Sierra Monitor Corporation TR-002 manual New Wafer Processing Laboratory, Methane Hydrogen

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

		METHANE			HYDROGEN
		PPM in	% Gas		PPM in	% Gas
		air	in air		air	in air
(%LEL)	100%	50,000	5.0		40,000	4.0
Limit				Typical HIGH
				Alarm Setpoint
Explosive
	60%	30,000	3.0	EMERGENCY	24,000	2.4
Lower				Typical LOW
				Alarm Setpoint
	20%	10,000	1.0	WARNING	8,000	0.8

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 high-high alarm can be used as an indication that the concentration is very dangerous. “Zone voting,” where two or more sensors must be in alarm before activating a zone relay, can be used as a way to neutralize a faulty sensor and to take action specific to the area that the gas hazard exists.

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.

Record-Keeping. Record-keeping becomes an important element of response to alarms. Sensor status reports provide a valuable reference to assess the severity and breadth of the gas hazard. Real-time and post-event analysis can be generated. Calibration reports give data for monitoring sensor performance over time. Sensor history reports are useful for satisfying regulatory requirements and providing data on gas exposure. It is the transfer of information provided from the system to plant personnel that enables gas risk management to be effective.

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 microprocessor-based controller. System configuration is user programmable to allow the system to be customized to the specific application requirements. SENTRY has extensive data management and internal diagnostics that provide information required to help manage gas leak problems.

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.

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