines running between the gas shed and the shield huts
Flow restriction allowing this isolation is provided 1) by an Excess-Flow valve (maximum flow= 4 SLPM, nominal flow = ~1 SLPM) in each gas supply just prior to gas-line entry to the gas shed from the bottle-storage area, and 2) by the mass-flow control system which meters the gas delivered out of the gas shed to the two shield houses.
This analysis will document that the HAWGS should be classified under "Gas System Class 0" except for the outdoors on-line bottle and storage area which is "Class I". Following the decision flowchart in Figure 1 of the referenced document, we begin each case by calculating the inventory of Ethane in the subject area.
1) Bottle on-line / storage area: (this is an outside fenced-in area):
Volume of 1 bottle of ethane = 410 SCF
Q =(410 SCF)*(0.0283 m3/SCF)*(1.26 kg/m3)*(0.36)
= 5.27 per bottle,
where the factor (0.36) is the "gross heat of combustion of ethane relative to that of hydrogen".
2 Bottles on-line plus 2 bottles (max) in ready storage Q= (4 * 5.27) = 21.1
Flowchart:Box 1: Q > 0.6 kg? YESBox 2: Inside?: NO
Box 5: Q > 200 kg? NO
Box 6: d < sqrt(2+2Q) = 6.6m? NO
-->Class I.
2) Gas Mixing Room within the Gas Shed[5]
|
System Component |
Ethane Volume (SCF) |
|
~50 feet of 1/4" Copper Tubing and Misc. plumbing at <45 psig : V < [pi]r2l P =
(3.14)(0.1252)(50)((1/144 ft/in)2)(4
Atm) |
0.07 |
|
Mixing bubbler and alcohol buffer tank (50%
C2H6 at 15 psig) |
0.5 |
|
~20 feet of 1/2" Plastic Tubing (50% C2H6 at 15 psig) V < [pi]r2l P =
(3.14)(0.252)(20)((1/144 ft/in)2)(2
Atm)= |
0.03 |
|
Total |
0.6 ft3 |
Q= V *[rho] * h= (0.6 ft3) (0.0283 m3/SCF)(1.26 kg/m3)(0.36)= 0.007 << 0.6
This implies Risk Class 0.
3) Electron Arm Shield House
|
System Component |
Ethane Volume (SCF) |
|
~50 feet of 1/4" Plastic Tubing and Misc. plumbing at ~0 psig: V < [pi]r2l P =
(3.14)(0.1252)(50)((1/144 ft/in)2)(1
Atm)(50%) |
0.02 |
|
~50 feet of 1/2" Plastic and Copper Tubing, Misc. Plumbing (50% C2H6 at 15 psig) V < [pi]r2l P =
(3.14)(0.252)(20)((1/144 ft/in)2)(2
Atm)(50%) |
0.08 |
|
VDCs: 2 each at 0.65 x 2.5 x 0.075 m3 (50%
C2H6) |
4.2 |
|
Total |
4.3 ft3 |
Q= V * [rho] * h= (4.3 ft3) (0.0283 m3/SCF)(1.26 kg/m3)(0.36)= 0.05 << 0.6
This implies Risk Class 0.
4) Hadron Arm Shield House
|
System Component |
Ethane Volume (SCF) |
|
~100 feet of 1/4" Plastic Tubing and Misc. plumbing at ~0 psig: V < [pi]r2l P =
(3.14)(0.1252)(50)((1/144 ft/in)2)(1
Atm)(50%) |
0.04 |
|
~100 feet of 1/2" Plastic and Copper Tubing, Misc. Plumbing (50% C2H6 at 15 psig) V < [pi]r2l P =
(3.14)(0.252)(20)((1/144 ft/in)2)(2
Atm)(50%) |
0.16 |
|
VDCs: 2 each at 0.65 x 2.5 x 0.075 m3 (50%
C2H6) |
4.3 |
|
FPPs: 1cm dia x 100cm long x 5200 straw tubes ~ 408 liters (50% C2H6) |
7.2 |
|
Total |
11.7 ft3 |
Q= V * [rho] * h
= (11.7 ft3) (0.0283 m3/SCF)(1.26 kg/m3)(0.36)= 0.15 << 0.6
This implies Risk Class 0.
5)Interconnecting Tubing
|
System Component |
Ethane Volume (SCF) |
|
2 each, ~700 feet 0.5" OD Supply Tube, 15 psig, 50% C2H6 V= 700*(0.252)*(3.14)*(1/144)*(2
Atm) |
3.8 |
|
2 each ~700 feet 1" OD Exhaust Tube, ~0 psig, 50% C2H6 V= 700*(0.52)*(3.14)*(1/144)*(1
Atm) |
7.6 |
|
Total |
11.4 ft3 |
= (11.4 ft3) (0.0283 m3/SCF)(1.26 kg/m3)(0.36)= 0.15 << 0.6
This implies Risk Class 0.
For the sake of this calculation the flow circuit has been simplified and the total length of tubing overestimated. While it might be argued that this gas volume in the supply/return lines should be considered part of the gas shed or shield hut inventories, it is clear that doing so would have no effect on the results of the hazard class determinations.
To: P. HuntFrom: Brian Kross
Date: August 26, 1994
Subject: Alcohol bubbler refrigerators in Physics Department's gas systems
In response to your concerns about the refrigerators in the Physics Department's gas system let me assure you that we have addressed these concerns. Before coming to CEBAF I have had 14 years experience designing electrical devices including designing explosion-proof and flammable storage freezers and refrigerators for the Lab-Line Instrument company. Due to that experience, I was asked to serve on the group at Fermilab that oversaw all handling of flammable gases at the lab.
The refrigerators in these systems are used to control the amount of alcohol that goes into solution in the gas stream. By controlling the temperature of the alcohol, the vapor pressure is controlled and therefore the amount of alcohol in the gas stream. The gas and the alcohol are in sealed vessels or tubes that pass through the refrigerator and are not open anywhere in the system. 1/2 to 1% are typical alcohol amounts that go into a 50/50 Argon/ Ethane mix. The only way that there would be gas in the refrigerator would be if there would be a leak in the system. All of systems were assembled with stainless steel tubes and highly reliable VCR fittings. The systems were checked for leaks when finished and will be rechecked periodically. This section of the gas system is further protected by a pressure relief that vents outside the cabinet. The gas systems operate in strongly ventilated gas sheds that prevent the build up of flammable gas in the event of a system failure.
The refrigerators selected were chosen without defrost, fans, door heaters, and lights. The Thermostats were moved to the exterior of the cabinet and the penetrations sealed with RTV. This essentially makes them flammable storage refrigerators. I did look into commercially available flammable storage refrigerators and found the one we needed, the least expensive available, was $1900. This seemed a frivolous expense compared to the $90 we did spend. Even at that I received complaints about the costs of these systems. Also one needs to considers that the mass flow valves also required for these systems are not available as explosion proof units. It would be rather wasteful to increase your costs by a factor of 20 to get flammable storage capability and place the unit 8 inches from a non explosion- proof device.
I believe that the first and best path to mitigate flammable gas system hazards are to make every effort that these systems be as leak-tight as possible. If there are no leaks there are no hazard. The next step is to build the system, as we have, such that there is a free flow of air around it, so there can be no accumulation of flammable gases in the event of a leak. Step three is to limit as much as reasonably possible, the amount and proximity of sources of ignition. One should also as much as possible minimize the amount of energy available to a flame by keeping the system as small as possible. In Halls A & C the amount of gas in the system at any time is small and poses little risk even in the event of catastrophic failure.
If you have further questions please call.
CC. L. Cardman, S. Majewski
The DYNAMASS flow control system of the Hall-A Wire chamber Gas System controls the mass flow of each component gas. It does not, of its own, control the gas delivery pressure. According to the manufacturer, as of January, 1996, there is no device available which could allow it to do this.
Gas consumptions by the Hall-A wire chambers (VDCs and FPPs) are controlled by needle valves (one for each chamber circuit) on the delivery control panel within each shield house. Needle valves deliver a constant flow if the differential pressure across the valve is constant. Thus it is necessary to provide an approximately constant supply pressure in the gas manifold upstream of the needle valves.
This is accomplished in the Hall-A system by sensing the pressure of the supply manifold and reducing (stopping) the total supply flow if the pressure is too high or increasing (restoring) the flow if the pressure is too low. Pressure higher than expected from this regulation system causes the gas interlock system to generate an alarm and to completely stop the supply of gas until an operator corrects the problem and resets the system.
Two pressure-sensitive switches are installed in the back of the gas mixing rack in the gas shed. They monitor the outlet pressure from the mixing system. The Primary Pressure Control Switch selects "high" or "zero" flow (where high should be programmed to provide more gas than needed by the chambers). The Overpressure Alarm Switch operates somewhat above 16 psig, and trips the gas interlock system if opened.
The DYNAMASS supports four "PROGRAMS": A,B,C,D. The external wiring shown below, when in "Auto" mode, selects program C when high flow is needed, and D when zero flow is needed.
(Please refer to the copy included with the HAWGS Manual in the Hall-A Counting Room)
(Please refer to the copy included with the HAWGS Manual in the Hall-A Counting Room)
(Please refer to the copy included with the HAWGS Manual in the Hall-A Counting Room)
(Please refer to the copy included with the HAWGS Manual in the Hall-A Counting Room)
The following people are familiar with the Hall-A Wire Chamber Gas System and may be consulted for assistance. For after-hours (nights and weekends), the current Hall-A call-in list should be consulted.
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Howard Fenker |
JLab |
x7431 |
849-7431 |
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Jack Segal |
JLab |
x7242 |
849-7242 |
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Bogdan Wojtsekhowski |
JLab |
x7191 |
849-7191 |
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The following pages are a collection of specification sheets for various components used in the fabrication and installation of the Hall A Wire Chamber Gas System. It is not intended to be complete, but reasonable effort has been made to provide a library of whatever information was included with purchased parts.
[3] Note that if not all three gas supply circuits are in use, the pressure switch corresponding to the unused gas(ses) must be bypassed electrically. This is usually done inside the pressure switch housing.
[4] Program C is hard-wired as the "normal-flow" program for this system. It is the only program used to mix gas for the detectors under automatic pressure control. Program D is hard-wired as the "zero-flow" program. All flows in program D MUST remain set to zero.
[5] Note that the O.D. is used in this and following tubing volume calculations, as it has no meaningful impact on the conclusions drawn and this method avoids any complication arising from differing tube wall thicknesses.
