October 29, 2012

Considerations for Placing Fixed-Point Combustible Gas Sensors

Hi all, this is Geoff Wilson, product manager with Net Safety Monitoring. At the Emerson Exchange event this year in Anaheim, CA, I presented a talk titled, “Fixed Gas and Flame Detection Best Practices – Who, What, and Where.” In today’s blog post I am going to expand on one of the topics discussed in that talk – considerations when locating your fixed point combustible gas sensors.

This material is primarily intended for the installation of catalytic bead sensors; however, most considerations outlined can also be applied to infrared detection technologies. If you are using other sensing technologies, consult the relevant manual for more specific information about your particular sensor. This information is included for informational purposes only and is intended to be used only as a guide for the most important considerations and is not an exhaustive review. It is not a substitute for common sense and solid safety engineering and judgment. Because there are so many variables that must be taken into account, there are no hard and fast rules.

A sensor is a point monitor, which means it can only monitor gas at a single point. It does not cover an area. If gas from a leak does not reach the sensor, it will not be detected. The selection of the number of sensors to install and the location for each sensor is one of the most critical factors to be considered for overall system effectiveness. Placing the sensor in the wrong location will defeat the purpose for which it is intended. One of the most important concepts to keep in mind is that the sensor should be placed between the potential leak and the ignition source for combustibles, or between the leak and the likely location of people for toxic gases. For complicated installations, it is good practice to prepare drawings showing all potential leak sites. The locations should be graded in terms of their potential for harm.

Vapor Density of Gases to Be Monitored
Sensors should be located near the ground for gases or vapors which are heavier than air. You should consider not installing them closer than 18 inches above the ground so they will not be as likely to collect mud and water. To detect heavy gases, some companies do not allow sensors to be installed higher than 36 inches. Sensors should be located near the ceiling or roof, or an exit fan to detect gases that are lighter than air. You cannot rely, however, on heavier or lighter gases always behaving in a predictable manner. Even inside a building, air currents can create anomalies. Be especially aware of areas which could become potential gas pockets.

Dispersion of Gas/Vapor
Generally, sensors should be located as close as practical to any potential leak source. Liquids of low volatility, in particular, may require the location of the sensor in the immediate area of the vapor source. Liquids with high flash points or slow rates of dispersion take a long time to produce readings if the sensors are located any distance from the spill or leak.

Air Currents
Locate the sensor where prevailing air currents would be likely to contain the maximum concentration of the gas being monitored. Consider the possibility of changes in wind direction at different times of the day or during different seasons. Your local weather information center should have data on the direction and velocity of prevailing winds during the yearly cycle.

Air Speed
Catalytic bead sensors operate based on a diffusion principle. If the air speed past the sensor is too great, it may disrupt the normal flow of air in and out of the housing. With a catalytic bead sensor, it may also cause turbulence inside the sensor housing, causing the reference and active sensing elements to be cooled differentially. You should be concerned if the normal air flow past the sensor is greater than 5 miles per hour, 7.3 feet per second or 437 feet per minute. If you are placing the sensor outside where it will be subjected to occasional strong winds, these winds could cause a false alarm. The Net Safety Hydrophobic Filter has been shown to aid in reducing these false alarms.

Even small structures, such as piping and equipment, between the possible leak source and the proposed sensor location, can change the normal flow of air. All obstructions should be evaluated fully.

Where coverage of an entire area is desired, pay particular attention to the grade, floor and operating levels, and air flow from heating/ventilation systems. Pay attention not only to possible leak sites but to all penetrations and other points of entry where gas can be introduced into a closed area. Some companies have designated 400 square feet as the maximum area to be covered by one sensor. This number may not be appropriate for you. For installations where coverage of specific equipment is desired, consider placing sensors no closer than 12 inches and no more than 5 feet away from the anticipated leak source. Sensitivity of the sensor can be controlled, to a degree, by moving the sensor closer to the source for more sensitivity. A sensor between two vessels that are close together can often serve to monitor both. For locations in fresh air ducts or in or near exhaust systems, possible areas of concern are high or variable air flow, possible sensor poisons or contaminants, and high levels of humidity or temperature.

Poisons affect a catalytic bead sensors’ response & longevity. Poisons can coat the active bead of the sensor and prevent the oxidation process from happening, which stops or reduces the sensitivity of exposure to combustible gas vapors. The impact of poisoning depends on the poison type, concentration, and time of exposure. Known catalytic bead sensor poisons are shown, but not limited to, in the list below:

  • silicone oils, greases, resins (RTV adhesive)
  • halogens (halon, chlorine, fluorine, bromine, Freon)
  • phosphate esters
  • tetraethyl lead
  • trichlorobenzene
  • acid and PVC vapors
  • other corrosive materials

Moisture Protection
Sensors should be installed where they are protected from immersion or direct contact with water, i.e., where the floor is hosed down, steam cleaning is done, or water levels/table rises in a drainage ditch. In the case of water contact, steam or hosing, or blowing rain, the available Hydrophobic Filter will help protect the sensor. In the case of rising water levels, the water protection adapter will protect the sensor from damage for moderate rises in water level.

Temperature Limitations
All sensors and electronics have ambient temperature limitations. The installation of the sensor must be within the operating range of each. If the sensors operate outside of their published operating range, the overall life span of the sensor could be degraded significantly.

Vibration can be damaging to the sensor and may void the warranty. Anchor the sensor to a wall or firm base rather than to a vibration source such as a motor housing. A length of flexible conduit used between the sensor and the pipe conduit can also give vibration protection.

Sensor wiring should be separate from other high current AC or DC wiring. If EMI or RFI is a problem, the transmitter has an earth ground screw. This is the only earth ground on the system. If it is used, it should be well grounded and separate from other grounds. Connectors to the transmitter should be tight and free of corrosion. High resistance wire splices can cause an unstable zero. If possible, splices should be avoided.

Good instrument wiring practices are essential to good sensor installation. Review the wire size and allowable distances described in the owner’s manual before making a final location commitment. Before deciding how far to locate the sensor from the transmitter, review the allowable distance.

Sensors that need periodic calibration should have their transmitters installed in a location permitting reasonable access. A calibration line, located next to the transmitter, should be run to the sensor to aid in the calibration process. Consideration needs to be given to the length and size of the calibration line as the flow of calibration gas may need to be increased.

Sensor Orientation
Some sensors are orientation sensitive. The catalytic bead sensor must be installed in a vertical position, with the sensing element pointed towards the floor. If the sensor is not mounted this way, the sensor may not work properly. In addition, the sensing element may get wet and fail, and the flame arrestor will become more easily clogged.

Conduit, Seals, and Drain Plugs
It is mandatory that good conduit installation practices are observed. To comply with the requirements for equipment installed in Class I, Division 1 areas, an EYS seal is required within a specified distance of a junction box. Once the wiring has been pulled, the seals are potted. This prevents a flame front from travelling down the conduit when the cover is removed from the conduit box.

Dust Protection
Sensor dust covers should be used if sensors are mounted in dirty or dusty environments.

Local, state and federal codes for electrical installation and regulations should be followed.

Other Exposures
Besides protecting workers in an environment, consideration should be given to other areas which may need protection, such as roads, housing, adjacent plants, public buildings, community activities, etc. In addition, you may wish to consider protection from hazards which might be caused by adjacent plants.

October 12, 2012

Measuring the Quality of Landfill Gas with Gas Chromatography

Hi everyone. This is Michael Gaura, product marketing manager with the Gas Group. I recently wrote an article for Waste Age Magazine about extracting valuable gas from landfills and people seem interested in the topic. I thought I’d share some of this data with you here.

To utilize landfill gas (LFG) in plant operations such as fueling boilers, power generators and turbines, or to sell it to a natural gas network, the landfill operator must understand the components present in the gas stream as well as their concentrations. A gas chromatograph makes this analysis possible. A typical gas chromatograph used in landfill operations is shown in Fig 1.

Figure 1

Methane and carbon dioxide (CO2) are often the two components with the highest concentration levels found in LFG applications. Oxygen, nitrogen and water are typically found also at percent levels. Other components, such as ammonia, silanes and hydrogen sulfide (H2S) also may be present at concentrations of several hundred ppm.

Knowing the concentrations of each of the typical components determines the appropriate method of gas treatment, blending or usage. The concentrations also may be required as part of permitting, as defined by a local air quality board or environmental agency.

Some general guidelines for landfill gas usage include:

  • Landfill gases with high-energy content and low impurities may be sold into a natural gas network.
  • Equipment fueled by LFG meeting a defined range of energy values (CV or BTU) will ensure efficient and reliable operation. Excursions outside of these defined ranges will result in equipment damage or reduced performance.
  • If the non-methane components of the LFG are too high, it may be too “lean” for proper engine performance, requiring the addition of natural gas or another fuel source.
  • If the H2S content is higher than the equipment rating, pipeline specification or regulatory agency-permissible level, it will need to be removed. The only way to safely determine the continuous H2S level is to use an on-line analyzer, as manual testing may miss variances or present unsafe concentrations for human exposure.
  • Common H2S removal methods include water washing, amine absorption, pressure swing adsorption (PSA) or turbo expander usage. Regardless of the system used, monitoring both the inlet “sour” gas and the outlet “sweet” gas is required to ensure efficient operation of the H2S removal process. A sudden change, or even a gradual increase, of the H2S concentration in the sweet gas, indicates that the removal system requires inspection or maintenance. A process gas chromatograph that monitors the H2S concentration at the H2S removal outlet can identify off-specification gas early, preventing contamination of an entire batch.
  • High oxygen (O2) levels can indicate that a serious operational safety risk is present or may indicate an issue with the landfill’s degradation process.

To read the entire article, please click HERE.

Does your plant have the potential to reuse any of these valuable gases? If you have any questions, let me know.