Coping with Confined Spaces

March 1, 2008
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For many individuals, the commute to and from work is the most dangerous part of their workday. But people who enter and work in confined spaces face deadly occupational hazards every day.

Confined space hazards are often imperceptible, and conditions in a confined space can change quickly. To protect these workers, comprehensive training before entering the space — and constant vigilance once inside — are imperative.

Know the dangers
Confined space hazards include mechanical equipment with moving parts, engulfment and slips and falls. But atmospheric hazards are the number one cause of fatalities in confined spaces. Most confined space accidents can be attributed to ignorance of the hazard potential.

In July 2007, a dairy farmer in Bridgewater, Virginia, was attempting to transfer manure from a small pit to a larger one when the pipe transferring the manure became clogged. The farmer climbed into the pit to unclog the blockage. “It was probably something he had done a hundred times,” a friend said. “There was gas in there and he immediately succumbed.” Emergency workers who responded to the scene speculate that a farmhand climbed in to rescue his employer and he too collapsed. The farmer’s wife and two daughters also perished in would-be rescue attempts.

Since most atmospheric hazards are imperceptible to human senses, it is necessary to assess the confined space atmosphere with an appropriate gas detector. Assume that the least favorable conditions exist during every entry.

Be aware of oxygen depletion or enrichment, toxic/poisonous and combustible gases. These conditions may be present prior to entry, or they can arise suddenly any time during occupation of the space. Often, activity in or near a confined space can contribute to changes in atmospheric conditions, thus the need for continuous personal monitoring.

Assess the conditions
If possible, avoid entering confined spaces. If entry is unavoidable, assess the following prior to any entry: access and egress routes; restricted areas within the space; potential atmospheric, physical and mechanical hazards; the work to be performed; the number of people who will be involved; ventilation requirements; initial interrogation and continuous monitoring of the atmosphere; escape and rescue plans.

Use an appropriate gas detector to monitor the environment. A basic understanding of the gases that could be encountered and the hazards they present is essential. Most entries involve interrogating the atmosphere for oxygen concentration, toxic contaminants and combustible gases. The most common configuration in a portable gas detector used for routine confined space entry includes oxygen, combustible (%LEL), hydrogen sulphide and carbon monoxide sensors.

But not all applications are the same, and the types of sensors selected should reflect the known and potential atmospheric hazards associated with the confined space. For broad-range detection of toxic contaminants, add a photoionization (PID) detector or a metal oxide semiconductor sensor (MOS). For detection of carbon dioxide or combustible gases in anaerobic environments, add an appropriate non-dispersive infrared sensor. Many detectors available today offer a wide selection of sensor possibilities.

Get a representative sample
All gases have a vapor density, and when compared to normal air, some gases are heavier, some are lighter and some have a comparable density. Based on vapor density, gases in a confined space will stratify, so it’s important to obtain a representative sample at various levels prior to entry. Sampling at the ceiling, mid-height and bottom of a confined space must be done before an “all clear” for entry can be declared.

The most primitive method still in use for obtaining a representative air sample from a confined space is actually lowering a portable detector into the space on a rope, retrieving it and checking peak readings. But detectors can be physically damaged if they swing against walls, hit a ladder wrung or come into contact with water or muck at the bottom.

Remote sample draw systems are more commonly used for obtaining “pick hole” samples before opening a confined space cover and for sampling the atmosphere at a specific point or level. Two types of remote sample draw systems are generally available, motorized pump and manual hand aspirated squeeze bulb. The motorized pump provides an easy method for drawing a sample from a remote location. Motorized pumps can be integral or external to the portable safety gas detection instrument.

In order for an instrument to accurately determine the concentration of oxygen and other contaminant gases that may be present in a confined space, the sample must first reach the sensors and enough time must be allowed for the sensors to fully stabilize their readings. Be sure to follow the manufacturer’s instructions. Any failure in the sample draw system — such as leakage, pump failure, improper assembly or absorbance of contaminants in the system being used — can lead to dangerously inaccurate readings. Since most failure modes will produce readings lower than actual concentrations, proper testing of the sample draw system is critical.

Inside the space
Once a confined space has been declared safe and workers enter, continuous monitoring of the atmosphere is essential. Ideally, all confined space workers should have a personal multi-gas detector as part of their everyday equipment. Standard four-gas detectors are extremely compact and lightweight. The smaller the detector, the less likely employees will oppose donning yet another piece of protective equipment. The ideal confined space gas detection package would include an appropriate multi-gas detector with either a built-in or external motorized pump for initial interrogation and sentry monitoring, and individual small, lightweight diffusion multi-gas detectors for each employee.

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