Gas Risk Management – A Safer Approach to Monitoring for Hazardous Gases
numbers of sensors, it may be economical to apply the calibration gas automatically. A method to check sensor span voltage versus acceptable tolerance should be available either through calculation or as a system report. Advanced systems can prompt “calibration due” according to a user configured calibration frequency.
Response To Alarms. Response to alarms begins with preplanning – in plant and outside the plant perimeter. Next, training is essential to insure all plant
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| METHANE |
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| HYDROGEN | ||
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| PPM in | % Gas |
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| PPM in | % Gas |
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| air | in air |
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| air | in air |
(%LEL) | 100% |
| 50,000 | 5.0 |
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| 40,000 | 4.0 |
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Limit |
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| Typical HIGH |
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| Alarm Setpoint |
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Explosive |
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60% |
| 30,000 | 3.0 |
| EMERGENCY |
| 24,000 | 2.4 | ||
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Lower |
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| Typical LOW |
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| Alarm Setpoint |
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| 20% |
| 10,000 | 1.0 |
| WARNING |
| 8,000 | 0.8 | |
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Fig. 3 Alarm Setpoints for Methane and Hydrogen
personnel understand their function and have internalized their responsibilities. Part of this training includes identifying false alarms.
Relay action usually involves a low alarm (warning light and siren and ventilation to reduce the gas concentration) and high alarm (emergency light and siren and a process action). Process action involves turning off, or isolation of, the source of the gas and shutdown of process equipment. A
The final element of responding to alarms concerns evacuation procedures, both in plan in the local vicinity, and requesting outside assistance such as the fire department. Events that trigger these actions are based on the hazard assessment. Recent catastrophes and near catastrophes more than demonstrate the need for this requirement.
New Wafer Processing Laboratory
A new wafer processing laboratory located in California had extensive Hydrogen and toxic gas piping and equipment utilizing those gases. The facility was designed to minimize the possibility of a gas leak and further measures were taken to contain rather than expel any gas that might accidentally leak. For example, all gas piping was run through a NEMA duct to prevent gas from being uncontrollably dispersed into the room.
The facility installed both a Hydrogen and toxic gas monitoring system. It was necessary for the Hydrogen and toxic gas monitoring system to be independent due to the capabilities of the sensors. The remainder of this discussion will focus on the Hydrogen monitoring system; however, much of the discussion also applies to the toxic gas monitoring system.
The hazardous gas monitoring system used was the Sierra Monitor SENTRY Gas Monitoring System. The system uses a sophisticated
Sensor Locations. One of the first questions to be addressed as part of the system specification is how many sensors are required and where should they be located? There are no rules or formulas to answer these questions. In general, it is important to locate sensors close to the likely sources of leaks and in areas where a leaked gas might lighter than air, thus dictating whether sensors be located below or above the leak. With Hydrogen being lighter than air, sensors are always located in the ceiling as a backup to sensors located near the source of the leak.
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA
Visit our Web Site at: http://www.sierramonitor.comE-Mail: sales@sierramonitor.com Rev. A1