Sierra Monitor Corporation manual Technical Reprint TR-002

Technical Reprint TR-002

For some gases, excursions are permitted where the gas concentration may increase above the TLV for a limited period of time, i.e. the allowable excursion duration. The ceiling limit is the maximum concentration of a contaminant allowed without regard to duration. For example, in California, Hydrogen Sulfide has a TLV of 10 ppm with an excursion limit of 20 ppm for minutes in an 8-hour period. Its ceiling limit is 50 ppm.

Oxygen Deficiency. Where purging or processing with an inert gas as helium, argon, or nitrogen is employed, a leak may result in a depletion of the oxygen concentration in an area occupied by plant personnel. As the concentration oxygen drops, asphyxiation will result. OSHA has established 19.5 percent oxygen in air as a lower limit of a safe environment.

Contamination of Process Gas. If process gas is leaking out of a pipe, then some room air will be leaking into the pipe even through the process gas is under pressure. This leak-in air of course affects purity and can possibly have some detrimental effect on yield.

Components of a Gas Risk Management System

A gas monitoring system must provide information to insure that effective and comprehensive decisions are made in a timely fashion. It is not sufficient to sound an alarm after a low alarm limit has been reached. To be most effective in managing the risk of gas exposure, sufficient information is required to obtain an accurate assessment of the situation in order to make good decisions. Data management is a key ingredient of a gas risk management system.

Within this perspective, each situation imposes objectives and constraints on the gas monitoring system design, operation, response to alarms, maintenance, and record-keeping requirements. These considerations must address the risks identified in a hazard assessment and must be formulated into a risk management plan.

Hazard Assessment. Hazard assessment is the first step in system design. Field studies can help determine the what, where, when of hazardous situation and possible exposure. The characteristics of the hazard in terms of its ignitability, reactivity, corrosivity, and toxicity can be determined. In turn, the risk to human health as reflected in standards set by RCRA, OSHA, and EPA can be estimated. Correspondingly, NFPA and insurance company guidelines can help estimate risk to facilities. The impact of environmental factors can also be assessed. This is not to say that gas risks can be accurately quantified; however, the information obtained in a hazard assessment provides a basis to better address the identified gas risk.

System Specification and Design. System specification and design is a key output of the risk management planning process. Design begins with sensor type selection and location. Sensor performance should be evaluated in terms of speed of response, concentration range, resolution drift, ease of calibration, and interfering gases. In general, the controller, wiring, backup power and alarm response must meet the duty performance criteria established in the plan.

Day-To-Day Operation. Day-to-day operation of a gas monitoring system focuses on data review procedures. It is important to use fixed systems not only as an indicator of an alarm event, but also to provide an early notification of a leak. A system clock is becoming a more important function to document events accurately. Day-to- day operation also focuses on system reliability. Automatic self-monitoring of unit functions enhances reliability by providing an ongoing indication of operability. Regular maintenance should confirm the internal diagnostics.

One person calibration is an important feature in simplifying calibration and reducing labor requirements. This process can be further simplified if all adjustments are made automatically after application of calibration gas, which can be accomplished with microprocessor- bases controllers. Where there are a large

	8				5.6 - 7.3
	7
	6
CONCENTRATION (% Gas in Air)	5.0 - 5.3
	5
	4	4.0
	3		2.0 - 2.2
	2			1.2 - 1.4
	1
	0
	Methane	Hydrogen	Propane	Toluene	Methanol

Fig. 2. The lower explosive limit varies substantially for different gases.

Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042

Visit our Web Site at: http://www.sierramonitor.comE-Mail: sales@sierramonitor.com Rev. A1