On the Lookout
Proper use and maintenance of permanent and portable combustible gas detection equipment can save lives and reduce costs
- By Paul W. Schmitt
- Sep 01, 2005
There are two standards that apply to municipal water and wastewater treatment plants: NFPA 820 Standards for Fire Protection in Wastewater Treatment and Collection Facilities, 2003, and OSHA 1910.148 Appendix E: Sewer System Entry. While both standards apply, they address different facets of the treatment facility.
The National Fire Protection Association (NFPA) standard concerns itself with fire and explosive hazards that are inherent within the treatment facility. The Occupational Health and Safety Act (OSHA) regulation applies to all confined space entry by personnel within the facility. Compliance with each standard requires specialized equipment. For compliance with the NFPA standard, permanent fire and gas detectors are employed; for compliance with the OSHA regulation, portable instruments, self-contained breathing apparatuses, and respirators are used. The equipment selected to meet these standards must be carefully designed and properly installed. Improper design or installation may have potentially hazardous consequences.
Explosive Gas Hazards
The NFPA 820 standard does not explicitly state the nature of explosive hazards in wastewater treatment facilities. There are, however, normally only two primary gases of concern: methane and petroleum vapor. Both of these gases are explosive in relatively small concentrations in air. Methane becomes explosive at 5 percent by volume, and the explosive concentration of petroleum vapor varies depending on the particular petroleum product present. For example, gasoline becomes explosive at 1.4 percent by volume while kerosene is explosive at 0.7 percent by volume.
Not only are these gases explosive, but each gas has a particular vapor density relative to air. Gasoline and kerosene are heavier than air, while methane is lighter than air. The vapor densities of these gases require different locations for the gas sensors in order to ensure effective detection. When monitoring methane, the gas sensor should be mounted as high as possible, whereas for most petroleum vapors, mounting the sensor as low as possible allows the earliest possible detection.
Unfortunately, due to potential flooding or poor accessibility, it may not be possible to mount sensors in the ideal location. In these instances, a sample draw pump can be used to bring the gas to the sensor, which allows it to be located in a protected or more accessible area.
Specific Areas for Monitoring Gas
There are many areas within a wastewater treatment facility where gas buildup and explosive conditions may exist. Enclosed pumping stations, flow equalization tanks, coarse and fine screening rooms, grit rooms, and sludge-blending rooms and holding tanks are a few of the areas where gas detection is employed. Other areas include enclosed anaerobic digesters, digester control buildings, processor rooms, storage tanks, and any underground piping or tunnels for natural-gas or sludge-gas piping.
Wet and dry wells and lift stations also require gas detection, but they present unique challenges. These areas may contain not only explosive gases but also hydrogen sulfide. Hydrogen sulfide, which has the odor of rotten eggs, is a very toxic and corrosive gas that may require monitoring as well. Wet and dry wells and lift stations may also be subject to periodic flooding. Some gas detectors are damaged when submerged in water, so the location of sensors in these areas must be carefully considered.
If personnel will be entering a wet well, the oxygen concentration should be monitored. If the wet well is a confined space as defined by OSHA (see sidebar: "What is a Confined Space?"), then oxygen monitoring is required. It is conceivable that, within a wet well or lift station, all three gases: combustible gas, hydrogen sulfide, and oxygen, may need to be monitored. Fortunately, there are pre-engineered sample-draw systems available that can perform this monitoring combination.
Benefits of Monitoring
Worker Safety. The most important benefit of monitoring for oxygen and toxic and explosive gases is worker safety. Sometimes insurance carriers will even require gas detection in order to mitigate their exposure to personal injury or wrongful death lawsuits.
Facility Protection. Besides the obvious benefit of increased worker safety, there are also other benefits of gas monitoring. A methane explosion can damage and destroy physical property and interrupt plant service, forcing end-users to find alternate ways to dispose of waste. Hydrogen sulfide will corrode any exposed metal, concrete, or electrical equipment, so monitoring for this gas to mitigate its presence will pay dividends in longer equipment life.
Additionally, many treatment facilities were built in areas that were previously isolated but are now surrounded by residential developments. Gas monitoring helps ensure that the facility is a "good neighbor" by preventing obnoxious and explosive gases from migrating out of the facility toward homes.
Some municipalities that handle wastewater treatment from mostly industrial customers monitor for various solvents and petroleum vapors. This is done to assure compliance with applicable local regulations and to protect the digestion process in addition to monitoring for an explosive atmosphere.
Financial Implications of Gas Monitoring
Initial System Cost. There are two elements that define the initial cost of a gas monitoring system: 1) gas detector and hardware cost, and 2) installation expenditures.
When designing and procuring a gas detection system many engineers minimize the hardware cost, not realizing that the installation cost of that equipment may put them over budget. With properly chosen quality equipment and system design, the installation cost can be kept to a minimum. There are many design criteria that need to be considered when specifying and installing oxygen and toxic and combustible gas detection equipment. These include:
- Possible flooding of the area to be monitored
- Wiring between the sensor and its transmitter
- Alarm signaling
- Access for calibration of the gas detection equipment
- Alarm event procedures
- Sensor placement
- Area classification
- Personnel training
Much of today's gas detection equipment can connect directly to an existing controller or data acquisition system, eliminating the need for an intermediate control panel. This not only saves additional hardware costs, but it reduces wiring and installation costs as well.
Maintenance and Ownership Costs. After the equipment is installed, maintenance is required to keep it functional. There are two sources of costs once the equipment is installed: 1) checking/calibration of the system, and 2) replacement of exhausted sensor elements.
By far, the largest and most costly aspect of gas detection is the periodic checking and calibration of the equipment. All gas-detection equipment must be challenged, and its operation verified, on a routine basis. Failure to perform this routine confirmation will compromise the integrity of the system's safety aspects.
Checking and calibrating are not difficult. Many times it is forgotten because of more pressing process issues that are facing the maintenance staff. Calibration kits are available that contain certified test gases that will challenge the sensor to see if its readings are proper. If they are within an acceptable range, then a full calibration is not required. Many operators assume that a full calibration is always needed, when a simple gas check (or "bump test") is all they need. (see sidebar: "Bump Tests vs. Full Calibrations")
Cylinders of certified test gas are relatively inexpensive, and challenging of the gas sensors should be done as often as necessary to ensure complete confidence in the accuracy of the unit.
System Configuration and Set Up
Many gas detection systems are calibrated and configured at the factory, so the user simply needs to mount the unit, apply power, and perform an initial calibration. But there may be other parameters that the user wishes to implement, such as additional alarm levels, variations on alarm sounds, integration to existing PLC (programmable logic control) systems, or unique calibration requirements.
The most important set-up operation is the initial calibration performed after applying power to the unit. Even though the sensor may have been calibrated at the factory, it is important to calibrate the sensor in its operating environment. The factory calibration was done in an ideal and dry environment that may not be representative of the installation area. Also, when initially calibrated by the user, some sensors collect their initial operating parameters to determine their end-of-life set point. Some manufacturers use "time deployed" as a measure of end-of-life, but with modern electronics it is easy to compare initial sensor parameters and other operating parameters to determine true sensor end-of-life.
What is a Confined Space?
Standard 1910.146 of 29 CFR Subpart J of the Occupational Safety and Health Standards defines a "confined space" as a space that:
- Is large enough and so configured that an employee can bodily enter and perform assigned work;
- Has limited or restricted means for entry or exit (for example, tanks, vessels, silos, storage bins, hoppers, vaults and pits are spaces that may have limited means of entry); and
- Is not designed for continuous employee occupancy.
Bump Tests vs. Full Calibrations
There are two methods of verifying gas sensor calibration: a functional check or "bump test," and a full calibration. Each is appropriate under certain conditions.
A bump test is a means of verifying calibration by exposing the sensor to a known concentration of test gas. The sensor reading is then compared to the actual quantity of gas present (as indicated on the cylinder). If the sensor's response is within an acceptable range of the actual concentration, then its calibration is verified.
If the bump test results are not within the acceptable range, then a full calibration must be performed. A full calibration is the adjustment of the sensor's reading to coincide with a known concentration (generally a certified standard) of test gas. In most cases, a full calibration is only necessary when an instrument does not pass the bump test or after it has been serviced.
This article originally appeared in the 09/01/2005 issue of Environmental Protection.
Paul W. Schmitt is a senior design engineer for Mine Safety Appliances Co. He can be contacted at (412) 736-5941.