Measuring and testing – Gas analysis – Gas of combustion
Reexamination Certificate
2001-01-31
2003-08-12
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas analysis
Gas of combustion
C073S023200, C073S031050, C073S863000, C422S083000, C422S094000
Reexamination Certificate
active
06604405
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to monitoring systems having a continuously operating gas analyzer. More particularly, the present invention relates to area monitoring systems having “dilution” source for supplying oxygen to a continuously operating combustible gas sensor.
The most common atmospheric danger in industry today is oxygen deficiency. Oxygen may be below the naturally occurring level of 20.9% by volume either due to consumption or displacement. Consumption sources include internal combustion engines and biological consumption sources, such as aerobic organisms. Displacement of oxygen can occur by the introduction of heavier-than-air gases into a confined space. Explosive gas dangers are the second most common hazard. Methane gas can result from the decomposition of organic matter in wastewater treatment plants and landfills. Natural gas leaks (methane) is a perpetual urban problem in sewers and subways. Propane gas can leak out of storage tanks or distribution piping.
Carbon monoxide and hydrogen sulfide are the two most commonly found toxic gases. Hydrogen sulfide is a byproduct of the decomposition of organic matter and can be present in significant concentrations in wastewater treatment plants and landfills. Hydrogen sulfide is also found in mining and oil fields, or wherever water comes in contact with elemental sulfur.
Federal regulations require that employers protect their workers from unsafe breathing atmospheres. The most common places where the atmosphere may not be safe are “confined spaces”, for example wells, tanks, vessels, vaults, and unventilated rooms. Where access to such spaces is required and it is undesirable or impossible to spot test the atmosphere of such space prior to each entry, continuously operating area monitoring systems are frequently utilized. Other applications for area monitoring systems where unsafe atmospheres exist include steel mills, warehouses and parking garages (carbon monoxide danger), fertilizer manufacturing (ammonia danger), and plating operations (hydrogen cyanide).
Atmospheric sampling is conventionally accomplished by either a diffusion method or manual/continuous sample-draw. Diffusion allows for sampling the atmosphere only in the immediate area of the detector. Random air currents serve to deliver the detected gas to the sensor face. Sample-draw systems bring the gas sample from a remote location through tubing or pipes to the gas sensor. Manual sample-draw systems use a hand actuated aspirator bulb to pump the sample. Continuous sample-draw systems include the use of a battery-powered or mains-powered motorized sample draw pump.
A common application for industrial safety gas detection is the detection of explosive gases in reaction vessels that contain atmospheres largely comprised of nitrogen. Reaction vessels used for the refinery of petroleum products contain layers of catalyzing beds. These beds must be periodically replaced with new catalyzing material. The old catalyzing material is laden with volatile hydrocarbons that present an explosion risk. To eliminate the risk of explosion while workers are removing the old catalyst, the vessel is filled with nitrogen while the workers wear supplied air respirators or Self-Contained-Breathing-Apparatus. The atmosphere is still monitored for the presence of explosive gases in case the nitrogen purge is lost. Monitoring instrumentation is typically located outside the hazardous location and is monitored by dedicated personnel utilizing continuous sample-draw gas detection equipment.
The most common technology for monitoring for explosive gases are catalytic or “hot bead” gas sensors. The sensors are constructed by coating tiny coils of platinum wire with a ceramic material, and then doping the coils or “beads” with a catalyst. In operation, sufficient current is directed through the sensor such that the surface temperature of the bead exceeds the temperature at which explosive gases will combust in the presence of oxygen and the catalyst. The temperature of the bead is elevated by heat released by the combustion of the explosive gas. The elevated temperature of the bead is reflected by the increased electrical resistance of the coil of platinum wire. Direct-reading instrumentation use this increased resistance to signal the presence of explosive gas. Hot bead sensors typically require about 10% oxygen concentration to operate.
Conventional monitoring systems utilize a “dilution orifice” to combine fresh air with the sample stream to provide ample oxygen for the combustible gas sensor to operate. In practice, a fitting with an orifice open to the atmosphere is placed in the sample-draw tubing near the gas detector. By design, the orifice has a restriction to air flow about equal to the restriction provided by the length of tubing and any filtration that may be in the sampling system. Often times the orifice is adjustable so that the ratio of sample to dilution by fresh air is 1:1, causing the indicated readings to be halved. The operator must mentally multiply readings as indicated by the gas detector by two to arrive at the true sample readings.
The first of three problems with the use of the dilution orifice is that the dilution ratio can change by unknown amounts as the instrument is being used, thus affecting the indicated readings. The dilution ratio will change as the effective restriction of the sample filter changes as the sample filter becomes soiled. Subtle differences between the ambient pressure in the vicinity of the dilution orifice inlet and the ambient pressure in the vicinity of the sample tube inlet will also change the dilution ratio. Significant pressure changes can occur due to process requirements, inerting, and effects of wind.
The second problem with the use of a dilution orifice is that the user must remember whether the dilution orifice is in use and that the actual readings are twice the indicated readings. The user may forget that the dilution orifice is in use, as it may need to be removed from time to time to make straight un-diluted readings.
The third problem with the use of a dilution orifice is that daily calibrations of the gas measuring equipment must be made with the actual length of sample tube and the actual sample filtration system. This can be burdensome since it is often inconvenient to perform calibrations at the worksite and inconvenient to dismantle the sampling system and bring it along with the instrument to an office or laboratory for calibration.
SUMMARY OF THE INVENTION
Briefly stated, the invention in a preferred form is a monitoring system which comprises sample, dilution, and test subsystems. The sample subsystem includes a gas probe assembly defining a first end of a sample line and disposed in an area or adjacent to an object to be monitored. A sample pump mounted in the sample line provides a positive motive force for drawing a sample. A first sample pressure detector senses the pressure in the sample line intermediate the sample pump and the second end and provides a first sample pressure signal which is proportional to the sample flow rate. The dilution subsystem includes a dilution line having a first end vented to atmosphere. A dilution pump mounted in the dilution line provides a positive motive force for drawing the dilution air. A dilution pressure detector senses the pressure in the dilution line intermediate the dilution pump and the second end and provides a dilution pressure signal which is proportional to the dilution flow rate. The test subsystem includes a test line having a first end in fluid communication with the second ends of the sample and dilution lines. At least one gas sensor senses the presence of a gas in the test line and provides a gas signal proportional to the level of sensed gas in the test line.
Preferably, the sample subsystem also includes a second sample pressure detector for sensing the pressure in the sample line intermediate the first end and the sample pump. A particulate and hydrophobic filter mounted in the sample line intermediate the first en
Goossens Arnaud
Pawlyk Michael A.
Saubestre Paul
Whynall Jeffrey M.
Alix Yale & Ristas, LLP
Bacou USA Safety, Inc.
Larkin Daniel S.
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