Toxic gases, such as Hydrogen Sulfide and Sulfur Dioxide, can injure and kill quickly. Therefore, mitigation of these hazards must be fast and sure. The United States Occupational Safety and Health Administration (OSHA) and other authorities take detection of these gases very seriously.
The American National Standards Institute/ American Petroleum Institute (ANSI/API) and ISA industry standards form much of the basis of the OSHA criteria for proper personnel protection. In fact, API (the United States governing body of offshore safety) requires that all toxic gas detection devices conform to the standard ISA92.0.01. As you design and update safety plans to protect offshore personnel, consider the requirement and the standard.
The ANSI/API and ISA standards establish user criteria for performance, installation, operation, and maintenance of toxic gas detectors. This article provides general guidance for how to employ the standards to device performance criteria and proper installation. The goal is to lay groundwork for a smooth approval process and to promote improved safety.
The standard ISA92.0.01 is a widely accepted standard for toxic gas detection. It defines performance requirements for toxic-gas detection instruments. The standards are in place, in part, to avoid the use of fast-responding detectors that are not accurate, accurate detectors that are not fast, or detectors that drift excessively over time resulting in a loss of sensitivity or false alarm.
From the ISA standard1: “2.1 This Standard addresses the details of construction, performance, and testing of portable, mobile, and stationary electrical gas-detection instruments operating at ambient temperatures and pressures used to provide a warning of the presence of toxic gases in air. Instruments used to detect flammable (explosive) concentrations of toxic gas(es) when the LEL exceeds the TLV are excluded.”
The table below is a summary of sections of the ISA standard. As device evaluation is performed for safety projects, these requirements should be strongly adhered to.
The ISA standard also demands a certain measure of repeatability. Repeatability compares the expected sensor performance with the actual sensor performance. Specifically, repeatability is the percent error between the benchmark calibration value and the reading generated by a second application of calibration gas. The readings are taken over the same temperature, humidity, flow rates, and exposure times. The reading demonstrates the ability of the detector to recover after sensing the toxic gas.
One of the best ways to know if a product is performing at or beyond the standard requirements is to examine the third-party test results. If a manufacturer’s products have undergone testing by an independent testing authority to the ISA92.0.01 standard, results are documented and verifiable.
Proper Device Installation
You can choose the best available performance-tested technology, but if your gas detectors are too few or incorrectly placed, they might never see the hazards and will not provide effective protection. To assist with protection and promote compliance with the requirements2 for operations, the United States Minerals Management Service has created guidelines (NTL No. 98-16) that are helpful across industries where H2S is a risk.
The guidelines state “one sensor per 400 square feet of deck area and a sensor within 10 feet of equipment where atmospheric concentrations of H2S could reach 20 ppm.” The guidelines go on to suggest conducting a design analysis that includes dispersion modeling to determine if a more effective or a more efficient placement of sensors exists.
The guidelines also cover the topic of sensor calibration and specify that a sensor tolerance of 2 ppm or 10 percent during a functional test is acceptable. But if a sensor is used with a higher test tolerance, the activation point must be adjusted so that the sensor alarm will activate at no higher than 22 ppm H2S atmospheric concentrations.
Additional thought is required beyond the guidelines, however. How many gas detectors is enough and where should they sit? Experience, job-site analysis, and standards recommendations help determine the answers.
Consider the following factors for every installation:
- Because hydrogen sulfide is a highly toxic gas, think about where people will have initial contact with it. That area is a prime location for an H2S gas detector. Consider vapor density. Hydrogen sulfide is slightly heavier than clean air. It tends to settle at the lowest available points, unless it is heated, mixed with lighterthan- air gases, or prevented from settling by air move ment.
- How rapidly will the hydrogen sulfide gas diffuse into the air? Select a location for the detector as close as practical to an anticipated source.
- Think also about ventilation characteristics of the immediate area. Due to air movement, the gas might accumulate more heavily in one area than another. Detectors should be placed where concentrated accumulation of hydrogen sulfide gas is anticipated. Also, consider the fact that some ven tilation systems do not operate continuously, and there fore areas with poor circulation should be evaluated for toxic gas accumulation.
- Place the detector away from potential sources of con tamination.
- Prevent the buildup of contaminants on the filter.
- Make sure the detector is accessible for testing and cali bration.
- Avoid exposure to excessive heat or vibration, which can cause premature failure of electronic devices.
Smoother Project and Safer Conditions
Designing an effective toxic gas detection plan represents a critical part of a full life-safety solution. Ensuring that a performance-tested detector is used in proper installation conditions means your project will move more smoothly. And most importantly, the likelihood of danger to personnel through a safety incident will be reduced.